Mauricio Galleguillos

Frédéric Jacob

Laurent Prévot

Carlos Faúndez

Aline Bsaibes

Estimation of actual evapotranspiration over a rainfed vineyard using a 1-D water transfer model: A case study within a Mediterranean watershed

Keywords: Heterogeneous landscape, HYDRUS-1D, Vadose zone Water table

The current study aims to evaluate the capabilities of soil water balance modeling to estimate ET for very different conditions of rainfed grapevine water status, within a vineyard landscape that depicts heterogeneities in canopy, soil and water table conditions. We calibrated the HYDRUS-1D model against measurements of the soil moisture profile within seven contrasted sites, we validated HYDRUS-1D simulations against ET estimates derived from eddy covariance (EC) measurements within two contrasted sites, and we analyzed the temporal dynamics of the HYDRUS-1D ET simulations throughout almost two growth cycles for the seven sites. The calibration of HYDRUS-1D was correctly achieved, with a relative RMSE of 20% on average. Validation of HYDRUS-1D simulations against EC measurements was satisfactory, with RMSE values of about 40 W m -2 at the hourly timescale and 0.5 mm d -1 at the daily timescale. HYDRUS-1D was able to provide consistent time series of ET within the seven contrasted sites and throughout the two growth cycles. We conclude that HYDRUS-1D simulations can be used as an alternative to EC measurements within rainfed vineyards, to alleviate experimental efforts for device cost and maintenance. Further, HYDRUS-1D simulations can be used for characterizing spatial variabilities and temporal dynamics, assessing impact of pedological conditions and land use on ET, or validating remote sensing retrievals over regional extents.

Introduction

For ages, grapevine (Vitis Vinifera L.) has been an important rainfed monoculture in Mediterranean regions, occupying large areas that have mostly extended over France, Italy and Spain (OIV, 2007). Mediterranean grapevine monoculture has direct implications in terms of land use, jobs and incomes [START_REF] Lereboullet | Socio-ecological adaptation to climate change: a comparative case study from the Mediterranean wine industry in France and Australia Agriculture[END_REF]. Nowadays, water use in Mediterranean regions is characterized by increasing competition between different purposes, which can affect wine growing through groundwater availability. Meanwhile, the competition may be sharpened under the influence of global warming [START_REF] Tóth | Future of wine grape growing regions in Europe[END_REF].

The grapevine organoleptic properties depend upon root zone soil moisture. Under unrestrictive water availability, grapevine can generate excessive vegetative growth that competes with assimilate accumulation within grapes. This process induces an excess of canopy vigor, generating in turn changes in the fruit zone microclimate with an increase in humidity and disease occurrence [START_REF] Smart | Sunlight interception by vineyards[END_REF][START_REF] Zahavi | Effect of grapevine training systems on development on powdery mildew[END_REF][START_REF] Pellegrino | A model-based diagnosis tool to evaluate the water stress experienced by grapevine in field sites[END_REF][START_REF] Liang | Regional characteristics of anthocyanin and flavonol compounds from grapes of four Vitis Vinifera varieties in five wine regions of China[END_REF]. Conversely, grapevine can experience severe water stress for long periods in summer, when evaporative demand is maximal and rainfall is negligible. In such situations, root zone moisture is strongly reduced, and assimilate production decreases with grape yield and quality [START_REF] Pellegrino | Towards a simple indicator of water stress in grapevine (Vitis Vinifera l.) based on the differential sensitivities of 18 vegetative growth components[END_REF][START_REF] Griesser | Severe drought stress is affecting selected primary metabolites, polyphenols, and volatile metabolites in grapevine leaves (Vitis Vinifera cv. Pinot noir)[END_REF]. These processes underline the necessity to quantify grapevine water status throughout the growing cycle, in order to improve grape production and water resource management within vineyard landscapes.

Quantifying grapevine water status requires knowledge of actual evapotranspiration (ET) that directly drives both surface soil moisture through soil evaporation and root zone soil moisture through plant transpiration [START_REF] Montes | A three-source SVAT modeling of evaporation: application to the seasonal dynamics of a grassed vineyard[END_REF]. However, estimating vineyard evapotranspiration is challenging because of the architectures of aboveground canopy and subsurface root system that are typified by trellis and row structures. Thus, it is necessary to discriminate soil evaporation and plant transpiration [START_REF] Heilman | Soil and canopy energy balances in a west Texas vineyard[END_REF]Kool et al., 2014a), to account for shadow effects in accordance to sun position and row orientation [START_REF] Riou | Consommation d'eau de la vigne en conditions hydriques non limitantes[END_REF][START_REF] Heilman | Effects of trellising on the energy balance of a vineyard[END_REF][START_REF] Zarco-Tejada | Assessing vineyard condition with hyperspectral indices: leaf and canopy reflectance simulation in a row-structured discontinuous canopy[END_REF], and to pay attention to the combined effect of wind direction and canopy geometry [START_REF] Heilman | Soil and canopy energy balances in a west Texas vineyard[END_REF][START_REF] Zhang | Comparison of three evapotranspiration models to Bowen ratio-energy balance method for a vineyard in an arid desert region of northwest China[END_REF][START_REF] Ortega-Farias | Parameterization of a two layer model for estimating vineyard evapotranspiration using meteorological measurements[END_REF].

Nowadays, ET measurements over land surfaces commonly rely on the eddy covariance (EC) method [START_REF] Zitouna-Chebbi | Assessing the consistency of eddy covariance measurements under conditions of sloping topography within a hilly agricultural catchment[END_REF], 2015). The EC method has been successfully used over irrigated and rainfed vineyards [START_REF] Li | Vineyard evaporative fraction based on eddy covariance in an arid desert region of northwest china[END_REF][START_REF] Li | Variability in energy partitioning and resistance parameters for a vineyard in northwest china[END_REF]Galleguillos et al., 2011aGalleguillos et al., , 2011b;;Kool et al., 2014a;[START_REF] Montes | Comparing Landsat-7 ETM+ and ASTER Imageries to Estimate Daily Evapotranspiration Within a Mediterranean Vineyard Watershed[END_REF] while some studies relied on other micro-meteorological methods such as the Bowen ratio-energy balance [START_REF] Heilman | Soil and canopy energy balances in a west Texas vineyard[END_REF][START_REF] Spano | Estimating sensible and latent heat flux densities from grapevine canopies using surface renewal[END_REF][START_REF] Yunusa | Evapotranspiration components from energy balance: sapflow and microlysimetry techniques for an irrigated vineyard in inland Australia[END_REF][START_REF] Zhang | Comparison of three evapotranspiration models to Bowen ratio-energy balance method for a vineyard in an arid desert region of northwest China[END_REF]. Overall, most of the existing studies involved longterm experiments, and provided important outcomes for better understanding the temporal dynamics of ET throughout the grapevine growth cycle. However, the use of micrometeorological methods is limited by experimental efforts in terms of device cost and maintenance, especially when addressing heterogeneous landscapes that require networks of measurement plots across areas spreading over several square-kilometers.

Another way to estimate ET consists of using mechanistic models based on the resolution of mass and energy balances, along with in-situ measurements of ET drivers such as micrometeorological variables or soil moisture (e.g., [START_REF] Verstraeten | Assessment of evapotranspiration and soil moisture content across different scales of observation[END_REF]Kool et al., 2014b). Existing studies for irrigated and rainfed vineyards relied on the Penman-Monteith formalism and its extensions to dual and multiple source modeling [START_REF] Sene | Parameterisations for energy transfers from a sparse vine crop[END_REF][START_REF] Ortega-Farias | Latent heat flux over cabernet sauvignon vineyard using the Shuttleworth and Wallace model[END_REF], 2010;[START_REF] Zhang | Comparison of three evapotranspiration models to Bowen ratio-energy balance method for a vineyard in an arid desert region of northwest China[END_REF], possibly including the coupling with a soil water balance to address the growth cycle [START_REF] Montes | A three-source SVAT modeling of evaporation: application to the seasonal dynamics of a grassed vineyard[END_REF]. These studies reported good performances for the proposed approaches, but some of them underlined the restricted applicability to other conditions, mainly because of changes in soil hydrodynamic and canopy aerodynamic properties. The proposed approaches either excluded the soil water balance or describe it very coarsely, without paying much attention to water fluxes within the unsaturated zone and especially to capillary rises caused by water table fluctuations. A first attempt to address water fluxes within the unsaturated zone was conducted by Kool et al. (2014a), but the study focused on soil evaporation in an arid area without water table fluctuation.

As compared to either direct measurements based on the EC method or to indirect measurements based on energy balance modeling, the use of water balance modeling is an alternative for the monitoring of ET within heterogeneous vineyards. First, it does not require elaborated information about aerodynamic properties. Second, it permits the consideration of water table fluctuations that strongly drive the functioning of grapevine within rainfed vineyards [START_REF] Guix-Hébrard | Influence of water table depths on the variation of grapevine water status at the landscape scale[END_REF]. Third, it relies on measurements of the soil moisture profile to calibrate model parameters related to soil properties, thus alleviating experimental efforts in terms of device cost and maintenance. Indeed, measurements of the soil moisture profile can be replicated across large spatial extents by using reliable techniques such as time domain reflectometry (TDR) or neutron probe (NP), as reviewed by [START_REF] Dobriyal | A review of the methods available for estimating soil moisture and its implications for water resource management[END_REF].

The purpose of the current study was to evaluate the capabilities of soil water balance modeling to estimate ET for contrasted conditions of grapevine water status, within a vineyard landscape that depicts heterogeneities in terms of canopy, soil and water table conditions. For this, we evaluated the calibration of a soil water balance model over in-situ measurements of the soil moisture profile, we validated the model simulations against independent EC measurements, and we analyzed the model simulations at hourly and daily timescales throughout the grapevine growth cycle. The paper is structured as follows. We first detail the materials and methods, including the study area and the in-situ measurements (Section 2), as well as the estimation of ET by means of EC measurements and of soil water balance modeling (Section 3). We second report the calibration results (Section 4.1), the validation results (Section 4.2), and the analysis of the temporal dynamics for simulated ET throughout the growth cycle (Section 4.3). We third discuss the results in the light of previous studies (Section 5), and we finally conclude on both the study outcomes and the future challenges.

Materials and data

We calibrated the soil water balance model against soil moisture measurements, and we analyzed the model simulations over seven sites that depicted a gradient in grapevine water status. We validated the soil water balance model against EC measurements for two sites amongst the seven, where the two sites depicted very contrasted grapevine water status. We present hereafter the data collection within these seven sites.

