interactions between talc particles and water and organic solvents

JOURNAL OF COLLOID AND INTERFACE SCIENCE 194, 183–193 (1997)ARTICLE NO. CS975103

Interactions between Talc Particles and Water and Organic Solvents

H. Malandrini,* F. Clauss,† S. Partyka,‡ and J. M. Douillard‡,1

*Sanofi Recherche, 371 rue du Prof. Joseph-Blayac, 34184 Montpellier Cedex 04, France; †Luzenac Europe, 2 pl. Edouard Bouilleres,B.P. 1162, 31036 Toulouse Cedex, France; and ‡LAMMI, ESA 5072 CNRS, Case 015, Universite Montpellier II,

2 Place Eugene Bataillon, 34095 Montpellier Cedex 05, France

Received May 12, 1997; accepted July 24, 1997

The contact angle method has been used to characterizeThree talc samples have been studied by adsorption and immer- the surface intensive properties of solids. Indeed, the contact

sion methods after a classical characterization of their properties. angle between a drop of a liquid probe and the solid surfaceThe combination of adsorption isotherms and of immersion mea- studied is linked to solid/ liquid, solid/vapor, and liquid/surements allows the calculation of enthalpies and entropies of

vapor interfacial tensions through Young’s equation:adhesion. The studied talcs are characterized as ‘‘middle energy’’solids. The differences between the particle shapes of the differentsamples are shown to be of great importance, indicating a linkage gLV cos u Å gSL 0 gSV. [1]between cristallinity and surface properties. The whole results areexplained by the influence of intermolecular forces such as acid–

Unfortunately, the use of this equation is limited becausebase interactions in the interfacial layer. q 1997 Academic Press

solid / liquid and solid /vapor interfacial tensions cannot beKey Words: adsorption; immersional enthalpy; organic solvents;measured independently, and a second interfacial equationsurface tension; talcites; water.has been sought. Essentially two models allow the interpre-tation of contact angle measurements: the surface tensioncomponents model (STC) (5) and Neumann’s equation ofINTRODUCTIONstate (6) . The surface tension values calculated from thesemodels lie in the range 20–100 mJ m02 . These two methods

Divided minerals have been used as diluents of manufac-give values of solid surface tension which are low com-

tured materials such as polymers or paints for many years.pared with those determined by direct measurement of the

Recently, the formulation of composite materials with spe-cleavage, possible only in very special circumstances (7) .

cial properties has required more advanced inorganic sub-For example, mica, which is a lamellar mineral, gives a

stances. Clay minerals possess naturally attractive surfacesurface tension of 4500 mJ m02 using this experimental

properties of fillers, and talc substances, therefore, which domethod (3) . These large differences must result from the

not need complex treatments are of particular interest.ambient conditions of the experiment. Contact angles are

Talc is used as filler in many industrial sectors, includingmeasured in air saturated with vapor of the probe liquid,

the polymer, paint, paper, and cosmetic industries. Adhesionwhereas cleavage work is determined in a vacuum. In other

between the talc filler and the matrix is essential in thewords, the surface tension calculated from contact angles

control of the behavior of the composite material. This adhe-are obtained neglecting vapor adsorption on the solid, and

sion depends on the surface properties of the filler and ofthe energetic term linked to this adsorption: the surface

the matrix (1, 2) . The purpose of this work is to determinepressure. Furthermore, Neumann’s model is limited to low

the surface energy properties of some talc samples.energy solids and the liquids used must have a higher sur-

At equilibrium and at constant temperature, pressure, andface tension than the solid studied (6) .

composition, the surface properties of a solid phase dependIn this work, to overcome these problems, the affinity of

on only two thermodynamic parameters: surface area anddifferent kinds of talc for some liquid probes by microcalori-

surface tension (3). Simple methods allow the surface areametric technique has been determined in order to measure

of solids to be determined to a good approximation: forthe wetting, the immersion, and the adhesion properties of

example nitrogen adsorption associated with the BET equa-these fillers. Moreover, the surface pressure terms of differ-

tion (4) . Until now, however, there was no valuable methodent vapor probes on these solids have been determined. Fi-

for the determination of the surface tension of solids.nally, one interfacial model has been used with a view toestimating the surface tensions of the talc samples studied

1 To whom correspondence should be addressed. from the microcalorimetric results.

183 0021-9797/97 $25.00Copyright q 1997 by Academic Press

All rights of reproduction in any form reserved.

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184 MALANDRINI ET AL.

