Lilya Boudriche

Rachel Calvet
email: calvet@mines-albi.fr

Boualem Hamdi
email: bhamdi99@yahoo.fr

Henri Balard
email: h.balard@wanadoo.fr

Surface properties evolution of attapulgite by IGC analysis as a function of thermal treatment

Keywords:

a b s t r a c t At low temperatures, thermal treatment of attapulgite clay causes a loss of mass due to the elimination of linked, zeolitic and crystallized water molecules. At high temperatures, there is a junction of particles until total melting of the attapulgite fibers. This transformation reduces significantly the volume of the porous network resulting in the drop in the value of specific surface area and causing important changes in the textural, structural and surface properties of the raw attapulgite. The inverse gas chromatography shows sensitivity of the attapulgite sample to the effect of temperature through the decrease of the dispersive component of the surface energy d s . The evolution of the nanomorphological index, a measurement of the surface nanorugosity, follows the appearance of surface roughness on carbonates with the increase of the temperature. The surface heterogeneity, determined by calculating the distribution functions of adsorption energies also underlines the morphology changes and the formation of new phases under the effect of temperature.

Introduction

Clays are used in various application areas such as ceramic, paper, paint, adsorbent, catalyst, etc. [START_REF] Murray | Traditional and new applications for kaolin, smectite, and palygorskite: a general overview[END_REF][START_REF] Murray | Applied clay mineralogy today and tomorrow[END_REF][START_REF] Murray | Overview -clay mineral applications[END_REF]. However, it is know that clay minerals are materials sensitive to thermal stress, and may undergo many changes with a consumption or release of energy with or without loss of mass. In this context, many studies were made to assess to the thermal transformations of some clays [START_REF] Melo | Effects of thermal and chemical treatments on physical properties of kaolinite[END_REF][START_REF] Michot | Thermal conductivity and specific heat of kaolinite: evolution with thermal treatment[END_REF][START_REF] Carroll | Solid-state NMR characterisation of the thermal transformation of a Hungarian white illite[END_REF][START_REF] Pingxiao | The microstructural study of thermal treatment montmorillonite from Heping, China[END_REF][START_REF] Laureiro | Dehydration kinetics of Wyoming montmorillonite studied by controlled transformation rate thermal analysis[END_REF][START_REF] Temuujin | The effect of water vapour atmospheres on the thermal transformation of kaolinite investigated by XRD, FTIR and solid state MAS NMR[END_REF].

Our study investigates the changes, under temperature effect, of a fibrous clay from Algerian origin, called "attapulgite". Although the dehydration and dehydroxylation of the attapulgite clays were studied in detail by thermogravimetric techniques [START_REF] Frost | Controlled rate thermal analysis and differential scanning calorimetry of sepiolites and palygorskites[END_REF][START_REF] Gonzalez | Thermal investigation of acid-activated attapulgites: influence of isomorphic substitution in the octahedral sheet[END_REF][START_REF] Gonzalez | Structural and textural evolution under thermal treatment of natural and acid-activated Al-rich and Mg-rich palygorskites[END_REF], but there is no previous study on the evolution of surface properties of the solid after heat treatment. So, in this study, we focused on studying the evolution of surface properties of attapulgite by implementing a well-established analytical technique, the inverse gas chromatography [START_REF] Balard | Study by inverse gas chromatography of the surface properties of milled graphites[END_REF][START_REF] Comard | Coupling of inverse gas chromatography at infinite dilution (IGC-ID) with a controlled modification of a solid surface[END_REF][START_REF] Kellou | Surface properties of dimethylsilylated silicas, assessed using IGC at finite concentration[END_REF][START_REF] Hamdi | IGC characterization of surface energy and morphology of two natural fillers: kieselguhr and bentonite[END_REF][START_REF] Huang | Investigation on interfacial interaction of flame retarded and glass fiber reinforced PA66 composites by IGC/DSC/SEM[END_REF][START_REF] Nastasovic | Determination of glass temperature of polymers by inverse gas chromatography[END_REF] already used after acid treatment of the same solid [START_REF] Boudriche | Effect of acid treatment on surface properties evolution of attapulgite clay: an application of inverse gas chromatography[END_REF]. It must be notice that only few studies by inverse gas chromatography were conducted on heat treated clays. We can quote the works of Brendlé [START_REF] Brendlé | Suivi par chromatographie gazeuse inverse de la transformation de la goethite en hématite au cours d'un traitement thermique[END_REF] who used IGC to follow the surface properties variations that occur when goethite transforms into hematite upon heat treatment.

Another study made by Hamdi [START_REF] Hamdi | Variation de l'énergie superficielle d'une bentonite par traitement chimique et thermique[END_REF] on a bentonite allows to note that the dispersive component of the surface energy ( d s ) slightly increases for heat treatments less than 300 • C. The increase can be attributed to the elimination of the physically adsorbed water molecules and molecules present between lamellar sheets of the montmorillonite which let appear free active sites. Beyond 300 • C, the values of the d s decrease significantly because the lamellar system of the montmorillonite is completely destroyed.

Theoretical part

Inverse gas chromatography (IGC) is an extension of conventional analytical gas chromatography. It permits, whatever the nature of the solid examined in the chromatographic column, to obtain information about its surface properties.

Depending on the amount of injected probe, the IGC can be divided into two modes: infinite dilution (IGC-ID) [START_REF] Balard | Determination of the acid-base properties of solid surfaces using inverse gas chromatography: advantages and limitations[END_REF] and finite concentration (IGC-FC) [START_REF] Conder | Physicochemical Measurements by Gas Chromatography[END_REF][START_REF] Balard | Estimation of the surface energetic heterogeneity of a solid by inverse gas chromatography[END_REF].

Inverse gas chromatography at infinite dilution (IGC-ID)

The inverse gas chromatography at infinite dilution (IGC-ID) involves the injection of very small amounts of probe molecules (at the limit of detector sensitivity) in order to consider only the interactions between the solid-surface and an isolated probe. The IGC-ID gives different information according to the nature of the injected probe.

The dispersive component of the surface energy ( d s )

This parameter indicates the ability of the solid surface to have non-specific interactions with probe molecules. Experimentally, the plot of the variation of the adsorption free energy versus the number of carbon atoms for an alkanes series leads to a straight line and from its slope is calculated the Gibbs energy adsorption of a methylene group G CH 2 a . The d s is then calculated from the Dorris and Gray [START_REF] Dorris | Adsorption spreading pressure and London force interactions of hydrocarbons on cellulose and wood fiber surfaces[END_REF] relation:

d s = G CH 2 a 2 4N 2 a 2 CH 2 CH 2 (1)
where d s the dispersive component of the surface energy (mJ/m 2 ), N the Avogadro's number, a CH 2 the molecular area of one methylene group (0.06 nm 2 ), CH 2 the surface energy of polyethylene ( CH 2 = 35.6 + 0.058(293 -T )) (mJ/m 2 ).

The nanomorphological index I M ( t )

Established by injecting branched or cyclic alkanes, this value delivers information about the regularity of the solid surface at the scale of the injected probe. Indeed, accessibility to the surface of a solid depends strongly on the probes encumbrances (cyclic and branched probe) and the surface roughness of the analyzed solid can be due to the existence of a porosity in the particles or to a superposition of two crystal planes which reveals structural defects giving to the surface a roughness at the nanoscale. The nanomorphology index I M ( t ) [START_REF] Balard | Determination of the acid-base properties of solid surfaces using inverse gas chromatography: advantages and limitations[END_REF] for a branched or cyclic alkane is obtained by application of the following Eq. ( 2):

I M ( t ) = e -G M a /RT (2)
where -G M a is the departure of the representative point of the bulky alkane from the reference n-alkane straight line.

The specific component of the surface energy (I sp )

Specific interactions include all types of interactions except the dispersive ones; they are obtained by injecting polar probes [START_REF] Balard | Determination of the acid-base properties of solid surfaces using inverse gas chromatography: advantages and limitations[END_REF]. The difference between the global free energy of the injected polar probe and the straight line of alkanes probes represents the contribution of specific interactions G sp a also noted I sp :

G a = G d a + G sp a (3)
It can be noticed that to calculate the I M ( t ) and the I sp , the variation of the adsorption free energy is plot versus a topological index t proposed by Brendlé [START_REF] Brendlé | A new topological index for molecular probes used in inverse gas chromatography for the surface nanorugosity evaluation. 1. Method of evaluation[END_REF] based on the Wiener index.

A limit of IGC-ID is reached for solids having a heterogeneous surface for which only the high energy sites mainly contribute to the retention times. So, IGC-FC can be carried out to overcome this limitation.

Inverse gas chromatography at finite concentration (IGC-FC)

In inverse gas chromatography at finite concentration (IGC-FC), some microliters of liquid probe are injected into the column containing the solid to be analyzed, in order to cover approximately with a probe mono-layer the surface of the solid. Then a strong asymmetric chromatographic peak is obtained when ideal, nonlinear conditions are fully fulfilled. Due to the presence of very energetic sites of adsorption, it is quite generally observed that a non-negligible part of the injected probe is not necessary eluted in time for the return of the signal to the initial base line. In order to assess this irreversibly adsorbed amount, the temperature of the chromatograph oven is increased up to the conditioning temperature, leading to appearance of a secondary small peak. Then irreversible part of the adsorption phenomena could be assessed from the ratio of the area of thermodesorption peak onto the total area of the chromatogram and have defined [START_REF] Balard | Study by inverse gas chromatography of the surface properties of milled graphites[END_REF] an irreversibility index (I irr ) according to Eq. ( 4):

I irr = S th S rv + S th (4)
where S rv the surface of the main chromatographic peak and S th the surface corresponding to the thermodesorption peak.

