Plan
Comptes Rendus

Geomaterials / Géomatériaux
Transport coefficients of saturated compact clays
[Coefficients de transport pour des argiles compactes saturées]
Comptes Rendus. Géoscience, Volume 338 (2006) no. 12-13, pp. 908-916.

Résumés

The coefficients that characterize the simultaneous transports of mass, heat, solute and current through compact clays are experimentally and theoretically determined. The role of a characteristic length scale that can be derived from conductivity and permeability is illustrated for the electrokinetic coefficients. The macroscopic Soret coefficient in clays was found five times larger than in the free fluid, presumably because of extra couplings with electrical phenomena.

Les coefficients qui caractérisent le transport simultané de masse, de chaleur, de soluté et de courant au travers d'argiles compactes sont déterminés expérimentalement et théoriquement. Le rôle d'une longueur caractéristique qui peut être déduite de la conductivité et de la perméabilité est illustré pour ce qui concerne les coefficients électrocinétiques. Le coefficient de Soret macroscopique dans les argiles est cinq fois plus grand que dans le fluide libre, probablement à cause de couplages supplémentaires avec les phénomènes électriques.

Métadonnées
Reçu le :
Accepté le :
Publié le :
DOI : 10.1016/j.crte.2006.06.008
Keywords: Compact clays, Electro-osmotic coefficient, Soret coefficient
Mot clés : Argiles compactes, Coefficient électro-osmotique, Coefficient de Soret

Marcin Paszkuta 1 ; Maria Rosanne 1 ; Pierre M. Adler 1

1 Sisyphe, 4, place Jussieu, 75252 Paris cedex 05, France
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     author = {Marcin Paszkuta and Maria Rosanne and Pierre M. Adler},
     title = {Transport coefficients of saturated compact clays},
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     pages = {908--916},
     publisher = {Elsevier},
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     number = {12-13},
     year = {2006},
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     language = {en},
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Marcin Paszkuta; Maria Rosanne; Pierre M. Adler. Transport coefficients of saturated compact clays. Comptes Rendus. Géoscience, Volume 338 (2006) no. 12-13, pp. 908-916. doi : 10.1016/j.crte.2006.06.008. https://comptes-rendus.academie-sciences.fr/geoscience/articles/10.1016/j.crte.2006.06.008/

Version originale du texte intégral

1 Introduction

The ability of clay soils to act as semipermeable membranes that inhibit the passage of electrolytes may be of great value. Clays exhibit membrane properties when charged ionic species are excluded from the pores, while uncharged species have a relatively free movement. Clays are therefore capable of inducing coupled transport such as osmosis, electro-osmosis, electrodialysis, and thermodiffusion.

The major purpose of this work was to experimentally determine the coefficients that characterize these couplings in compact clays. More precisely, let us consider a porous medium filled by an electrolyte solution with a concentration n (defined as the number of molecules per m3). When a pressure gradient ∇P, an electric field E, a concentration gradient ∇n, and a temperature gradient ∇T are simultaneously applied to the porous medium, four fluxes are generated, namely a flow characterized by the seepage velocity U (m s−1), a current density I (A m−2), a solute flux Jd which is the number of particles per unit surface and per unit time (m−2 s−1), and an heat flux Jq (W m−2). Close to equilibrium, when gradients are not too large, the problem is linear, and fluxes are proportional to these gradients (see [4]). Then, with respect to the solvent-fixed reference plane, one can write:

I=γ11E+γ12(P)+γ13(n)+γ14(T)(1a)
U=γ21E+γ22(P)+γ23(n)+γ24(T)(1b)
Jd=γ31E+γ32(P)+γ33(n)+γ34(T)(1c)
Jq=γ41E+γ42(P)+γ43(n)+γ44(T)(1d)
A distinction is generally made between the diagonal phenomenological coefficients γij with i=j, and the non-diagonal ones with ij. Because of the Onsager symmetry properties, the non-diagonal coefficients are not all independent.

A literature search shows that measurements of these coefficients for clays (and especially for compact clays) were not frequently performed. Interesting contributions concerning electrokinetic coefficients are from Elrick et al. [5], Gronevelt et al. [6] and Sherwood and Craster [19]. To the best of our knowledge, only Thornton and Seyfried [20], and Lerman [10] have contributed to thermodiffusion. A recent review of all of these coefficients was made by Horseman et al. [7].