MATERIALS AND METHODS

Solids

Talcs are very attractive mineral for the study of the behav-ior of lamellar substances, because they are pure, without sub-stitutions, without charge, and so without interlayer cations.Talc is a trioctraedric phyllosilicate with three hydrated layers.The structural theoretical formula is Mg3Si4O10(OH)2 (8–10).

The elemental layer is composed by an octahedral planeof brucite confined in two tetrahedral sheets of silicate. Theinterlayer distance is 9.36 A and the crystal system is triclinicor monoclinic (11, 12). The layers are expected to be linkedby Van der Waals forces between surface oxygen atoms.

Three kinds of talc supplied by Luzenac Europe have beenstudied. The first, from a Spanish deposit ( talc A), is formed

FIG. 2. (a, b) Electron micrographs of the French talc (talc B). In (a) ,the bar represents 10 mm. In (b), the bar represents 1 mm.

of a disorganized entanglement of lamellae, small comparedto the particle diameter (see Fig. 1) . The second comes froma French deposit ( talc B) and has a regular stacking of largelamellae (see Fig. 2) . The last one from an Italian deposit( talc C) is formed by very regular and organized stackingof large lamellae compared with the particle diameters (seeFig. 3) . Ones can see a visible evolution from talc A to talcB and talc C. The phenomenological parameter linked tothis evolution can be called ‘‘lamellarity.’’ It depends onthe sheet sizes compared to the particle diameters and onthe regularity of stacking.

The specific surface areas of our samples (Table 1) havebeen determined by the volumetric adsorption of nitrogenvapor at 77 K with an Analsorb 9011 intrument and applica-tion of the BET equation (4). Before measurement the sam-ples were kept under vacuum (better than 1003 Torr) atFIG. 1. (a, b) Electron micrographs of the Spanish talc (talc A). In (a),

the bar represents 1 mm. In (b), the bar represents 100 nm. 1807C overnight.

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185TALC, WATER, AND SOLVENT INTERACTIONS

16). For the adsorption experiments in a liquid phase, n-butanol was used. All the organic solvents are HPLC grades(purity ú99.5%), dried on molecular sieves Prolabo 3A.Water was distilled after deionization on ion exchange res-ins. The different surface characteristics of these liquids havebeen taken in the literature (Table 6).

Immersion Microcalorimetry

A Calvet differential microcalorimeter was used (17),allowing the determination of the immersion enthalpy tohigh precision (better than 3%). Before the experiment, thesolid is outgassed under vacuum at high temperature (thesame treatment as for BET analysis) . All the experimentaldetails are listed elsewhere (17).

Adsorption Microcalorimetry

The Microscal flow microcalorimeter allows the determi-nation of the displacement enthalpy of a liquid probe to highprecision (better than 2%) (18). Before the wetting of thesolid with the solvent, the solid is outgassed under vacuumat ambiant temperature.

Adsorption

The vapor adsorption isotherms were performed with ahomemade adsorption device, built around a M.K.S. gaugeand a Sartorius SD3V Balance ({0.1 mg). The solid samplewas maintained before the experiment at a temperature of1507C, under a vacuum better than 1005 Torr, for 5 h. Theprecise experimental conditions and results are presentedelsewhere (19).

THEORETICAL BACKGROUND

FIG. 3. (a, b) Electron micrographs of the Italian talc (talc C). In (a) , Immersion Microcalorimetrythe bar represents 10 mm. In (b), the bar represents 1 mm.

The thermodynamics of immersion have recently beenreviewed (20, 21). The immersion experiment gives the

The mineralogical and chemical compositions have been change of enthalpy resulting from the creation of a solid/determined by Luzenac Europe, using thermogravimetric liquid interface, starting from a solid interface and a bulkanalysis and atomic force absorption after dissolution in acid. liquid. The thermodynamic system is composed of the fol-(The silicon formed during the dissolution in acid is titrated lowing elements: the surface of the outgassed solid, a pureby colorimetric technic after melting with soda.) The chemi-cal and mineralogical compositions are reported in the Ta-

TABLE 1bles 1 and 2. One can observe the presence of other minerals,Specific Surface Areas, BET Constants, and Mineralogicalessentially chlorite and dolomite (13, 14).