The elution characteristic point method

Applying "the elution characteristic point method" (ECP) to the reversible isothermal part of the experimental chromatogram, the desorption isotherm of the molecule probe can be obtained from a single chromatographic peak [START_REF] Conder | Physicochemical Measurements by Gas Chromatography[END_REF]. The first isotherm derivative is directly related to the net retention time of each point of diffuse front of the chromatogram by Eq. ( 5):

∂N ∂P L,tr = D c (t r -t 0 ) mRT (5)
where for a given characteristic point, N is the number of adsorbed probe molecules, P is the partial pressure of the probe at the output of the column -directly related to the height of the signal, t r is the retention time and t 0 the retention time of CH 4 -a molecule not retained by the solid surface, D c the corrected flow rate, m the mass of the solid contained in the column, L the column length.

The integration of Eq. ( 5) gives access to the desorption isotherm which allows the determination of the Henry's and the BET constants, and of the specific surface area.

But the surface heterogeneity will influenced shape of the desorption isotherm. Intuitively, the residence times of the adsorbed molecules will depend of the energy of the adsorption site. Hence, the shape analysis of the isotherm will give access to the distribution function of the adsorption energies.

Distribution function of the adsorption energies (DF)

Estimation of the surface heterogeneity of a solid through the adsorption energy distribution function is generally based on a physical adsorption model which admits that the global isotherm may be considered as a sum of local isotherms of adsorption on isoenergetic domains (patchwork model).

Then, the distribution function of the relative abundance of each type of domain, having the same characteristic energy of interaction is given by the following integral Eq. ( 6):

N(P m , T m ) = N 0 εmax ε min Â(ε, P m , T m ) (ε)dε (6)
where N(P m , T m ) is the number of molecules adsorbed at the pressure Pm and temperature Tm of measurement, N0 is the number of molecules needed for the formation of a monolayer, Â (ε, Pm,Tm) is the local isotherm, ε is the adsorption energy of a site, and (ε) is the distribution function (DF) of the sites seen by the probe. The range of adsorption energies is included between minimal (ε min ) and maximal (ε max ) values. The resolution of the integral Eq. ( 6) has been presented by Balard [START_REF] Balard | Estimation of the surface energetic heterogeneity of a solid by inverse gas chromatography[END_REF].

Materials and methods

Material

Attapulgite, or palygorskite, is a fibrous Mg-clay, which exhibits microchannels oriented longitudinally with the fibers (Fig. 1). The chemical composition of this clay mineral varies within certain limits according to its origin. The main variation is that an A1 3+ cation can replace Mg 2+ in the octahedral sheet [START_REF] Gonzalez | Structural and textural evolution under thermal treatment of natural and acid-activated Al-rich and Mg-rich palygorskites[END_REF].

The natural attapulgite used in this study was obtained from the Ghoufi's region, located at the Northeast of Algeria. This clay has a fibrous morphology with presence of cubic inclusions attributed to carbonates [START_REF] Boudriche | Effect of acid treatment on surface properties evolution of attapulgite clay: an application of inverse gas chromatography[END_REF]. The raw attapulgite was first coarsely milled and sieved to obtain a size fraction less than 1 mm allowing easier analysis, measurements, and treatments to be made in laboratoryscale processes.

Thermal treatment of attapulgite

The heat treatment of the clay was performed by means of a tubular furnace, controlled by a timer-controller type Eurotherm Controls, in a ceramic crucible. The heat treatment was realized in a temperature range from 100 • C to 1000 • C with a heating rate of 5 • C/min under nitrogen rate flow. After each increase of 100 • C in temperature, cooling is performed in an inert atmosphere.

Only temperatures leading to modifications in the solid properties are commented in this paper.

Characterization methods

Before IGC analysis, several complementary techniques were used to characterize our samples before and after heat treatment: a. The thermogravimetric analysis (TGA) coupled with differential thermal analysis (DTA) provides information about the mass and enthalpy changes according to a heating rate. The coupling of these two techniques is often used to identify changes in the analyzed material. In our study, the device used was a TG-DTA SETARAM 92-16. Thermal analysis was done with a temperature rise of 5 • C/min from 30

• C to 1000 • C.
b. The dilatometric analysis provides access to dimensional changes of a sample as a function of temperature and time. The analysis was done by means of SETSYS-TMA 18 Setaram dilatometer, from 30 • C to 1000 • C with a ramp of 5 • C/min. c. The density of the obtained solids was determined by a helium pycnometer (Accupyc 1330-Micrometrics). d. The specific surface areas, BET constants, meso and micro-pore volumes were determined using nitrogen adsorption at liquid nitrogen temperature (ASAP 2010, Micromeritics) by applying the BET, BJH, and t-plot methods, respectively. The samples were out gassed at 105 • C for 24 h at a pressure of 6.6 × 10 -3 mbar. e. The particle morphology was investigated using a scanning electron microscope (Philips XL 30 model ESEM-FEG) operating at 3 kV. f. For the analysis of silicon, samples were mineralized by alkaline fusion with sodium hydroxide. For the analysis of the following elements: Al, Mg, Ca, Fe, samples were mineralized by wet way in acid media. After mineralization, all the elements were then analyzed by ICP-AES. g. Phase identification was performed on a Panalytical X'PERT Pro diffractometer (Philips) (CuK radiation) with a step size of 0.017 • for 2Â angles ranging from 5 • to 80 • . The 2Â values were analyzed with the X'Pert High Score software and compared with the ICDD (International Centre for Diffraction Data) database. h. The IR spectra of activated attapulgite were obtained using a Perkin Elmer 560 Fourier transform infrared (FTIR) spectrophotometer with a resolution of 2 cm -1 and a scan number equal to 64. The spectral study was extended over the range 4000-400 cm -1 . The analysis was done on KBr pellets with 1% of treated sample, using a hydraulic press by applying a pressure of 73.5 kPa.

The IGC measurements were performed with two GC devices (Agilent 7890 A and 6890), fitted with sensitive flame ionization detectors (FID). Helium was used as the carrier gas with a flow rate of 30 mL/min measured with an electronic flowmeter (Flow500-Agilent). The temperatures of the injector and detector were respectively 130 • C and 200 • C. The columns were filled with a mixture of attapulgite powder and glass beads so as to lower the pressure drop (<1 bar) through the columns in order to avoid leaks at fittings.

The IGC-ID study was done with an oven temperature of 130 • C, using stainless steel columns of 20 cm in length, with 6.35 mm in internal diameter. The columns were conditioned overnight at 150 • C. The probes used were linear alkanes (hexane C6, heptane C7 and octane C8), a branched alkanes, the 2,3,4trimethylpentane (2,3,4-TMP) and 2,5-dimethylhexane (2,5-DMH), a cyclic alkane, the cycloctane (cycl8) and polar probes (chloroform and dichloromethane). These probes had a chromatographic grad (>99%) purity.

For IGC-FC, the chromatographic columns were 10 cm in length with 6.35 mm or 3.18 mm in internal diameter. The conditioning temperature was 130 • C and analysis temperature was depending on the probe used, according to the Conder criterion [START_REF] Conder | Physicochemical Measurements by Gas Chromatography[END_REF] which states that the contribution of probe vapor to the flow of carrier gas does not exceed 5% of the initial flow at the maximum of the chromatographic peak. Three probes were examined in IGC-FC, the octane (C8) at 53 • C, the isopropanol (IP) at 43 • C and the 1,4dioxan (DX) at 51 • C. We chosed an apolar probe, octane, to study the surface morphology and two polar probes to study the surface chemistry. These probes had a chromatographic grade (>99%) purity and were placed on a molecular sieve to avoid the water effect. The chromatograms were treated using a home-made software from the Adscientis Society (Wittelsheim, France).

Results and discussion

4.1. Thermal analysis 4.1.1. TGA-DTA analysis TGA-DTA analysis (Fig. 2) indicates that attapulgite undergo four losses in mass of about 25.93% of the initial mass.

The losses in mass occur respectively in the following ranges: 30-160 • C (5.26%), 160-280 • C (2.19%), 280-550 • C (3.77%) and 650-800 • C (14.70%). The first mass loss is quite quick, and corresponds to the removal of bound water (on the surface) and a part of the zeolitic water (located in the channels). The second water loss corresponds to the total elimination of the zeolitic water. Up to 280 • C, the phenomenon is perfectly reversible and the clay behaves as an excellent absorbent towards water. When the temperature increases, it causes between 280 • C and 550 • C a slow departure of crystallized water (coordinated to cations situated along octahedral sheets) with an evolution towards a rafter structure. It should be noted that an attapulgite in this state is still able to adsorb water by capillarity [START_REF] Rautureau | Etude par microscope électronique des différents états d'hydration de la sépiolite[END_REF]. The fourth water mass loss is produced by dehydroxylation of Mg-OH groups in attapulgite [START_REF] Frost | Controlled rate thermal analysis and differential scanning calorimetry of sepiolites and palygorskites[END_REF][START_REF] Khomari | Comparison of attapulgites from different sources using TG/DTG and FTIR[END_REF]. The elimination of water molecules at these temperatures is accompanied by a partial destruction of the crystal structure of attapulgite [START_REF] Preisinger | Sepiolite and related compounds: its stability and application[END_REF]. The release of the carbon hydroxide must be also considered in the fourth mass loss because beyond 500 • C; the carbonates present in the attapulgite are thermically unstable and their decomposition releases the CO 2 [START_REF] Hamdi | Etude physico-chimique de la surface des diatomites[END_REF].

Several authors studied the thermal analysis of this clay [START_REF] Gonzalez | Thermal investigation of acid-activated attapulgites: influence of isomorphic substitution in the octahedral sheet[END_REF][START_REF] Kuang | Dehydration and rehydration of palygorskite and the influence of water on the nanopores[END_REF][START_REF] Mifsud | Etude de l'eau dans la palygorskite à l'aide des analyses thermiques[END_REF] and they observed the mass losses at the same temperatures.

Dilatometric analysis

Three retractions (volume change) are observed from the dilatometric analysis (Fig. 3).