Study site: the Peyne watershed

The study took place within the Peyne watershed that is located in the Languedoc-Roussillon region of southern France (43.49 • N, 3.37 • E, 80 m asl). The area is mainly devoted to vine growing (70%) while cereals, alfalfa and scrublands constitute the main additional land uses. Annual rainfall ranges from 400 to 1300mm, distributed in autumn and spring with large inter-annual variability. Annual ET obtained with the Penman-Monteith method [START_REF] Allen | Crop Evapotranspiration. Guideline for Computing Crop Water Requirements[END_REF] is close to 1100mm [START_REF] Trambouze | Measurement and modelling of the transpiration of a Mediterranean vineyard[END_REF]. Over the period, averaged values of annual rainfall and annual ET were 634 mm and 1110 mm, respectively.

Vineyards are located over relatively flat terrains with a 4% slope on average, whereas 90% of these terrains have slopes less than 10%. Soils depict large heterogeneities in texture, depth and parent material, which leads to very different conditions in terms of soil moisture and rooting depth. Additionally, water table within the rooting zone depict a large range of conditions, including absent, transient or permanent water tables during the growth cycle. Overall, both soil and water table conditions induce very contrasted situations in terms of grapevine water status.

Experimental plots and calendar

We assessed the capabilities of soil water balance modeling to estimate ET over seven sites, features of which are listed in Table 1. The seven sites had similar trellis structures, with averaged values of 2.5 m, 1 m and 1.5 m for row spacing, max row width and canopy height, respectively. Sites 1-5 and 7 corresponded to one field, with size ranging from 0.03 to 0.09km 2 . Site 6 had a 0.15 km 2 size, and spread over nine fields including vineyards by 90% in surface area. The experiment lasted from August 2007 (middle of growing season) to October 2008 (end of harvest period).

Table 1. Characteristics of the seven sites selected within the Peyne watershed. The "Devices" column indicates the (1) number of locations within each site for measuring soil moisture profiles with NP and TDR devices, and (2) the collection of eddy covariance (EC) measurements when applicable. A soil was considered shallow when the underlying parental material was above a 2.5-m depth. Each location of NP data was equipped for piezometric measurements. Water table conditions are related to winter and spring seasons. An absent water table (respectively permanent water table) means a piezometric level below (respectively above) the 2.5-m depth throughout the year. A seasonal water table means a piezometric level above the 2.5-m depth during winter and spring.

Data collection and related devices

To constrain the soil water balance model on atmospheric forcing, hourly values of air temperature and relative humidity, wind speed, solar irradiance and rainfall were collected within Site 6 with a CIMEL ENERCO 400 station (CIMEL Electronics, Paris, France) by following meteorological standards apart from wind speed (measured at 2 m height rather than 10 m). These hourly meteorological data were further used to estimate short-grass reference evapotranspiration (ET 0 ) by means of the Penman approach for standard conditions [START_REF] Valiantzas | Simplified versions for the penman evaporation equation using routine weather data[END_REF].

To constrain the soil water balance model on water fluxes within the unsaturated zone, the seven sites were monitored bi-weekly and after each event of heavy rainfall for the soil moisture profiles and the water table level. Vegetation canopy structure was also monitored regularly. The data collection was conducted around solar noon, and it included three items. First, soil moisture profiles were sampled every 0.2 m to the 2.5-m depth with a Vectra 503-DR CPN neutron probe (NP) (Vectra, la Verrière, France), and completed for the upper 0.15 m using a Soil Moisture Equipment TRASE 6050 time domain reflectometry (TDR) sensor (Soil Moisture Equipment Corp., Santa Barbara, CA). The number of locations for collecting soil moisture profiles (Table 1) varied according to site size and soil heterogeneity derived from a pedological map. For each location, we beforehand characterized thickness and texture for soil horizons by using in-situ observations along with expert knowledge, and we ascertained soil surface properties using additional archive information from former studies in the same region [START_REF] Trambouze | Measurement and modelling of the transpiration of a Mediterranean vineyard[END_REF][START_REF] Bsaibes | Évaluation d'une approche multi-locale d'estimation spatiale de l'évapotranspiration[END_REF]. Second, the water table level was monitored to the 2.5-m depth with manual piezometric devices. The monitoring was continuous on Site 6 and regular on the other sites for which the two-week frequency observations were linearly interpolated. Third, vegetation structures were monitored for grapevine canopy structure (height, thickness and width), over one (Site 1-5 and 7) or more (Site 6) fields. Following [START_REF] Trambouze | Caractérisation Et éléments De Modélisation De l'évapotranspiration réelle De La Vigne à l'échelle De La Parcelle[END_REF], the monitoring was conducted in winter and summer, and next interpolated at the daily timescale. Row orientation and spacing were measured once.

To validate the soil water balance model simulations, EC devices were set up for direct ET measurements within two sites that differed much in canopy structure, soil hydrodynamic properties and water table dynamics. We set up a temporary flux station on Site 7, whereas a permanent flux tower was installed on Site 6 in the context of the environmental research observatory OMERE (a French acronym for the Mediterranean Observatory of Water and the Rural Environment, http://www.umr-lisah.fr/omere). To measure wind speed components and air temperature, each flux station was equipped with a R.M. Young 81000 3D sonic anemometer (R.M. Young Co., Traverse City, MI, USA). To measure air humidity, the flux station on Site 6 was equipped with a LiCor LI7500 fast hygrometer (LiCor Inc., Lincoln, NE, USA), and the flux station on Site7 was equipped with a Campbell KH2O fast hygrometer (Campbell Scientific Co., USA). For Site 6 (respectively Site 7), that included nine (respectively one) fields, the sensors were setup 6.2 m (respectively 2.5 m) above ground, and the acquisition frequency was set to 20 Hz to avoid spectral loss for wind speed values up to 6 m s -1 (respectively 3 m s -1 ).

All instruments were manufacturer-calibrated. The NP was calibrated by accounting for soil type and moisture. Calibration was performed against gravimetric soil moisture data at each depth of NP measurement (soil density was estimated using a Campbell DR 501 gamma probe, Campbell Scientific Co., USA), and the calibration residual error was 0.04 m 3 m -3 (15% relative).

Methods: estimating actual evapotranspiration (ET)

Direct estimation from eddy covariance measurements

For Site 6 and 7, sensible and latent heat fluxes were calculated at the hourly timescale from the EC measurements. The latter were preprocessed by applying the whole set of instrumental corrections proposed by the ECPACK version 2.5.20 library [START_REF] Van Dijk | The Principle of Surface Flux Physics: Theory, Practice and Description of the ECPACK Library[END_REF] along with the double rotation correction [START_REF] Kaimal | Atmospheric Boundary Layer Flows, Their Structure and Measurement[END_REF]. Fluxes were calculated along with tolerance intervals, about 20% and 12% for Sites 6 and 7, respectively. For energy balance closure, the sum of convective fluxes and available energy agreed within 80% for Site 7. Daily ET was finally calculated as the sum of hourly ET.

We evaluated the daytime footprints of the EC measurements using the approach proposed by [START_REF] Horst | Footprint estimation for scalar flux measurements in the atmospheric surface-layer[END_REF] for scalar fluxes. We calculated the footprints as ellipsoids located a few meters from which 90% of the convective fluxes originated. For site 7 that was a homogenous vineyard, the footprint extensions always remained within the field, and the EC station was located few meters far from the neutron probe (NP) access tube. For site 6 that included nine vineyard fields, a sensitivity analysis showed that the footprints had a median size of 0.12 km 2 and thus included the nine neutron probe access tubes [START_REF] Bsaibes | Évaluation d'une approche multi-locale d'estimation spatiale de l'évapotranspiration[END_REF].

Indirect estimation from HYDRUS-1D model simulations

For each of the seven sites within the Peyne watershed, ET was indirectly estimated by calibrating a soil water balance model over time series of soil moisture profiles measured with NP and TDR devices. We choose the HYDRUS-1D model [START_REF] Simunek | Recent developments and applications of the HYDRUS computer software packages[END_REF] as the soil water balance model, because it includes the state of the art for water and solute movements in the vadose zone. HYDRUS1D has been widely used for various agricultural studies, including water and nitrogen fluxes under different crops [START_REF] Ramos | Two-dimensional modeling of water and nitrogen fate from sweet sorghum irrigated with fresh and blended saline waters[END_REF][START_REF] Phogat | Evaluation of water movement and nitrate dynamics in a lysimeter planted with an orange tree[END_REF], water fluxes and wetting patterns under drip irrigation (Patel and Rajput,2008;Kandelous and Simunek,2010;[START_REF] Kandelous | Evaluation of subsurface drip irrigation design and management parameters for alfalfa[END_REF][START_REF] Arbat | Drip Irriwater: computer software to simulate soil-wetting patterns under surface drip irrigation[END_REF]Kool et al., 2014a), solute transports under saline irrigation [START_REF] Ramos | Field evaluation of a multicomponent solute transport model in soils irrigated with saline waters[END_REF], or water, salinity and nitrate dynamics under drip irrigation [START_REF] Phogat | Seasonal simulation of water, salinity and nitrate dynamics under drip irrigated mandarin (Citrus reticulata) and assessing management options for drainage and nitrate leaching[END_REF]. HYDRUS-1D includes several options for upper and lower boundary conditions, to address various conditions. Water fluxes and solute transport are solved using the Richards equation and the advective-dispersive equation, respectively.

For each of the seven sites within the Peyne watershed, HYDRUS-1D was calibrated over the soil moisture profiles measured between August 2007 and October 2008, by constraining the model simulation over a single period that spread from the beginning of March 2007 to the beginning of October 2008. We started the simulation period a few months before the experimental period so that the simulation process was numerically leveled off when calibrating HYDRUS-1D simulations with the measured soil moisture profiles. We detail hereafter the implementation of HYDRUS-1D, including the forcing and calibration strategies, as well as the obtaining of model simulations for soil water balance and ET at the hourly and daily timescale.

Water flux and domain discretization

HYDRUS-1D simulates water fluxes within the unsaturated zone by using the Richards equation in one dimension [START_REF] Richards | Capillary conduction of liquids through porous media[END_REF].

whereq is volumetric water content (m 3 m -3) , h is soil water potential also called pressure head (m), t is time (s), z is vertical ascending coordinate (m), K is unsaturated hydraulic conductivity (m s -1 ), and S(h) is water uptake by roots (m 3 m -3 s -1 ). For each horizon, retention curve q (h) and unsaturated hydraulic conductivity K (h) were characterized using the van Genuchten functions (van Genuchten, 1980):

where these functions depend upon residual (qr) and saturated (qs) soil moisture, saturated hydraulic conductivity Ks, effective water content Se, bubbling pressure a and pore-size distribution index n.

