domain: chim spi

text:
F Lanoue

F Veron

I Pasquet

K Kiryukhina

V Baco-Carles

O Vendier

Ph Tailhades
email: philippe.tailhades@univ-tlse3.fr

S N Monteiro

Treatment of AlSi12 feedstock with water to manufacture multi-material parts by laser powder bed fusion process: A potential route for the fabrication of inorganic 3D metamaterials

Keywords:

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Treatment of AlSi12 feedstock with water to manufacture multi-material parts by laser powder bed fusion process: A potential route for the fabrication of inorganic 3D metamaterials

Introduction

Laser-Powder Bed Fusion (L-PBF) is an additive manufacturing process that has already attracted interest from the aerospace, automotive, medical and jewelry industries [START_REF] Yap | Review of selective laser melting: materials and applications[END_REF] and continues to grow. Conventional L-PBF machines however, have only one powder supply tank. As a consequence in their basic version they cannot produce parts in which the spatial distribution of several materials can be controlled [START_REF] Rafiee | Multi-material 3D and 4D printing: a survey[END_REF]. At best, it is possible to produce composites that randomly combine several materials at the micro-or nanoscale [START_REF] Yu | Particle-reinforced metal matrix nanocomposites fabricated by selective laser melting: a state of the art review[END_REF][START_REF] Koopman | Additive manufacturing of a steel-ceramic multimaterial by selective laser melting[END_REF][START_REF] Pasquet | A multimaterial based on metallic copper and spinel oxide made by powder bed laser fusion: a new nanostructured material for inert anode dedicated to aluminum electrolysis[END_REF]. This is a major drawback that limits the use of the conventional L-PBF process [START_REF] Zhang | Additive manufacturing of functionally graded material objects: a review[END_REF]. To overcome this difficult technological constraint [START_REF] Th | Mass flow characterization of selective deposition of polymer powders with vibrating nozzles for laser beam melting of multi-material components[END_REF] and to be able to build parts in which discrete or gradual changes of materials are found [START_REF] Ch | Multi-Material Processing. Next step in laser-based powder bed fusion[END_REF], special machines have been designed. They can be classified into the following four categories [START_REF] Ch | Recent progress and scientific challenges in multimaterial additive manufacturing via laser-based powder bed fusion[END_REF].

The blade-based dissimilar powder spreading technique [START_REF] Andriani | Review on melting of multiple metal materials in additive manufacturing[END_REF] deposits two materials each contained in a feed compartment. These dissimilar materials can only be layered but cannot be arranged in the same plane. This process only produces multilayer structures along the vertical axis, which also results in rough interfaces between the two materials.

The ultrasonic-based dissimilar powder spreading method uses two silos containing different powders. The latter allows for local and programmed spreading of controlled quantities of one or other powder, thanks to an ultrasonic device. This process leads to the production of 3D multi-material parts [START_REF] Hinduja | Characteristics of the bond in Cu-H13 tool steel parts fabricated using SLM[END_REF]. However, the slowness of the manufacturing process remains a weak point.

A third technology, called electrophotographic spreading of dissimilar powder, is inspired by the principle of desktop laser printers. It allows to deposit locally, in thin layers, particles from different toners [START_REF] Benning | Proof of concept of a novel combined consolidation and transfer mechanism for electrophotographic 3D printing[END_REF]. The laser fusion makes it possible to combine different materials in the same plane and to manufacture parts composed of at least two materials distributed in the three directions of space. Spreading systems based on a similar principle, in which the powders are supported on the surface of a drum by purely mechanical forces, are adaptable to various L-PBF or binder jetting machines [START_REF] Neirinck | Powder deposition systems used in powder bed-based multimetal additive manufacturing[END_REF]. They are currently marketed by Aerosint [14] and offer the possibility of building multi-material parts in three dimensions, without generating a significant mixing of the powders used.

Finally, the fourth variant of these multi-material processes is the 'Blade + ultrasound' hybrid method for spreading dissimilar powders. In this case, a blade spreads the powder making up the main volume of the part, which is then fused by the laser. A portion of the untreated powder is removed by a micro-suction system and replaced by a second powder supplied by ultrasonic pipettes. The latter is in turn fused by the laser to form the second material of the part under construction [START_REF] Ch | 3D Printing of multiple metallic materials via modified selective laser melting[END_REF]. The recycling of the residual powders is difficult due to their mixing, but this process allows the use of several pipettes and the construction of parts with more than two different materials.

Very recently, colloid nebulization has also been proposed to fabricate multi-materials by L-PBF process [START_REF] Mussatto | Laser-powder bed fusion in-process dispersion of reinforcing ceramic nanoparticles onto powder beds via colloid nebulization[END_REF]. Small quantities of tungsten carbide can be added to 316-L alloy by this method. However, with the exception of the recently introduced Aerosint devices, which transform the L-PBF process into a true Multi-Material Additive Manufacturing (MMAM) technique, machines operating on the concepts described briefly above, are not commercially available at this time.

The most studied materials for L-PBF process are iron alloys, but aluminum alloys are also receiving much attention, especially those containing silicon and/or magnesium [START_REF] Kusoglu | Research trends in laser powder bed fusion of Al alloys within the last decade[END_REF][START_REF] Lathabai | Additive manufacturing of aluminium-based alloys and composites[END_REF][START_REF] Aboulkhair | 3D printing of aluminium alloys: additive manufacturing of aluminium alloys using selective laser melting[END_REF]. For L-PBF, the controlled combination in three dimensions of space, of aluminum or its alloys with another metallic or ceramic material, obviously poses similar problems to those mentioned above. Indeed, the bibliography only reports work on ceramic-reinforced aluminum composites [START_REF] Minasyan | Laser powder-bed fusion of ceramic particulate reinforced aluminum alloys: a review[END_REF] or iron/iron-aluminum alloy multilayers [START_REF] Goll | Additive manufacturing of soft magnetic materials and components[END_REF]. However, a new approach has recently been proposed [START_REF] Veron | Selective laser powder bed fusion for manufacturing of 3D metal-ceramic multi-materials assemblies[END_REF]. It consists in chemically activating an aluminum alloy powder with an alkaline base, to give it a higher reactivity to oxygen. Thus, from this single powder, a conventional machine operating in air or an oxygen-enriched atmosphere, can either fuse at low energy density, zones in the metallic state, or oxidize and sinter other zones in ceramic form, by simply applying a higher laser energy density to them. Metal and ceramic can thus be combined in all three directions of space.

Aluminum reacts with pure water by releasing heat and hydrogen [START_REF] Wefers | Oxides and hydroxydes of aluminum[END_REF][START_REF] Wang | A review on hydrogen production using aluminum and aluminum alloys[END_REF]. This exothermic reaction can be easily achieved at room temperature with nano-sized aluminum powders [START_REF] Kader | A novel method for generating H 2 by activation of the μAl-water system using aluminum nanoparticles[END_REF]. Aluminum is therefore potentially a material for large-scale energy storage [START_REF] Haller | Combined heat and power production based on renewable-aluminum-water reaction[END_REF] or for small mobile devices [27], as the hydrogen generated upon contact with water can be converted into electricity by fuel cells. But beyond these potential technological applications, the reaction of water with aluminum could be used to activate AlSi12 alloy powders, implemented in the multi-material L-PBF process proposed by Veron et al. [START_REF] Veron | Selective laser powder bed fusion for manufacturing of 3D metal-ceramic multi-materials assemblies[END_REF]. The main advantage of chemical activation with pure water would be absence of sodium or potassium pollution from the alkaline base, used to etch the starting metal powder. The ease of recycling liquid effluents, almost exclusively made up of water, as well as the potential valorization of the hydrogen gas produced, would also be significant advantages of this original mode of activation. However, the powders intended for a L-PBF process, made of an aluminum alloy on the one hand, and grains of a few tens of micrometers on the other hand, differ strongly from the pure nanometric aluminum most often used for energy devices. The activation of micrometric powders by water therefore requires specific experimental conditions, which remain to be developed. In particular, the activation must be controlled so as not to degrade the total volume of the grains and to affect only their surface.

As multi-material additive manufacturing by L-PBF is a major issue, it is interesting to evolve the process proposed by Veron et al. considering its virtues related to the use of a single powder and the use of conventional L-PBF machines. The first objective of the present work was therefore to search for suitable processing conditions allowing the activation of micrometric AlSi12 powders, by a simple reaction with water. It was thus necessary to find activation conditions allowing the oxidation of the powders and their sintering in the form of ceramics, under the effect of a high energy density brought by the laser of the L-PBF machines. But it was also necessary that these powders could be melted for lower laser energy densities, in order to form mechanically resistant metallic zones.

This work also has a second objective. It is to demonstrate that from a single activated powder and by implementing a conventional L-PBF machine, it is possible to fabricate complex ceramic-metal assemblies or periodic structures alternating ceramic and metal. Inspired by photonic [START_REF] Joannopoulos | Photonic crystals: molding the flow of light[END_REF][START_REF] Yablonovitch | Photonic band-gap crystals[END_REF], magnonic [START_REF] Nikitov | Spin waves in periodic magnetic structures: magnonic crystals[END_REF][START_REF] Gulyaev | Ferromagnetic films with magnon bandgap periodic structures: magnon crystals[END_REF] or phononic [START_REF] Kushwaha | Acoustic band structure of periodic elastic composites[END_REF] crystals, these structures could constitute new 2D or 3D metamaterials, likely to have technological applications in the field of microwave or mechanics.

Materials and methods

Feedstock and additive manufacturing machine

The basic feedstock was an AlSi12 powder from 3D Systems (PS2585-18) with the eutectic composition. This powder contains aluminum and silicon phases as revealed by X-ray diffraction. Such a powder is made of almost spherical particles with a median size close to 4.5 μm according to calculations made from the specific surface area (see appendix 1 equation λ). The basic feedstock was chemically treated by pure water. The process carried out is described and studied in the Result and Discussion part.

The powder bed laser fusion experiments were performed under air. A 3D Systems ProX200 machine was used. The laser beam emitted by PROX200 has a wavelength of 1070 nm and it is focused on a spot of 75 μm in diameter. Its power can be regulated from 30 to 300 W by step of 3 W.