This paper is organized as follows. Section 2 is devoted to the theoretical determination of the coupling coefficients. The local transport equations corresponding to the electrical potential, the ionic concentrations, and the velocities are solutions of the Poisson–Boltzmann, the convection diffusion and the Stokes equations; these equations are numerically solved and integrated to obtain the macroscopic fluxes. Numerical results for various porous media correspond to the analytical solutions valid for circular Poiseuille flows when the capillary radius is replaced by a suitable length scale.

The material, the experimental procedure and the data are described in Section 3. Two samples of argilite extracted from a Callovo-Oxfordian formation were characterized. Permeability, conductivity, electro-osmotic coefficient, effective diffusion coefficient, osmotic coefficient, coefficient related to “membrane potential” and Soret coefficient measurements are described. Results are considered as functions of solute concentration, porosity and temperature in non-isothermal conditions. Data relative to the electrokinetic coefficients are compared to numerical and analytical results derived by [2] and [12].

2 General

The coefficients γij defined by Eq. (1) can be derived by analysing the phenomena on the pore level. Such an analysis was performed for instance by [12] when there is no temperature gradient imposed on the medium. Let us summarize it briefly. Under such circumstances, transport is governed by three types of coupled equations. First, the concentration ni(R,t) of each ith ion species is given by a convection–diffusion equation

nit+(DinieziDikTniψ+niu)=0i=1,2,,N(2)
where k is the Boltzmann constant (=1.38×10−23 m2kgs−2K−1), t the time, Di the diffusion coefficient of ion i, e the absolute value of the electron charge, zi the valency, ψ the electric field, and u the fluid velocity.

Then, u and the pressure p(R,t) are governed by modified Stokes equations, since the Reynolds number is in most cases much less than unity:

u=0,μ2u=p+ρψ(3)
where μ is the fluid viscosity and ρ the charge density.

Finally, ψ(R,t) is solution of the Poisson equation

2ψ=ρεel=eεeli=1Nnizi(4)
where εel is the fluid permittivity.

To be solved, the set of Eqs. (2) to (4) is subject to the following boundary conditions on the solid/fluid interface S:

νji=0,u=0andψ=ζ(5)
where ν is the outward normal to the solid surface, and ζ the zeta potential.

These equations were linearized around equilibrium, made dimensionless, and solved by [9] (and the references therein) for homogeneous porous media that could be represented as spatially periodic media. The coefficients γij (i,j=1,,3) can be derived by integrating the local fields over the unit cell.

The only cases which can be solved analytically, are the ones of a plane channel or of a circular tube of radius Rc. Let us recall the coefficients obtained in the latter case by Coelho et al. [8] and Marino et al. [9] (and references therein):

γ11=γ11μe2(εelkTκ)2=12(D1+D2)ζκλI0(κRc)((D1D2)I1(κRc))+2ζ[λ3κRcI0(κRc)I1(κRc)](6a)
γ12=γ21=γ21μεelζKκ2=ζ[2I1(κλ)κλI0(κλ)1](6b)
γ13=γ13μeεel(kT)2=12(D1D2)ζ(D1+D2)I1(κλ)κλI0(κλ)(6c)
γ23=γ23μκ2kT=0(6d)
γ33=γ33μe2εel(kT)2=12(D1+D2)ζ(D1D2)I1(κλ)κλI0(κλ)(6e)
where κ−1 is the Debye–Hückel length.

It is important to note that most numerical results obtained for various types of porous media were shown to be very close to the analytical results (6) when the radius of the tube is replaced by the characteristic length Λ defined by

Λ=(12.5Kσσ)12(7)
where σ is the fluid conductivity (measured before any contact with argilite) and σ the macroscopic conductivity of the medium filled by the electrolyte. The definition of this length scale was inspired by the length scale Λ introduced by [9].

3 Experimental

The phenomenological coefficients γij were measured in a series of one-dimensional experiments performed on porous specimens of thickness l and cross sectional diameter S separating two compartments each of volume V filled by aqueous solutions at different temperatures, pressures and electrical potentials with different concentrations of the same electrolyte. The concentration n expressed in m−3 is related to the molar concentration C expressed in mol m−3 by

C=n/Na(8)
where Na is the Avogadro number (=6.02×1023 mol−1).