Compositions of the Studied Talc Samples

Liquids BETMineralogical composition

Two kinds of liquid have been used in the microcalorime- Talc Astric experiments: apolar liquids, n-heptane and isooctane samples (m2 g01) CBET Talc Chlorite Dolomite(2,2,4 trimethyl pentane), expected to interact only by dis-

A 6.5 433 97.0 1.0 1.0persive forces (15), and two polar liquids, water and for-B 1.9 198 93.5 5.0 1.0mamide, characterized by their hydrogen bonds. Water isC 3.0 189 94.5 2.5 1.0

considered to be amphoteric and formamide is basic (15,

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TABLE 2 is the enthalpy of wetting per unit surface area. In a moreChemical Compositions of the Studied Talc Samples general case of wetting,

Components A B C

0DWH* Å gSV 0 gSL 0 TS Ì(gSV 0 gSL)ÌT Dp

SiO2 61.3 61.9 60.00Al2O3 0.20 0.90 0.85Fe2O3 0.20 0.60 0.90

Å gLV cos u 0 cos uTS ÌgLV

ÌT Dp 0 TgLVS Ì cos u

ÌT DpMgO 30.80 31.20 31.20CaO 0.30 0.20 0.41K2O 0.01 õ0.10 õ0.10Na2O 0.07 0.02 õ0.10 Å H *LVcos u 0 TgLVS Ì cos u

ÌT Dp . [5]TiO2 õ0.10 õ0.10 õ0.05

Starting from this relation, it is possible to determine anexpression for the contact angle:liquid (or a mixture) , and its vapor (or a vacuum). Per unit

surface area the general relation is

cos u Å (DWH)

H *LV

/ TgLV(Ì cos u /ÌT )p

H *LV

. [6]

0DH* Å gSeff0 gSL 0 TS Ì(gSeff

0 gSL)

ÌT Dp , [2]

Adding H*LV to (3) ,where DH* is the experimental enthalpy per unit surfacearea, gSeff is the effective surface tension of the solid, T the 0DimmH* / H *LVtemperature, and p the pressure. The subscripts S, L, and Vrefer, respectively, to the solid, liquid, and vapor states, and

Å g 7S 0 gSL 0 TS ÌDg

ÌT DpgSL is the interfacial tension between the solid and liquidphases.

The results depend on the experimental conditions. In the/ (gSV 0 gSL) 0 TS ÌgLV

ÌT Dp [7]case of pure liquids and pure solids, there are two limitingcases. First, when the solid particles are immersed into theliquid from the vacuum, the relationship per unit area is Å (g 7S / gLV 0 gSL) 0 TS ÌDg

ÌT Dp 0 TS ÌgLV

ÌT Dp

Å 0DadhH*, [8]0DimmH* Å g 7S 0 gSL 0 TS Ì(g 7s 0 gsl )ÌT Dp

where DadhH* is the enthalpy of adhesion per unit surfaceÅ H 7S 0 HSL , [3]area.

where DimmH* is the thermodynamical enthalpy of immer- Vapor Adsorption onto a Solidsion and g 7S is the minimal surface tension of the solid in

This work is concerned with physisorption (the term phy-the vacuum.sisorption is used here for reversible adsorption). The evolu-Second, the vapor pressure is close to saturation, implyingtion of the Gibbs free energy of adsorption per unit surfacethat the solid is fully covered by vapor molecules. Then thearea with the vapor pressure is described by the surfacesystem follows Young’s equation. In the case of perfectpressure p of the adsorbed film on the surface:wetting one obtains

0DadsG* Å p Å g 7S 0 gSV. [9]0DWH* Å gSV 0 gSL 0 TS Ì(gSV 0 gSL)

ÌT Dp

From the integration of a vapor adsorption isotherm, withthe Gibbs equation, it is possible to calculate the surfaceÅ gLV 0 TS ÌgLV

ÌT Dp Å H *LV, [4]pressure term,

where H *LV is the reduced surface enthalpy of the liquid (notp Å RT *

p

0

G dln p , [10]taking into account the chemical potential term Gm7) . DWH*

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Models

In the absence of chemical reactions at the surface, theadsorption forces involve mainly dispersion interactions(functions of r06 where r is the surface–molecule distance)and electrostatic interactions (functions of r03) . The choiceof an interfacial model to describe these interactions is diffi-cult, because all authors have pertinent arguments to defendtheir point of view. Nevertheless, if one observes the evolu-tion of the immersion enthalpy of various solids in differentkinds of liquids, giving a direct idea of solid–fluid interac-tions, some trends appear.

Analysis of Eq. [3] suggests that immersion enthalpy de-pends mainly on the solid, because many theories indicatea small value of the solid/ liquid enthalpy compared to the

FIG. 4. Immersion enthalpy of some solid samples, expressed insolid surface enthalpy.mJ m02 .