Analytical techniques

The crystalline structure of the samples was studied using Bruker AXS D4 Endeavor diffractometer equipped with a 1D LynxEye detector. Kα radiations (λKα 1 = 0.15405 nm and λKα 2 = 0.15443 nm) emitted by a copper anode, were used as X-ray source. The Kβ ray was eliminated by a nickel filter. A second diffractometer was used (Brucker D8 Endehavior), that has a LynxEYE detector and the same wavelength as before. The Xray source is a copper anode with the following Kα radiations: λKα 1 = 0.15405 nm and λKα 2 = 0.15443 nm. No Kβ filter is used for this device.

The microstructure of samples and patterns written by laser, were observed by optical (Optical numerical microscope VHX 1000 from Keyence) and scanning electron (Jeol JSM 6510LV) microscopes. Surface roughness analyses were carried out with Sensofar S-NEOX focused variation microscope. The chemical composition of samples was analyzed by Energy Dispersive Spectroscopy (EDS) with a Brucker Quantax electron microscope analyser. A focused ion beam combined with field effect emission gun (SEM/FIB FEI Helios 600i) associated with Oxford Instruments AZTEC Advanced electron microscope analyser, were used to prepare cross-sections from green pellets prepared by uniaxial compaction at 750 MPa of raw or activated metal AlSi12 powder. This equipment made it possible to carry out chemical and microstructural analyzes. Another similar system (Helios Nanolab G3UC from Thermofisher) was also used for some experiments.

The optical properties of the AlSi12 alloy powders, before and after chemical treatment, were analyzed by an Agilent Cary 5000 spectrophotometer equipped with a PbSmart NIR detector. The detection range is from 175 nm to 3300 nm. The specular mode which includes diffuse reflection and specular reflection, has been considered. Infrared spectrometry analyses were carried out between 400 and 4000 cm -1 , using a Nicolet 6700 apparatus from Thermofisher Scientific. The specific surface area of the powders was measured by a Tristar 3020 of Micromeritics, using the Brunauer, Emmett and Teller model (BET). The adsorbed gas was nitrogen. The density of metal and ceramic manufactured parts was measured with an Archimedes balance.

X-ray Computed tomography (CT) was carried out using a Phoenix V |tome| x M microfocus CT system from Baker Hughes. The scan was performed at 140 kV and the voxel size was 8 μm. The data was processed using Baker Hughes reconstruction software and VGSTUDIO MAX application suite (from Volume Graphics) was used for virtual cross-sections.

Results and discussion

Feedstock chemical treatment

As indicated in the introduction, the first objective of this work is to transform the surface of AlSI12 alloy powders with pure water. This treatment should lead to an activated powder with properties similar to those treated with sodium hydroxide [START_REF] Veron | Selective laser powder bed fusion for manufacturing of 3D metal-ceramic multi-materials assemblies[END_REF]. Thanks to the increase of the reactivity towards oxygen and of the absorbance at the laser wavelength of the L-PBF process, the activated powder should then allow the manufacture under air, of parts with ceramic zones, mainly made of alumina, and metallic zones, based on AlSi12 alloy [START_REF] Veron | Selective laser powder bed fusion for manufacturing of 3D metal-ceramic multi-materials assemblies[END_REF](Fig. 1).

The reactivity with water of the commercial AlSi12 powder (cf. Materials and Methods) was then tested. The micrometric size of its grains does not allow, at room temperature, to initiate a significant reaction with water, in agreement with the observations of Rosenband and Gany [START_REF] Rosenband | Application of activated aluminum powder for generation of hydrogen from water[END_REF]. The hydrogen evolution is only observed after an initiation time in order of 10 min to nearly 1 h, depending on the water temperature. It then follows a chemical attack of the metal part which, in the case of pure aluminum, is described by the following chemical equation [START_REF] Wang | A review on hydrogen production using aluminum and aluminum alloys[END_REF][START_REF] Kader | A novel method for generating H 2 by activation of the μAl-water system using aluminum nanoparticles[END_REF]:

2Al + 6H 2 O → 2Al(OH) 3 +3H 2 (1)
Due to the less reactivity of silicon, this reaction could be written as follow for AlSi12 powder (by assimilating AlSi12 to Al0.88Si0.12):

2Al 0.88 Si 0.12 +5.28H 2 O → 1.76Al(OH) 3 +0.24Si + 2.64H 2 (2)
This equation is supported by microscopic observations, which reveal that metallic silicon remains inside the hydroxide aluminum layer (Fig. 2).

When the reaction is stopped even after a short time, a change in color of the dry powder is observed. The initial grey color becomes black-brown to black, depending on the progress of the activation. This change in optical absorption in the visible is also evident at the wavelength of the laser (1070 nm) used by the L-PBF process (see for instance Fig. 3). It increases significantly as a function of the progress of the activation reaction, which can thus be measured. The specific surface area of the powder also increases with time of contact with hot water (Fig. 4), because hydroxide layer formed on surface of the grains is porous. Thus, the thicker it is, the greater the overall specific surface area of the activated powder. The specific surface area can therefore characterize the progress of the activation. The values of optical absorption and specific surface area are strongly correlated as shown in Fig. 5. This also highlights that the optical absorption varies greatly for weak activations, while it tends to stabilize when the reaction is more advanced. Optical absorption is therefore only relevant for qualifying very weakly activated powders, unlike the specific surface area whose sensitivity extends over a wider domain. The optical measurement gives however information about the interaction of the laser with activated powder. The more the powder absorbs the light from the laser, the easier it will be to heat and to oxidize it.

Optical absorption at 1070 nm and specific surface area measurements, therefore make it possible to compare the activated powders with each other. They also allow to have a qualitative view of undergone modifications by AlSi12 powder grains during treatment with hot water. However, to characterize activation process more finely and to know more precisely the stages and their kinetics, as well as to quantifying the surface modifications generated, other experiments have been implemented.

A follow-up of the evolution of hydrogen, variation in pH and temperature, was carried out thanks to the setup of Fig. 6. This device has the advantage of measuring the volume of hydrogen released with a quite good precision. At room temperature, a volume of hydrogen of only 1 ml displaces a mass of 1 g of water, which is very easy to measure with a laboratory balance. Moreover, according to equation ( 2), 1 ml of hydrogen is generated by 3.38. 10 -5 mol of AlSi12, i.e. 9.2. 10 -4 g. This means that it is possible to detect the attack of less than 0.1 % of metal.

Experiments at different temperatures, using the experimental setup in Fig. 6, revealed several main results. Hydrogen is released only after an initiation period of a few minutes, regardless of the initial water temperature (Fig. 7). This period is shorter the higher the temperature. It seems reasonable to think that the initiation time of the reaction corresponds to the chemical attack of the alumina layer, which generally protects the grains of aluminum or its alloys [START_REF] Trunov | Effect of polymorphic phase transformations in Al 2 O 3 film on oxidation kinetics of aluminum powders[END_REF][START_REF] Kovarik | High temperature transition aluminas in δ-Al 2 O 3 / θ-Al 2 O 3 stability range: review[END_REF][START_REF] Mirhashemihaghighi | Corrosion protection of aluminum by ultra-thin atomic layer deposited alumina coatings[END_REF]. The kinetics of hydrogen evolution after initiation tends logically to increase with temperature. The chemical attack of the alloy powder is even very sensitive to the temperature. For the concentration of 33 g L -1 of alloy in water, hydrogen is produced at 62 • C, at a rate of approximately 1.5 cm 3 min -1 after only 8 min, whereas at 43 • C the gas production is only 0.1 cm 3 min -1 after more than 45 min. These kinetics are also slightly modified by the concentration of the metallic powder in water (Fig. 7), but at 55 • C between 50 and 400 g.L -1 , the impact on the kinetics of the reaction is very small.

During the experiments, the exothermicity of aluminum oxidation tends to slightly increase the temperature of the alloy suspension in water. This was particularly observed for the study of very concentrated suspensions. But this temperature increase, limited to about one or two degrees for a short period of time, does not have a strong effect on the kinetics of the oxidation reaction. The pH of the metal suspension, however, seems to have a stronger influence. As shown by Fig. 8, the reaction starts above pH7.2 and is related to a continuous increase in pH. This observation tends to confirm the interpretation of Kader et al. [START_REF] Kader | A novel method for generating H 2 by activation of the μAl-water system using aluminum nanoparticles[END_REF], who attribute to the increase of the pH, the main role in the acceleration of the metal attack.

The quantities of aluminum transformed into aluminum hydroxide on the surface of the grains are evaluated from the measurement of the hydrogen produced (see appendix 1). The mass in grams of attacked aluminum can indeed be estimated from equation α in appendix 1, by 0.81 times the volume in liters of hydrogen produced: knowing that from equation ( 2), 1 g of attacked aluminum affects about 1.14 g of Al 0.88 Si 0.12 alloy.

M (Al transformed) ≃ 0.81V Hydrogen (g) (3)
With some approximations, it is also possible to deduce the average thickness t of hydroxide layer at the surface of the grains (see appendix 1, equation ν to ο). As an illustration, estimates of the total thicknesses of the hydroxide layers and the silicon they contain, are plotted as a function of the contact time with water for several experimental conditions (Fig. 9). These thicknesses increase more strongly with increasing temperature, the concentration of the powder in the water only playing a secondary role. Experiments were also carried out at 42 • C. The results obtained have not been plotted in Fig. 9 to preserve clarity. It should be noted, however, that the hydrogen release only begins after about 50 min of contact time with water. The estimated thickness of the hydroxide layer is only about 1 nm after 1 h of experiment.