For each soil horizon, values of residual and saturated soil moisture were derived from minimum and maximum soil moisture values measured with the NP and TDR devices throughout the experiment. For topsoil (first horizon), the three other inputs of van Genuchten functions (Ks, a and n) were obtained from previous observations within the Peyne watershed [START_REF] Trambouze | Measurement and modelling of the transpiration of a Mediterranean vineyard[END_REF][START_REF] Bsaibes | Évaluation d'une approche multi-locale d'estimation spatiale de l'évapotranspiration[END_REF]. For deeper horizons, these three inputs were determined using the module of HYDRUS-1D inverse mode as a calibration procedure. The latter consists of minimizing, over the simulation period, the differences between simulated and measured profiles of soil moisture. The initial guess for the three inputs were derived either from literature values wherever possible [START_REF] Trambouze | Measurement and modelling of the transpiration of a Mediterranean vineyard[END_REF][START_REF] Bsaibes | Évaluation d'une approche multi-locale d'estimation spatiale de l'évapotranspiration[END_REF], or from the ROSETTA module of HYDRUS-1D otherwise.

We simulated the soil water balance over the soil portion between 0 and 2.5-m depth. We split this soil portion into a limited number of horizons in accordance with in situ observation and expert knowledge (between two and four horizons, including surface), and we next discretized the whole soil portion into 251 layers for numerical simulations.

Boundary conditions

For the upper boundary conditions, meteorological forcing included rainfall, soil potential evaporation Ep, and plant maximal transpiration Tv. Rainfall was measured by the meteorological station (Section 2.3). Tv was adjusted to potential transpiration, which was within the confidence interval reported by [START_REF] Trambouze | Measurement and modelling of the transpiration of a Mediterranean vineyard[END_REF] who suggested a 10% larger value for grapevine. Both Tv and Ep were derived from short-grass reference evapotranspiration (ET 0 ) using the Riou model designed for grapevine canopies [START_REF] Riou | Consommation d'eau de la vigne en conditions hydriques non limitantes[END_REF][START_REF] Bsaibes | Évaluation d'une approche multi-locale d'estimation spatiale de l'évapotranspiration[END_REF] and previously validated in Site 6 by [START_REF] Trambouze | Measurement and modelling of the transpiration of a Mediterranean vineyard[END_REF]. Riou model infers Tv as a fraction of ET 0 by assuming that the ratio Tv/ET 0 equals the ratio Rv of solar irradiance absorbed by grapevine leaves to that absorbed by the whole grapevine canopy:

where Rg is solar irradiance, r is grapevine canopy reflection coefficient over the solar domain, set to 0.2 by following [START_REF] Bsaibes | Évaluation d'une approche multi-locale d'estimation spatiale de l'évapotranspiration[END_REF], and Rv is geometrically derived from solar position and canopy structure. Finally, soil potential evaporation is determined as:

For the lower boundary conditions, we set them to free drainage when the water table was absent, and to variable pressure head at the water table level according to piezometric measurements otherwise.

Root water uptake S(h)

The root density was distributed according to both in situ observations and previous studies [START_REF] Trambouze | Measurement and modelling of the transpiration of a Mediterranean vineyard[END_REF][START_REF] Bsaibes | Évaluation d'une approche multi-locale d'estimation spatiale de l'évapotranspiration[END_REF]. The average root profile extended between 0.25 and 2.2 m, with 75% of cumulated root density between 0.25 and 1.25 m. Since HYDRUS-1D could not accommodate horizontal changes in root profile (e.g., below the grapevine rows and the inter-rows), we assumed that the root profile was homogeneous within each site, following [START_REF] Trambouze | Caractérisation Et éléments De Modélisation De l'évapotranspiration réelle De La Vigne à l'échelle De La Parcelle[END_REF].

For each soil layer with a given root density, root water uptake S(h) corresponds to the regulation of plant maximal extraction by pressure head. Plant maximal extraction is plant maximal transpiration normalized by the relative root density of the layer (i.e., the ratio of layer root density to total root density). This means that plant maximal transpiration is distributed over the soil layers proportionally to their root density, and is locally controlled according to soil moisture (Simunek and Hopmans,2009).

The regulation of plant maximal extraction is expressed using the Feddes function [START_REF] Feddes | Simulation of field water use and crop yield[END_REF]. The latter is a trapezoid-like function with a linearly increasing uptake between h1 (pressure head below anaerobic conditions without uptake) and h2 (pressure head at maximum uptake), a plateau with maximum uptake between h2 and h3 (pressure head at maximum uptake), and a linearly decreasing uptake between h3 and h4 (pressure head at wilting point). Following [START_REF] Wesseling | Meerjarige Simulaties Van Grondwateronttrekking Voor Verschillende Bodemprofielen[END_REF], the h1 and h2 parameters were set to -0.001mm and to -0.0025mm, respectively. Following [START_REF] Trambouze | Measurement and modelling of the transpiration of a Mediterranean vineyard[END_REF], h3 and h4 parameters were set to -10000 mm and to -150000 mm, respectively.

Actual evapotranspiration

Actual soil evaporation is derived from soil potential transpiration Ep by using the Neuman conditionality that modulates Ep according to surface pressure head and soil moisture [START_REF] Neuman | Finite Element Simulation of Flow in Saturatedunsaturated Soils Considering Water Uptake by Plants[END_REF]. The threshold value for the Neuman conditionality was adjusted to -10190 mm (respectively +10 mm) for the minimum (maximum) soil pressure head that correspond to complete dryness (wetness at runoff initiation).

Actual plant transpiration is estimated as the sum of root water uptake over all layers [START_REF] Vrugt | One-, two-and threedimensional root water uptake functions for transient modelling[END_REF]. Thus, HYDRUS-1D is based on a macroscopic approach that consists of deriving actual plant transpiration through the formulation of root water uptake S(h) in a multiple layer framework.

Finally, actual evapotranspiration was estimated as the sum of actual soil evaporation and actual plant transpiration. Simulated ET values at the hourly timescale were aggregated at the daily timescale. The hourly timescale was related to changes in sunlit and shadow effects within vineyards rows, and the daily timescale was related to dynamics of soil water balance.

Results

Assessing calibration consistency

We first evaluated the consistency of the HYDRUS-1D calibration by analyzing the calibration residual errors, i.e., the differences between the measured and the simulated soil moisture profiles. The analysis was conducted by considering the calibration residual errors as a function of soil depth, the calibration residual error from one site to another and throughout the simulation period, and the differences between measurements and simulations for soil water storage, where the latter was derived from the integration of soil moisture profiles.

Over all dates and all locations of soil moisture profile measurements, the average profile of root mean square error (RMSE) between measured and simulated soil moisture decreased with depth, from 0.06 (top) to 0.03 m 3 m -3 (bottom). Similarly, the average profile of relative RMSE (i.e., the ratio of absolute RMSE to observed mean value, labeled RRMSE) decreased from 30% to 10%, with an overall value of 20% (Fig. 1). The largest RMSE and RRMSE values were observed around the 1-m depth that corresponded to the maximum root density. The RMSE and RRMSE profiles varied much from one date to another and from one location to another, with values ranging from 0 to 0.15 m 3 m -3 and from 0% to 60%, respectively. The lowest and largest values were observed for Sites 4 and 1, respectively. For all measurement locations, the RMSE and RRMSE values systematically decreased with simulation time. When addressing soil water storage, we observed an overall 10% relative difference between measured and simulated values. Fig. 2 displays a typical example of comparison between measurements and simulations, for two sites that differed much in terms of water availability (i.e., Site 3 was typified by an absent water table and Site 5 was typified by a seasonal water table). As illustrated with the two examples of Fig. 2, the measurements and simulations depicted very similar temporal dynamics, regardless of considered site. We did not identify any systematic over-or under-estimation, neither for a given site throughout the simulation period, nor for a specific period over the seven sites. Fig. 2 indicates that the sites depicted stable soil water storage from January to June 2008, with a peak in June 2008 after heavy rainfall, whereas soil water storage rapidly decreases from July because of both grapevine transpiration and lack of heavy rainfall.

ValidatingHYDRUS-1Destimatesofevapotranspiration against EC measurements

The validation of HYDRUS-1D estimates of evapotranspiration against independent estimates from EC measurements was conducted at both the hourly and daily timescales.

At the hourly time scale, the validation was conducted over daytime hours since nighttime ET is negligible at the daily timescale. When considering the whole simulation period, the RMSE between EC and HYDRUS-1D estimates was 44 W m -2 (54% relative) for Site 6 with intermediate water status related to a shallow water table and silty/clay loam soils. For Site 7 with severe water stress in summer because of clay soil along with the lack of a water table, the RMSE was 40 W m -2 (44% relative). For both sites, linear regressions between EC and HYDRUS-1D estimates provide slopes and offsets that were close to 1 and 0, respectively, which indicated that systematic errors were very low (Fig. 3). However, we observed large scatters around the regression lines, which indicated that unsystematic errors were large. When analyzing the diurnal course of RRMSE, we observed that the latter was lower between 11 AM to 3 PM for Site 6 (RRMSE<45%) and between 12 AM to 4 PM for Site 7 (RRMSE < 40%). When addressing the daily timescale, RMSE between EC and HYDRUS-1D estimates was 0.57 mm d -1 (33% relative) for Site 6, and 0.4 mm d -1 (18% relative) for Site 7, as displayed in Fig. 4. RMSE was stable throughout the simulation periods, which induced a larger RRMSE between August and December 2007 that depicted lower ET values, as compared to the [January-October] 2008 interval. Similar validation results were obtained when restricting the time interval to grapevine growth periods, with RMSE values of 0.62 mm d -1 (29% relative) and 0.49 mm d -1 (22% relative) for Site 6 and 7, respectively. The temporal dynamics of EC and HYDRUS-1D estimates of daily ET are displayed in Fig. 5 for Site 6 throughout the simulation period. This comparison was possible over Site 6 because the data collection was continuous throughout the experiment (Section 2.3). It is shown that HYDRUS-1D simulations and EC estimates depicted very similar temporal dynamics, for both dry and wet conditions. However, large differences were observed in September 2007 with an underestimation of EC estimates by HYDRUS-1D simulations during a very dry period, and in June 2008 with an overestimation of EC estimates by HYDRUS-1D simulations after heavy rainfall. Fig. 4. Validation of HYDRUS-1D estimates against EC estimates for ET at the daily timescale when considering Sites 6 (left) and 7 (right). R 2 is the determination coefficient, slope and offset result from linear regression between X and Y data (continuous line). Dotted line is the 1:1 line. Nash coefficient is also reported. Hyphen-dotted lines correspond to tolerance intervals given by ECPACK for EC estimates.