To clarify this point, some immersion microcalorimetricmeasurements determined on different kinds of solids (de-scribed in Refs. (19, 20, 37)) are reported in Fig. 4. Indeedwhere R is the perfect gas constant, T the temperature, Gthe enthalpy values increase from ‘‘low energy’’ solidsthe amount adsorbed, and p the vapor pressure. At saturation(PTFE) to ‘‘high energy’’ solids (silica and illite) , whatever(p Å p 7) , p is maximum and noted pe .the liquid used. The immersion enthalpy depends also onThe liquefaction of the adsorbed vapor on the solid sur-the nature of the fluid, indicating a large variation of solid–face, at constant pressure and temperature, leads to a solid/liquid enthalpy.liquid interface. The Gibbs free energy linked to the process

In apolar liquids like n-heptane, the immersion enthalpiesof adsorption and liquefaction of the vapor is identical toof all the solids are very close, suggesting similar values ofthe Gibbs free energy of immersion of a perfectly outgassedVan der Waals interactions between all types of solids andsolid in a pure liquid:alkanes.

In polar liquids, the evolution of the immersion enthalpiesDimmG* Å gSL 0 g 7S [11]are more complex. In case of apolar solids, the so-called

Å (gSL 0 gSV ) / (gSV 0 g 7S ) ‘‘hydrophobic,’’ the interaction between the solid and theliquid is mainly repulsive. Consequently the solid/ liquid en-Å 0gLV cos u 0 pe . [12]thalpy HSL is high and the immersion enthalpy is low. Incase of polar solids, hydrogen bonds are created, and the

The Gibbs free energy of immersion can then be deter-attraction between the solid and the liquid is very strong,

mined from measurements of surface pressure and contactimplying the solid/ liquid enthalpy to be low and therefore

angle. In the case of perfect wetting of the solid surface bythe immersion enthalpy is large.

the liquid, the Gibbs free energy of immersion is reducedThe opposite classification of the immersion enthalpies in

to the following form:water and formamide between polar solids can be explainedby some differences in the acido-basicity of solids. For ex-

DimmG* Å 0gLV 0 pe . [13] ample, illite presents a higher surface acidity than silica:hence, illite interacts strongly with the most basic liquid,

The expression of the free enthalpy of adhesion is formamide.All these results can be explained by using the STC mod-

DadhG* Å gSL 0 gLV 0 g 7S . [14] els. According to such models, in the case that both the solidand the liquid are apolar, or if only one of them is apolar,

So, one obtains the interaction energy depends only on apolar forces, there-fore the immersion enthalpy is low. However, if both the

DadhG* Å DimmG* 0 gLV, [15] solid and the liquid are polar, the resulting interaction energydepends on the sum of apolar and polar forces, consequently,

where DadhG* is the enthalpy of adhesion per unit surface the immersion enthalpy is higher.area. In the case of perfect wetting, one has In conclusion, two ideas result from this phenomenologi-

cal analysis. First, the immersion enthalpy evolution is inagreement with an interpretation of the solid/ liquid interac-DadhG* Å 0pe 0 2gLV. [16]

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TABLE 3Immersion Enthalpies of the Talc Samples in n-Heptane, Isooctane, Water, and Formamide and Adhesion Enthalpies

of the Talc Samples in n-Heptane and Water

0DimmH (mJ m02) 0DadhH (mJ m02)Talc

samples n-Heptane Isooctane Water Formamide n-Heptane Water

A 84 84 356 531 133 475B 109 103 321 416 158 440C 151 111 311 345 200 430

tion in terms of apolar and polar contributions. Second, the gABi Å 2(g/i rg0i ) 1/2 . [18]

solid/ liquid term is never negligible. These two points arein complete agreement with STC models. The Lifshitz–Van der Waals adhesion free enthalpy

DadhGLW12 , resulting from the interfacial interactions between

The Surface Tension Components Model of Van Oss– the apolar intermolecular forces of the two condensedChaudury–Good (VCG) (22–27) phases, is given by the following equation:

Following the STC model and the approach of Fowkes,DadhG

LW12 Å 02(gLW

1 rgLW2 )1/2 . [19]the surface tension can be represented by a sum of an apolar

component gLWi and a polar component gAB

i :The Lewis acid–base adhesion free enthalpy, between

two condensed phases, is due to the interaction between thegi Å gLWi / gAB

i , [17]basic component (or acidic) of one phase and the acidic (orbasic) component of the other:The apolar component of the surface tension combines all

the Liftshitz–Van der Waals intermolecular forces and thepolar component of the surface tension regroups all the DadhG

AB12 Å 02((g/1 rg

02 ) 1/2 / (g01 rg

/2 ) 1/2 ) [20]