Scanning microscopy of sections obtained by FIB (see Materials and Methods), of activated powder grains, highlights the transformed surface layers (Fig. 10). Their average thickness varies according to the experimental conditions of activation, but it is lower than those observed by Veron et al. [START_REF] Veron | Selective laser powder bed fusion for manufacturing of 3D metal-ceramic multi-materials assemblies[END_REF], which varied from 300 nm to 2 μm, depending on the sodium hydroxide concentration. This result shows that activation with water is milder than activation with sodium hydroxyde. However, in Fig. 10, it can be seen that when the grains are

Table 1

AlSi12 powder activation conditions and main characteristics of these powders. in contact with each other, the activated layers are compacted. They are thus thinner than in the grains whose surface, adjacent to a hole in the metal compact, is not compressed. The surface of the grains also shows a significant roughness visible on the areas cut by the FIB, as well as on the areas observed perpendicular to their surface. All these observations associated with specific surface area modifications (Fig. 4 and Table 1), confirm that the activation layer is porous. The simplifying assumption of a perfectly dense layer, used in the calculation of the average thickness (appendix 1), does not strictly correspond to reality. The calculated thicknesses (for instance 3 nm for the sample activated in water at 55 • C for 15 min and studied in Fig. 10) are thus lower than the average thicknesses measured on the electron microscopy images in the compressed zones (about 15 nm according to Fig. 10c) and, a fortiori, in the regions bordering a hole in the pellet (more than 200 nm in Fig. 10b). Some such regions are visible in Fig. 10a andb. They also show the surface roughness of the activated grains in areas with little or no compaction. The order of magnitude of the activated thicknesses obtained by calculation or evaluated from electron microscopy images being the same, the previously proposed activation mechanism seems however consistent. Given the very low thickness of these layers, it was difficult to precisely determine their nature. For the thickest layers, analysis by IR spectrometry and X-ray diffraction confirms however, the presence of bayerite-type aluminum hydroxide.

Several 400 g batches of powders activated with pure water under different conditions, were prepared for use in the manufacture of parts, by the L-PBF process. Two main objectives were targeted:

-to demonstrate that powders weakly activated by water and free of impurities containing sodium, improve the quality of the metal parts compared to those obtained by Veron et al. [START_REF] Veron | Selective laser powder bed fusion for manufacturing of 3D metal-ceramic multi-materials assemblies[END_REF], -to demonstrate that obtaining ceramic-metal multi-materials parts comprising well-densified ceramic zones, is possible with these powders.

It is known that unactivated powder (Specific surface area ≃ 0.5 m 2 g -1 ) can produce quality metal parts by L-PBF. Therefore, to achieve the first objective, it seems preferable to activate the powder only very little. However, this activation must be "sufficient" to also be able to obtain a

Table 2

Comparison of the relative densities and the roughness of the parts made from the reference AlSi12 alloy powder and the C powder (2.5 m 2 g -1 ). ceramic, with reasonable L-PBF manufacturing parameters [START_REF] Veron | Selective laser powder bed fusion for manufacturing of 3D metal-ceramic multi-materials assemblies[END_REF]. It was therefore chosen to explore activation conditions leading to the production of powders with a moderate specific surface area, typically less than 5 m 2 g -1 . Table 1 shows the production conditions and characteristics of the activated powders that will be used for the L-PBF process. These powders will allow the optimization of manufacturing parameters for metal parts on the one hand and ceramic parts on the other hand.

Manufactured parts by L-PBF of activated AlSi12 powders

Metal parts

The L-PBF process was applied to the previously selected powders. First, parallelepipedic metal parts of 10 × 10 × 8 mm 3 were prepared in order to determine the favorable conditions to obtain a well-densified, mechanically consistent material with moderate roughness. The thickness of the powder beds was set at 50 μm and the hatch spacing at 100 μm, standard values for the used AlSi12 alloy powders. On the other hand, the laser power and the scanning speed were adjusted to values from 54 to 225 W and 100-1200 mm s -1 , respectively. The results obtained are presented in a synthetic way (Fig. 11) by the evolutions of the relative density, as a function of the surface energy density E (J.mm -2 ) brought by the laser. This surface energy density is defined by:

E = P v.h (4)
with P the laser power, v the scanning speed and h the hatch spacing.

It should be noted that below a laser power of approximately 60 W and even 90 W when scanning speeds are high (greater than or equal to 800 mm s -1 ), a coherent part cannot be obtained. The very synthetic Fig. 11 does not show this result, which must however, be considered.

Fig. 11 shows that the best densifications are obtained for the lowest activated powders (A and C in Table 1), with specific surfaces areas of the order of 2 m 2 g -1 , to which a surface energy density between 1 and 2.5 J mm -2 has been applied. Stronger activation indeed, leads to a higher specific surface area which reflects a greater quantity of hydroxide formed. During laser exposure, this hydroxide transforms into oxide, which tends to prevent the sintering of the metal particles, because of wettability problems encountered in different metal-ceramic interfaces [START_REF] Wang | Computational and experiments exploration of convection on Cu filling characteristics of multiple aspect-ratio micro throughholes[END_REF]. For similar reasons, the hydroxide phase is also responsible for an increase in surface roughness. We chose to select the activated powder C(55 • C, 2.5 m 2 g -1 ) for the rest of the work. The best arithmetic roughness that it allows to reach is certainly a little higher than those obtained with the powder A (Ra = 35 against 25 μm), but the relative densities to which it allows to reach, are higher. Moreover, its greater specific surface area (2.5 against 1.9 m 2 g -1 ) will be more favorable to the oxidation of the metal, necessary for the formation of the ceramic.

The precise additive manufacturing conditions used for this powder are then as follows:

-Laser power P: 135 W -Scanning speed v: 800 mm s -1 -Hatch spacing: 100 μm Using these conditions, the activated powder produces parts with a quality approaching that made with conventional AlSi12 powder, as shown in Fig. 12 and the data collected in Table 2. X-ray diffraction (Fig. 13) also shows that parts made from pure water-activated powder, with a surface area of 2.5 m 2 g -1 , are predominantly aluminum and silicon, from the eutectic AlSi12 alloy, as with parts obtained with commercial AlSi12. A very minor secondary phase is present, however, in the parts derived from the activated powder, as revealed by two additional small peaks. These peaks are attributed to the (400) and (440) diffraction planes of γ-Al 2 O 3 oxide (see JCPDS reference n • 00-010-0425). It is not surprising that the temperature rise during laser melting, leads to the formation of an aluminum oxide by dehydration of the hydroxide layer formed on the surface of the particles. The chemical reaction involved is simply:

2Al(OH) 3 →Al 2 O 3 +3H 2 O (5)
However, the stabilization of the gamma variety of aluminum oxides, which usually occurs only as a high surface-to-volume ratio material, was not expected. The existence of strong mechanical stresses in the manufactured parts is very probably at the origin of the stabilization of this metastable phase.

Despite the few differences observed between the parts from activated or commercial powders, we can therefore conclude at this stage, that the first objective set in part 3.1 has been achieved.

We note that the relative density of 88 % for parts made from an activated powder remains low. The formation of oxide and hydrogen bubbles during the decomposition of water resulting from the dehydration of the hydroxide on the grain surface, are most likely responsible for the residual porosity. To reduce it, dehydration of the hydroxide of the raw material in the form of boehmite AlO(OH) or aluminum oxide, would have to be implemented. Conventional annealing or sintering by Spark Plasma Sintering could also rectify this defect, however affecting the initial dimensions of the part.

Ceramic parts

In order to achieve the second objective displayed in section 3.1, more energetic manufacturing conditions were applied to the activated powder, in order to ensure its oxidation and sintering into a properly densified ceramic. This approach was dictated by the pioneering work of Veron et al. [START_REF] Veron | Selective laser powder bed fusion for manufacturing of 3D metal-ceramic multi-materials assemblies[END_REF] who had however, practiced sodium hydroxide activation, which was much more severe than that implemented in the present work. Samples of 7 × 7x4 mm 3 surrounded by 1.5 mm of metal and resting on 4 mm thick a metal base, were therefore fabricated with different P, v and h. Phase diagrams P = f(v.h) were plotted (Fig. 14) to summerize the results of the different test arrays.

For high powers and low v.h products, i.e. for high surface energy densities, oxide and oxide nitride are mainly obtained. These are corundum alumina (α-Al 2 O 3 , JCPDS file n • 01-089-3072(C)) and aluminum oxide nitride (4AlN-96Al 2 O 3 , JCPDS file n • 00-020-0043 (N)) in which traces of metallic silicon may remain (Fig. 15). The alumina phase is predominant, but the severe heating conditions during manufacturing allow a partial reaction of nitrogen in the air, with aluminum. In the resulting oxide-nitride, however, nitrogen is a very minor constituent compared to oxygen. Starting from the oxide zone (Fig. 14) and moving in the diagram along a diagonal from the upper left corners to the lower right corners, one encounters a zone in which alumina and metal alloy can coexist, and then a zone in which only metal is identified. In contrast to the observations made by Veron et al. in experiments with sodium hydroxide activated powders, the transitions between the ceramic and metallic zones are therefore less abrupt. Moreover, the boundary separating these zones corresponds to a line of higher energy densities (about 70 J mm -2 ) than during sodium hydroxide activation (36 J mm -2 [START_REF] Veron | Selective laser powder bed fusion for manufacturing of 3D metal-ceramic multi-materials assemblies[END_REF]). This result was expected since water activation is weak and therefore, cannot have the same oxidation-facilitating effect as sodium hydroxide activation.

Ceramic-metal multi-materials and « metamaterials »

The process implemented to produce the metal or ceramic parts, as described above, is obviously very interesting for the production of multi-material parts. It is all the more interesting as it opens the way to the manufacturing of parts comprising at least two different materials deposited at will, in the three directions of space, using only the L-PBF technology and a single feedstock.

Thermal cycling tests were performed on such multi-materials between -55 • C and +125 • C with heating and cooling rates of 2 propagation, even after 200 cycles. The cohesion between these materials with very different characteristics, in particular their expansion coefficient, is thus strong. To date, however, a more in-depth study intended to demonstrate by electron microscopy, the development of dislocations or other modifications at the scale of atomic rows, has not been carried out.

The cohesion between the ceramic and the metal seems ensured by an "entangled" interface over a thickness of approximately 100 μm as shown in Fig. 16. In addition, during the construction of the ceramic areas of the parts, oxidation cannot stop sharply at the interface with the metal. There is necessarily a narrow band in which a metal-ceramic composite exists. The latter certainly allows a progressive accommodation of the expansion coefficients of ceramics and metal. This zone therefore also contributes to preserving the interfaces and avoiding the propagation of cracks that thermomechanical shocks could cause, given that metal has an expansion coefficient approximately three times greater than that of ceramic (α ceramic # 8 K -1 , α AlSi12 # 22 K -1 ).