The first one occurs between 30 • C and 200 • C, and is due to the evaporation of bounded and zeolitic water molecules, previously observed in TGA-DTA analysis (Fig. 2). It is very low, 0.6% of the initial height (1.75 mm). The second retraction, a little higher than the first one, 2.8%, happens between 200 • C and 400 • C, it can be attributed to the evaporation of crystallized water out of the structure of attapulgite. The highest volume change takes place between 750 • C and 900 • C, corresponding in TGA-DTA curve to a mass loss of 14.7%. In this case, there is a densification of the material and the shrinkage is estimated of about 437.55 m in the measuring cell, representing 24% of the initial height. This consolidation of attapulgite particles under the effect of heating is called sintering [START_REF] Traore | Sintering of a clay from Burkina Faso by dilatometry influence of the applied load and the pre-sintering heating rate[END_REF][START_REF] Khalfaoui | Sintering mechanism and ceramic phases of an illitic-chloritic raw clay[END_REF]. The consolidation of a material could be emphasized using a helium pycnometer.

Textural analysis

Density measurement by helium pycnometer

The values of average densities confirm the sintering of the attapulgite under the effect of the increase of the heat temperature (Table 1). The density equal to 2.574 for the raw material increases gradually to 3.038 after the heating to 1000 • C.

Another characteristic of the material that will certainly be affected by the sintering is the surface morphology in terms of specific surface areas and porosity measured by nitrogen adsorption at 77 K.

Surface analysis by nitrogen adsorption

Table 2 contains the specific surface areas, BET constants, meso and micro-porous volumes obtained from nitrogen adsorption isotherms under heat treatment effect.

The texture of attapulgite evolves significantly during the heat treatment. The specific surface area decreases as the temperature increases up to 2 m 2 /g at 1000 • C. It is linked to the disappearance of the micropores since 200 • C while the mesopores only disappear at 1000 • C. A similar change was obtained for attapulgite from India [START_REF] Sidheswaran | Heat-induced structural modifications in palygorskite[END_REF] since its initial surface area decreases from 186 m 2 /g to 140 m 2 /g at 200 • C, it decreased further at 350 • C to 123 m 2 /g, and at 500 • C it reached 129 m 2 /g. Sidheswaran [START_REF] Sidheswaran | Heat-induced structural modifications in palygorskite[END_REF] attributed the decrease in surface area at high temperatures to the formation of two crystalline phases: forsterite (MgSiO 4 ) and enstatite (MgSiO 3 ).

Another study [START_REF] Frini-Srasra | Effect of heating on palygorskite and acid treated palygorskite properties[END_REF] on attapulgite from Tunisia, led to different results. Indeed, the specific surface area of the attapulgite initially equal to 67.4 m 2 /g increased successively, to 74.2 m 2 /g at 350 • C, then, to 85.9 m 2 /g at 550 • C, before decreasing to 60.7 m 2 /g at 750 • C. The increase in the observed surface was attributed to the elimination of water molecules, leading to the appearance of a new porosity while the decrease of the surface was interpreted by the coalescence of particles under the effect of temperature.

In this study, the continues decreasing in the surface area, whatever the heat treatment temperature, can be linked to the disappearance of the porous volume, more exactly the microporous volume, caused by the coalescence of particles from 200 • C. It can be supposed reasonably that the ability to observe the departure of water from the measurement of specific surface area depends on the degassing conditions.

Then, it was interesting to observe by scanning electron microscopy the effect of the heat treatment on the morphology of attapulgite.

Morphological analysis by SEM

The SEM micrographies (Fig. 4) represent the morphological evolution of attapulgite fibers and carbonates before and after heat treatment at 400 • C, 800 • C and 1000 • C.

At 400 • C, SEM analysis indicates that the fibers and carbonates are still preserved. From 800 • C, there is an alteration of carbonates which appear roughness, contrary to the initial state. At the same temperature, some attapulgite fibers appear slightly affected because they melt. A total disappearance of the fibrous morphology happens at 1000 • C, because of their complete fusion while the carbonates inclusions have distinctly granular appearance.

Chemical structure 4.4.1. Determination of chemical composition by elemental analysis

Cations present in the attapulgite sample before and after heat treatment at 400 • C, 800 • C and 1000 • C were determined by elemental analysis (Table 3).

The decrease in the carbon percentage with the temperature increasing is caused by the release of carbon oxides, indeed, at 500 • C, calcite (CaCO 3 ) and dolomite CaMg(CO 3 ) 2 are thermally unstable and decompose respectively into CaO, CO 2 and MgO, CaCO 3 , CO 2 . Similar results were obtained by Hamdi [START_REF] Hamdi | Etude physico-chimique de la surface des diatomites[END_REF] during calcination of a diatomite containing calcite, at the same temperature. In addition to the decomposition of carbonates, the decomposition of organic carbon must be considered in the clay matrix. Other elements ratios increase as the consequence of the carbon ratio decrease. Indeed, it is very difficult to induce dealuminization at temperatures below 800 • C, so the presence of aluminum, either in free form (Al 2 O 3 ) or combined with silica (aluminosilicate), displays a thermal stability [START_REF] Kosuge | Effect of SiO2 addition on thermal stability of mesoporous c-alumina composed of nanocrystallites[END_REF][START_REF] Novakovic | The thermal stability of porous alumina/stainless steel catalyst support obtained by spray pyrolysis[END_REF]. On the other hand, the ferric species are stable beyond 800 • C.

In order to confirm the decomposition of carbonates, X-ray diffraction analysis was made on samples.

Mineralogical analysis by X-ray diffraction

Fig. 5 shows the diffraction patterns of raw and calcined attapulgite at 400 • C, 800 • C and 1000 • C.

At 400 • C, the disappearance and the decrease in the intensity of some peaks reflect a decrease in the solid crystallinity. However, attapulgite and carbonates, like dolomite CaMg(CO 3 ) 2 and calcite CaCO 3 seem to be resistant to heating at 400 • C. At 800 • C, the amorphization is complete, and at this temperature the crystal structure of attapulgite is entirely destroyed with the disappearance of attapulgite and carbonates and appearance of iron oxide and quartz.

At 1000 A study of different hydration states of sepiolite [START_REF] Rautureau | Etude par microscope électronique des différents états d'hydration de la sépiolite[END_REF] showed that the major disorganization in the structure occurs with the departure of crystallized water (at about 500 • C) and results in a phase where the elements are rotated as illustrated in Fig. 6.

At high temperature, the sepiolite reaches the dehydroxylation stage and this transformation is achieved at around 850-875 • C, corresponding to a total reorganization of the cell with the formation of enstatite.

In our study, when the temperature increases, the color of the raw attapulgite changes from light brown to brick red at 900 • C. This color change means that isomorphous substitutions by iron can occurs within the structure (Si 4+ can be replaced by Fe 3+ and Mg 2+ by Fe 2+ ), which gradually makes the treated surface composed essentially of iron oxide (Fig. 5). The powder calcined at 1000 • C is gray, this can be attributed to a complete elimination of organic matter, as was demonstrated for a kaolinite of Brazil [START_REF] Melo | Effects of thermal and chemical treatments on physical properties of kaolinite[END_REF], calcined at 550 • C during 8 hours. This removal of organic matter was accompanied by a color change from dark brown before treatment to light gray.

In summary, the XRD analysis of heat treated samples indicates that attapulgite is undergoing profound changes, at temperatures above 400 • C.

The Fourier Transform Infrared Spectroscopy is also of interest to bring information on the evolution of the chemical functionality. FTIR analysis supports the elimination of water molecules. In details:

The hydroxyl vibrational bands at 3612 cm -1 and 3544 cm -1 (Fig. 7) decrease in intensity until it disappears completely at 800 • C, forming a broad band centered at 3450 cm -1 . The first one is attributed to the contribution of stretching vibrations of OH in (Fe, Mg)-OH and (Al, Mg)-OH and the second one to the stretching vibration of OH associated with Al 3+ cations in dioctahedral coordination Al 2 OH and OH stretching vibration of water coordinated with Mg along the fibers [START_REF] Blanco | Differences between one aluminic palygorskite and another magnesic by infrared spectroscopy[END_REF]. This evolution confirms the elimination of water molecules (linked, and zeolitic crystallized) and the dehydroxylation of Mg-OH groups occurring at high temperatures.

Fig. 8 represents the spectra of FTIR attapulgite heated at 400 • C, 800 • C and 1000 • C, in the region 1500-400 cm -1 . Fig. 8. FTIR spectra in absorbance units of attapulgite before and after heat treatment at 400 • C, 800 • C and 1000 • C, in the 1500-400 cm -1 region.

In the region of low wavenumbers (Fig. 8), the band at 1652 cm -1 corresponding to the bending vibrations of H 2 O [41] also disappears when the temperature increases.

The band at 1423 cm -1 , characteristic of carbonates [START_REF] Garcia-Romero | Characteristics of a Mg-palygorskite in Miocene rocks, Madrid basin (Spain)[END_REF], also decreases in intensity as the temperature increases, due to the thermal instability of carbonates.

The bands between 1200 cm -1 and 900 cm -1 for Si-O vibration [START_REF] Suárez | FTIR spectroscopic study of palygorskite: influence of the composition of the octahedral sheet[END_REF][START_REF] Mendelovici | Infrared study of attapulgite and HCl treated attapulgite[END_REF] decrease in intensity until they disappear at 800 • C revealing a broad band centered at 1030 cm -1 . This variation reflects the partial destruction of the crystal structure of attapulgite from 800 • C also observed by XRD.

The IR spectrum obtained at 1000 • C reveals no new absorption peaks.

After reviewing the textural, structural and morphological properties evolution of the attapulgite under heat treatment, we focused on the inverse gas chromatography, a technique to follow the modifications of surface properties. The same samples have been examined by IGC at infinite dilution or finite concentration. 4.