The relationships between the coefficients γij and the measurable clay parameters can be summarized by:

(γ11γ12γ13γ14γ21γ22γ23γ24γ31γ32γ33γ34γ41γ42γ43γ44)=(σγ12γ13γ14γ12KμωKμkTnkTγ13nkTγ23D¯nD¯STTγ14TkTkT2n2γ33STλ)(9)
where σ is the electrical conductivity, K the permeability, μ the viscosity, ω the coefficient of osmotic efficiency, kT the thermo-osmotic coefficient, D¯ the macroscopic diffusion coefficient, ST the Soret coefficient and λ the thermal conductivity.

3.1 Materials

The material, supplied to us by ANDRA, is a natural compact argilite extracted in eastern France from a Callovo-Oxfordian formation. The wellbore referred to as EST104 was sampled at two different levels. One at 483.6 m (EST104/2364) and the other at 505.15 m (EST104/2487). Both blocks were used to obtain solid cylinders of thickness 3 mm and diameter 12 mm, or powder. A SEM analysis of the argilite powder shows an heterogeneous structure with many aggregates; because of the careful crushing process to obtain powder from the original block, the average grain radius ranges from 1 to 10 μm. The variability of the samples is discussed in this issue by [14].

The solute was sodium chloride supplied by SIGMA (purity 99.5%). The solvent was pure water filtered by an HPCL Maxima unit. The concentration C was ranging between 10−4 and 10−2 mol l−1.

The samples were equilibrated with the solutions for about a month and any additional and undesirable ion that appears during this initial equilibration phase can be eliminated. During the experiments, HPCL chromatography was performed and the concentration of any extra ions such as calcium was found negligible when compared to sodium.

The zeta potential ζ was estimated by measuring the electrophoretic mobility of clay particles in electrolyte solutions. Since the ratio between κ−1 and the particle dimension is small, the Smoluchowski formula [8] can be used for all particle shapes with an estimated precision of 10%:

ζ=μueεel(10)
where ue is the electrophoretic mobility.

Results for ζ in various NaCl solutions are displayed in Fig. 1. |ζ| is an increasing function of the concentration; it changes by only 7 mV (−18 to 25 mV) over a concentration range of 10−4 to 10−2 moll−1 at a constant pH value equal to 5.

Fig. 1

The zeta potential ζ as a function of NaCl concentration C. Data are for: EST104/2364 (■) and EST104/2487 (●). The precision is about 15%.

Potentiel ζ en fonction de la concentration C en NaCl. Les données sont pour : EST104/2364 (■) et EST104/2487 (●). La précision est d'environ 15%.

3.2 Experimental determination of the coefficients K, σ and γ21 (=γ12) in isothermal conditions

3.2.1 Experimental procedure

The experimental cell is detailed in [11] and [13]. The experiments were carried out at atmospheric pressure. The sample, whether it is the original compact rock or the powders, can be compacted or not by means of two porous disks. This compaction limits the swelling that is likely to occur with the addition of sodium chloride. Therefore, various porosities can be obtained for the same sample.

The permeability K was measured by generating a steady flow by means of a constant pressure difference ΔP2.4×104 Pa (0.24 bar). The mass of the outgoing water was measured during a given time in order to calculate the flow rate. Then, K was determined by using Darcy's law. Measurements were found reproducible.

The sample conductivity σ was measured by a classical method by imposing a constant dc voltage ΔV1 V between two bronze plates at a vanishingly small pressure difference (ΔP=0). Then, Ohm's law was used.

The electric current was observed to decrease during the measurement time when the electric potential difference is set, probably due to the formation of polarization layers on the electrodes. Simultaneously, a water flow rate Qe was induced by ΔV and γ21 was obtained by:

γ21=QeSlΔV(11)
Qe was determined from the slope of the straightline obtained from the linear fit of the collected liquid mass as a function of time during the first three minutes before it starts decreasing due to polarization.

3.2.2 Results and discussion

Results for permeability are displayed in Fig. 2. Note that the permeability obtained for the argilite original sample is in good agreement with the powder permeability.

Fig. 2

The permeability K as a function of porosity ε. Data are for: argilite powder (●) and argilite original sample (▵). The precision is about 34% and 24% for small and large porosities, respectively.

Perméabilité K en fonction de la porosité ε. Les données sont pour : la poudre d'argilite (●) et l'échantillon initial (▵). La précision est d'environ 34% et 24% pour les petites et grandes porosités.