However, this progressive and tangled interface, could be a drawback for some technological applications requiring sharper transitions between two materials. The components for microwave electronics, a priority technological application, could potentially be concerned. It will therefore be necessary in the future to adapt the manufacturing conditions in these interfacial areas to find the best compromises.

Using the developed process, it was possible to fabricate coaxial structures enclosing a conductive core in a ceramic, itself surrounded by metal (Fig. 17). All the fabricated devices conducted electricity through the sample (10 mm thick) in their central and peripheral part. Despite the color differences observed in the ceramic block due to some silicon precipitates, no electrically conductive areas were found in it. The dielectric character and its homogeneity within the ceramic were therefore satisfactory.

The process studied in this work has also favored the construction of even more complex structures, including a 2D and especially 3D periodicity, to get closer to the realization of metamaterials. This was done on small centimeter parts. Ceramic parts encircled with metal were thus built with metallic spheres placed periodically inside. These spheres were either 1.5 or 2 mm in diameter and were flush with the ceramic surface as shown in Fig. 18 for the case of 1.5 mm spheres. The truncation of each sphere presented on the surface of the ceramic, is a disc of the order of 1 mm in diameter. The detailed fabrication conditions of these parts are given in a schematic (Fig. 19). A hexagonal strategy was used to fabricate the majority of the ceramic and metallic parts. Two hoops, one ceramic and one metallic, were constructed to ensure the geometric shape of the samples.

The real part obtained (Fig. 18b) presents a rather high surface roughness (Ra >80 μm). However, the small metal balls that are flush with the ceramic are clearly visible. According to the construction file (Fig. 18a), the flush metal discs should have a diameter of 0.94 mm and be 1.45 mm apart. On the real part, we find very close values (Fig. 18c), which demonstrate the good conformity of the fabricated metal-ceramic structure, with the digital file. On the other hand, the high surface roughness does not allow to clearly observe the circular geometry of the metal discs section, as drawn in Fig. 18a. Using similar conditions, other ceramic parts, this time consisting of three superimposed layers of metallic spheres or half-spheres of 1 mm diameter, were fabricated. The arrangement of the metallic entities is face-centered cubic symmetry (Fig. 20). The metal spheres truncated by half are clearly visible on the real part (Fig. 20d). However, they are again embedded in a ceramic with a high roughness (Ra >80 μm). X-ray tomography allows us to observe the metallic inclusions located in the core of the ceramic piece (Fig. 21). However, this observation is disturbed by the presence of some silicon precipitates whose X-ray contrast is similar to that of the AlSi12 alloy. The geometric arrangement and overall dimensions of the "buried" metallic features are consistent with the digital object. Differences in dimensions of less than a tenth of a millimeter, have been observed in the (x,y) plane. It should be noted, however, that the spherical metallic entities are not perfectly fabricated. The ceramic seems to have penetrated these porous spheres. Although they are inscribed in a circle, their sections revealed by the tomography, are thus jagged, (Fig. 21).

There is still progress to be made to improve the geometric quality of surfaces and interfaces. An improvement will notably involve the search for more efficient laser exposure strategies (contours of ceramic or metal zones, local reflows, compaction of powder beds, etc.). Because powders play a key role in the quality of parts obtained by laser fusion, other powder particle sizes could also be advantageously used.

Microwaves extend from 300 MHz to 300 GHz, i.e. between wavelengths (in vacuum or air) of about 1 m to 1 mm. The three-dimensional periodic metal patterns, realized on a millimeter scale in a dielectric ceramic matrix, thus give hope for the future realization of real 3D metamaterials, useable in a large part of the microwave domain. However, a considerable amount of work remains to be done to achieve truly functional metamaterials. The construction strategy of the metallic inserts must indeed be better mastered so that they have very well-defined geometries and that their density is high. It will also be necessary to define precisely by calculation, the periodic structures or particular architectures interesting for a given spectral range, to realize them by the developed L-PBF process and to test in real operating conditions, the manufactured parts.

Conclusion

The proposed L-PBF process allows the fabrication or parts comprising, in selected areas of the space, a ceramic or a metallic alloy from a single powder chemically activated by pure hot water. Three main phases condition the successful manufacture of multi-material parts by this process:

-the chemical activation of the starting alloy powder, -the fusion and densification of metallic zones, -the oxidation and sintering for ceramic zones.

Each of these phases has been studied and has been the subject of initial improvements. The chemical activation has been controlled in order to obtain powders allowing both the correct fusion of the metallic zones and the oxidization of the activated powder and sintering for the ceramic zones. The L-PBF parameters were also adjusted to take full advantage of the activated powders.

The results obtained allowed to approach the realization of functional multi-material structures. Thus, small coaxial conductors have been fabricated. Beyond these relatively simple parts, periodic metalceramic structures (2D and 3D) have been successfully built. Even if the geometrical regularity and the density of the metallic entities included in the ceramic still need to be improved, the scales of size and periodicity that can be reached by the implemented process, seem to open the way to the subsequent fabrication of metamaterials for microwave applications. The corresponding volumes are then:

for Al transformed : v A = 0.81.V H2 volumic mass of Al = 0.81.V H2 2.7 = 0.3V H2 (eq. γ)

for remaining metallic Si : v S = 0.112.V H2 volumic mass of Si = 0.112.V H2 2.33 = 0.048.V H2 (eq. δ)

The volume fraction of transformed Al and remaining metallic Si compared to the total volume VofAl 0.88 Si 0.12 is: (eq. ε)

v A + v S V =
If D(μm)is the average grain diameter, the average volume of an Al 0.88 Si 0.12 grain in μm 3 is:

4 3 π [ D 2
] 3 (eq. ζ)

If e = thickness of transformed Al + remaining metallic Si, the corresponding volume in a grain is :

4 3 π [ [ D 2 ] 3 - [ D -2e 2
] 3 ]

(eq. η)

This volume is equal to the fraction of the degraded Al 0.88 Si 0.12 multiplied by the volume of the grain, that gives:

4 3 π [ [ D 2 ] 3 - [ D-2e 2
] 3 ] = 0.922.

V H2 M . 4 3 π [ D 2
] 3 (eq. θ)

From the previous equation e(μm)can be calculated:

e = D 2 - D 2
( 1 -0.922.

V H2 M

)1

3

(eq. ι)

The average grain diameter D, if the grains are assumed to be spherical, can be calculated from the specific surface area of the powder: Remarks: e = 0 when the grain is not degraded (V H2 = 0), e is close to the radius of the grain when the grain is totally transformed i.e for V H2 = 1.09 liter and M = 1g in agreement with the chemical equation( 2): 2Al 0.88 Si 0.12 +5.28H 2 O→1.76Al(OH) 3 +0.24Si+2.64H 2 (2b)

Specific
According to eq (2), 1 cm 3 ofAl 0.88 Si 0.12 gives about 3cm 3 of hydroxide and silicon, assuming that the starting Al 0.88 Si 0.12 alloy has a molar mass of 27 g and a density of 2.65 g cm -3 on the one hand, and the hydroxide and the metallic silicon resulting from the activation reaction, have an atomic mass and a density close respectively to 78 g and 2.4 g cm -3 for the hydroxide and 28 g and 2.33 g cm -3 for the silicon, on the other hand. Assuming perfect dense layers and t is the thickness of the hydroxide and silicon layer, the ratio "Volume of hydroxide layer/Volume of degraded alloy layer" is such that: that gives: t ≃ 3e (eq. ο)

The real hydroxide layer observed by electron microscopy, would then be close to 3e if it is perfectly dense or thicker if it is porous.

F

  Lanoue, F Veron, I Pasquet, K Kiryukhina, V Baco-Carles, O Vendier, Ph Tailhades To cite this version: F Lanoue, F Veron, I Pasquet, K Kiryukhina, V Baco-Carles, et al.. Treatment of AlSi12 feedstock with water to manufacture multi-material parts by laser powder bed fusion process: A potential route for the fabrication of inorganic 3D metamaterials. Journal of Materials Research and Technology, 2023, 27, pp.4772 -4786. 10.1016/j.jmrt.2023.10.254. hal-04305028

Fig. 1 .

 1 Fig. 1. Diagram of the manufacturing process for metal ceramic parts.

Fig. 2 .

 2 Fig. 2. View of a FIB cross-section of two compacted grains from a powder activated at 80 • C for 3 min 30 s. Silicon precipitates, which appear dark grey in the aluminum grains and in hydroxide layers, have not been attacked by hot water.

Fig. 3 .

 3 Fig. 3. (a) Optical absorption at 1070 nm of compacted powders activated for different times in water at 80 • C (concentration of AlSi12 powder in water = 400 g.L -1 ) (b) Commercial AlSi12 powder before and after activation (photography in the visible spectral range).

Fig. 4 .

 4 Fig. 4. Specific surface area of powders activated for different times in water at 80 • C (concentration of AlSi12 powder in water = 400 g.L -1 ).

Fig. 5 .

 5 Fig. 5. Optical absorption at 1070 nm versus specific surface area for different powders activated with water.

Fig. 6 .

 6 Fig. 6. Experimental setup dedicated to the measurement of hydrogen release, pH and temperature variation during the activation of AlSi12 powder.

Fig. 7 .

 7 Fig. 7. Hydrogen evolution with time per 1 g of AlSi12 powder, for different water temperatures (diamonds) and concentrations of the water suspension (circles) (10 g of AlSi12 in the appropriate volume of water for the given concentrations).

Fig. 8 .

 8 Fig. 8. Typical hydrogen evolution per 1 g of AlSi12 versus pH of the suspension (concentration of the metallic suspension in water 33 g L -1 , temperature 55 • C).

Fig. 9 .

 9 Fig. 9. Estimated average thickness t of aluminum hydroxide layer at the surface of the grains versus water contact time for different concentrations and temperatures of the suspension.

Fig. 10 .

 10 Fig. 10. Typical images obtained after cutting metallic pellets by focused ion beam: (a) AlSi12 activated in water at 43 • C for 92 min (Sw = 7 m 2 g -1 ) the direction of compaction is according to the arrow, (b) AlSi12 activated in water at 55 • C for 15 min (Sw = 2.9 m 2 g -1 ), (c) detail of the image (b) showing the thickness of the passivation layers subjected to perpendicular pressure after compaction of the powder.