Lewis acid–base (or donor–acceptor) intermolecular forces.This point is obviously an approximation, alkanes having an From the Dupre equation (DadhG12 Å g12 0 g1 0 g2) andacid–base character, but it is possible to accept it. considering that the total free enthalpy can be represented

In the VCG model, the Lewis acid g/i and the Lewis by the sum of both the Lifshitz–Van der Waals and thebase g0i contributions of the polar surface tension are inde- Lewis acid–base contributions, the following expression ofpendent and not additive. The resulting Lewis acid–base the interfacial tension between two condensed phases is ob-contribution is described by a geometric mean of these two tained:components:

g12 Å g1 / g2 0 2((gLW1 rgLW

2 )1/2

/ (g/1 rg02 ) 1/2 / (g01 rg

/2 ) 1/2 ) . [21]

RESULTS AND DISCUSSION

Microcalorimetry

The immersion enthalpies in n -heptane, water and for-mamide of our talc samples are reported in Table 3 andFig. 5.

The wetting enthalpies by water on the talc sample sur-faces after thermal treatment and precoverage by water vapornear the saturation vapor pressure have been determined.The experimental results have been divided by the specificsurface areas reported in Table 1. Comparing these resultsto the pure water reduced surface enthalpy (H *LV Å 119 mJm02) , we conclude (see Eq. [5]) that there is a contact angleFIG. 5. Immersion enthalpy of the three talc samples, expressed in

mJ m02 . of water on talc. From the experimental results, and Eq. [6] ,

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TABLE 4 ing that lateral surfaces are composed of SiOH and MgOH,Wetting Enthalpies and Contact Angle Calculated they can interact strongly with polar solvents by strong hy-

from the Equation (6) for the Different Talc Samples drogen bonds. It is the reason why this sample gives thelargest immersion enthalpies in water and formamide. In

Talc 0DWH u 0DadsH (but/hept) HSP contrast, the talc sample C, which has the highest lamellaesamples (mJ m02) (7) (mJ m02) HSA (%)

sizes and consequently the lowest lateral surface percentage,gives the lowest immersion enthalpies in water and for-A 104 29 160 5.3 82

B 65 57 115 1.1 58 mamide. Effectively, the basal surfaces are constituted onlyC 61 59 95 1.4 47 by siloxane groups which cannot form strong hydrogen

bonds. S. Yariv (28) has developed more precisely the originNote. Displacement enthalpies of n-butanol from n-heptane, hydrophilic

of these hydrophilic/hydrophobic properties of the lateralsurface areas (HSA), and hydrophilic surface percentage (HSP) of the threeand basal surfaces for minerals. In the same way, L. Michottalc samples.et al. (29) consider that lateral surfaces of talc are perfectlywet by water, and from wetting enthalpy measurements, theyhave calculated that the contact angle of water on basalthe contact angle of water on each talc sample has been

calculated and reported in Table 4. surface of talc is 807. Finally, it appears that the talcs aremade up of surface patches of opposite properties. Some areFirst, such values are characteristic of highly polar solids.

Moreover, one observes, in the apolar solvent (n-heptane), high energy surfaces and the others are low energy ones.The lateral surfaces with sites which can form strong hydro-a decrease of the immersion enthalpies with the sample la-

mellarity. The order of this gradation is reversed in case of gen bonds are the more hydrophilic; basal surfaces are hy-drophobic and specially in case of talc because talcs has nopolar solvents, water, and formamide.

When the lamellarity parameter changes from one sample substitutions of silica by aluminum or iron.Adsorption microcalorimetry has been used in order toto one another, the basal and lateral surface area ratios

change. Apparently, it is an important effect, influencing the estimate the hydrophilic surface percentage of our talc sam-ples, following the Grosjek–Partyka method (18). If a solidimmersion enthalpy results.

The case of the n-heptane, which is apolar, is difficult to immersed in an hydrophobic liquid is placed in the presenceof hydrophilic molecules, then these molecules adsorb onunderstand. We have noticed in a previous paragraph the

weak influence of the polarity of the solid on the immersion the hydrophilic sites of the solid surface, displacing the liquidmolecules of the hydrophobic solvent. Then the correspond-enthalpies in apolar solvents. Nevertheless, one can observe

an increase in the immersion enthalpies in n-heptane with the ing enthalpy produced is characteristic of the hydrophilicityof the solid surface.sample lamellarity. An effect coming from the orientation of

the solvent molecules on the solid surface can be suspected. The displacement of n-butanol from n-heptane solutions(concentration 2 g liter01 and 10 g liter01) has been chosenAccordingly, one can suppose that n-heptane molecules are

more organized on the regular, large, and plane basal sur- as a model system to characterize the hydrophilic surfacesite energies of our talc samples. Effectively, in n-heptane,faces of talc C than on the disorganized and short basal

surfaces of talc A. To verify this orientation effect on the n-butanol is adsorbed by its alcohol functional group on thehydrophilic sites of the talc. Moreover, a reference solid,solid/ liquid interactions, the immersion enthalpies of our

talc samples in a very branched alkane, isooctane, have been hydrophilic on all this surface, allows the hydrophilic surfacepercentage of the studied solid to be calculated from thedetermined.