In order to study the effects of the compaction pressure and κ, σ was measured for several porosities ε at fixed C, and for several C at fixed ε. It is more convenient to represent the experimental results in terms of the electric formation factor F, which is defined as:

F=σfpσ(12)
where σfp is the fluid conductivity after permeation. σfp was found equal to σ for C larger than 10−3 mol l−1; however, for smaller concentrations, the two values were different. Results are displayed in Fig. 3. F is a decreasing function of ε and is independent of C.

Fig. 3

The electric formation factor F (▴) and the diffusive formation factor F¯ (■, □, ●, ○) as a function of the porosity ε. Data are for: EST104/2487 (●, ○) and EST104/2364 (■, □). Empty symbols are for C¯10−3 moll−1. Full symbols are for C¯>10−3 moll−1. These data are compared to numerical results () obtained for various types of random packings [15]. The solid line is an approximate fit by Archie's law ε−2.

Facteur de formation électrique F (▴) et facteur de formation diffusif F¯ (■, □, ●, ○) en fonction de la porosité ε. Les données sont : EST104/2487 (●, ○) et EST104/2364 (■, □). Les symboles vides sont pour C¯10−3 moll−1. Les symboles pleins sont pour C¯>10−3 moll−1. Ces données sont comparées aux résultats numériques () obtenus pour les différents empilements aléatoires [15]. Le trait plein est une approximation par la loi d'Archie ε−2.

In Fig. 3, these data are seen to be in very good agreement with the solution of the Laplace equation for various random packings [1]. They can be gathered by an Archie's law, F=ε−2 when 0.1ε0.75. Therefore, σ is essentially dominated by geometrical effects.

γ21 was measured by the same procedure as σ. γ21 is independent on C in the studied range; the mean value was found to be equal to (3.3±1.2)×10−9m2V−1s−1 for ε=0.6. At a fixed C=10−4 moll−1, γ21 increases with ε.

In Fig. 4, all results are gathered in the dimensionless form γ21 as functions of κΛ, and compared to the numerical data recalled in Section 2. It is remarkable to note that the experimental data cluster around a single curve with very little dispersion. Moreover, the comparison with numerical results is very successful; therefore, the experimental data are also well approximated by Eq. (6b), which is a conclusion of high practical interest (cf. [15] for a detailed discussion).

Fig. 4

The reduced coupling coefficient γ21 as a function of κΛ. Data are for: argillite powder (●), numerical results obtained for various types of random packings (). The dotted line is the least-square fit of Eqs. (7) and (6d).

Coefficient de couplage réduit γ21 en fonction de κΛ. Les données concernent : la poudre d'argillite (●), les résultats numériques () obtenus pour les différents empilements aléatoires. Le trait discontinu représente l'approximation aux moindres carrés – Éqs. (7) et (6d).

3.3 Experimental determination of the coefficients γ13, γ23, γ33 and γ34

3.3.1 Experimental procedure

The experimental cell was described extensively in Refs. [13,16,17].

Two types of experiments have been performed; the samples were either compacted under different pressures by exerting a compaction pressure during the experiment (EST104/2487) or they were precompacted under a pressure equal to 46 bar before the experiment started (EST104/2364). Thermal regulation with a precision of ±0.5°C was imposed by inserting coils of silicone tubings connected to two thermostats. The imposed temperature in each reservoir was measured by heat resistances. The temperature difference ΔT is ranging between 5 and 30 K.

The cell was initially assembled with either the less concentrated solution or the more concentrated one and the clay paste was brought to equilibrium with these concentrations. The minimal imbibition time is about 200 h. As a result, the sample was equilibrated with the solution and any additional and undesirable ion that appears during this initial equilibration phase can be eliminated.

Each experiment was divided into two periods. In the first one, a concentration gradient was imposed at a constant temperature of 25 °C. ΔC, ΔP and ΔΨm were measured as functions of the time t. The second period starts after approximatively 200 h when a temperature difference was superposed over the concentration gradient, either in the same direction as the concentration gradient (TC>0) or in the opposite direction (TC<0). Again, ΔC, ΔP and ΔΨm were measured as functions of t. T was also recorded in order to control its stability.