1 a

 1 The activation time can be defined by the contact time of the powder with the water since the powder was introduced into the reactor.

Fig. 11 .

 11 Fig. 11. Metallic sample fabricated by L-PBF process with different activated powders: relative density versus surface energy density brought by the laser.

Fig. 12 .

 12 Fig. 12. Comparison of small parts made with the powder C and the reference AlSi12: a) Part design and sizing. b) Part manufactured with activated powder. c) Part manufactured with reference AlSi12.

Fig. 13 .

 13 Fig. 13. Comparison of X-ray diffraction patterns for L-PBF parts coming from activated (2.5 m 2 g -1 ) and reference AlSi12 powders.

Fig. 14 .

 14 Fig. 14. Phase diagrams of samples obtained for different values of laser power P and different products of hatch spacing h and laser scanning speed v.

Fig. 15 .

 15 Fig. 15. X-ray diffraction pattern of a ceramic part (AOxN = Aluminum Oxide Nitride).

Fig. 16 .

 16 Fig. 16. Ceramic-metal interface: (a) and (b) General view and corresponding oxygen mapping of the corner of a ceramic square surrounded by metal; (c) Detail of the interface and evaluation of the thickness of the ceramic and metal interpenetration area.

Fig. 17 .

 17 Fig. 17. Image of a coaxial conductor made by L-PBF with a powder activated by pure water at 55 • C.

Fig. 18 .

 18 Fig. 18. Schematic section of the fabricated bi-material part (a), view of the real part in a direction perpendicular to the manufacturing plane (b), detail of the surface of the real part (c).

Fig. 19 .

 19 Fig. 19. Schematic representation of manufacturing parameters and strategies used for ceramic parts comprising a single layer of metal spheres, distributed in a square planar array.

Fig. 20 .

 20 Fig. 20. Representation of the manufactured bi-material part (a) associated with two cross-sectional views of the metal balls inserted into the ceramic matrix (b) and (c). View of the actual part at the same scale, in a direction perpendicular to the manufacturing plane (d).

Fig. 21 .

 21 Fig. 21. X-ray tomography of a part comprising a face-centered cubic arrangement of metallic spheres embedded in an alumina-based ceramic. The section observed by tomography corresponds to the plane of the balls completely embedded in the ceramic part (see Fig.20).

surface area = 6 ρ

 6

  (D-2e)+3(D-2e) 2 ) ( 4e 2 -6De+3D 2 ) = 3 (eq. ξ)When e ≪ D and t ≪ D (i.e.for low water activation), typically for t ≥ e and t and e < D 50 , we have :( 4t 2 +6t(D-2e)+3(D-2e) 2 ) ( 4e 2 -6De+3D 2 ) →1

  .Al atomic mass= 0.03 . 27.V H2 = 0.81.V H2 (g) .Si atomic mass= 0.004 . 28.V H2 = 0.112.V H2 (g) (eq. β)

	0.88.	2 2.64	.	V H2 22.4	(eq. α)
	the mass of remaining metallic Si:
	0.12.	2 2.64	.	V H2 22.4

Acknowledgments

The authors would like to thank Marie-Claire Barthélémy, Vincent Baylac, Yann Borjon-Piron at Cirimat for their help in surface area measurements, additive manufacturing support and logistic, and for the microscopy observations. They also thank Patrick Nguyen from Elemca for its technical support with the focused ion beam preparations. They are also grateful for the advice and experimental help provided by Yohann Thimont and Diane Samelor (Cirimat) for the optical measurements.

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: No Appendix 1 Al 0.88 Si 0.12 : calculation of the thickness of the transformed layer at the periphery of the grains after activation by water


Jesús F Ontiveros

Maxime Nollet

Christel Pierlot

Veronique Nardello-Rataj
email: veronique.rataj@univ-lille1.fr

Revisiting the Influence of Carboxylic Acids on Emulsions and Equilibrated SOW systems using the PIT-Slope Method

Keywords: PIT-slope method, fatty acids, carboxylic acid, sorbitan monooleate, emulsion, microemulsion

The influence of various carboxylic acids (from C 2 to C 18 ) on equilibrated and emulsified surfactant/oil/water (SOW) systems is investigated. The PIT-slope method, based on the influence of an additive on the PIT of the C 10 E 4 /n-octane/10 -2 M NaCl (aq) reference emulsion, shows that from butanoic to hexadecanoic acids, the PIT of the system is decreased. A breakdown on dPIT/dx (considering the molar fraction x) or a minimum on dPIT/dC (taking into account the weight percentage C) is obtained with octanoic acid. The phase behavior of the C 10 E 4 /carboxylic acid/n-octane/0.01M NaCl (aq) system maintaining a molar fraction of carboxylic acid x CA = 0.2 shows that from octanoic acid, the solubility of the system is clearly enhanced explaining the breakdown obtained in the PIT-slope. The Span 80/carboxylic acid/oil/10 -2 M NaCl (aq) system at a water-to-oil ratio equal to 1 is able to form O/W emulsions in the presence of some carboxylic acids, of which the alkyl chain modifies the partition of the different oligomers of Span 80. Finally, the PIT-slope method is applied to mixtures of lauric acid/sodium laurate providing a linear behavior of the PIT-slope dPIT/dx as a function of the acid/salt ratio.

INTRODUCTION

The world production of fatty acids (FAs) from natural fats and oils represents about 4 million metric tons per year. Most fatty acids of vegetable origin have a straight-chain with an even number of carbon atoms while odd-numbered fatty acids are most commonly found in bacteria, some plants and animals [START_REF] Řezanka | Odd-numbered very-long-chain fatty acids from the microbial, animal and plant kingdoms[END_REF]. Chain lengths typically range commonly from 12 to 24.

FAs are simple in structure and can be saturated or unsaturated, named MUFA for the monounsaturated and PUFA for the polyunsaturated ones. In some animals, fatty acids may be more complex with branched chains or other functional groups.

Fatty acids are ultimately consumed in a wide variety of end-use industries (rubber, plastics, detergents...). They are used to modify polyesters providing alkyd resins for coatings.

They are also widely introduced in pharmaceutical, food and cosmetic formulations as additives. The alkaline salts of fatty acids provide surfactant molecules which have been used in soap formulations since Roman civilization [START_REF] Toedt | Chemical composition of everyday products[END_REF]. The presence of carboxylic acid groups in oil, as naphthenic and resins molecules, is at the origin of the so-called alkaline flooding process for enhanced oil recovery [START_REF] Mendez | Surfactant-oil-water systems near the affinity inversion. Part XI. pH sensitive emulsions containing carboxylic acids[END_REF][START_REF] Rivas | Microemulsion and optimal formulation occurrence in pH-dependent systems as found in alkaline-enhanced oil recovery[END_REF]. Description of aqueous phase behavior and pH dependence of these compounds has largely been described in the literature [START_REF] Cistola | Phase behavior and bilayer properties of fatty acids: hydrated 1: 1 acid-soaps[END_REF][START_REF] Cistola | Ionization and phase behavior of fatty acids in water: application of the Gibbs phase rule[END_REF][START_REF] Edwards | Aggregate structure in dilute aqueous dispersions of oleic acid/sodium oleate and oleic acid/sodium oleate/egg phosphatidylcholine[END_REF][START_REF] Douliez | Self-Assembly of Bilayer Vesicles Made of Saturated Long Chain Fatty Acids[END_REF]. Because of the pH dependence of the carboxylic function, their influence on Surfactant/Oil/Water (SOW) systems is important. Indeed, under alkaline conditions, they can play the role of a surfactant with a more or less important effect at the interface while under acidic conditions, they are more lipophilic, changing thus the polarity of the oil depending on their concentration. However, as weak acids, both forms generally coexist over a certain range of pH.

Several studies dealing with S/O/W ternary systems and fatty acids and/or their salts at equilibrium have also been reported in the literature. According to the Winsor nomenclature, the behavior of microemulsions [START_REF] Winsor | Binary and multicomponent solutions of amphiphilic compounds. Solubilization and the formation, structure, and theoretical significance of liquid crystalline solutions[END_REF] can include systems with two phases (Winsors I and II), three phases (Winsor III) or one single phase behavior (Winsor IV). For the two-phase behavior, when the microemulsion is in equilibrium with an excess oil phase, the system is called Winsor I. When the microemulsion is in equilibrium with an excess water phase, the system is Winsor II. In the early studies of S/O/W equilibrated systems, Schulman et al. [START_REF] Schulman | Mechanism of formation and structure of micro emulsions by electron microscopy[END_REF] studied oleic acid and its salt with different cations using alkanes, benzene or kerosene as oily phases. Ekwall et al. [START_REF] Ekwall | Solutions of alkali soaps and water in fatty acids, Kolloid-Z[END_REF][START_REF] Ekwall | Two types of micelle formation in organic solvents[END_REF] published detailed phase diagrams of sodium octanoate/oil/water systems, using alcohol or fatty acids as third component. Mendez et al. [START_REF] Mendez | Surfactant-oil-water systems near the affinity inversion. Part XI. pH sensitive emulsions containing carboxylic acids[END_REF] [START_REF] Bravo | Phase behavior of fatty acid/oil/water systems: effect of the alkyl length chain acid[END_REF] have studied the influence of pH and alkyl chain length on the phase behavior of FA/oil/alcohol/NaCl aq systems. They have shown that it is possible to obtain a three-phase behavior, maintaining a constant quantity of NaOH in the system, by changing the concentration of the fatty acid. For each fatty acid, it is possible to determine the pH to achieve the so-called optimal formulation, i.e. the formulation at which the minimum interfacial tension and the maximal solubilization occurs. The longer the alkyl chain of the acid is, the higher the pH of the aqueous phase must be to obtain the optimal formulation. In addition, when the system is triphasic, i.e. Winsor III, for an equal amount of FA, the microemulsion middle phase increases in volume when the alkyl chain increases.