The experimental results are reported in Table 3. The three following relation:talc samples give close immersion and adhesion enthalpyvalues in isooctane. The gap between the immersion enthal- DdisplHbut /hept ( test)

DdisplHbut /hept ( ref)Å

Ashydrophilic( test)

As ( ref). [22]

pies in n-heptane and in isooctane is nil for talc A and itincreases strongly when the lamellarity of the studied sam-

DdisplHbut /hept ( test) and DdisplHbut /hept ( ref) are respectively theples increases. These results show a large influence of theintegral displacement enthalpy of n-butanol from n-heptanesurface geometry of the adsorbed molecule on the interactionby unit mass on the studied and reference solids.energy.Ashydrophilic

( test) is the hydrophilic specific surface area of theIt has been shown from the wetting enthalpy measure-ments that water gives large contact angle values on talc. It studied solid and As ( ref) is the specific surface area of the

reference solid.was obvious. However, the immersion enthalpies of talc inwater are of the same order of magnitude as hydrophilic A silica sample (Rhone Poulenc X015 LS; specific surface

area, 22.0 m2 g01) , studied in detail (20), has been chosensolids such as quartz, hydrophilic silica, and clay minerals.The lamellae of talc A have the smallest sizes; therefore, as reference. The integral adsorption enthalpy of n-butanol

from n-heptane solution (10 g liter01) is 197 mJ m02 .the ratio basal surface/ lateral surface is the lowest. Consider-

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TABLE 5 The cross-sectional areas of both the water and the n-Monolayer Adsorbed Quantities Nm, Cross-Sectional Area of heptane molecules are in the range of classical values for

Adsorbed Molecules s at the Monolayer Coverage, and BET Con- clay minerals and oxides (30) and they are very close tostant (CBET) of Water and n-Heptane Vapors on Talc B the values calculated from the liquid densities, respectively,

s(water) Å 12.1 A2 and s(n-heptane) Å 63.1 A2 .Nm s pe 0DimmG 0DadhG From the vapor adsorption isotherms the surface pressureVapor (mol g01) (A2) CBET (mJ m02) (mJ m02) (mJ m02)

near saturation, of n-heptane and of water vapor on talc Bn-Heptane 5.7 55 12 30 50 70 have been calculated with Eq. [10]. The values obtained areWater 17.3 18.2 54 123 162 234 reported in the Table 5.

In the literature, talcs are supposed to be low energy sol-Note. Surface pressure values of water and n-heptane and immersion and

ids, and consequently the surface pressure is considered asadhesion free enthalpies in n-heptane and water of the talc B.negligible. Nevertheless, it appears that talc gives high sur-face pressure, particularly with water vapor. This point con-

The experimental results are given in Table 4. firms the strong hydrophilicity of talcs shown previously.Very high hydrophilic surface percentages are obtained. Some authors are in complete disagreement with the com-

Nevertheless, the supposed hydrophilic sites of talcs are mon idea that the surface pressure term could be generallySiOH and MgOH, i.e., sites that can form strong hydrogen neglected. Kelebek et al. (31) have determined a surfacebonds, and the reference silica sample has only SiOH as pressure terms in the range 20–150 mJ m02 for varioushydrophilic sites. In fact, the hydrophilic surface areas ob- hydrophobic solids, like graphite and carbon. Furthermore,tained have to be associated with an equivalent specific sur- high surface pressure values of water vapor on solids canface area of the reference silica sample. It is difficult to find be found in the literature (32, 33), for example on illite,a perfect reference solid, adapted to the study of talcs, be-

pe Å 190 mJ m02 , on kaolinite, pe Å 220 mJ m02 , oncause the reference solid must possess the same kinds of montmorillonite, pe Å 147 mJ m02 , and on alumina, pe Åhydrophilic sites as talcs (SiOH and MgOH) and in the same 202 mJ m02 .ratio. Nevertheless, these results give useful information; The surface pressure term describes the decrease of thethey help to classify our solid according to their hydrophil- surface tension of a solid, consecutive to the vapor adsorp-icity. They confirm that a large part of the talc surface, but tion, and is defined by Eq. [9] in which both the surfacenot all the surface is hydrophilic. tension and the solid/vapor interfacial tension are positive.