The dispersive component of the surface energy decreases when the temperature increases from 164 mJ/m 2 before treatment to 119 mJ/m 2 at 800 • C. This drop is attributed to the disappearance of structural defects in which the alkanes probes can establish strong dispersive interactions with the solid surface by insertion.

However, despite the disappearance of the porosity and structural defects measured respectively by nitrogen adsorption (Table 2) and by the d s values (Table 4), the nanomorphological indexes I M ( t ) obtained with branched or cyclic probes decrease with increasing temperature, indicating the appearance of a roughness surface. Then, the evolution of the I M ( t ) can be attributed to the morphology of the carbonates, as observed by SEM analysis (Fig. 4), which becomes gradually roughness at high temperature.

The specific component of surface energy is not affected by thermal treatment meaning that the surface chemistry remains preserved.

4.5.2. Inverse gas chromatography at finite concentration (IGC-FC) 4.5.2.1. Study of desorption isotherms. In Table 5 are reported the values of specific surface areas values measured by nitrogen at 77 K and those obtained by IGC-FC, using: octane, isopropanol and dioxan probes. The irreversibility indexes of the same samples are also reported.

Whatever the probe used, the surface area decreases significantly under the effect of the temperature reaching values between 20 and 30 m 2 /g at 800 • C, depending of the injected probe. This is explained by the coalescence of particles occurring at high temperatures and which removes the roughness and surface porosity.

Before and after thermal treatment at 400 • C, specific surface area values with nitrogen are higher than those measured with organic probes. It was attributed to the approximation in the calculation of molecular areas of organic probes which are established from adsorption isotherms, generally, on nonporous solids. At 800 • C, the disappearance of the porosity and surface roughness under the thermal effect, makes the uncertainty in the calculation of molecular areas weak, because the surfaces are almost flat, leading to practically identical surface areas whatever the nature of the probe, this tends to prove that it is the positioning of the organic probes on the surface of solid which is the reason of differences observed for the samples treated at 400 • C. Another explanation for the difference between the surfaces with nitrogen and those measured by IGC-FC, may be the amount of the irreversible adsorption which is not taken into account in IGC-FC for the calculation of specific surfaces areas. The irreversibility indexes were used to quantify the irreversible adsorption, which increases with increasing the temperature of thermal treatment, indicating a greater affinity of these probes to the surface of the heated attapulgite. Taking into account the irreversible adsorption, corrected specific surface areas measured with the organic probes before and after treatment at 400 • C (Table 6) are again lower than with nitrogen. Irreversible adsorption is not the only factor responsible of the low values of the specific surface areas measured with organic probes but also the uncertainty on the actual molecular area of the probes. 4.5.2.2. Assessment of the surface energetic heterogeneity. The heterogeneity of surface energy of attapulgite samples before and after thermal treatment was estimated by IGC at finite concentration conditions, choosing three probes with different polarities. The surface heterogeneity was evaluated by determining the adsorption energy distribution functions of the three probes according to the method of Rudzinski and Jagiello, by applying the technique of multiple derivations developed by Balard [START_REF] Balard | Estimation of the surface energetic heterogeneity of a solid by inverse gas chromatography[END_REF]. So, Fig. 9 shows the adsorption energy distribution functions of n-octane (a) isopropanol (b) and 1,4-dioxan (c) on the surface of attapulgite before and after thermal treatment at 400 • C and 800 • C.

At 400 • C, the distribution functions with the three probes remain preserved comparatively to those of the raw attapulgite, on the contrary the distribution function (FD) of the sample treated at 800 • C show the appearance of two shoulders at high energies, which are more pronounced with polar probes. The weak shoulders observed with the octane, a probe sensitive to the morphology changes, indicate that the attapulgite morphology is affected by the melting of the fibers, while with polar probes, the two shoulders are attributed to the formation of the new phases as confirmed by X-ray diffraction.

Conclusion

After submitting attapulgite clay to a thermal treatment in 100-1000 • C interval, two temperature ranges are distinguished from our study:

Below 500 • C, mass loss is mainly due to a departure of water molecules (related, zeolitic and crystalized water) and the fibrous morphology is preserved despite a rearrangement of the structure.

Above 500 • C, there is a coalescence of particles (sintering) reducing the porous volume network and consequently, the specific surface area. At high temperatures, new crystalline phases appear and the fibrous morphology is significantly affected due to the complete melting of the attapulgite fibers.

By IGC-ID, it is observed a decrease of the dispersive component of the surface energy and correlatively a decrease of the nanomorphological index emphasizing the appearance of a roughness of the carbonates surface.

The specific surface areas measured in IGC-FC reflect the approximation in the calculation of molecular areas of organic probes established from adsorption isotherms, generally, on non-porous solids. The differences on specific surface areas measured with nitrogen and organic probes on the raw solid and the solid treated until 400 • C, disappear on the attapulgite treated at 800 • C which surface becomes flat.

The distribution functions also evolve with the thermal treatment indicating with octane modifications on the surface morphology and with isopropanol or 1,4-dioxan formation of new phases.

This study proves that inverse gas chromatography (IGC) is a convenient and powerful analysis method, in particular for the study of the evolution of physicochemical surface properties of a solid at a molecular scale under a thermal treatment, and it is a very complementary technique to other conventional techniques.

Fig. 1 .

 1 Fig. 1. Polyhedral representation of attapulgite [27].

Fig. 2 .

 2 Fig. 2. TGA-DTA analysis of attapulgite.

ΔFig. 3 .

 3 Fig. 3. Dilatometric analysis of attapulgite (initial height: 1.75 mm).

Fig. 4 .

 4 Fig. 4. Micrographs of attapulgite before and after calcination at 400 • C, 800 • C and 1000 • C.

Fig. 5 .

 5 Fig. 5. Attapulgite diffraction patterns before and after heat treatments at 400 • C, 800 • C and 1000 • C (Att = attapulgite, D = dolomite, C = calcite, For = forsterite, Aug = augite).

Fig. 6 . 1 Fig. 7 .

 617 Fig.6. Schematic structure of anhydrous sepiolite showing the rotation of the elements of the sheets relative to each other[START_REF] Rautureau | Etude par microscope électronique des différents états d'hydration de la sépiolite[END_REF].

4. 5 .

 5 Determination of surface properties by inverse gas chromatography 4.5.1. Inverse gas chromatography at infinite dilution (IGC-ID) Parameters like the dispersive component of the surface energy, d s , the nanomorphological index, I M ( t ), and the specific component of the surface energy, I sp , obtained by IGC-ID at 130 • C, on attapulgite samples before and after thermal treatment are gathered in Table

Fig. 9 .

 9 Fig. 9. Adsorption energy distribution functions of the n-octane (a) isopropanol (b) and 1,4-dioxan (c) probes of the initial and thermal treated attapulgite at 400 • C and 800 • C, measured respectively at 53 • C, 43 • C and 51 • C.

Table 1

 1 Density values of attapulgite before and after heat treatment, measured by helium pycnometer.

	Samples	Average density (g/cm 3 )
	Raw attapulgite	2.57
	200 • C	2.65
	400 • C	2.77
	800 • C	2.97
	1000 • C	3.04

Table 2

 2 Values of specific surface areas (aBET), BET constants (CBET), volumes of micro-and meso-pores attapulgite samples heat treated at 200 • C, 400 • C, 800 • C and 1000 • C.

		aBET	CBET	Vmeso	V micro
		(m 2 /g)		(cm 3 /g)	(cm 3 /g)
	Raw attapulgite	125.1	436	0.302	0.018
	200 • C	88.3	170	0.271	0.007
	400 • C	81.5	173	0.340	0.006
	800 • C	28.6	176	0.256	-
	1000 • C	1.6	28	-	-

Table 3

 3 Chemical composition analysis of raw and heat-treated attapulgite at 400 • C, 800 • C and 1000 • C.

	Element (%)	Elemental analysis
		Raw attapulgite	400 • C	800 • C	1000 • C
	Si	16.63	20.21	22.89	25.93
	Mg	6.35	7.01	7.98	8.53
	Al	4.12	5.05	5.8	6.28
	Ca	8.62	4.84	5.74	5.90
	Fe	2.33	2.72	3.15	3.40
	C	5.03	2.41	0.44	<0.30

  2Â), 52(2Â) and 57(2Â) are related to the formation of new crystalline phases such as the augite Ca(Fe,Mg)Si 2 O 6 and the forsterite MgSiO 4 . These phases are formed following the topotactic transformations.

	Counts
										Before thermal	treatment
										400°C
										800°C
		For	For	Aug		Aug		For+Aug	For	For	1000°C
	6	16	26	36	(2θ)	46	56	66	76

• C, new peaks located at 28(2Â), 30(2Â), 31(2Â), 36(2Â), 45(

Table 4

 4 Variations of the d s , IM ( t ) and Isp values of attapulgite samples, before and after thermal treatment at 400 • C and 800 • C, obtained at an oven temperature of 130 • C.

		d s (mJ/m 2 ) ± 4%	IM ( t ) ± 5%			Isp (kJ/mol) ± 4%
		C6, C7, C8	2,3,4-TMP	2,5-DMH	Cycl 8	CH2Cl2	CHCl3
	Raw attapulgite	164	0.69	0.67	0.22	17	14
	After thermal treatment
	400 • C	143	0.68	0.61	0.24	16	13
	800 • C	119	0.43	0.34	0.18	18	13

Table 5

 5 Values of the specific surface areas (aBET) and irreversibility indexes (I irr ) of raw and thermal treated attapulgite, obtained by nitrogen adsorption and IGC-FC.