Thus, the longer the alkyl chain, the more the FA solubilizes both water and oil. Researches concerning carboxylic acids have not been limited to equilibrated systems and several studies have also been published on emulsions [START_REF] Friberg | The system: Water-p-xylene-l-aminooctane-octanoic acid, Kolloid-Z[END_REF][START_REF] Levy | Characterization of diazepam submicron emulsion interface: Role of oleic acid[END_REF][START_REF] Lovell | Less-painful emulsion formulations for intravenous administration of clarithromycin[END_REF][START_REF] Sherwin | Effect of Fatty Acid Type on Dispersed Phase Particle Size Distributions in Emulsion Edible Films †[END_REF][START_REF] Jeong | Phase diagram and emulsion properties of n-octyl β-d-glucoside/water/octanoic acid containing pseudo-ceramide mixtures[END_REF], most of them related to formulations for pharmaceutical, cosmetics or food applications.

Recently, Ontiveros et al. have proposed a simple and convenient method for the classification of surfactants [START_REF] Ontiveros | A simple method to assess the hydrophilic lipophilic balance of food and cosmetic surfactants using the phase inversion temperature of C10E4/n-octane/water emulsions[END_REF][START_REF] Ontiveros | Structure-interfacial properties relationship and quantification of the amphiphilicity of well-defined ionic and non-ionic surfactants using the PIT-slope method[END_REF]. The method is based on the perturbation of the Phase Inversion Temperature (PIT) through addition of a second surfactant. The concept of PIT, introduced by Shinoda et al. [START_REF] Shinoda | The correlation between phase inversion temperature in emulsion and cloud point in solution of nonionic emulsifier[END_REF][START_REF] Shinoda | The effect of the size and the distribution of the oxyethylene chain lengths of nonionic emulsifiers on the stability of emulsions[END_REF], characterizes the change of the affinity of polyethoxylated fatty alcohol surfactants, noted CiEj, towards water, oil and interface when they are heated or cooled. Starting from the C 10 E 4 /n-octane/water reference system, with a PIT close to 25 °C, the linear decrease or increase evolution of the PIT through addition of a second surfactant is quantified through the so-called PIT-slope which affords a robust classification of surfactants compared to the reference C 10 E 4 .

Based on these previous studies, we have investigated the influence of the alkyl chain length of a series of saturated carboxylic acids from C 2 to to C 18 and oleic acid C 18:1 on the PIT-slope, dPIT/dx CA (i.e., the PIT variation versus the molar fraction of the acid). The effect of the lauric acid/sodium laurate ratio on dPIT/dx CA has also been studied using the same method. The discussion is completed with the phase behavior of C 10 E 4 /CA/n-octane/water systems, in order to verify the data obtained with the emulsified system and to evaluate the impact of carboxylic acids into the solubility capacity of microemulsion. Finally the influence of the carboxylic acids into the morphology of Span 80/CA/oil/water model emulsion is studied. In this emulsion, both surfactant (Span 80) and oil phase (Creasil IH CG)

are mixtures, a model system with ingredients often used in cosmetics to point-out the complex behavior of these formulations.

MATERIALS AND METHODS

Chemicals

Pure tetraethyleneglycol monodecyl ether (C 10 E 4 ) reference surfactant was synthesized according to a method described elsewhere [START_REF] Gibson | Phase-transfer synthesis of monoalkyl ethers of oligoethylene glycols[END_REF][START_REF] Lang | Nonionic surfactant mixtures. I. Phase equilibria in CE-HO and closed-loop coexistence[END_REF]. Its purity was assessed by NMR and GC analyses (> 99%) and by comparing its cloud point temperature [START_REF] Schlarmann | Correlation between film properties and the purity of surfactants[END_REF]

PIT-slope method

The method has been described in our previous papers [START_REF] Cistola | Ionization and phase behavior of fatty acids in water: application of the Gibbs phase rule[END_REF][START_REF] Edwards | Aggregate structure in dilute aqueous dispersions of oleic acid/sodium oleate and oleic acid/sodium oleate/egg phosphatidylcholine[END_REF] (see Supporting Information). It consists in measuring the PIT deviation of the C 10 E 4 /n-octane/water reference system through addition of small amounts of an additive, i.e. the carboxylic acids (CAs) in the present work.

The results are interpreted in terms of the PIT-slope which can be expressed in molar fraction dPIT/dX CA or in weight percentage dPIT/dC CA . The carboxylic acid molar fraction X CA in the surfactant/carboxylic acid mixture is given by Eq.1:

( was experimentally previously determined by Queste et al. [START_REF] Queste | The EACN scale for oil classification revisited thanks to fish diagrams[END_REF] (See supporting information).

RESULTS AND DISCUSSION

Effect of carboxylic acids chain length on SOW systems

The influence of carboxylic acids (CAs) at the interface as a function of the CA chain length was determined using the PIT-slope method. Thus, the PIT of the 3 wt.% C 10 E 4 /n-octane/10 -2 M NaCl (aq) reference system was measured in the presence of increasing amounts of 12 linear carboxylic acids (CAs) from acetic acid (C 2 ) to stearic acid (C 18 ) and one unsaturated compound, i.e. oleic acid. Acetic acid C 2 is water-soluble and slightly increases the PIT, it enhances the hydrophilicity of the system as expected. For longer CAs from C 4 to C 8 , the PIT significantly decreases with the chain length. From decanoic acid C 10 , the PIT value still diminishes but this decrease is less important than the strongest effect observed until C 8 . In the range of the studied concentrations, i.e. below 1 wt.%, there is a linear dependence between the PIT and the concentration. The slope of PIT with dPIT/dC CA (weight percentage) or dPIT/dX CA (molar fraction) corresponds to the ability of the carboxylic acid CA to modify the phase inversion temperature of the reference system. If the slope is negative, the acid decreases the PIT, at contrary a positive slope reflects an increase of the PIT. Figure 2 shows a rapid fall of dPIT/dC CA as a function of the total number of carbon atoms of the acid, the lowest dPIT/dC corresponding to the C 8 . When the slope is represented by considering the molar fraction X CA , there is not a minimum value with C 8 but a discontinuity in the tendency. The slopes determined from the PIT versus CA concentration plots for the 11 investigated acids are reported in the supplementary material.

The change of the PIT reflects the activity of the carboxylic acids at or near the interfacial layer, modifying the spontaneous curvature and the rigidity of the interfacial film formed by the C 10 E 4 surfactant molecules. For short CAs like acetic and propionic acids, the influence on the interfacial film is weak leading to slightly positive and negative values of dPIT/dC or dPIT/dX CA , respectively. Other CAs increase the lipophilicity of the system, however octanoic acid establishes a boundary in the effect of these compounds on the PIT. We have already reported such a similar evolution of the PIT with linear alcohols, using both the 7 wt.% Brij30/n-octane/10 -2 M NaCl (aq) [START_REF] Ontiveros | Dramatic influence of fragrance alcohols and phenols on the phase inversion temperature of the Brij30/n-octane/water system[END_REF] and the 7 wt.% C 10 E 4 /n-octane/10 -2 M NaCl (aq) systems [START_REF] Ontiveros | Classification des tensioactifs et huiles biocompatibles par mesure de la température d'inversion de phase (PIT) et comparaison des diagrammes de phases[END_REF]. In both cases, the minimum observed for dPIT/dC was reached with heptanol, and this alcohol defines the zone in which the alcohols act as lipophilic linkers. That means that its polar head is not enough efficient to lead the alcohol to the interface but it contributes to create an order between the alkyl chain of the surfactant and the alkane [START_REF] Graciaa | Improving solubilization in microemulsions with additives. 1. The lipophilic linker role[END_REF][START_REF] Graciaa | Improving solubilization in microemulsions with additives. 2. Long chain alcohols as lipophilic linkers[END_REF]. Compared to alcohols, the carboxylic acids are more polar and the fact that the minimum for dPIT/dC for acids is obtained with the C 8 is not casual. The additional carbon in the alkyl chain compensates for the higher polarity of the carboxylic group compared to the OH in alcohols.

From the values of their dPIT/dx CA , most acids are more hydrophobic than C 10 E 4, i.e the interfacial activity of lauric acid is comprised between that of dodecyl isosorbide (dPIT/dx = -54) and dodecyl glycerol (dPIT/dx = -43) [START_REF] Ontiveros | Structure-interfacial properties relationship and quantification of the amphiphilicity of well-defined ionic and non-ionic surfactants using the PIT-slope method[END_REF]. The weight concentration values dPIT/dC CA indicate a lipophilic tendency higher than the values obtained for all surfactants of the sorbitan alkanoates which are commercial lipophilic surfactants (i.e. Span).

Phase behavior of the C 10 E 4 /carboxylic acid/n-octane/water systems

In order to go deeper into the influence of carboxylic acids on SOW systems, the phase behavior evolution of the C 10 E 4 /n-octane/water system through addition of the carboxylic acids was also investigated at equilibrium. The phase behavior of the C 10 E 4 /n-octane/water system is well-known with a critical temperature (noted T*) and concentration (noted C*) at f w = 0.5 equal to 25.6 °C and 10.8 wt.% respectively [START_REF] Queste | The EACN scale for oil classification revisited thanks to fish diagrams[END_REF][START_REF] Burauer | On microemulsion phase behavior and the monomeric solubility of surfactant[END_REF][START_REF] Pizzino | Bidimensional Analysis of the Phase Behavior of a Well-Defined Surfactant (C 10 E 4 )/Oil ( n -Octane)/Water-Temperature System[END_REF]. When the concentration of the surfactant is lower than C* and the surfactant is not pure, the temperature at which the ethoxylated surfactant presents the same affinity for oil and water in a S/O/W equilibrated system is called T HLB and differs from T* [START_REF] Kunieda | Phase behavior in systems of nonionic surfactant/water/oil around the hydrophile-lipophile-balance-temperature (HLB-temperature)[END_REF][START_REF] Antón | Surfactant-oil-water systems near the affinity inversion Part IV: Emulsion inversion temperature[END_REF][START_REF] Ben Ghoulam | Quantitative Effect of Nonionic Surfactant Partitioning on the Hydrophile-Lipophile Balance Temperature[END_REF].

A temperature scan for the 3 wt.% C 10 E 4 /n-octane/water system has thus been carried out without CA and in the presence of the C 6 to C 14 acids at a molar fraction x CA of 0.2, under equilibrated conditions using temperature as a scan variable.