The total measured potential difference ΔΨm induced by a concentration gradient consists of the electrode potential difference ΔΨN, and of the so-called membrane potential difference ΔΨ. Since ΔΨN is given by the Nernst equation, in isothermal conditions, ΔΨ can be expressed as:

ΔΨ=ΔΨmRTFlnC1C2(13)

Fig. 5 shows typical evolutions of ΔC, ΔΨm and ΔP with t. ΔC, ΔΨm and ΔP vary linearly with t and decrease very slightly.

Fig. 5

A typical example of the evolution with time of the measured parameters: (a) ΔC=C1C2. (b) ΔP. (c) ΔΨ(=ΔΨmeasΔΨN) (○) obtained by subtracting from the measured potential difference ΔΨmeas (), the electrode potential difference ΔΨN (□). For period I, only ∇C is imposed and T=298 K. For period II, ∇T is superimposed over ∇C.

Exemple typique d'évolution temporelle des paramètres mesurés : (a) ΔC=C1C2. (b) ΔP, (c) ΔΨ (=ΔΨmesΔΨN) (○), obtenu en soustrayant la différence de potentiel d'électrode ΔΨN (□) à la différence de potentiel mesurée. Pour la période I, seul ∇C est imposé et T=298 K. Pour la période II, ∇T est superposé à ∇C.

The weights md and mw of the dry and wet sample are measured after the experiment in order to determine the porosity of the clay sample. The sample was dried in an oven at 100 °C during 24 h before the measurement of md.

3.3.2 Determination of γ13, γ23 and γ33 in isothermal conditions

During period I, in the absence of any thermal gradient, ΔC, ΔP and ΔΨ decrease with the same rate. Therefore, the ratios ΔΨΔC and ΔPΔC are constant, in agreement with the theory [6].

Since the seepage velocity is zero (U(t)=0) in the present setup, and since there is no externally applied electrical field (I(t)=0), the values of γ13 and γ23 can be deduced from the experimental values ΔPΔC and ΔΨΔC by using Eqs. (1a) and (1b)

γ13=γ12ΔPΔC+γ11ΔΨΔC(14a)
γ23=γ22ΔPΔCγ21ΔΨΔC(14b)
The global macroscopic diffusion coefficient D¯ can be determined by considering the evolution of the concentration profile (cf. [16]). D¯ is related to γ33. For non-isothermal conditions, the Soret coefficient was determined as described in [17].

3.3.3 Analysis of γ13, γ23 and γ33 in isothermal conditions

D¯ can be analysed by defining a diffusion formation factor:

F¯=DfD¯(15)
where Df is the diffusion coefficient in an infinite medium. These data are plotted in Fig. 3. If there were no influence of surface charges on the ions, one should obtain the same results for F and F¯. This is not quite the case. D¯ is relatively independent of concentration, but it depends on the sample; surface effects are indeed more important for EST104/2364.

The surface effects on D¯ can be more precisely analysed by introducing the coefficient α33:

α33(γ33D1+D2γ33(ζ=0)D1+D2)1ζ=(D1D2)(D1+D2)I1(κRc)κRcI0(κRc)(16)
It was numerically shown to depend only on κΛ by [12]. However, such a representation was not successful for the experimental data, which were found to be of the same order of magnitude, but with important differences, possibly due to experimental errors. The most satisfactory display for this quantity is given in Fig. 6 as a function of concentration.

Fig. 6

The coupling coefficient α33 as a function of the average concentration C¯. Data are for: EST104/2487 (●) and EST104/2364 (■).

Coefficient de couplage α33 en fonction de la concentration moyenne. Les données sont : EST104/2364 (■) et EST104/2487 (●).

The coefficient γ13, related to the membrane potential, can be analyzed more precisely with the help of (6c) in order to eliminate the bulk effects; it may be written as:

α13(γ13D1D2γ13(ζ=0)D1D2)1ζ=(D1+D2)(D1D2)I1(κRc)κRcI0(κRc)(17)
Hence, division of γ13 by (D1D2) and F cancels the influence of the molecular diffusion coefficients and of the restricted geometry, respectively. However, the same remarks as for α33 could be made and the representation in terms of κΛ was not successful. In its stead, Fig. 7 displays α13 as a function of concentration.

Fig. 7

The coupling coefficient α13 as a function of the average concentration C¯. Data are for: EST104/2487 (●) and EST104/2364 (■).

Le coefficient de couplage α13 en fonction de la concentration moyenne. Les données sont : EST104/2364 (■) et EST104/2487 (●).