		Adsorption N2	IGC-FC
			C8 (53 • C)		IP (43 • C)		DX (51 • C)
		aBET (m 2 /g)	aBET ± 2.8 (m 2 /g)	I irr ± 0.2 (%)	aBET ± 4.5 (m 2 /g)	I irr ± 0.6 (%)	aBET ± 3.6 (m 2 /g)	I irr ± 0.1 (%)
	Raw attapulgite	125.1	114.5	0.6	70.9	8.3	71.0	7.8
	After thermal treatment
	400 • C	84.4	81.7	1.2	67.1	12.8	61.1	9.3
	800 • C	28.6	29.9	5.3	21.8	18.7	25.1	12.8

Table 6

 6 Corrected values of specific surface areas (acorr) attapulgite samples before and after thermal treatment compared to those measured with nitrogen.

		aBET (m 2 /g)	acorr (m 2 /g)
		N2	C8 (53 • C)	IP (43 • C)	DX (51 • C)
	Raw attapulgite	125.1	115.2	76.9	76.5
	After heat treatment
	400 • C	84.4	82.7	75.6	66.7
	800 • C	28.6	31.5	26.1	28.2

Appendix A. Supplementary data

Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2012.02.015.


S Bensalem

B Hamdi

Sylvie Del Confetto

M Iguer-Ouada

Alain Chamayou

H Balard

Rachel Calvet

S Del Confetto

Characterization of chitosan/montmorillonite bionanocomposites by inverse gas chromatography

published or not. The documents may come

• XPS patterns show the chitosan intercalation in the montmorillonite layered.

• Surface parameters are determined by inverse gas chromatography.

• Surface parameters are sensitive to chitosan intercalation in montmorillonite structure.

• The increase in the chitosan mass ratio leads to the progressive disappearance of high energy sites.

g r a p h i c a l a b s t r a c t

The distribution function obtained by inverse gas chromatography at finite concentration on the bionanocomposite shows the disappearance of the shoulders at high energy after intercalation/adsorption of chitosan in montmorillonite galleries.

Keywords: Chitosan Montmorillonite Bionanocomposite Inverse gas chromatography XPS Surface heterogeneity a b s t r a c t Montmorillonite was modified with different amounts of chitosan to obtain chitosan/montmorillonite bionanocomposites (CSMMT). The structural and surface characteristics of the raw materials and bionanocomposites were analyzed. XPS results showed that the obtained bionanocomposites had MMT-rich surfaces with chitosan essentially intercalated in the host clay galleries. IGC at infinite dilution confirmed that the increase of CS mass ratio in the bionanocomposites significantly affected the surface properties. IGC at finite concentration revealed a clear evolution in the surface heterogeneity in chitosan/montmorillonite bionanocomposites by the progressive disappearance of shoulders of high energy with the increase of CS mass ratio in MMT.

Introduction

Bionanocomposites based on biopolymers and clays have received considerable attention over the last decade due to their new structure and functional properties. They are formed by the combination of natural polymers and clays and show at least one dimension on the nanometric scale [1]. Thus, the resulting materials are characterized by properties inherent to both types of components [2]. One of the most extensively-used filler materials in bionanocomposite is the smectite class of aluminum silicate clays, of which the most common representative is montmorillonite (MMT).

Montmorillonite (MMT) is a 2:1 type aluminosilicate. Its crystalline structure presents an alumina octahedral between two tetrahedral layers of silica [3]. The layers of MMT are negative in nature and this charge can be neutralized by a counter cation (generally the Na + cation), which is situated in the gap between the layers. This cation can be modified by cationic exchange reactions with other cations such as Nickel, Cobalt and Zinc [4], cationic surfactants [5], salts of fatty acids [6] and polymers [7].

Biopolymers commonly used in bionanocomposites are proteins, phospholipids, biopolyesters such as polylactic acid, nucleic acids, and polysaccharides such as starch, cellulose, xanthan gum or chitosan (CS). CS is the second most abundant natural polysaccharide in the world after cellulose.

CS is a linear cationic poly-␤-(1,4)-2-amino-2-deoxyglucopyranose with a variable degree of N-deacetylation. It is mainly extracted from shrimp bio-waste [8]. Under slightly acidic conditions, most of the amino groups of CS are protonated and the resulting CS contains multiple cationic charges. CS is considered one of the most valuable polymers for environmental, biomedical and pharmaceutical applications due to its properties of biodegradability, biocompatibility, antimicrobial effects, good film formation, and high positive charge [9,10]. Furthermore, the polyelectrolyte positive charges of CS are able to intercalate MMT and other smectites by ion-exchange reactions [11,12], the resulting bionanocomposites showing interesting structural and functional properties [2].Since chitosan-montmorillonite bionanocomposites exhibit good functional and mechanical properties, they are widely used as active phases in sensors for potentiometric determination [13], for the removal of heavy metals such as Cobalt [14], for dyes removal [15] and as antimicrobial materials [16]. It is, therefore, of fundamental and technological importance to determine their surface chemical composition and surface energy because these physicochemical properties are directly correlated with the strength of the CS-MMT clay interaction.

The surface chemical composition can be determined by an appropriate surface analytical technique such as X-ray photoelectron spectroscopy (XPS), whereas the surface energy of powders is best assessed by inverse gas chromatography (IGC).

Inverse gas chromatography (IGC) is a commonly-used method for physicochemical characterization of the surface properties of divided solids and in the study of adsorption phenomena. IGC has already been implemented in the analysis of clays, such as attapulgite [17], bentonite [18], smectite [19], of polymers like chitin [20] and of various nanocomposites, such as montmorillonite/PVC [21], montmorillonite/polypyrole [22] and rectorite/thermoplastic polyurethane [23].

In this study, various bionanocomposites based on CS and MMT were prepared in a liquid phase. The influence of CS mass ratio on the bionanocomposites powders was examined by different techniques and compared to those of the raw materials, the pure MMT Na + and CS. The surface composition of the bionanocomposites was determined by XPS. IGC at finite and infinite dilution was implemented to determinate surface free energy, acid-base properties and to examine surface energetic heterogeneity of raw materials and bionanocomposites.

Theory of inverse gas chromatography (IGC)

Inverse gas chromatography at infinite dilution (IGC-ID)

IGC is an extension of conventional analytical gas chromatography. It provides information about surface properties, regardless of the nature of the solid examined in the chromatographic column.

IGC can be carried out in two ways: at infinite dilution (IGC-ID) and at finite concentration (IGC-FC). IGC-ID consists in the injection of negligible amounts (vapor) of well-identified molecules, called probes, into a column filled with the solid that is being tested. At infinite dilution (Henry's domain), the interactions between the probe and the solid material are calculated from the retention times, and the surface properties of the solid are determined.

The dispersive component of surface energy, s d , is calculated when n-alkanes are injected as probes. The assumption is that only dispersive interactions exist between n-alkanes and the solid. Experimentally, the s d parameter is determined from the slope of the straight line obtained by the plot of the variation of the adsorption free energy versus the number of carbon atoms for an alkane series [24].

The nanomorphological index I M ( t ) can be calculated by injection of cyclic alkanes and this parameter gives information about the regularity of the solid surface at the molecular scale [START_REF] Balard | Acid-Base Interact[END_REF].

• The specific component of the surface energy, I sp , can be determined by injections of polar probes, and gives information about polar interactions developed by the solid surface [START_REF] Balard | Acid-Base Interact[END_REF]. Experimentally, the parameter I sp is determined from the difference between the global free energy of the injected polar probe and the straight line of alkane probes.

Inverse gas chromatography at finite concentration (IGC-FC)

In the case of IGC-FC, a few microliters of a liquid probe are injected into a column containing the solid to be analyzed, in order to provide an approximately mono-layered coverage on the surface of the solid. The results of this technique reflect the interactions of probe molecules with all the sites on the solid surface.

The desorption isotherm

Experimentally, the "elution characteristic point" method (ECP) was used, allowing the acquisition of the desorption isotherm from a unique chromatographic peak. The first derivative of the adsorption isotherm can be readily calculated from the net retention time of each characteristic point of the diffuse real front of the chromatogram, according to Conder's equation (Eq. ( 1)) [26].

∂N ∂P

L.t = JD c (tr -t 0 ) mRT (1)
where, for a given characteristic point, N is the number of desorbed probe molecules, P the partial pressure of the probe at the output of the column (directly related to the height of the signal), t r the retention time, t 0 the dead retention time of methane (a non-adsorbed molecule), D C the corrected flow rate, m the mass of the solid in the column, L the column length, R the universal gas constant and T the oven temperature. Integration of Conder's equation provides the desorption isotherm, the pressure of the probe being directly related to the height of the signal for each point of the diffuse real front of the chromatogram. The specific surface area and BET constant were determined using the inverse Langmuir calculation method [27].

Due to the presence of highly energetic sites of adsorption, it is generally observed that a non-negligible part of the injected probe is not necessarily eluted in time for the return of the signal to the initial base line. In order to assess this irreversibly adsorbed amount, the temperature of the chromatograph oven was increased up to the conditioning temperature, leading to the appearance of a small secondary peak. This gives an assessment of the irreversible part of the adsorption phenomenon from the ratio of the area of the thermodesorption peak with respect to the total area of the chromatogram, giving an irreversibility index (I irr ), as previously defined by Balard [27].

The surface heterogeneity

A solid surface is always heterogeneous; this heterogeneity can be generated by the morphology (as with a lamellar structure), by the faces of a crystal lattice structure, or by chemical groups. The chromatogram obtained in the IGC-FC conditions allows access to the surface heterogeneity through the calculation of the adsorption energy distribution function for a given probe. Depending of the nature of the probe (an alkane or a polar probe), the morphological or chemical heterogeneity is determined from the analysis of the isotherm shape. The calculation of the distribution function is based on a physical patchwork model given that the global isotherm can be considered as the sum of local isotherms of adsorption on isoenergetic domains.