As shown previously, the PIT of the C 10 E 4 /n-octane/water system is impacted by the presence of CAs. The behavior at equilibrium when adding a carboxylic acid is also affected and the temperature needed to obtain a three-phase system (i. Addition of carboxylic acids from C 6 to C 14 to the C 10 E 4 /n-octane/water system changes not only the temperature but also the shape of the volume fraction-temperature difference diagram of the reference system. At T HLB , the presence of carboxylic acids increases the microemulsion phase volume of the systems in the following order C 14 > C 12 > C 10 > C 8 .

Conversely, hexanoic acid does not change the limits of the volume fraction-temperature difference diagram of the reference system. At temperatures far away from T HLB, the frontiers of acid systems converge with the reference system (in both WI and WII phase behaviors).

Temperature range in which the WIII systems are obtained dramatically decreases for longer acids, from 6 °C for the reference system to less than 0.5 °C for the C 14 . This decrease on the temperature extent of the WIII behavior and the significant increase of the volume of the microemulsion phase for C 12 and C 14 shows that the reference system tends to a monophasic WIV microemulsion in the presence of these acids. It is noteworthy that to attain the Winsor IV system with the C 10 E 4 /n-octane/water system, 10 wt.%. of C 10 E 4 is required. Hence, a similar effect is obtained by addition of only 0.2 molar fraction of long chain fatty acids (> C 8 ) with only 3 wt.% C 10 E 4 instead of 10 wt.%.

As described for alcohols in the earliest 90's by Graciaa et al. [START_REF] Graciaa | Improving solubilization in microemulsions with additives. 2. Long chain alcohols as lipophilic linkers[END_REF], the addition of some carboxylic acids generate a synergistic effect that allows to increase the water/oil interfacial activity of the surfactant. This increase is probably due to the segregation of the fatty acid between the alkyl chain of the surfactant and octane, inducing a more organized transition from the surfactant palisade to the oil bulk phase. This is the so-called lipophilic linker effect [START_REF] Graciaa | Improving solubilization in microemulsions with additives. 1. The lipophilic linker role[END_REF][START_REF] Graciaa | Improving solubilization in microemulsions with additives. 2. Long chain alcohols as lipophilic linkers[END_REF]. Such a behavior matches well with the change on the dPIT/dx CA tendency observed in Figure 2 from octanoic acid. In that sense, our findings complete the behavior reported by Lohateeraparp et al. [START_REF] Lohateeraparp | Study of alcohol-free microemulsion systems containing fatty acids as cosurfactants[END_REF] with the sodium dodecyl sulfate (SDS)/carboxylic acid (CA)/n-hexane/NaCl (aq) system, using the salinity as a formulation variable. The increase of alkyl chain decreases the salinity required to achieve WIII systems, analogous to the temperature decrease observed for the C 10 E 4 /CA/n-octane/W system. Alcohols can be easily replaced by CA not only when using ionic surfactant systems (SDS) but also with non-ionic surfactants (C i E j ). Contrary to the continuous increase of solubility with the alkyl chain reported in Figure 3, in the SDS/CA/n-hexane/NaCl (aq) system, the increase of solubility with the alkyl chain of CA is asymptotic, probably "because gel formation prevents further enrichment in the solubilization" [START_REF] Lohateeraparp | Study of alcohol-free microemulsion systems containing fatty acids as cosurfactants[END_REF].

Influence of carboxylic acids on the emulsification of the Span 80/oil/water systems

Fatty carboxylic acids are not able to form stable emulsions by themselves but strongly influence the behavior of SOW emulsified systems. Their influence has thus also been studied as additives on Span 80/oil/water systems (0.5 wt.% of span 80, f w = 0.5). Two oils were selected: n-octane and the commercial Creasil-IH CG, a mixture of isohexadecane isomers, used in cosmetic formulations. The systems without carboxylic acid produce O/W emulsions at low concentrations of surfactant (0.5% wt.) with both oils. Even if Span 80 is a lipophilic surfactant, at low concentrations, the partition of the different molecules is enhanced and a net hydrophilic behavior is obtained as described in the literature for ethoxylated surfactants [START_REF] Ben Ghoulam | Quantitative Effect of Nonionic Surfactant Partitioning on the Hydrophile-Lipophile Balance Temperature[END_REF][START_REF] Graciaa | The partitioning of complex surfactant mixtures between oil/water/microemulsion phases at high surfactant concentrations[END_REF][START_REF] Graciaa | The selective partitioning of the oligomers of polyethoxylated surfactant mixtures between interface and oil and water bulk phases[END_REF]. Indeed, for a Span/Cod Liver + sec-butanol/water (1 wt.% NaCl) system, Salager et al. [START_REF] Salager | Surfactant-Oil-Water Systems Near the Affinity Inversion: Part VII: Phase Behavior and Emulsions with Polar Oils[END_REF] reported an O/W emulsion at f w = 0.5, using 2 wt.% of the sorbitan surfactant. To study the effect of the CAs, the conductivity of the emulsions were measured (Figure 4). Addition of 0.5% wt. of carboxylic acid clearly changes the morphology of the emulsions. For n-octane, all systems produce W/O emulsions except lauric and tetradecanoic acids. The "hydrophobicity" of the oily phase can be quantified by the Equivalent Alkane Carbon Number (EACN), the number of carbon atoms of the linear alkane that has its optimal formulation at the same conditions of the studied oil [START_REF] Cayias | The utilization of petroleum sulfonates for producing low interfacial tensions between hydrocarbons and water[END_REF][START_REF] Cash | The application of low interfacial tension scaling rules to binary hydrocarbon mixtures[END_REF]. The increase of the CA chain length from 6 to 16 should produce a monotonic change of the EACN with an inversion of the emulsion from W/O to O/W when the EACN of the system reaches the initial value without carboxylic acid. When the alkyl chain of CA increases up to 12, an O/W emulsion is obtained.

However, the fact that Span 80 is not a well-defined surfactant but a mixture (as most of commercially available surfactants) increases the complexity of its behavior. Span 80 contains fatty acid mono, di and tri-esters of sorbitan and the alkyl chain is a mixture of oleic, linoleic, linolenic and palmitic acid [START_REF] Wang | Analysis of sorbitan ester surfactants. Part I: High performance liquid chromatography[END_REF]. Partitioning of all these compounds between the oily phase and the interface, when the carboxylic acid changes, appears to be determinant to re-obtain W/O emulsions. This hypothesis is schematized in When the balance between the polar head and the alkyl chain of the carboxylic acid is reached, the CA preferentially localizes at the interface (as n-butanol for alcohols), changing the Span 80 partition (Figure 5A andB). When the alkyl chain of the carboxylic acid increases, the CA plays the role of a linker between the alkyl chain of the oil and the surfactant. The most lipophilic oligomers of Span 80 are segregated into the bulk oily phase and the O/W system is obtained (Figure 5C). Longer alkyl chains are solubilized into the oily phase and the lipophilic oligomers of Span 80 are preferentially distributed in the interfacial film, thus accounting for the formation of W/O emulsions observed when adding 0.5% wt.

hexadecanoic acid to the 0.5% wt. Span 80/octane/water (Figure 5D).

This "retrograde" inversion has already been reported for ethoxylated surfactants when using alcohol concentration [START_REF] Salager | Retrograde transition in the phase behavior of surfactant-oil-water systems produced by an alcohol scan[END_REF] or oil nature [START_REF] Ysambertt | Retrograde transition in the phase behaviour of surfactant-oil-water systems produced by an oil equivalent alkane carbon number scan[END_REF] as formulation scan variables. Salager [START_REF] Salager | Retrograde transition in the phase behavior of surfactant-oil-water systems produced by an alcohol scan[END_REF] and

Ysambertt [START_REF] Ysambertt | Retrograde transition in the phase behaviour of surfactant-oil-water systems produced by an oil equivalent alkane carbon number scan[END_REF] have shown that the partitioning of the surfactant produces anomalous inversions in S/O/W equilibrated systems. Furthermore, the partitioning of Span 80 would be affected by its concentration and by the carboxylic acid concentration as well.

It is well known that an increase of EACN enhances the hydrophilicity of the system producing a WII→WI transition [START_REF] Salager | Optimum formulation of surfactant/water/oil systems for minimum interfacial tension or phase behavior[END_REF][START_REF] Bourrel | A correlation for phase behavior of nonionic surfactants[END_REF]. The behavior obtained with the mixture of isohexadecane isomers is similar except that O/W emulsions are observed for decanoic and lauric acids. This result agrees with the main hypothesis: as the iso-hexadecane has a higher EACN (13.5) than that of n-octane (8), the O/W emulsion is easily obtained (the alkyl chain of the CA required is smaller). The retrograde inversion is observed for the iso-hexadecane emulsion when adding the tetradecanoic acid, even if it will be expected that this system need a higher carboxylic acid chain that that observed for octane emulsion (hexadecanoic acid).

This behavior could be promoted by the different isomers of hexadecane present at the Creasil-IH CG.

Influence of the carboxylic acid/sodium carboxylate ratio on dPIT/dx CA

As carboxylic acids are weak acids, there is always a mixture of the acid and the carboxylate forms in solution depending on the pK a of the acid and on the pH of the solution. The pK a of shorter carboxylic acids is near to 5 (4.79 for acetic acid) and increase until 8.1-8.2 for tetradecanoic acid [START_REF] Kanicky | Cooperativity among Molecules at Interfaces in Relation to Various Technological Processes: Effect of Chain Length on the pK a of Fatty Acid Salt Solutions[END_REF]. It was thus of interest to assess the influence of the sodium salt of the carboxylic acid as well as of its amount on the PIT of the reference system. Two methods have been used. The first one consisted in mixing directly the acid and its corresponding sodium salt and measuring the resulting dPIT/dx CA of the mixture at different ratios. The second one was to change progressively the pH of the aqueous phase by addition of NaOH until one equivalent in order to form the carboxylate in situ. Figure 6 shows the influence of the lauric acid/sodium laurate ratio on the PIT evolution of the 3wt.% C 10 E 4 /n-octane/10 -2 M NaCl (aq) system. As expected, the higher the molar fraction of sodium salt, the higher the dPIT/dx CA is. The last point on molar fraction on Figure 6A (for every molar ratio different to zero) is the maximal molar fraction of the acid/salt mixture that allows the phase inversion by heating/cooling. Indeed, addition of sodium laurate, even at a molar ratio of 0.25, induces at some critical concentration the formation of O/W emulsions insensitive to temperature, as expected for ionic surfactants-C i E j mixtures [START_REF] Ontiveros | Structure-interfacial properties relationship and quantification of the amphiphilicity of well-defined ionic and non-ionic surfactants using the PIT-slope method[END_REF]. When the salt ratio increases, the limit of the molar fraction to obtain the temperature insensivity decreases. Independently of the ratio of the mixture salt/acid, the quantity of sodium laurate to obtain the temperature insensitivity is approximately the same. Indeed, when the molar fraction of sodium laurate is higher than 0.026, an O/W emulsion is produced no matter the temperature of the system or the salt/acid ratio. Figures 6B shows a very good linearity of dPIT/dx as a function of the molar ratio of lauric acid/sodium laurate. If the linearity between mixtures of a same family of non-ionic surfactants could be expected, it was not obvious for mixtures of a carboxylic acid and its salt as it complicates the patterns in phase behavior [START_REF] Mendez | Surfactant-oil-water systems near the affinity inversion. Part XI. pH sensitive emulsions containing carboxylic acids[END_REF][START_REF] Bravo | Phase behavior of fatty acid/oil/water systems: effect of the alkyl length chain acid[END_REF] and, as mentioned before, their mixtures with the C 10 E 4 /n-octane/10 -2 M NaCl (aq) system produces at some critical concentration O/W emulsions insensitive to temperatures variations. Results from Figure 6B suggest that acid and salt effects are independent and dPIT/dx quantifies both effects at the same time.