Following the previous conclusion, under ambient condi- The interfacial tension is the lowest term, and it decreasestion, i.e., in the presence of vapors, high energy talc surfaces when the solid/ liquid affinity increases. Consequently, theadsorb these vapors spontaneously, apparently essentially solid surface tension in the vacuum must be greater than thewater vapor on hydrophilic sites, then the solid surface en- surface pressure term of all kinds of vapor on the solidergy will decrease with this adsorption. surface. Nevertheless, the literature so-called surface ten-

One can suspect from contact angle measurement on a sions of talc, determined from the application of the VCGnonoutgassed solid that the high energy sites of the surface model to contact angle measurements, are gS Å 30.7 mJ m02

will be covered by adsorbed water vapor and that the solid (34), gS Å 36.6 mJ m02 (35), and gS Å 44.8 mJ m02 (36),behaves apparently as a low energy solid over the whole to be compared with the surface pressure of water on talcsurface. B obtained here: pe Å 123 mJ m02 .

We recall here that in case of our results (37–42), ob-Vapor Adsorption Isothermstained with perfectly outgassed solids, the starting state is

The water and n-heptane vapor adsorption isotherms at the surface tension of a solid in equilibrium with its own277C of sample B are of type II. The adsorbed volumes at vapor: g 7s . Then, in contrast with the contact angle tech-the monolayer coverage and the C constant for each vapor nique, in the initial state, there are no physisorbed impuritieshave been calculated from the BET equation (Table 5). The which can modify the solid surface energy.cross-sectional area s of the adsorbed molecules on the solidsurface at monolayer coverage are determined by using the The Talc Surface Tensionadsorbed monolayer quantities Nm, the specific surface area

It is possible to link the thermodynamics of the immersionAs obtained from the BET method applied to a nitrogenprocess and the VCG model by combining Eqs. [9] , [13],vapor adsorption isotherms with (Table 5)and [21]:

s(A) Å As (A 2 g01)

Nm(mol g01)rN, [23]

0DimmG Å 0gLV / 2((g 7LWS rgLW

L )1/2

/ (g 7/S rg0L )1/2 / (g 70S rg/L )1/2 ) . [24]where N is the Avogadro number.

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TABLE 6 Therefore, knowing the apolar surface tension, from theSurface Tension and Surface Tension Components immersion enthalpies in the two polar solvents and the f

of n-Heptane, Isooctane, Water, and Formamide (43–47) coefficient, the acid and basic components of the surfacetension are determined (Table 7).

gLWL g/L g0L gLV These results show that talc is highly polar, essentiallyLiquid probes mJ m02 or mN m01

because of its strong surface acidity. This point is not reallyn-Heptane 20.4 0 0 20.4 a surprise, effectively, the strong surface acidity of the clayIsooctane 18.6 0 0 18.6 minerals like kaolinite (48, 49), attapulgite (48), and mont-Water 21.5 25.5 25.5 72.5 morillonite (48) has been known for a long time. SimilarFormamide 39.0 2.28 39.6 58.0

results have been shown for silica–alumina and silica–mag-nesia catalysts by pyridine or ammonia adsorption experi-ments (50–53). It appears that silica–alumina is more acid

However, from immersion microcalorimetry only the im- than silica–magnesia. Kaolinite and illite which are alumin-mersion enthalpy and not the free enthalpy of immersion ium–silicate clays have a higher Lewis acid component de-can be determined. These two thermodynamic values are termined by our method (41) (g 7/illite Å 350–460 mJ m02 ,linked by the following equation:

g 7/kaolinite Å 240–340 mJ m02) than talc, which is a magne-sium–silicate clay. Generally, the surface acidity is attrib-uted to the hydrogen of the surface hydroxyl groups (50)DimmH Å DimmG 0 T

ÌDimmG

ÌT. [25]

of AlOH, MgOH, and SiOH. Obviously, the surface acidityof these talcs increases with their surface hydroxyl group

This equation can be written at constant temperature, content, i.e., when the lateral surface area percentage in-creases.