However, IGC-FC analyses are generally made above room temperature and the condensation approximation is then no longer correct. Rudzinski [28] proposed another method allowing the calculation of the so-called DFRJs (distribution function in the Rudzinski-Jagiello model) through a limited development of the even derivatives of the DFCA (distribution function in the condensation approximation), corrected for the lateral interaction energy. Balard [27,29] described a signal treatment method based on Fourier's transform to calculate the DFRJ up to the order 4.

The surface heterogeneity can be quantified by the surface heterogeneity index, I hete , expressed in Eq. ( 2):

I hete = 100 A exp -A homo A exp (2)
Where A exp is the area under the experimental DF and A homo is that under the homogeneous DF. This allows the comparison of the computed DF with the DF describing a homogeneous surface that fits the left descending branch of the experimental DF towards the lowest interaction energies [27].

The distribution functions were computed using special software from the ADSCIENTIS company (Wittelsheim, France).

Materials and methods

Materials

Montmorillonite (MMT) used as raw material was obtained from the Maghnia deposit (western Algeria), and provided by the ENOF Company (Algeria). Its cation exchange capacity of 86.5 × 10 -5 eq/g was determined according to the sodium acetate saturation method. A quantity of 20 g of MMT clay was suspended in 500 mL of NaCl 1 M. After several hours of stirring, the suspension was left to decant and the supernatant was discarded. The process was repeated 3 times. Then the solid was washed with de-ionized water to remove chloride ions (until AgNO 3 test was negative). The resulting chloride-free suspension was diluted in more de-ionized water, homogenized and left to decant in a test tube for 24 h. Then the upper 20 cm was collected (i.e. clay sample fraction < 2 m). The product was dried at 358 K to yield MMT Na + .

Chitosan (CS) (Degree of Deacetylation = 80% determined by 1H NMR and molecular weight 293000 g/mol determined by Size exclusion chromatography) was purchased from France-Chitine, France.

The acetic acid used for dissolution of CS was provided by the Merck Company and had 99% purity. The probes employed for IGC were apolar alkanes: n-pentane (C5), n-hexane (C6), nheptane (C7), n-octane (C8); a cyclic alkane: cycloheptane (Cy7) and polar probes (dichloromethane CH 2 Cl 2 and isopropanol (IP)). These probes had a chromatographic grade (>99%) purity.

Preparation of bionanocomposites

Chitosan-montmorillonite (CSMMT) bionanocomposites were prepared according to the protocol described by Darder [2].

MMT Na + suspension was prepared by dispersing appropriate amount of MMT Na + into 100 mL of de-ionized water. CS powder was dissolved into a 1% v/v acetic acid solution at 303 K for 6 h in order to obtain an aqueous chitosan solution using magnetic stirring at 800 rpm. The pH of solutions was adjusted to 4.9 with a NaOH solution (0.1 M). Afterwards, the CS solutions were slowly added to the clay suspension at 333 K under continuous stirring with CS to MMT mass ratios of 0.1/1, 0.5/1, 1/1 and 2/1, respectively. The CSMMT mixtures were continuously stirred for 48 h and then washed with distilled water to remove the acetate salt that remained in the solution. Finally the obtained CSMMT bionanocomposites were dried at 333 K for 24 h at atmospheric pressure, and ground to powder.

Analysis

X-ray diffractograms were obtained using a PANanalytical X'Pert Pro MPD diffractometer (Bragg-Brentano set-up). Diffraction data were acquired by exposing the powder samples to Cu-K␣ radiation, which has a wavelength of 1.5418 Å. The generator was set to 45 kV and to a current of 40 mA. The data were collected over a range of 5-25 • 2 with a step size of 0.03 • 2 and a nominal time per step of 100 s, using the scanning X'Celerator detector.

Thermogravimetric analyses (TG/DTA) of samples were performed using an TGA-DSC 111 Setaram analyzer with a nitrogen flow rate at 50 mL/min and a heating rate of 278 K/min from ambient temperature to 773 K.

Particle morphology was investigated using a scanning electron microscope (SEM) (Philips XL 30 model ESEM-FEG) operating at 10 kV.

The IR spectra of samples were obtained by transmittance on a KBr disc using a Nicolet iS10 Fourier transform infrared (FTIR) spectrophotometer from the ThermoFisher Scientific Company. About 2-3 mg of the sample was mixed with 100 mg of dry potassium bromide and the mixture was submitted to a 73.5 kPa pressure using a hydraulic press to obtain a translucent disc. The samples were scanned from 4000 to 400 cm -1 wave numbers with a resolution of 4 cm -1 .

The specific surface areas were measured from nitrogen adsorption isotherms obtained at 77 K with a 3Flex Surface Characterization Analyser (Micromeritics) applying the BET method. The samples were previously out-gassed under vacuum at 303 K for 24 h.

Elemental analyses of MMT Na + , CS and CSMMT bionanocomposite samples were investigated by XPS. Spectra were obtained with a KRATOS Axis Ultra X-ray photoelectron spectrometer (Kratos Analytical, Manchester, UK) equipped with a monochromated Al K␣ X-ray (h = 1486.6 eV) operating at 120 W. The samples were pressed into a Cu tape fixed on a holder, and then pumped overnight in the fast-entry lock at 10 -9 mbar before introduction in the analysis chamber. Spectra were collected at normal take-off angle (90 • ) and the analysis area was 700 × 300 m 2 . The spectra were acquired in the constant analyzer energy mode, with a pass energy of 160 and 20 eV for the survey and the narrow regions, respectively. Charge correction was carried out using the C 1 s core The core levels were reported from the peak maxima and had an experimental error of ±0.01 eV. These analyses were conducted by the LCPME in Villers-lès-Nancy (France).

The IGC experiments were performed with two Agilent 7890A and 6890 gas chromatographs, equipped with a flame ionization detection (FID) system. Helium was used as the carrier gas with a flow rate of about 30 mL/min measured with an electronic flowmeter (Flow500-Agilent). The temperatures of the injector and detector were respectively 423 K and 443 K. Methane was used as a non-interacting probe.

The IGC-ID study was made with oven temperatures of 383 K for both MMT Na + and CSMMT bionanocomposites. For CS, the temperature was fixed at 313 K. The MMT and bionanocomposite columns were conditioned overnight at 393 K and the CS column at 373 K. The chromatographic columns were 5 cm in length with 6.35 mm in internal diameter for MMTNa + and CSMMT bionanocomposites.

For IGC-FC, the chromatographic columns were stainless steel tubes of 5 cm in length and 6.35 mm inside diameter. The conditioning temperature was 403 K and the analysis temperature depended on the probe used, according to the Conder [26] criterion which states that the contribution of probe vapor to the total flow of carrier gas at the maximum of the chromatographic peak should not exceed 5% of the initial flow. Two probes were analyzed in IGC-FC, octane (C8) at 333 K and isopropanol (IP) at 313 K.

The injections of each probe were repeated three times and three columns were made with each sample, so as to take into account the heterogeneity of the solid. The quantities after the sign (±) in the values tables correspond to the standard deviation from duplicate measurements.

Results and discussion

Thermogravimetric analysis

The results of TGA of MMT Na + , CS and CSMMT bionanocomposites are given in Table 1. Two significant mass losses are observed for CS, namely 10 and 44.8%. These two mass losses were attributed respectively to water loss and to decomposition of the polymer via deacetylation-depolymerization. Similar results have been reported by other researchers [30,31]. MMT Na + shows a 9.5% mass loss between 293 and 453 K, which is associated to water desorption from the interlayer space and a thermic stability between 453 and 723 K.

However, CSMMT bionanocomposites show mass losses ranging from 4.2 to 9.3% between 453 and 723 K. This result indicates the high thermal stability of bionanocomposites. This thermal stability is attributed to the thermal insulation behavior induced by the CS insertion between MMT Na + layers and to changes in the dynamics of molecular motion in bionanocomposites [32].

BET surface analysis

Values of specific areas and BET constants obtained by N 2 adsorption are given in Table 2.

The results clearly show a decrease in the specific surface area of bionanocomposites with the increase of CS mass ratio. This decrease can be attributed to the compact packing of the CS molecules in the interlayer space, resulting in the blocking of pores, which prevents the adsorption of nitrogen molecules [33].

At the same time, the values of C BET generally decrease with the decrease in the specific surface area. The interactions between nitrogen and MMT Na + are lower in the presence of CS.

FT-IR analysis

The FT-IR spectra of CS, MMT Na + and CSMMT bionanocomposites are shown in Fig. 1.

The absorption bands from FT-IR spectrum of MMT Na + were assigned as follow: 3600 and 3412 cm -1 correspond to OH stretching vibrations in molecular waters and in MMT Na + to Si-OH and Al-OH bonds, 1640 cm -1 corresponds to the bending vibrations of water and at 1044 cm -1 to Si-O stretching in (SiO) 4-tetrahydra.

The IR spectrum of CS Exhibits -OH stretching vibration at around 3419 cm -1 and bands at 2919 and 2841 cm -1 assigned to aliphatic C H stretching [33]. An absorption band with a maximum at 1594 cm -1 belongs to free primary amino group bending (NH 2 ) at C2 position of glucosamine, which is a major group present in CS. The peak at 1651 cm -1 is assigned to the acetylated amino group in CS. Another peak at 1383 cm -1 is attributed to a C H bending vibration.

Bionanocomposites possess characteristic bands at approximately 2925 and 2832 cm -1 corresponding to the stretching vibrations of CS C H. The band attributed to the bending vibration of the CS C H methylene group (1380 cm -1 ) is also observed in the spectra of the bionanocomposites. The peak at 1594 cm -1 of the NH 2 groups in CS was shifted to 1563 cm -1 in CSMMT 2/1 bionanocomposite spectrum, corresponding to the deformation vibration of the protonated amine group (-NH +

3 ) of CS resulting from its dissolution in acetic acid. This -NH + 3 group will interact with the negatively-charged sites of MMT Na + and will be at the origin of strong interactions between the two components.