Lauric acid acts as a lipophilic additive of the mixture, and the non-ideal behavior with C 10 E 4 is only due to the presence of the ionic surfactant. Table 1 shows the values of the PIT for the C 10 E 4 /n-octane/water system when the molar fraction of lauric acid and sodium laurate is equal to 0.02 at different mixture ratios. It is noteworthy that the PIT linearly increases by adding directly NaOH as well as by mixing directly the salt and the acid. However, by comparing the PIT values obtained by the two methods, there are some discrepancies. The values obtained when mixing the acid and the salt are slightly higher than the values obtained by adding sodium hydroxide, and the difference is higher when the ratio increases. Lauric acid is predominantly in the oil phase of the system C 10 E 4 /n-octane/water and the real sodium laurate/lauric acid ratio is difficult to predict as reported by Mendez et al. [START_REF] Mendez | Surfactant-oil-water systems near the affinity inversion. Part XI. pH sensitive emulsions containing carboxylic acids[END_REF]. In order to reduce the PIT difference, the addition of NaOH must be followed by an additional time of equilibration that allows the complete reaction and partition of reactants and products. Irrespective of the method, interaction of sodium laurate and lauric acid on the PIT of the reference system is complementary. Higher hydrophilicity of the systems using the sodium salt can be adjusted by changing the salt/acid ratio. Linearity of the dPIT/dx is analogous to the hydrophilic lipophilic balance (HLB) linearity [START_REF] Griffin | Classification of surface-active agents by "HLB[END_REF] observed with the Tween/Span mixtures.

CONCLUSION

The role of carboxylic acids in SOW systems is complex and it can be compared in some aspects to the alcohols effect. They are able to modify both the polarity of the oil phase and also the interfacial layer behavior. PIT-slope experiments allow to demonstrate that most of them (except acetic acid) enhances the lipophilicity of the C 10 E 4 /n-octane/water emulsion. The temperature scan for equilibrated systems at a fixed molar fraction of carboxylic acid of 0.2 confirms the PIT-slope results. Even if all acids modify the temperature at which the three phase behavior is obtained, hexanoic acid changes only slightly the third phase volume of the reference system while from octanoic to tetradecanoic acid, there is a dramatic increase of solubility. Alcohols can thus be easily replaced by fatty acids, not only in ionic SOW formulations as pointed out by Lohateeraparp et al. [START_REF] Lohateeraparp | Study of alcohol-free microemulsion systems containing fatty acids as cosurfactants[END_REF] but also in nonionic systems.

Emulsification of the Span 80/0.5% CA/octane or isohexadecane/water systems at f w = 0.5

shows that different carboxylic acids are able to modify not only the EACN of the oily phase but also the partition of the surfactant, producing an unexpected inversion behavior. Finally, the influence of mixtures of lauric acid and sodium laurate at different ratios on the PIT of the reference C 10 E 4 /n-octane/water emulsion exhibits a linear behavior. Temperature insensitive O/W emulsions are obtained when the fraction of sodium laurate is higher than 0.026.

3 . 4 .

 34 and MW CA are the mass and molecular weight of the carboylic acid respectively and m C10E4 and MW C10E4 are the mass and molecular weight of the surfactant of the reference system, i.e. C 10 E 4 . The water weight fraction f w is equal to 0.5 in all systems. For the experiments carried out with mixtures of carboxylic acid and sodium salt, the molar fraction is defined as: Phase behavior of equilibrated C 10 E 4 /carboxylic acid/n-octane/NaCl aq systems 1.5 g samples were prepared in 2 mL vials by weighing successively the compounds. NaCl aqueous solution and n-octane had the same weight (f w = 0.5) in each sample, carboxylic acid was added until reaching a molar fraction X CA = 0.2 and the surfactant total concentration was 3% wt. The resulting 3 wt.% C 10 E 4 /carboxylic acid/n-octane/10 -2 M NaCl aq systems were gently mixed to maximize the water/oil contact while avoiding emulsification. The vials were then placed in a thermostated bath at the required temperatures T ± 0.1 °C until phase separation and equilibrium were reached, at least for one day and up to several days in the vicinity of transition zones. Visual inspection of the vials allowed the determination of the phase behavior in order to classify them in Winsor scale. Emulsification of the Span 80/carboxylic acid/oil/NaCl aq systems 8.5g samples were prepared in 20 mL vials by weighing successively the compounds. Span 80 was preliminary dissolved into the oily phase, either octane or Creasil IH CG (isohexadecane). the NaCl (10 -2 M) aqueous solution and the oil (octane or Creasil IH CG) had the same weight (f w = 0.5) in each sample. Hexanoic, octanoic, decanoic, lauric, tetradecanoic and hexadecanoic acids were used at 0.5 wt.%. The 0.5 wt.% Span 80/0.5% carboxylic acid/oil/10 -2 M NaCl (aq) resulting systems were gently mixed to maximize the water/oil contact while avoiding emulsification All systems were prepared and left to equilibrate at least for 18h. The samples were then emulsified using an Ultra-Turrax at 6000 rpm for 30 s and their conductivity was measured simultaneously. The alkane carbon number for octane is 8. For the Creasil IH CG, the Equivalent Alkane Carbon Number (EACN = 13.5)

Figure 1

 1 describes the linear evolution of the PIT values as a function of the weight concentrations for some acids in the studied range of concentrations (< 1 wt.%), limited by the solubility of the acids and by the practical lowest temperature (≈ 10 °C) that can be experimentally reached. Results for all the acids are given in Supporting Information.

Figure 1 .

 1 Figure 1. Phase inversion temperature (PIT) of the 3 wt.% C 10 E 4 /n-octane/10 -2 M NaCl (aq) system as a function of weight percentage of carboxylic acid (CA). Acetic acid (C 2 ), butyric acid (C 4 ), octanoic acid (C 8 ), tetradecanoic acid (C 14 ) and oleic acid (C 18:1 ). Dotted lines indicate the linear fitting.

Fig. 2

 2 Fig. 2 Evolution of dPIT/dX CA (°C) and dPIT/dC CA (°C/wt. %) with increasing alkyl chain length of carboxylic acid CA for the 3 wt.% C 10 E 4 /CA/n-octane/10 -2 M NaCl (aq) system.

  e. WIII) shifts to lower values. In order to compare all the studied CAs, we have plotted, on Figure 3, the volume fraction of the different phases as a function of the difference T-T HLB , where T HLB is the temperature of the three phase system with the same weights of oil and water in the middle phase microemulsion.

Figure 3 .

 3 Figure 3. Volume fraction of the oil, microemulsion and water phases as a function of T-T HLB

Figure 4 .

 4 Figure 4. Conductivity and emulsion type of the Span 80/oil/water system in the presence of

Figure 5 ,Figure 5 .

 55 Figure 5. Schematic representation of droplet interface for different carboxylic acids added to

Figure 6 .

 6 Figure 6. A) Phase inversion temperature (PIT) vs. molar fraction x of lauric acid + sodium

  have shown that some fatty acid (FA)/O/W systems exhibit an unusual retrograde transition, i.e. WII WIII WI WIII WII when NaOH is added. Indeed, pH raises first increases the proportion of

fatty acid sodium salt (i.e. hydrophilic surfactant) at the interface thus promotes WII WIII WI transitions. Then, a further pH increase does not change the acid/carboxylate equilibrium and the rise of Na + concentration results in the classical salinity driven WI WIII WII transitions. Bravo et al.

Table 1 .

 1 PIT of the 3 wt.% C 10 E 4 /sodium laurate/lauric acid/n-octane/10 -2 M NaCl (aq) system in the presence of different molar ratios of the lauric acid/sodium laurate mixtures determined 1)

	𝑆𝑜𝑑𝑖𝑢𝑚 𝑙𝑎𝑢𝑟𝑎𝑡𝑒 𝑆𝑜𝑑𝑖𝑢𝑚 𝑙𝑎𝑢𝑟𝑎𝑡𝑒 + 𝐿𝑎𝑢𝑟𝑖𝑐 𝑎𝑐𝑖𝑑	0	0.25	0.5	0.75	1
	PIT (°C) (sodium laurate + lauric acid)	22.6	24.4	27.2	29.2	31.4
	PIT (°C) (addition of NaOH)	22.6	24.1	25.7	27.0	28.2

by mixing directly lauric acid and sodium laurate and 2) by adding NaOH to the lauric acid-based system. In all cases the molar fraction of lauric acid and sodium laurate mixture is 0.02.

ACKNOWLEDGMENTS

Chevreul Institute (FR 2638), Ministère de l'Enseignement Supérieur et de la Recherche, Région Nord Pas de Calais and FEDER are acknowledged for supporting and funding part of this work. The authors would like to thank Prof. Jean Marie Aubry who kindly provided helpful comments for the discussion.