Seasonal ET variability over study sites

When focusing on HYDRUS-1D estimates of daily ET throughout the simulation period, we observed large temporal dynamics for the seven sites, with seasonal variations of daily ET values between 0 and 6.5 mm d -1 . Further, the ET temporal dynamics differed much from one site to another, and we accordingly classified the seven sites into three groups. Group 1 included Sites 2 and 3 that experienced severe water stress during the end of summer (particularly in 2007 with daily ET values lower than 1 mm d -1 ) and large evaporative rates during spring with ET values close to the short-grass reference evapotranspiration ET 0 after heavy rainfall (June 2008 with daily ET ranging between 4 and 6.5 mm d -1 ). Group 2 included Sites 1, 6 and 7 that experienced water stress during the end of summer, with daily ET values around 1 mm d -1 , and constant evaporative rate for the rest of the simulation period. Group 3 included Sites 4 and 5 that depicted constant ET rates throughout the simulation period, with daily ET values between 2 and 4 mm d -1 , even during very dry periods.

The ET temporal dynamic is illustrated in Fig. 6 where three sites were selected as representative of each group above-listed. In this figure, we can identify severe water stresses during August and September 2007 for Site 3 (Group 1) with a low evaporative rate over more than 40 days. A similar dynamic was observed for Site 6 (Group 2) but for 10 days only during September 2007. Conversely, Site 4 (Group 3) continuously depicted an evaporative rate very close to ET 0 during the same period. We observed contrary situations for the same sites during a period of unrestrictive water availability in 2008 that followed heavy rainfall. Indeed, we observed large evaporative rates over short periods, particularly for Site 3 (Group 1), whereas ET for the three sites almost reached ET 0 after rainfall.

Cumulative statistics for the seven sites, as shown in Table 2, were consistent with the results discussed above. The three groups could be identified when considering cumulated ET during the full grapevine vegetation period, with Sites 2 and 3 (Group 1) depicting the lowest ET cumulated values; Sites 1, 6 and 7 (Group 2) depicting intermediate ET cumulated values; and Sites 4 and 5 (Group 3) depicting the largest ET cumulative values. For the driest and wettest sites, ET cumulated and daily values over full vineyard vegetation period showed large differences, with ET cumulated values of 255 mm and 565 mm, and ET daily value of 1.46 mm d -1 and 2.87 mm d -1 , which corresponded to a factor of two between these two sites.

Discussion

The profile of calibration residual error, as expressed through RMSE between measured and simulated soil moisture profiles, was explained by the temporal dynamics of water fluxes within the unsaturated zone. Low RMSE values for deep layers were ascribed to slower exchanges related to drainage and capillary rises, whereas large RMSE values for root zone and soil surface were ascribed to faster exchanges related to plant transpiration and soil evaporation, respectively. The 10% overall RRMSE value between simulated and measured profiles was considered acceptable since the error on NP measurements was around 15% relative, while the RMSE values were similar to those reported in previous studies that addressed the performances of HYDRUS within irrigated and heterogeneous crops [START_REF] Skaggs | Comparison of HYDRUS-2D simulations of drip irrigation with experimental observations[END_REF][START_REF] Phogat | Evaluation of water movement and nitrate dynamics in a lysimeter planted with an orange tree[END_REF][START_REF] Phogat | Seasonal simulation of water, salinity and nitrate dynamics under drip irrigated mandarin (Citrus reticulata) and assessing management options for drainage and nitrate leaching[END_REF]. We explained the RMSE decrease during the simulation period by the stabilization of the numerical process, which justified the setting of a simulation starting date a few months before the measurements of soil moisture profiles (Section 3.2). The lack of an evident link between the RMSE values and the site features was consistent with the lack of systematic under/overestimation when analyzing the temporal dynamics of soil water storage from HYDRUS and EC estimates, which underlined the relevance of the proposed approach. Indeed, HYDRUS-1D calibration performances were similar throughout the simulation period and across a large range of grapevine water status, where both site and periods were typified by large differences in soil and/or water table conditions [START_REF] Guix-Hébrard | Influence of water table depths on the variation of grapevine water status at the landscape scale[END_REF].

At the hourly timescale, the validation results for HYDRUS-1D estimates against EC estimates were similar to those reported in previous studies over irrigated vineyards [START_REF] Ortega-Farias | Latent heat flux over cabernet sauvignon vineyard using the Shuttleworth and Wallace model[END_REF], 2010;Kool et al., 2014a), and the resulting accuracy on HYDRUS-1D estimates was close to that regularly quoted for further applications [START_REF] Seguin | IRSUTE: a minisatellite project for land surface heat flux estimation from field to regional scale[END_REF][START_REF] Kalma | Estimating land surface evaporation: a review of methods using remotely sensed surface temperature data[END_REF]. The same applied to the validation results at the daily timescale, with similar values as compared to those reported in previous studies over irrigated and rainfed vineyards [START_REF] Trambouze | Comparison of methods for estimating actual evapotranspiration in a row-cropped vineyard[END_REF][START_REF] Bsaibes | Évaluation d'une approche multi-locale d'estimation spatiale de l'évapotranspiration[END_REF][START_REF] Ortega-Farias | Latent heat flux over cabernet sauvignon vineyard using the Shuttleworth and Wallace model[END_REF], 2010), and with a resulting accuracy close to that required for further applications. This underlined the relevance of the proposed approach, since the validation relied on comparing two independent methods for ET estimation, i.e., the first one based on near surface turbulent fluxes (EC) and the second one based on water transfers in the unsaturated zone (HYDRUS-1D). An important outcome is that the calibration of HYDRUS-1D over times series of measured soil moisture profiles can be used as an alternative to EC measurements. Indeed, the approach based on HYDRUS-1D can assist in alleviating experimental efforts for device cost and maintenance, since measurements of soil moisture profile can be replicated across large spatial extents. This is all the more interesting that the considered study area depicts large heterogeneities in terms of pedology or water table dynamics [START_REF] Guix-Hébrard | Influence of water table depths on the variation of grapevine water status at the landscape scale[END_REF] and in terms of canopy conditions such as row orientation, canopy thickness and interrow weeding (Galleguillos et al., 2011a(Galleguillos et al., , 2011b)).

When validating HYDRUS-1D estimates against EC estimates at both hourly and daily timescale, the larger differences we observed for Site 6 as compared to Site 7 was ascribed to the experimental conditions. Site 6 had a 0.15 km 2 size, and spread over nine fields including vineyards by 90% in surface area, whereas Site 7 had a 0.09 km 2 size and spread over one field only (Section 2.2). Besides, the EC footprint was designed in accordance to the site extent, since the measurement height for was larger (almost three times) for Site 6 than for Site 7. Therefore, the within-footprint variability in canopy conditions was larger for Site 6 as compared to Site 7, and the same applied for the within-footprint variability in evaporative rate.

The validation of HYDRUS-1D estimates at the hourly timescale showed large differences between model simulations and EC measurements during sunrise and sunset periods, which was consistent with the outcomes from Kool et al. (2014a) who reported a similar trend. This was ascribed to EC measurements that had difficulties in properly estimating evapotranspiration during sunrise and sunset periods, whereas several studies reported that energy balance fails under advection conditions [START_REF] Rana | Measurement and estimation of actual evapotranspiration in the field under Mediterranean climate: a review[END_REF][START_REF] Zhang | Comparison of three evapotranspiration models to Bowen ratio-energy balance method for a vineyard in an arid desert region of northwest China[END_REF][START_REF] Ding | A dynamic surface conductance to predict crop water use from partial to full canopy cover[END_REF]. However, these difficulties in estimating evapotranspiration were not considered critical, since they occurred during periods of low energy fluxes, with limited impacts at the daily timescale.

The three site groups we identified when analyzing the temporal dynamics of ET over the seven sites were not clearly correlated to the soil depth, soil texture and water table conditions. First, Group 1, 2 and 3 corresponded to shallow soils, shallow/deep soils, and deep soils, respectively. Second, Group 1 and 2 corresponded to absent/seasonal water tables, while Group 3 corresponded to seasonal/permanent water tables. Third, soil textures could be different within the same group (e.g., clay/gravels and silty/sandy for Group 1) or similar from one group to another (e.g., clay soils for Groups 1, 2 and 3). Therefore, we could not identify any evident link between ET temporal dynamics and soil depth/water table conditions, but a finer analysis required accounting for additional factors. Premature varieties might explain the large evaporative rates observed in spring 2008 for Sites 1, 3 and 6, whereas weeds were likely to induce ET values close to ET 0 after rainfall. For Site 3 with restrictive water availability, weeds might explain the larger values of daily ET as compared to Site 4 with a permanent water table. For Site 2, a premature variety might explain the large ET values in spring 2008 since there were no weeds. For Sites 4 and 5, the influence of a water table might explain the stable evaporative rates throughout the growth cycle, especially after a dry period in August and September 2007. Additionally, other factors such as farmer practices, including pruning or weeding, might have impacted on the temporal dynamics of evaporative rate, so that the ET rate was limited for the sites with water availability. For example, the pruning was necessary at Site 4, to prevent canopy vigor and thus obtain sufficient fruit quality.

Conclusions

The outcomes of the present studies are threefold. First, it was possible to calibrate HYDRUS-1D against measurements of soil moisture for a larger range of grapevine water status that resulted from very different soil and water table conditions. Second, HYDRUS-1D simulations after calibration can be used as an alternative to EC measurements within rainfed vineyards, to alleviate experimental efforts for device cost and maintenance, especially over a regional extent that depicts large heterogeneities in terms of pedology or water table conditions. Then, HYDRUS-1D simulations can be used for several applications such as characterizing spatial variabilities and temporal dynamics, assessing impact of pedological conditions and land use on evapotranspiration, or validating remote sensing retrievals over large regions. Third, HYDRUS-1D permitted the characterization of the temporal dynamics of evaporative rate over a large range of grapevine water status in space and time. It was possible to explain some coarse behaviors in relation to soil depth and water table conditions, but a finer analysis required accounting for additional factors such as variety precocity, weeds, as well as pruning and weeding practices that are necessary to ensure grape quality and yield.