XRD analysis

XRD analysis was also implemented for the characterization of polymer/clay nanocomposites. It allows the distance between the silicate plates to be measured and makes it possible to observe the intercalation of the polymer chains [34]. Fig. 2 illustrates XRD patterns of MMT Na + , CS and CSMMT bionanocomposites. Considering the XRD pattern of MMT Na + , the peak at 2 = 7.545 • corresponds to a d 001 spacing of 11.8 nm. Structural changes of the MMT Na + upon CS incorporation are clearly observed from this characteristic d 001 diffraction peak. The d 001 spacing increases slightly from 11.8 nm for MMT Na + to 14.65, 14.75, 14.80, 15.05, 15.45 nm for 0.1/1, 0.5/1, 1/1 and 2/1 CSMMT bionanocomposites respectively, indicating the MMT Na + interlayer expansion, which can be reasonably ascribed to the intercalation of CS in the MMT Na + interlayer region.

Moreover, the movement of the diffraction peak at 2 = 7.545 • for pure MMT to a lower angle for CS bionanocomposites indicates the formation of an intercalated nanostructure, whereas the peak broadening and intensity decrease suggest, for the CSMMT 2/1 sample, the presence of a disordered intercalated or exfoliated structure [11].

Surface chemical composition

The surface composition of MMT Na + , CS and CSMMT bionanocomposites was monitored by XPS, as this technique can probe the surface of samples and detect subtle changes in the chemical composition of the surface.

Fig. 3 displays XPS survey spectra of electron intensity as a function of bending energy for MMT Na + , CS and the CSMMT bionanocomposites.

For MMT Na + (Fig. 3a), the main peaks are Si2p (103 eV), Al2p (131 eV), C1 s (285 eV), Mg (KLL) (305 eV), Na (KLL) (500 eV), O1 s (531 eV), O (KLL) (979 eV), and Na1 s (1071 eV).

The XPS spectrum of CS (Fig. 3d) reveals that carbon and oxygen are the major elements, which occurs about 284 and 532 eV, respectively, and there is a small amount of nitrogen at 398 eV with some impurities.

While, CSMMT bionanocomposites have a survey scan similar to that of the corresponding untreated MMT Na + , with the MMT Na + characteristic peaks of Si, Al, Mg, a qualitative indication that the bionanocomposite surface is MMT-rich (Fig. 3b,c). The same results were obtained by Boukerma for polypyrrole/montmorillonite nanocomposites [22]. However, the scan reveal the Na + -CS cation exchange by the disappearance of the two peaks corresponding to Na at 500 and 1071 eV, in the bionanocomposites.

For a clearer understanding, the peaks of N1s and C1s spectra have been deconvoluted for MMT Na + , CS and CSMMT 2/1 samples (Figs. 4 and5).

Fig. 4a shows the deconvolution of N1s spectrum for CS sample, the broad at 399-402 eV includes two peaks at bending energies of 399.10 and 399.91 eV, respectively. The peak located at 399.10 eV is assigned to N atoms bonded with C atoms and the peak at 399.91 eV is attributed to C-NH + 3 bending. The peak of N1s in CS illustrates a larger contribution of neutral N and a relatively smaller contribution of positively charged N + .

In the case of CSMMT 2/1 bionanocomposite (Fig. 4b), the deconvolution of N1s spectrum shows the same two peaks as for CS. But the peak located at 399.91 eV assigned toNH + 3 , in CS has shifted to 402.17 eV. As indicated with IR spectroscopy results, this NH + 3 group is at the origin of strong interactions with the negatively charged sites of MMT Na + .

The assignment of deconvoluted high resolution XPS of C1 s peak in CS, MMT Na + and CSMMT 2/1 bionanocomposite is displayed in Fig. 5.

The peak of C1 s in MMT Na + results from the presence of organic carbon. The deconvolution of the C1 s spectrum in MMT Na + (Fig. 5a) leads to four peaks centred at 284.59, 285.74, 286.99 and 289. 14 [35]. The spectrum reveals the higher contribution of C H than other forms of carbon bonds.

eV attributed to C1 (C H/C C), C2 (C O/C OH), C3 (O C-O) and C4 (O C O), respectively

For CS, the C1 s spectrum (Fig. 5b) is deconvoluted in four peaks located at 284.61, 286.11, 287.76, 289. As regards the CSMMT 2/1 bionanocomposite, its spectrum (Fig. 5c) also reveals four bands, C1 (C H/C C), C2 (C O/C N/C O-C), C3 (C O) and C4 (N C O) at 284.62, 286.52, 288.17 and 289.27 eV, respectively. Yang [36] suggested that the carboxylate carbon peak present in MMT at 289.14 eV has been replaced by an amide peak at 289.27 eV, indicating that chitosan was adsorbed onto MMT Na + through the reaction of amino groups with carboxylate groups.

Table 3 shows that the weight percentages of nitrogen and carbon atoms, the major elements in the chemical composition of CS (except the oxygen), increase with the increasing CS mass ratio, while at the same time those of the Si, Al, Mg and Fe atoms characteristic of MMT Na + decrease slightly.

Table 3 The C/Si atomic ratio increases progressively with the increase of CS in bionanocomposites, namely 0.28, 0.30, 0.51, 0.59, The dispersive component of the surface energy, d s , the nanomorphological index, I M ( t ), and the specific interactions parameter, I sp , obtained by IGC-ID for MMT Na + , and four bionanocomposites at 383 K and CS at 323 K are listed in Table 4. The 323 K analysis temperature for CS was chosen due to its degradation at 383 K, and to its very fast elution above 323 K.

For MMT Na + , the high value of d s is typical of lamellar solids. It reflects the insertion of linear alkanes into the space between the sheets stacking of the phyllosilicate (Fig. 6). This value is in agreement with those obtained in the literature, for instance by Bandosz et al. [37], namely 155 mJ/m 2 at 423 K for untreated MMT or by Mravčáková [38], namely 216 mJ/m 2 for pure MMT at 423 K, or by Boukerma [22], 114 mJ/m 2 at 423 K for pure MMT.

The d s values of CSMMT bionanocomposites decrease with the increasing of the CS mass ratio. This drop may be related to the modification caused by the cation exchange reaction leading to the covering of the active groups of MMT Na + surface with long-chains CS biopolymer. So the active sites located in the space between the stacked sheets are no longer available for alkane adsorption. Furthermore, the results show that the values of the surface energy correlates with the values of the specific surface area (Fig. 7).

The low I M ( t ) of MMT Na + corresponds to size exclusion effects. The linear alkane is able to insert itself partially into the porous sites, whereas a cyclic probe such as cyclic alkane (Cy7) is excluded [39]. In the case of CSMMT bionanocomposites, the I M ( t )values obtained with the cyclic probe increase slightly with the increasing mass ratio of CS in MMT Na + , indicating the progressive disappearance of surface roughness or porosity by adsorption of CS.

Several polar and amphoteric probes, such as acetone and methyl acetate were injected, but due to their strong interactions with the MMT Na + surface, they could not be eluted through the chromatographic column even at 383 K oven temperature, testifying to the strong energetic character of the MMT Na + surface due to the presence of amphoteric silanol groups. Strzemiecka et al., also observed strong interactions between fragrance molecules and another aluminosilicate, zeolite [40].

Only the acidic dichloromethane probe was eluted and this was used to calculate the specific interaction parameterI sp . This parameter decreases slightly as the CS mass ratio increases. This decrease can be explained by the gradual disappearance of amphoteric sites like silanols consequent to CS adsorption on the MMT Na + surface or in the space between sheets, as supported by XPS.

. Study of desorption isotherms

The desorption isotherm was computed from a chromatogram obtained when the probe monolayer is reached at the solid surface, according to the ECP method. The characteristic parameters obtained by IGC-FC by injecting n-octane and isopropanol probes are listed in Table 5 and compared to those obtained with nitrogen adsorption.

It is observed that the specific surface areas measured with nitrogen adsorption are close to those measured with the octane probe.

The BET constants measured using nitrogen adsorption (Table 5) are higher than those obtained by IGC-FC with octane and isopropanol, reflecting a greater affinity of nitrogen to the surface of raw materials and CS/MMT bionanocomposites. This observation may be related to the difference in the analysis temperature, 77 k for nitrogen and 333 K for octane or 313 K for isopropanol, so the higher the temperature is, the weaker the interactions between the probe and the surface are, leading to a lower BET constant. Furthermore, it can be observed that whatever the probe used, the BET constant decreases with the increase of CS mass ratio testifying that N 2 and the injected probes interact less strongly with the CS/MMT bionanocomposites than with MMT. This behavior is due to the disappearance of high energy sites on MMT hidden by chitosan.

However, the specific surface areas measured with isopropanol are lower than those measured with nitrogen or n-octane. The I irr values (Table 6) obtained with isopropanol are higher than those with octane, reflecting a higher irreversible adsorption of isopropanol compared to the n-octane probe. Isopropanol interacts This irreversible adsorption at analysis temperature contributes to the decrease in the measured specific surface areas computed from the reversible part of the adsorption phenomenon.

Comparing the influence of CS mass ratio in bionanocomposites, it is observed that, whatever the probe, the values of I irr decrease persistently with the increasing CS mass ratio. This variation with n-octane, a probe sensitive to morphology, indicates the disappear-ance of surface roughness, while with the isopropanol, a probe more sensitive to functionality, the disappearance of high energy sites like silanols is more pronounced. The variations of I irr obtained with both probes can probably be attributed to the progressive adsorption of CS biopolymer chains inside and/or onto the layered clay, leading to a smoother surface with a covering of the porosity and a disappearance of polar groups.

Assessment of the surface energetic heterogeneity

The heterogeneity of the surface energy of CS, MMT Na + and CSMMT bionanocomposites was estimated by IGC-FC using the distribution functions for both n-octane and isopropanol probes (Fig. 8).