The coefficient γ31, related to γ13 (see [12]), gives the magnitude of the total solute flux induced by the application of an electric field.

In Fig. 8, we have plotted γ23 as a function of κΛ. It is obvious that γ23 is very close to zero over the whole range of κΛ, indicating that osmosis is negligible.

Fig. 8

The coupling coefficient γ23 as a function of κΛ. Data are for: EST104/2487 (●) and EST104/2364 (■).

Le coefficient de couplage γ23 en fonction de κΛ. Les données sont : EST104/2364 (□) et EST104/2487 (●).

The hyperfiltration (or inverse osmosis) coefficient γ32, which is related to γ23 (see [12]), is a filtration process by which, upon application of an hydraulic gradient, solute concentration increases on the high-pressure side of the porous medium and a dilution of solute appears on the low-pressure side.

3.3.4 Analysis of the Soret coefficient

Fig. 9 displays the Soret coefficient ST as a function of the applied temperature difference ΔT, the average concentration C¯ and the average temperature T¯. A linear temperature variation is likely to take place in the sample, since in (1d), γ43 which corresponds to the Dufour effect, is very small. Moreover, thermal equilibrium is very rapidly obtained compared to diffusion.

Fig. 9

The Soret coefficient ST in compact clays as a function of temperature difference ΔT for various C¯ and T¯: C¯=73 molm−3 and T¯=25°C (▵); C¯=73 molm−3 and T¯=33°C (◊); C¯=1 molm−3 and T¯=25°C (▵); C¯=1 molm−3 and T¯=33°C (□); C¯=54 molm−3 and T¯=25°C (●); C¯=30 molm−3 and T¯=25°C (○) and C¯=6 molm−3 and T¯=25°C (∗).

Coefficient de Soret ST dans les argiles compactes en fonction de la différence de température ΔT pour diverses valeurs de C¯ et T¯. Les symboles sont définis ci-dessus.

It is important to note that we have omitted the absolute value in ST and ΔT, due to the fact that (STΔT) is always positive, indicating that the solute concentrates in the warmer region. Values obtained for T¯ equal to 25 and 33 °C are approximatively the same. However, ST depends slightly on C¯. For C¯10−3 moll−1, ST is close to 5×10−3 K−1. For C¯>10−3 moll−1, ST is close to 1.3×10−2 K−1.

Let us now compare our results with values obtained in a free medium for sodium chloride [3]. In the free medium, ST is a slightly decreasing function of C. Indeed, for C=10−3 moll−1, ST is equal to 2.3×10−3 K−1 and for C=5×10−2 moll−1, equal to 1.6×10−3 K−1. Therefore, our results are 5 to 10 times larger. In [18], the macroscopic Soret coefficient has been shown to be equal to the value in the free fluid for mica and glass powders. This difference is likely to be due to the finely divided nature of argilite with electrical effects coupled with temperature effects on the smallest pore scales.

4 Concluding remarks

The objective of this study was to analyze the behaviour of a compact clay, namely argillite, submitted to concentration, pressure, potential and temperature gradients and to compare the data to theoretical analysis, whenever possible. The coefficients were measured as functions of C and ε. Permeability ranges between 10−18 and 10−15 m2 for ε between 0.35 and 0.6. The electrical formation factor is well represented by an Archie law ε−2. Moreover, the macroscopic diffusion coefficient D¯ ranges between 10−11 and 2×10−10 m2s−1.

The osmotic coefficient γ23 is close to zero for all concentrations, indicating that osmosis has not a strong effect on solute and fluid transport.

γ21 ranges between 10−10 m2V−1s−1 with a maximum value of 4×10−9 m2V−1s−1. The successful comparison with the analytical solution (6b) indicates that γ21 can be predicted in a simple way from K and F.

The coupling coefficient γ13 associated with membrane potential depends on the clay samples. Indeed, for EST104/2487, γ13 ranges between 5×10−31 and 10−29 Am2. Negative values correspond to values in a free medium. For argillite EST104/2364, γ13 ranges between 9×10−30 and 2×10−29 Am2 for κΛ<5, and between 10−29 and 5×10−29 Am2 for κΛ>5. Positive values indicate that the clay efficiency is about 25–30%.

Non isothermal experiments show that solute transfer is enhanced by thermal diffusion. The Soret coefficients range between 5×10−3 and 1.3×10−2 K−1. The positive sign indicates that solute concentrates in the warmer region.


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