Further investigations should focus on improving soil water balance modeling on several important issues related to soil conditions and vineyard structures. First, attention should be paid to non-equilibrium situations that are common for water fluxes within the vadose zone [START_REF] Flury | Susceptibility of soil to preferential flow of water: a field study[END_REF][START_REF] Jarvis | A review of non-equilibrium water flow and solute transport in soil macropores: principles, controlling factor and consequences for water quality[END_REF][START_REF] Köhne | A review of model applications for structured soils: a) Water flow and tracer transport[END_REF], and that can be a source of error for both the calculation of water balance and the inverse estimation of soil hydrodynamic parameters. Besides, accounting for hysteresis effects is likely to assist in improving modeling and calibration for silty and clay soils [START_REF] Fredlund | Estimation of soil suction from the soil-water characteristic curve[END_REF][START_REF] Osman | Soil water, irrigation and drainage[END_REF]. Second, a 2D model should be considered for including possible lateral water transfers and spatiotemporal differences in boundaries conditions (e.g., diurnal and seasonal courses of canopy shading effect within rows and inter-rows), which are key processes for rowstructured canopies affecting the water balance in the soil [START_REF] Ramos | Two-dimensional modeling of water and nitrogen fate from sweet sorghum irrigated with fresh and blended saline waters[END_REF][START_REF] Phogat | Evaluation of water movement and nitrate dynamics in a lysimeter planted with an orange tree[END_REF][START_REF] Phogat | Seasonal simulation of water, salinity and nitrate dynamics under drip irrigated mandarin (Citrus reticulata) and assessing management options for drainage and nitrate leaching[END_REF]Kool et al., 2014a). Third, coupling the plant functioning and the soil water balance modeling should assist in better characterizing plant influences on water transfers, whereas the use of ancillary information about leaf area index is likely to improve the estimation of grapevine ET 0 [START_REF] Allen | Evapotranspiración del Cultivo. Guías para la determinación de los requerimientos de agua de los cultivos[END_REF]. Then, the coupling with decision modeling would be valuable to account for farmer practices such as pruning and weeding.

Fig. 1 .

 1 Fig. 1. Profile of HYDRUS-1D residual calibration error, obtained by calculating RRMSE values between measured and simulated soil moisture profiles over all dates and all measurement locations. The dotted line represents the overall mean value, and the error bars represent for each depth the RRMSE standard deviation.

Fig. 2 .

 2 Fig. 2. Comparison of temporal dynamics for measured and simulated soil water storage between August 2007 and October 2008 when considering two contrasted sites. Water storage was calculated by integrating the measured and simulated soil moisture profiles at noon. Site 3 was typified by restrictive water availability because of absent water table, whereas Site 5 was typified by moderate water availability because of a seasonal water table.

Fig. 3 .

 3 Fig.3. Validation of HYDRUS-1D estimates against EC estimates for ET at the hourly timescale when considering Sites 6 (left) and 7 (right). R 2 is the determination coefficient, slope and offset result from linear regression between X and Y data (continuous line). Dotted line is the 1:1 line. The Nash coefficient is also reported.

Fig. 5 .

 5 Fig. 5. Comparison of temporal dynamics for EC and HYDRUS-1D estimates of daily ET, for Site 6 throughout the experiment. Rainfall is also indicated (right-hand axis).

Fig. 6 .

 6 Fig. 6. Comparison of ET simulations from HYDRUS-1D for three sites that differed much in soil and water table conditions, when selecting (a) a dry period in 2007 and (b) a wet period in 2008 with significant rainfall. Shortgrass reference evapotranspiration (ET0) is also indicated.

Table 2 .

 2 Statistics on HYDRUS-1D simulations of actual evapotranspiration ET over the seven study sites. Cumulated ET over the experimental period corresponds to cumulated ET from August 2007 to October 2008. Cumulated ET, averaged daily ET (Daily ET Mean), standard deviation on daily ET (daily ET STD) and coefficient of variation of daily ET (daily ET CV) for grapevine full vegetation period are statistical values obtained over both the [August 2007-October 2007] and the [June 2008-October 2008] periods.

Acknowledgments

The authors thank the LISAH technical staff for the experiment. This study benefited from grants by the Chilean National Commission for Scientific and Technological Research, and by the French National Institute for Agricultural Research. M. Galleguillos acknowledges the support of Project CONICYT/FONDAP/1511000. The authors are grateful to the Editorial Review Board of the Agricultural Water Management Journal who helped to improve the manuscript.


Olympe Longeras

Arnaud Gautier

Karine Ballerat-Busserolles

Jean-Michel Andanson
email: j-michel.andanson@uca.fr

Tuning critical solution temperature for CO 2 capture by aqueous solution of amine

Keywords:

Aqueous solution of amine is a mature technology to selectively absorb CO 2 from an industrial exhaust. Monoethanolamine (MEA) is still considered as the reference amine and unfortunately, the energy necessary to desorb CO 2 and recycle the solvent is limiting its use at industrial scale.

The main cost of the process is the energy needed to heat the solvent over 100 °C to desorb the CO 2 . Several options to decrease this expense have been considered including the use of phase separation to reduce the volumes to be treated. It was then proposed to use aqueous solution of an amine exhibiting a lower critical solution temperature (LCST) at an adequate temperature.

However, it is challenging to found a solvent able to absorb large quantity of CO 2 and having an LCST at a suitable temperature. Deep eutectic solvent (DES) composed of oleic acid (hydrophobic) and lidocaine (amphiphilic) can be separated from water with either temperature or CO 2 as stimuli. This DES has been used to adjust the critical solution temperature after sorption of CO 2 in an aqueous solution of demixing amine. By only changing the ratio of the two components of the DES, the critical solution temperature can be tuned from around 30 to 70°C.

The addition of this DES into an aqueous solution of demixing amine could then substantially help to adapt the critical solution temperature of a solvent to the require conditions of CO 2 capture process.

Introduction

Currently the industrial technologies for CO 2 capture processes are based on a CO 2 separation from post-combustion effluent by gas dissolution in chemical absorbent solutions. The technology is mature enough to be integrated into industrial sites without strong modification of the plant. [START_REF] Bui | Carbon capture and storage (CCS): the way forward[END_REF] The principle is based on CO 2 absorption-desorption cycles. Carbon dioxide contained in gaseous effluents is selectively absorbed by solvents. Aqueous solutions of alkanolamine are selective absorbents [START_REF] Rao | A Technical, Economic, and Environmental Assessment of Amine-Based CO2 Capture Technology for Power Plant Greenhouse Gas Control[END_REF] used in the decarbonation of natural gases. The affinity between the absorbent and the CO 2 has to be precisely adjusted: if the interaction is too weak, the CO 2 is not enough absorbed, however, if the interaction is too strong, the amount of energy to desorb will be extensive, the cycling process will be costly and not applicable at industrial scale.

Aqueous solution of monoethanolamine (MEA) is the benchmark for post-combustion capture (PCC). [START_REF] Zhang | Phase change solvents for post-combustion CO2 capture: Principle, advances, and challenges[END_REF] This amine has a strong affinity with CO 2 leading to favorable kinetic of absorption. In the cycling process with this type of system, the CO 2 desorption from an alkanolamine solution is highly energy consuming and not compatible with CO 2 remediation applications. It is thus necessary to imagine new breakthrough processes allowing substantial energetic cost reduction.

Tertiary alkanolamines such as methyldiethanolamines (MDEA) have therefore been proposed, [START_REF] Feyzi | Exergy analysis: A CO2 removal plant using a-MDEA as the solvent[END_REF] as the affinity between such amine and CO 2 is reduced, decreasing the energetic cost of the regeneration. However, in that case, the kinetic of absorption is unfavorable, and if the solvent is really efficient to treat gases containing high concentrations of CO 2 , it necessitates very large installations and solvent quantities to treat the fume from post-combustion processes.

Another option is to use an aqueous switchable solvent [START_REF] Zhang | Novel Thermomorphic Biphasic Amine Solvents for CO2 Absorption and Low-Temperature Extractive Regeneration[END_REF] possessing an LCST (lower critical solution temperature): while increasing the temperature, the solvent present a liquidliquid phase separation at its critical temperature. The topic of phase change solvents for PCC have been recently reviewed. [START_REF] Zhang | Phase change solvents for post-combustion CO2 capture: Principle, advances, and challenges[END_REF] In 2011 IFPEN proposed the DMX ® process based on the use of high capacity and chemically stable demixing amines to replace classical alkanol amines for CO 2 capture in industrial effluents. [START_REF] Raynal | The DMX™ process: An original solution for lowering the cost of post-combustion carbon capture[END_REF][START_REF] Raynal | Evaluation of the DMX Process for Industrial Pilot Demonstration -Methodology and Results[END_REF] Thermomorphic biphasic solvent (TBS) have been widely studied by Zhang et al. [START_REF] Zhang | CO2 absorption in biphasic solvents with enhanced low temperature solvent regeneration[END_REF][START_REF] Zhang | Carbon dioxide absorption into biphasic amine solvent with solvent loss reduction[END_REF][START_REF] Zhang | Improvement of Lipophilic-Amine-based Thermomorphic Biphasic Solvent for Energy-Efficient Carbon Capture[END_REF][START_REF] Zhang | Intensification of low temperature thermomorphic biphasic amine solvent regeneration for CO2 capture[END_REF][START_REF] Zhang | Development of an Energy-efficient CO2 Capture Process Using Thermomorphic Biphasic Solvents[END_REF][START_REF] Zhang | Thermodynamic Modeling for CO2 Absorption in Aqueous MDEA Solution with Electrolyte NRTL Model[END_REF][START_REF] Zhang | Thermodynamic modeling for CO2 absorption in aqueous MEA solution with electrolyte NRTL model[END_REF] and Tan [START_REF] Tan | Study of CO2 absorption into thermomorphic lipophilic amine solvents[END_REF]. The solvent is an aqueous mixture of lipophilic amines exhibiting a liquidliquid phase transition upon heating leading to a high carbon dioxide desorption at temperatures well below the boiling point of aqueous solutions. This technology uses heterogeneous absorbent solutions which becomes homogeneous upon carbon dioxide absorption and returns to two phases through heating. The organic phase acts as an extractive agent, removing the amines from the aqueous phase and thus favorably displacing the regeneration equilibrium. Zhang et al. have intensively worked on replacing steam stripping used for carbon dioxide release with nucleation or agitation techniques. [START_REF] Zhang | Carbon dioxide absorption into biphasic amine solvent with solvent loss reduction[END_REF][START_REF] Zhang | Intensification of low temperature thermomorphic biphasic amine solvent regeneration for CO2 capture[END_REF] They pointed out that the benefit of adding a hydrophobic organic solvent to reduce the operating temperature for carbon dioxide desorption. Mixed phase-changed solvents have been investigated by Svendsen and coworkers [START_REF] Arshad | Equilibrium Total Pressure and CO2 Solubility in Binary and Ternary Aqueous Solutions of 2-(Diethylamino)ethanol (DEEA) and 3-(Methylamino)propylamine (MAPA)[END_REF][START_REF] Ciftja | Experimental study on phase change solvents in CO2 capture by NMR spectroscopy[END_REF][START_REF] Monteiro | Kinetics of CO2 absorption in aqueous blends of N,N-diethylethanolamine (DEEA) and N-methyl-1,3-propane-diamine (MAPA)[END_REF][START_REF] Monteiro | VLE data and modelling of aqueous N,N-diethylethanolamine (DEEA) solutions[END_REF][START_REF] Pinto | CO2 post combustion capture with a phase change solvent. Pilot plant campaign[END_REF] as a part of the European Union project iCap. The process is very similar to DMX® developed by IFPEN. Screening investigations to evaluate the CO 2 absorption and desorption facilities of phase-change solvents have been completed. [START_REF] Xu | Solvents with Low Critical Solution Temperature for CO2 Capture[END_REF][START_REF] Ye | Screening and evaluation of novel biphasic solvents for energy-efficient postcombustion CO2 capture[END_REF] Since the initial work from Abbott and collaborators, deep eutectic solvents (DES) have gained a large interest from the scientific community. [START_REF] Abbott | Novel solvent properties of choline chloride/urea mixtures[END_REF][START_REF] Abbott | Deep Eutectic Solvents Formed between Choline Chloride and Carboxylic Acids: Versatile Alternatives to Ionic Liquids[END_REF] DES are mixtures with melting temperatures far below the melting temperature of the initial components. [START_REF] Zhang | Deep eutectic solvents: syntheses, properties and applications[END_REF] The existence of DES is generally explained by favorable specific interactions between the different chemicals of the mixture. Many DES are obtained by the mixing of a donor and an acceptor of hydrogen bond.