The DFs of n-octane obtained on MMT Na + , CS and CSMMT bionanocomposites are compared in Fig. 8A. The DF obtained with this linear alkane on the MMT Na + sample is bimodal. The first peak can be related to less energetic sites on the flat surface and the second one to high energy sites located in spaces between sheet stackings. On the other hand, the DFs obtained with CSMMT bionanocomposites samples are monomodal, the DFs obtained become more symmetrical as the CS mass ratio increases, testifying that morphology modification takes place after CS intercalation by a decrease in surface roughness.

The DFs of the isopropanol are different from those obtained by n-octane (Fig. 8B). In MMT Na + , the DFs are clearly bimodal, exhibiting a main maximum energy peak between 18 and 20 kJ/mol and a larger shoulder centred around 35-40 kJ/mol. According to Balard [START_REF] Balard | Acid-Base Interact[END_REF], the former corresponds to the alcohol/siloxane dispersive interactions and the later to the alcohol/silanol specific interactions, namely hydrogen bridge interactions.

However, the DFs of CSMMT bionanocomposites show a progressive disappearance of the shoulders at high energies with an increase in CS mass ratio, indicating that the sites having the highest energies on the surface or between galleries is replaced by CS chains.

The asymmetry of the DFs can describe the surface heterogeneity by means of an index of heterogeneity. The values were determined for n-octane and isopropanol (Table 7). The heterogeneity index values (I hete ) were much higher with isopropanol than with n-octane. These differences highlight the greater ability of isopropanol to detect surface heterogeneity by binding to a polar site.

The values of the heterogeneity index of the both injected probes decrease significantly. This decrease in the heterogeneity indexes is consistent with the progressive adsorption/intercalation of CS in the surface of clay.

Conclusion

Bionanocomposites based on CS and MMT Na + with different amounts of CS were prepared in a liquid phase and characterized in terms of structural and chemical properties by different techniques.

Among them, XRD patterns indicated that CS were intercalated in the MMT Na + galleries, and the interlayer spacing of clay increased after ion exchanging of Na + with CS. XPS analysis also showed that the surface of the bionanocomposites are MMT-rich despite the intercalation of CS in MMT.

Using IGC at infinite dilution, a surface characterization technique, it was observed that there is a decrease in the dispersive component of the surface energy and correlatively an increase in the nanomorphology index obtained with the cycloheptane probe on the bionanocomposites, testifying the disappearance of high energy sites with the increase in CS mass ratio.

IGC at finite concentration was used to obtain a more realistic description of the whole surface.

The distribution functions obtained with isopropanol, a probe sensitive to chemical functionality, confirmed the modification of MMT Na + after CS intercalation threw a progressive disappearance of the shoulders at high energy with an increase in CS mass ratio.

Fig. 1 .

 1 Fig. 1. FT-IR spectra of (a) MMT Na + , (b) CSMMT 0.1/1, (c) CSMMT 0.5/1, (d) CSMMT 1/1, (e) CSMMT 2/1 and (f) CS.

Fig. 2 .

 2 Fig. 2. XRD patterns of (a) MMT Na + , (b) CSMMT 0.1/1, (c) CSMMT 0.5/1, (d) CSMMT 1/1, (e) CSMMT 2/1 and (f) CS.

Fig. 3 .

 3 Fig. 3. XPS spectra of (a) MMT Na + , (b) CSMMT 0.1/1, (c) CSMMT 2/1 and (d) CS.

  31 eV, assigned to C1 (C H/C C), C2 (C O/C N/C O-C), C3 (C O) and C4 (N C O) respectively. The (C O/C N/C O-C) peak is more pronounced than the other peaks.

Fig. 4 .

 4 Fig. 4. N1s high resolution regions of (a) CS, (b) CSMMT 2/1.

Fig. 5 .

 5 Fig. 5. C1s high resolution regions of (a) MMT Na + , (b) CS, (c) CSMMT 2/1.

Fig. 6 . 4 . 6 .

 646 Fig. 6. SEM micrographs of MMT Na + .

Fig. 7 .

 7 Fig. 7. Plot of d s (at 383 K) versus aBET for CSMMT bionanocomposites.

4. 7 .

 7 Inverse gas chromatography at finite concentration (IGC-FC) 4.7.1

Fig. 8 .

 8 Fig. 8. Adsorption energy distribution functions of the n-octane (a1: MMT Na + , b1: CSMMT 0.1/1, c1: CSMMT 0.5/1, d1: CSMMT 1/1, e1: CSMMT 2/1 and f1: CS) and isopropanol (a2: MMT Na + , b2: CSMMT 0.1/1, c2: CSMMT 0.5/1, d2: CSMMT 1/1, e2: CSMMT 2/1 and f2: CS) measured respectively at 333 K and 313 K.

Table 1

 1 Results of thermogravimetric analysis.

	Samples	Weight loss	Weight loss
		at 293-453 K	at 453-723 K
		(%)	(%)
	MMT Na +	9.5	0.5
	CS	10.0	44.8
	CSMMT 0.1/1	4.5	4.2
	CSMMT 0.5/1	6.6	5.6
	CSMMT 1/1	5.5	7.4
	CSMMT 2/1	4.2	9.3
	line, setting the adventitious carbon signal (H/C signal) to 284.6 eV.

Table 2

 2 Values of specific surface areas (aBET), BET constants (CBET) for CS, MMT Na + and bio-nanocomposites.

	Samples	aBET (m 2 /g)	CBET
	MMT Na + CSMMT 0.1/1 CSMMT 0.5/1 CSMMT 1/1 CSMMT 2/1 CS	77.1 ± 4.3 65.8 ± 3.5 55.5 ± 2.9 37.1 ± 2.7 26.5 ± 2.1 4.1 ± 0.2	886 ± 42 575 ± 54 360 ± 25 231 ± 59 343 ± 43 12 ± 7

Table 3

 3 Surface chemical composition of MMT Na + , CS and bionanocomposites obtained by XPS.

		O	N	C	Na	Ca	Si	Al	Mg	Fe
	CS	38.08	5.87	53.58	-	1.61	0.86	-	-	-
	MMT Na +	48.99	-	6.12	3.83	0.95	26.63	10.2	2	1.28
	CSMMT 0.1/1	48.38	0.35	8.79	-	0.63	26.25	11.22	2.14	2.24
	CSMMT 0.5/1	47.96	0.84	12.86	-	0.35	25.42	9.89	1.36	1.32
	CSMMT 1/1	47.34	0.95	15.29	-	0.26	23.95	9.38	1.61	1.22
	CSMMT 2/1	46.41	1.46	16.98	-	0.29	23.44	8.73	1.57	1.12

Table 4

 4 Values of d s ,IM ( t ) and Isp of MMT Na + , CS and CSMMT bionanocomposites.

		d s	mJ/m 2 ⟨br⟩	IM ( t ) Cycl 7	Isp CH2Cl2 (kJ/mol)
	MMT Na + CSMMT 0.1/1 CSMMT 0.5/1 CSMMT 1/1 CSMMT 2/1 CS a	184 ± 4 160 ± 4 149 ± 4 119 ± 3 90 ± 3 46 ± 1	0.06 ± 0.01 0.08 ± 0.02 0.12 ± 0.03 0.20 ± 0.01 0.26 ± 0.02 0.98 ± 0.02	16.7 ± 0.7 15.5 ± 0.9 12.8 ± 0.6 12.7 ± 0.6 11.3 ± 0.5 10.2 ± 0.4

a Analyzed at 323 K.

Table 5

 5 Values of the specific surface areas (aBET) of MMT Na + , CS and CSMMT bionanocomposites samples by nitrogen adsorption and IGC-FC.

				IGC-FC
		Adsorption N2		C8 (333 K)		IP (313 K)
	aBET (m 2 /g)	CBET	aBET (m 2 /g)	CBET	aBET (m 2 /g)	CBET
	MMT Na + CSMMT 0.1/1 CSMMT 0.5/1 CSMMT 1/1 CSMMT 2/1 CS	77.1 ± 4.3 65.8 ± 3.5 55.5 ± 2.9 37.1 ± 2.7 26.5 ± 2.1 4.1 ± 0.2	886 ± 42 575 ± 54 360 ± 25 231 ± 59 343 ± 43 12 ± 7	72.1 ± 3.4 60.6 ± 3.5 49.3 ± 3.1 33.6 ± 2.9 27.1 ± 2.9 1.5 ± 0.1	17.0 13.6 14.1 10.2 10.3 6.2	69 ± 3.5 52 ± 3.2 43 ± 2.9 30 ± 3.0 24 ± 2.5 0.9 ± 0.1	23.1 22.5 16.3 16.2 12.5 8.3

Table 6

 6 Values of irreversibility indexes (I irr ) measured using n-octane and isopropanol probes on the studied samples.

		I irr(C8) 333 K	I irr(IP) 313 K
	MMT Na + CSMMT 0.1/1 CSMMT 0.5/1 CSMMT 1/1 CSMMT 2/1 CS	5.6 ± 0.3 3.7 ± 0.1 2.8 ± 0.2 2.1 ± 0.1 1.4 ± 0.2 0.2 ± 0.1	39.2 ± 4.1 37.3 ± 3.7 26.7 ± 3.9 21.4 ± 3.2 12.6 ± 2.1 4.2 ± 0.8

strongly with MMT Na + silanol groups through hydrogen bonds.

Table 7

 7 Heterogeneity index (I hete ) values measured by IGC-FC for n-octane and isopropanol on raw materials and bionanocomposites.

		I hete(C8) 333 K	I hete(IP) 313 K
	MMT Na + CSMMT 0.1/1 CSMMT 0.5/1 CSMMT 1/1 CSMMT 2/1 CS	32 ± 3 31 ± 3 26 ± 3 19 ± 3 15 ± 2 3 ± 1	49 ± 4 44 ± 4 36 ± 3 34 ± 3 21 ± 2 14 ± 1