DimmH Å DimmG / f , [26] A perfectly outgassed clay has acid sites similar to those ofalumino-silicate or magneso-silicate catalysts. It is generally

where f is a factor less than 1. admitted that clay minerals possess Lewis and Bronsted acidIt is well known that talc is perfectly wetted by alkanes sites which have catalytic properties. The catalytic properties

(35), so from Eq. [13] the free enthalpy of immersion has of clay minerals depend on the thermal treatment applied.been calculated and from Eq. [14] the corresponding adhe- In fact, if water is a catalyst poison, then the surface mustsion free enthalpy of talc B in n-heptane (Table 5) has been be outgassed to become reactive (49, 50, 54). So, on nonout-derived from the surface pressure term. In order to determine

gassed talcs (i.e., in ambient conditions) , the surface is morethe apolar surface tension component of that talc, the ob-

basic than acidic. The surface tension obtained from thetained free enthalpy of immersion and the n-heptane surface

application of the VCG model to contact angle measure-tension have been used in Eq. [24].

ments confirms that point. In that case the solid is not out-From previous studies (37), we have decided to attributegassed, so the polar surface tension components are verythat apolar surface tension component to all the talc samples.low and the basic component is higher than the acidic one:Effectively, the same free enthalpies of immersion in n-g 7/S É 0.1–2.4 mJ m02 and g 70S É 2.7–11.6 mJ m02

heptane for different kinds of talc, with various mineralogi-(34–36).cal or chemical composition and having undergone different

The surface tension of a solid covered by vapor is lowergrinding processes have been found. We conclude then thatthan the surface tension of a perfectly outgassed solid. Thisthe variation of immersion enthalpy observed here is dueis an important point, which explains the difference betweenonly to the entropic term, i.e., the steric effects due to thethe surface tension determined on an outgassed solid (fromlamellarity of the solid sample.adsorption measurements and immersion microcalorimetricSome difficulties appear in the case of water, because of

the presence of a contact angle on talc. From the surfacepressure term of water vapor on talc B, we have calculated

TABLE 7the free enthalpy of immersion (and the corresponding freeSurface Tension Components of the Three Studiedenthalpy of adhesion) of the talc B in water (Table 5) from

Talc SamplesEq. [12], using the contact angle estimated from the wettingmicrocalorimetric measurements. Talc gLW

S g/S g0SThen by dividing the experimental enthalpy of immersion samples (mJ m02) (mJ m02) (mJ m02)

by the calculated free enthalpy, a ratio f equal to 0.5 hasA 61 326 0been obtained for talc B/water. For water and formamide,B 61 159 11we will assume that the ratio f between the free enthalpy ofC 61 82 41

immersion and the enthalpy of immersion is the same.

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method) and on nonoutgassed solid (from contact angle sion enthalpy and the corresponding free immersion enthal-pies by using surface pressure measurements. At this stagemethod).

The problem actually sides in the surface pressure term. it appears clearly possible to obtain directly the solid surfacetension, and consequently the complete set of tensions in-This term is often neglected when the liquid has a higher

surface energy than the solid (55), i.e., when the liquid volved in the wettability process: solid–vapor, solid–liquid,and solid–solid interfacial tensions.does not spread spontaneously over the solid surface. This

assumption appears not to be justified. From all the surface tension components determined byour method, the conclusions are the following:Ambient air contains water vapor, and a nonneglecting

quantity is adsorbed on most solids. The surface energy is—Talcs are characterized by high energy surfaces on lat-changed by the adsorbed water vapor and becomes indepen-

eral surfaces and low energy surfaces on the basal surfaces.dent of the nature of the solid (56). Many authors have tried—Talcs are highly polar compounds essentially becauseto quantify this phenomena. B. Janczuk et al. (57) have

of their high surface acidity.shown the decrease of the surface tension of alumina and—In the presence of ambient moisture, the acid sites arequartz as a function of moisture. E. Chibowski et al. (58)

inactive.have calculated the surface tension of kaolinite from vaporadsorption isotherm on both a nonperfectly outgassed (ther- A comparison of these results with an ab initio calculationmal treatment õ1057C under vacuum) and on perfectly out- (62) of the surface energy taking into account the shape ofgassed kaolinite ( thermal treatmentú1257C under vacuum). the crystal could be an interesting topic.The results show that the outgassed sample is much morepolar and the surface tension component of the nonoutgassed

ACKNOWLEDGMENTSsample is very close to the surface tension of liquid water.It seems that high energy solids in ambient air have the This work has been supported by Luzenac Europe. We thank Dr. R.surface tension of the water adsorbed on its surface. This is Baeza (L.E.) and Dr. Deborah Jones (CNRS) for helpful discussions andan old idea (56). comments.

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interactions between talc particles and water and organic solvents

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