DES with specific properties can be designed to target particular physico-chemical properties: as example, hydrophobic DES has been developed for extraction of a broad range of molecules [START_REF] Van Osch | Hydrophobic deep eutectic solvents as water-immiscible extractants[END_REF] and ions [START_REF] Van Osch | Removal of alkali and transition metal ions from water with hydrophobic deep eutectic solvents[END_REF] from water. A first DESwater system with an LCST at 25°C has been recently observed. [START_REF] Longeras | Deep Eutectic Solvent with Thermo-Switchable Hydrophobicity[END_REF] This DES is composed of oleic acid (OA) and lidocaine (LD) which was first introduced by Bica et al. [START_REF] Bica | Liquid forms of pharmaceutical cocrystals: exploring the boundaries of salt formation[END_REF] The existence of the LCST has been explained by the variation of pKa of the amine: the temperature increase is inducing a fast decrease of the solubility of the amphiphilic lidocaine in water up to the phase separation. Although the system is fully miscible below 25°C, at 50°C only 5 wt% of water is present in the organic phase and less than 0.5 wt% of DES is detected in the aqueous phase. [START_REF] Longeras | Deep Eutectic Solvent with Thermo-Switchable Hydrophobicity[END_REF] Herein, we focus on aqueous solutions of a DES composed of lidocaine (LD) and oleic acid (OA). First, we found that the addition of carbon dioxide into an aqueous solution of DES (OA:LD) is triggering a switch from monophasic to biphasic. However, the quantity of CO 2 absorbed in the aqueous solution of DES (OA:LD) is irrelevant for the topic of CO 2 capture. We then decided to add another amine to increase the amount of absorbed CO 2. Several amines including monoethanol amine (MEA), piperidine, 2-methylpiperidine, 4-methylpiperidine, Nmethylpiperidine, N-ethylpiperidine were evaluated. The existence of an LCST between 25 and 80°C after addition of CO 2 for several compositions were tested for each amine. LCST was only observed for mixtures with N-methylpiperidine (NMPi) and N-ethylpiperidine. NMPi was selected as the most promising amine for CO 2 capture application. This tertiary amine with a pKa around 10, [START_REF] Weast | CRC Handbook of Chemistry and Physics[END_REF] is also increasing the CO 2 absorption compared to the LD with a pKa of around 8. [START_REF] Cassens | Solubility of Pharmaceuticals and Their Salts As a Function of pH[END_REF] Therefore, NMPi was selected to evaluate the ability of {DES (OA:LD) + water + Amine} mixtures to be efficient for CO 2 capture. Most studies using aqueous amine solutions for CO 2 capture are using concentrations of around 30 wt% of organic molecules. [START_REF] Pinto | CO2 post combustion capture with a phase change solvent. Pilot plant campaign[END_REF][START_REF] Bernhardsen | A review of potential amine solvents for CO2 absorption process: Absorption capacity, cyclic capacity and pKa[END_REF] Therefore in the present study, the experiments are realized with solvents composed of DES (OA:LD), NMPi and water and with fixe amount of water at 70 wt%. In particular, we studied the impact of DES and NMPi concentrations on the critical solution temperature. After this optimization step, we finally evaluated the quality of the separation of this new solution for carbon capture.

Results and discussion

Inducing a reversible phase separation by addition of CO 2 in aqueous solution of DES (OA:LD) While mixed with water at 20°C, only the DES (OA:LD) with initial concentrations of x OA between 0.45 and 0.60 are homogeneous. For lower concentrations of OA, the DES is partially solid, and for higher concentration of OA, the aqueous mixtures are biphasic liquidliquid at 20°C. [START_REF] Longeras | Deep Eutectic Solvent with Thermo-Switchable Hydrophobicity[END_REF] The addition of CO 2 into the aqueous DES were then realized for OA:LD molar ratios of 45:55, 50:50, 55:45 and 60:40. At 20°C, when CO 2 is bubbled at atmospheric pressure in an aqueous DES with 70 wt% of water, the mixture became white within few minutes under stirring.

The possibility to induce the phase separation by an increase of the temperature was avoided by keeping the samples at 20°C in water bath and under a vigorous stirring. After 1 hour when the flow of CO 2 and the stirring are stopped, a decantation in 2 liquid phases is observed. Then, stirring is restarted and N 2 is bubbled into the biphasic mixtures, and the systems are coming back to monophasic liquid in less than 1 hour. An example of these visual observations is given in CO 2 bubbling at 20°C was inducing the phase separation while bubbling N 2 was switching back the system to monophasic. [START_REF] Kohno | Dual stimuli-responsive phase transition of an ionic liquid/water mixture[END_REF] Figure 1: Pictures of DES (OA:LD)water mixture, before addition of CO 2 (1), after addition of CO 2

(2), and after addition of N 2 (3). The DES is composed of 60 mol% of OA and 40 mol% of LD and before the addition of CO 2 the mixture is composed of 30 wt% of DES and 70 wt% of H 2 O. The experiment was performed in a water bath thermostated at 20 °C. The vial is containing 2.5 ml of solution, and the aqueous phase is the bottom phase. White dashes have been added to help discerning the liquidliquid interface.

Switch hydrophobicity of this DES-water with temperature has already been investigated. [START_REF] Longeras | Deep Eutectic Solvent with Thermo-Switchable Hydrophobicity[END_REF] The system is composed of OA (an oil), water and LD. At low temperature, the amphiphilic lidocaine is making oil and water to remain together in a single phase. When the temperature is increased, the quantity of ions is decreased [START_REF] Longeras | Deep Eutectic Solvent with Thermo-Switchable Hydrophobicity[END_REF] and the solubility of neutral LD is much smaller than cationic LD. [START_REF] Cassens | Solubility of Pharmaceuticals and Their Salts As a Function of pH[END_REF] Therefore, the affinity between water and organic compounds is disrupted by the increase of temperature and the liquidliquid phase separation is induced with DES on one side and water on the other. The similar behavior is observed here with CO 2 absorption in DES-water at ambient temperature. Indeed, pH of solution is acidified when CO 2 is absorbed, thus solubility of LD in water is reduced initiating a phase separation.

For the considered concentrations of (OA:LD), the density of the DES is always between 0.94 and 0.96 g/cm 3 . [START_REF] Bica | Liquid forms of pharmaceutical cocrystals: exploring the boundaries of salt formation[END_REF] In the example shown in Figure 1, the volume ratio between the organic and aqueous phase is close to the ratio of water and DES. Nevertheless, for mixture with higher amount of OA, the volume of the aqueous phase tends to decrease. This could illustrate the nonideal separation between the DES and water. In particular, since the water volume is smaller than expected, the quantity of water in the organic phase should be significant.

A small increase of temperature and the addition of CO 2 are two stimuli that are inducing a phase separation between the DES (OA:LD) and water. In either case, the phase separation is reversible by decreasing the temperature or by removing the CO 2 . Depending on the application, CO 2 /N 2 stimulus could be more appropriate than changing the temperature. pKa of LD is 8.0 at 25°C, [START_REF] Cassens | Solubility of Pharmaceuticals and Their Salts As a Function of pH[END_REF] and because of its low pKa, this tertiary amine could not be considered as a good amine for CO 2 capture. [START_REF] Puxty | Carbon Dioxide Postcombustion Capture: A Novel Screening Study of the Carbon Dioxide Absorption Performance of 76 Amines[END_REF] Indeed, the mixture with 30 wt% of DES (2:1) can only absorbed around 0.3 wt% of CO 2 which is far from a benchmark system like MEAwater which are absorbing more than 10 wt% (g of CO 2 /g of solvent). [START_REF] Adeyemi | Novel Green Solvents for CO2 Capture[END_REF] It was then decided to add another tertiary amine with a higher pKa to improve the absorption of CO 2 . DES (OA:LD) -NMPiwater -CO 2 was selected after a screening over potential amines.

Impact of the composition of the solvent on the LCST in DES -NMPiwater -CO 2 mixtures

To evaluate CO 2 absorption in different conditions, the loading charge α is often used: α is the molar ratio of carbon dioxide per amine. Since aqueous solution of LD has not shown high ability to absorb the CO 2 , LD has not been considered for the calculation of the loading charge of CO 2 and only the concentration of NMPi has been taken into account:

This parameter is employed to describe CO 2 absorption. However,  is only utilized when a reactive amine is added to the solvent. To facilitate the comparison with other processes, CO 2 absorption can also be defined by the mass ratio (wt%): g of CO 2 / g of solvent.

CO 2 sorption experiments was first realized in a solution containing 70 wt% of water, 15 wt% of DES (OA:LD) and 15 wt% of NMPi. Flowing gaseous CO 2 above the solution for 45 min. induce a CO 2 sorption up to  around 0.6 which correspond to 4 wt% (g of CO 2 / g of solvent).

When bubbling directly CO 2 in the liquid phase, the sorption is faster and an  = 0.4 is reach in only 6 min. The addition of NMPi has been clearly improving the solubility of CO 2 ; it has increased from 0.3 wt% with DES to 4 wt% when replacing half of the DES by NMPi. After the CO 2 sorption, a vial containing 2 ml of sample is transfer into a water bath to measure the critical solution temperature while increasing the temperature up to 80°C. In the case of α = 0.40, a phase separation is observed at 43°C (Table 1, entry 1) while without CO 2 this mixture does not show a critical solution temperature up to 80°C.

The impact of the concentrations of each component was evaluated by modifying their concentrations. First, the ratio of DES and NMPi were changed while keeping the quantity of water constant at 70 wt% and the quantity of CO 2 at  = 0.4. Compared to the 70-15-15 (Table 1, entry 1), the critical solution temperature is lowered with other DES / NMPi ratios tested. With 10 wt% of DES, the critical solution temperature is at 28°C, while with 20 wt%, it is 36°C (Table 1,entries 2 and 3). Without NMPi (30 wt% DES 2:1 and 70 wt% H 2 O) the system is already biphasic at room temperature. In absence of DES (30 wt% of NMPi in 70 wt% of water) the system is also already biphasic in presence of CO 2 ( = 0.4) at room temperature. While keeping 70 wt% of water,  = 0.40 and OA/LD = 67/33, (Table 1, entries 1-5), the highest critical solution temperature observed is when using the same quantity of NMPi and DES; 15 wt% of each component. Therefore, the solvent composed of DES-NMPi-water 15-15-70 (wt%) is selected as an optimal composition of the solvent for CO 2 capture. Indeed, its LCST is appropriate for the application: above the typical temperature of CO 2 capture process (40°C) and not too high to reduce required energy. As the ratio of the two constituents of the DES can be modified, we decided to study the impact of their molar concentrations on the critical solution temperature after absorption of CO 2 (α = 0.40). While the phase separation was observed to be 43°C with the concentration of 67 mol% of OA (DES 2:1), changing the molar ratio of the two constituents has an obvious impact on the critical temperature. The temperature of the phase separation increases continuously from around 30 to 70°C when increasing the amount of OA from 60 to 80 mol% in the DES (Figure 2). All concentrations and temperatures are given in Table 1, entries 1 and 7 to 10. With lower concentration of OA (DES 1:1) the mixture is already biphasic at room temperature. It is worth to mention that for the range of concentrations considered in this work, oleic acid is the main substance of the DES and it is a fatty acid obtained from vegetal oil. Oleic acid is up to 80 wt% of the fatty acid composition of high oleic vegetable oils. [START_REF] Sakurai | The development and application of novel vegetable oils tailor-made for specific human dietary needs[END_REF] It is then interesting to note that it is possible to tune the property of the solution to have the separation at a specific temperature in order to adapt to the requirement of a chemical process. This adjustment of the critical temperature can be achieved simply by changing the composition of the solvent (ratio of DES/NMPi as well as ratio OA/LD) without changing any of the others parameters. We also studied the impact of the CO 2 concentration on the critical temperature. This investigation was realized with solvent composed of 70, 15 and 15 wt% of water, DES and NMPi, respectively, and with a 2:1 (OA:LD) molar ratio in the DES. Changing the CO 2 loading is also impacting the critical solution temperature. When increasing the CO 2 loading from 0.19 to 0.54, the temperature constantly decreases from around 65 to 27°C (Figure 3). Without CO 2 , the phase separation could not be observed up to 80°C. The decrease of the critical solution temperature with the addition of CO 2 is coherent with the observation of other tertiary aminewater mixture, where the addition of CO 2 is also inducing a phase separation at constant temperature in similar condition. [START_REF] Chen | A fatty acid solvent of switchable miscibility[END_REF] This is also in accordance with the behavior of aqueous solutions of alkyl-piperidines in presence of CO 2 (with similar concentrations of amine and CO 2 ). [START_REF] Coulier | Liquid-liquid phase separation of {amine -H2O -CO2} systems: New methods for key data[END_REF]

Evaluation of the separation in an optimized solution composed of DES (OA:LD) -

NMPiwater -CO 2

For a carbon capture application the kinetic of separation of two phases is a critical factor.

While considering the DES (2:1) -NMPiwater system (15, 15 and 70 wt%, respectively) with a sorption of CO 2 (α = 0.40) at 70°C, the separation of the two phases is occurring within few minutes. Above this temperature, separation of two phases is not improved (see supplementary information, Figure S1) and would require more energy. Thus, 70°C has been selected as optimal separation temperature. The kinetic of separation is naturally depending of several parameters including residual movement of the fluids and shape of the container. In the illustration presented in Figure 4, the apparition of the two phases occurs during the first minute, and the volume of the two phases is constant after only three minutes. The bottom phase (aqueous) is clear during the all experiment while it takes dozens of minutes for the top phase (organic) to become nearly transparent. The cloudiness of the phase is most probably corresponding to remaining droplets of water in the organic phases. DES and water have similar densities and the initial amount of water in the solution being 70 wt%, we could then expect that, if water and DES are well separated, the aqueous phase should be around 70% of the total volume. Nevertheless, as observed on Figure 4, the volume of the aqueous phase is smaller than the organic phase. Then, an experiment was realized in a NMR tube to assess the volume ratio of the two phases (see supplementary information Figure S1). In coherence with the picture of Figure 4, the volume of the organic phase was estimated to be 70% +/-3% of the total volume at 70°C. Water should then be poorly separated and present in large quantity in both phases.

The system was then evaluated by estimating the amount of chemical after 15 minutes at 70°C in both phases. Karl-Fischer titration was used to determine the quantity of water in aqueous and organic phases. Water is present in both phases in large quantity: water is the main compound in the two phases with 81 and 57 wt% in aqueous and organic phases, respectively. Quantifications of OA, LD and NMPi were obtained by quantitative proton NMR (q 1 H NMR, spectra are given in SI). In the aqueous phase, OA has not been detected and the quantity of LD is below 0.5 wt%.

In the organic phase, the ratio of OA and LD is 2:1, identical to the initial ratio, as expected because of the low concentrations of DES constituents in the aqueous phase. Unlike OA and LD, NMPi is poorly separated with 16 and 11 wt% in organic and aqueous phase, respectively.

Finally, the amounts of CO 2 present in both phases are obtained by IR spectroscopy. The CO 2 signal at around 2340 cm -1 has not be observed indicating the absence of physi-sorption, even at 40°C before heating, meanings that all absorbed CO 2 have reacted with the aqueous solution. One band is clearly visible at 1357 cm -1 when comparing solution before and after sorption as shown in Figure 5. This band is assigned to the carboxylate symmetric stretching vibration of HCO 3 -.

[39] Variation of the quantity of HCO 3 -in both phases is obtained using the intensity at this wavenumber. The phase separation induces an increase by 20% in the aqueous phase (green spectra in Figure 5), while the concentration decreases by 15% in the organic phase (black spectra). ) is also given as comparison (blue spectra). Dotted line at 1357 cm -1 corresponds to the signal used to estimate the quantity of CO 2 . IR spectra have been shifted for clarity reason.

Considering the concentration of CO 2 estimated from IR spectroscopy, we can evaluate that around 60% of the absorbed CO 2 remains in the organic phase and only 36% of the CO 2 is in the aqueous phase. The residual 4% could be explained by a possible small desorption of CO 2 during the heat from 40 to 70°C and also by the uncertainty of this rough methodology. For each compound, the estimation of the distribution between the two liquid phases is given in table 2. While the volume of the aqueous phase is 30%, the amount of CO 2 in this phase is 36% which correspond to a moderate improvement of its concentration with the phase separation. This modest separation is probably linked to the high concentration of water in the organic phase: H 2 O and HCO 3 -are similarly distributed between aqueous and organic phases. The majority of the CO 2 has clearly not been concentrated in aqueous phase and the phase separation induced by heating is inappropriate for a CO 2 capture process in the conditions tested in this work. These results could be compare to a well characterized biphasic system composed of water, CO 2 and three amines DMCA (N,N-dimethylcyclohexylamine), MCA (methylcyclohexylamine) and MAPA (3-methylaminopropylamine) with around 30 wt% of amine at 60°C, above the liquid -liquid separation. [START_REF] Tzirakis | Experimental measurement and assessment of equilibrium behaviour for phase change solvents used in CO2 capture[END_REF] In this case, concentrations of CO 2 in both phases are around 1 and 5 wt% in the organic and aqueous phase, respectively. [START_REF] Tzirakis | Experimental measurement and assessment of equilibrium behaviour for phase change solvents used in CO2 capture[END_REF] With such a large difference of concentration between both phases, the organic phase could be send back directly to the sorption unit without further treatment and the phase separation could be beneficial. In the case of DES (2:1), NMPi and water (15, 15, and 70 wt%), the difference of concentration of CO 2 between both phases is too small to apply this system in the field of carbon capture; 2.2 and 3.2 wt% of CO 2 in organic and aqueous phase, respectively. However, these primary results show the correct tendency. In fact, it is remarkable to observe that both OA and LD are very well concentrated in the organic phase while CO 2 and water are poorly accumulated in the aqueous phase.

Experimental details

Chemicals

Lidocaine (LD, 99% purity) and oleic acid (OA, 99% purity) were obtained from TCI chemical while N-methylpiperidine (NMPi, 99% purity) was purchased from Sigma-Aldrich.

CO 2 (99.995%) and N 2 (99,2%) were supplied by Air Products. The water used was monodistilled and the methanol-d 4 and D 2 O used for q 1 H NMR (99%) was purchased from Eurisotop.

DES preparation

DES (OA:LD) were prepared by heating the mixture at 80°C (above the melting temperature of the LD) for few minutes followed by a stirring to obtain a perfectly transparent liquid. DES and other liquid mixtures of 10 g were prepared with microbalance (precision of +/-0.05 mg).

Phase separation induced by CO 2 sorption in aqueous solutions

4 aqueous DES mixtures were prepared with 70 wt% of water. The compositions of DES were 45, 50, 55 or 60 mol% of OA. A magnetic stirrer and 2.5 g of the mixture were placed in a vial of 4 ml (with a septum) and was equilibrated at 20°C in a thermostated bath (Julabo, ME-18V) for at least 15 minutes. A first picture of each vial was taken after stirring vigorously the monophasic liquid phase. Images were obtained using a camera (Logitec, C920). Some small bubbles could be observed at this stage because of the stirring. At this first stage, the gas phase in the vial was air at atmospheric pressure. Then, two needles have been used to flush gases into the vials. The first needle was used to add gas (either N 2 or CO 2 ) and was immerged into the liquid phase, while the other was used to release any overpressure. The gas was added using a balloon. During the second step, the CO 2 was bubbled into the liquid under continuous stirring using a magnetic The quantity of water in both phases was estimated with a Karl-Fischer Coulometer DL32 from Mettler Toledo.

The relative concentration of CO 2 before separation and in both phase after separation at 70°C was obtained by ATR-FTIR spectroscopy. IR spectra were measured with a Nicolet 380 FT-IR spectrometer equipped with a single reflection diamond ATR cell from Specac (Golden Gate) and a DTGS detector. IR spectra were recorded at room temperature with a co-addition of 32 scans and a resolution of 4 cm -1 . Similar results (+/-2%) were obtained from two separates samples for all analyses (NMR, IR, KF). More technical details can be found elsewhere. [START_REF] Longeras | Design and Comprehension of New Deep Eutectic Solvents[END_REF][START_REF] Brugère | The Use of IR Spectroscopy to Follow the Absorption of CO2 in Amine Media -Evaluation of the Speciation with Time[END_REF][START_REF] Coulier | Experiments and Modeling for CO2 Capture Processes Understanding[END_REF]

Figure 1 .

 1 Figure 1. All tested ratio of DES (OA:LD molar ratios of 45:55, 50:50, 55:45 and 60:40) give the same behavior : monophasic before addition of CO 2 , biphasic liquidliquid after absorption of CO 2 at 20°C and monophasic again after flushing N 2 . These observations are comparable to those made on an ionic liquid based on tetrabutylphosphonium N-trifluoromethanesulfonyl leucine.

Figure 2 :

 2 Figure 2: Evolution of critical solution temperature while changing the ratio of OA and LD in the DES. Initial composition of all samples is 70 wt% of water, 15 wt% of DES and 15 wt% of NMPi and the loading of CO 2 is α = 0.40. Black line is only a guide for the eye.

Figure 3 :

 3 Figure 3: Impact of the amount of CO 2 on the critical solution temperature. Initial composition of all samples is 70 wt% of water, 15 wt% of DES (2:1) and 15 wt% of NMPi. Black line is only a guide for the eye.

Figure 4 :

 4 Figure 4: Pictures of the separation of the two phases without stirring at 70°C of a solvent with DES 2:1, NMPi and water (15, 15, and 70 wt%) after sorption of CO 2 (α = 0.40). After an equilibration of 15 minutes at 40°C (homogeneous transparent liquid), the vial containing 0.67 g of solution was place in a water bath at 70°C.

Figure 5 :

 5 Figure 5: Infrared spectra of sample with DES 2:1, NMPi and water (15, 15, and 70 wt%) before CO 2 sorption (purple spectra), after sorption of CO 2 (α = 0.40) and before phase separation (red spectra),

stirrer bar during 1

 1 hour. Bubbles appearing out the first needle are used to confirm that the gas is flushing slowly through the solution. The liquid was systematically becoming white during this step. The stirrer was then stopped and a second picture was taken after equilibration. Then, N 2 has been flushed slowly during 1 hour under stirring of the solution and a third picture has been taken for each sample to illustrate the switch back to monophasic liquid. Identical results were obtained by two separated experiments.CO 2 loadingpropanesulfonic acid sodium salt (98%, Aldrich) was used as standard in an 80/20 methanold 4 /D 2 O mixture as solvent. Trimethylsilyl (0.0 ppm, standard), methyl of NMPi (s, 2.63 ppm) and methyl of lidocaine (1.15 ppm) were used.

Table 1 :

 1 Critical solution temperature of mixtures after absorption of CO 2 . 1 : LCST: low critical solution

	temperature corresponding to the cloud point while heating. 2 :L-L correspond to a system which is always
	biphasic liquid -liquid between 25 and 80°C. 3 :L is a mixture which is always monophasic in this range
	of temperature.
	Entry	Solvent composition	DES ratio CO 2 loading LCST 1
		NMPi	DES	H 2 O	OA/LD	mol CO 2 /	(°C)
		(wt%)	(wt%)	(wt%)	(mol%)	mol NMPi
	1	15	15	70	67/33	0.40	43
	2	30	0	70	-	0.40	L-L 2
	3	20	10	70	67/33	0.40	28
	4	10	20	70	67/33	0.40	36
	5	0	30	70	67/33	-	L-L 2
	6	15	15	70	50/ 50	0.40	L-L 2
	7	15	15	70	60/40	0.40	31
	8	15	15	70	71/29	0.40	51
	9	15	15	70	75/25	0.40	55
	10	15	15	70	80/20	0.40	69
	11	15	15	70	67/33	0.00	L 3
	12	15	15	70	67/33	0.19	65
	13	15	15	70	67/33	0.25	56
	14	15	15	70	67/33	0.37	51
	15	15	15	70	67/33	0.47	33

Table 2 :

 2 Evaluation of the separation at 70 °C after 15 minutes calculated from the concentrations obtained from Karl-Fischer titration (H 2 O), q 1 H NMR spectroscopy (NMPi and DES) and IR spectroscopy (chemisorbed CO 2 ). The percentages given correspond to the proportion of chemical present in each phase considering a 70/30 volume ratio between organic and aqueous phases.

		Organic phase Aqueous phase
	Volumes	70%	30%
	CO 2	60%	36%
	H 2 O	62%	38%
	NMPi	77%	23%
	DES	99%	1%

Acknowledgments

The authors acknowledge the financial support received from the Agence Nationale de la Recherche of the French government through the program ''Investissements d'Avenir'' (16-IDEX-0001 . Giovanni Ramdani and Juzaimi Nurhazwani are acknowledged for their help during their graduations work in the preliminary screening of amines.

Conclusions

A first example of an aqueous DES solution with a phase separation induced by the addition of CO 2 at 20°C has been presented. In an aqueous solution of this DES based on OA and LD, bubbling N 2 into the biphasic liquidliquid system is switching back to a monophasic liquid.

The quantity of CO 2 absorbed into DES (OA:LD)water mixture is moderate and this system could not be employ for the issue of carbon capture. However, aqueous solution of DES (OA:LD) could still be employed as a switchable hydrophobic solvent, where temperature or CO 2 can be the stimulus inducing the switch.

Another amine (NMPi) was added into the solution to increase substantially the absorption of CO 2 . In the mixture with DES -NMPiwater -CO 2 , it is possible to tune the critical temperature with constant concentrations of water, DES, amine and CO 2 , simply by changing the (OA:LD) molar ratio of the DES. The temperature could be increase from around 30 to 70°C with higher concentrations of oleic acid. This possibility to adapt the critical temperature simply by changing the ratio of the two components of the DES is a practical way to adapt the property of the solvent to a specific application.

Unfortunately, the current system evaluated in this work does not have a good CO 2 separation.

Even if the DES is very well separated, the CO 2 has been concentrated by only 20% in the aqueous phase compared to the homogenous solution. This system is a pioneer for upcoming research on CO 2 capture by demixing solvent; it is illustrating the possibility to concentrate the CO 2 in the aqueous phase using a DES composed of an amphiphilic (LD) and a hydrophobic (OA) molecules. Our forthcoming project will then focus on the screening of many parameters; concentrations of OA, LD, amine, water and CO 2 as well as the possibility to test other DES and amine in order to found composition where the CO 2 and water are less present in the organic phase.

CO 2 sorption was realized under continuous stirring at 40°C and under constant pressure of 0.2 MPa of CO 2 using a high pressure syringe pump (Teledyn Isco 260D). When the CO 2 is in contact with the aqueous solution, the CO 2 is absorbed and increase the weight of the solution.

The CO 2 sorption was then calculated with the mass uptake of the sample after sorption. CO 2 was not bubbling directly into the liquid to prevent the loss of the solvent in the needle. The sorption was then slow as taking place only at the gasliquid interface. A typical CO 2 sorption with 5 g of solution (water, DES and NMPi with 70, 15 and 15 wt%, respectively) took about 40 minutes.

After the vial was opened shortly to replace the 0.2 MPa of CO 2 of the gas phase by air at atmospheric pressure. The typical uptake is around 160 mg of CO 2 with α = 0.5 for 5 g of solution. This experimental protocol was validated was the sorption of CO 2 of well-known MEA/H 2 O systems. The uncertainty of loading charges is estimated at 2%.

Visual determination of critical solution temperatures

Critical solution temperatures were observed using a 2ml sample in a thermostated water bath equipped with two insulated glass windows (Julabo ME-18V). The bath was heated at a constant rate of 0.25°C per minute up to 80°C using the controller of the thermostated bath. Temperature of the water bath was measured with a 100 Ω platinum resistance thermometer immerged in the water bath. During the heating, images were taken every two minutes by a camera (webcam, Logitec C920) and Yawcam software. The temperature was defined as the temperature of the first picture where the solution became cloudy. Uncertainty of critical temperatures is estimated at +/-1°C.

Quantification of chemicals in both phases

NMR spectra were recorded in Fourier transform mode with a Bruker AVANCE 400 spectrometer at 298 K. For the quantification, q 1 H NMR sequence (128 scans) according to references [START_REF] Pauli | Importance of Purity Evaluation and the Potential of Quantitative 1H NMR as a Purity Assay[END_REF][START_REF] Lee | Equilibrium between carbon dioxide and aqueous monoethanolamine solutions[END_REF] was used. Internal standard for q 1 H NMR measurement of the organic phase (with DES) was 1,4-dimethoxybenzene (1,4-DMB, Aldrich), recrystallized twice in cyclohexane.

Its purity (99.5%) was determined using q 1 H NMR sequence with malonic acid (gold reference Aldrich). Methanol-d 4 as solvent was used. Methoxy of the standard (s, 3.75 ppm), methyl of the NMPi (s, 2.50 ppm), aliphatic methyl of lidocaine and oleic acid, (t, 1.18 ppm and t, 0.92 ppm respectively) were used for the calculations. For the aqueous phase, 3-(trimethylsilyl)-