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Clays and Clay Minerals, Vol. 45, No. 1, 8-22, 1997.

M E C H A N I S M OF A D S O R P T I O N A N D D E S O R P T I O N OF W A T E R

V A P O R B Y H O M O I O N I C M O N T M O R I L L O N I T E : 3. T H E M g 2+, Ca 2+, Sr 2+ A N D Ba 2+ E X C H A N G E D F O R M S

J .M. CASES, 1 I. BI~REND, 1 M. FRAN(~OIS, 1 J .R URIOT, 2 L.J. MICHOT I AND E THOMAS l

)Laboratoire Environnement et Min6ralurgie et UA 235 du CNRS, BP 40, 54501 Vandoeuvre Cedex, France

2Centre de Recherches P6trographiques et G6ochimiques (CNRS-UPR 9046), BP 20, 54501 Vand0euvre Cedex, France

Abstract--The swelling of some well-defined Mg-, Ca-, Sr- and Ba- homoionic montmorillonites was studied in the domain of water relative pressures lower than 0.95. This involves the expansion of the crystal lattice itself, commonly known as the "interlamellar expansion" or "inner crystalline swelling". The initial freeze-dried clays were characterized by nitrogen adsorption-desorption volumetry and controlled transformation rate thermal analysis. The evolution of the structural and textural properties of these different clays at different stages of hydration and dehydration was investigated using water adsorption gravimetry, immersion microcalorimetry at different precoverage water vapor relative pressures and X-ray diffraction (XRD) under controlled humidity conditions. Large textural variations are observed in the dry state depending on the exchangeable cations. The 2-layer hydrate exhibits the most ordered layer stacking. Water is mainly adsorbed in the interlamellar space. With increasing water pressure, each homoionic species leads to a l-layer hydrate and, with the exception of Ba-montmorillonite, to a predominant 2-layer hydrate. The relative pressure corresponding to the formation of the 2-layer hydrate decreases with increasing hydration energy of the interlayer cation. For Ca-, Sr- or Mg-montmorillonites, simulation of XRD patterns leads to the definition of successive homogeneous states corresponding to the 2-layer hydrate. Furthermore, it yields the water filling ratio corresponding to the different hydration states during adsorption and desorption of water vapor.

Key Words--Adsorption, Desorption, Earth-Alkaline Cations, Immersion Microcalorimetry, Montmoril- lonite, Relative Humidity, Surface Area, X-ray Diffraction.

I N T R O D U C T I O N

The adsorption-desorption of water on homoionic montmori l loni tes and its relation to swell ing properties have been invest igated previously (Newman 1987). The amount of water adsorbed and the corresponding basal spacing are governed by both the size and the charge of the saturating cation, as well as by the value and localizat ion or distribution o f the charge o f the adjacent silicate sheets that the exchangeable cations neutralize. Textural features such as the extent in the ab plane or the thickness of layer stacks also play an important role in swelling. At low relat ive water vapor pressure, water sorption occurs both on the external surface and in the interlamellar space, causing inner crystal l ine swell ing through the hydration of the exchangeable cations o f the dry clay. F rom the desorption cycles of sorption isotherms, Na-montmori l loni te is known to form a l - layer hydrate and Ca-montmorillonite a 2-layer hydrate over a large range of water vapor relat ive pressures (B6rend 1991). Tarasevitch and O v c h a r e n k o (1975) d i s t inguished 3 d i f ferent cases: 1) cations such as Ca 2§ and Mg 2§ with high energy of hydration and octahedral coordinat ion leading to a predominant 2-layer hydration state; 2) cations such as Ba 2§ Li § or Na § with either a lower hydration energy or another coordinat ion leading to a predominant l - layer hydration state; 3) cations such as K § and Cs § with low hydration energy leading to a highly pre-

Copyright �9 1997, The Clay Minerals Society

dominant l - layer hydration state. In the case of cal- cium, the 2-layer hydrate predominates over an exten- sive range o f relat ive humidity. The spacing is stabi- l i zed by the fo rmat ion o f the dodecahydra t e (Ca(H20)12)2~. The basal spacing of the 2-layer hydrate o f Ca-montmori l loni te varies f rom 15.5 to 15 A for relat ive pressures decreasing f rom 0.9 to 0.2 (Glaeser and M6ring 1968), or f rom 15.7 to 15 A for P/Po vary- ing f rom 0.87 to 0.35 (Ormerod and N e w m a n 1983). For high relative humidities, the increase in water content corresponds mainly to mult i layer adsorption and capil lary condensation.

In water-saturated materials, large differences between the concentrat ions close to the clay surface and in the pores cause osmotic swelling. Two different states can be described: 1) clay-l iquid water system in the gel state; and 2) clay-l iquid water system in the fluid state, that is, pastes and sols. Quasi-crystals o f 4 to 5 layers separated by more than 35 ,~ have been reported in pastes o f Na-montmori l loni te for water vapor pressures lower than 106 Pa (Pons et al. 1981). Water is mainly located in the interlamellar space. The number of layers per particle increases and the interlayer spacing decreases with increasing intensity of the initial drying (Cases et al. 1992). In Ca-montmori l lonite, stacks o f 8 to 11 layers (number o f layers increasing with pressure) separated by about 3 to 4 layers o f water form the walls o f a network o f large pores saturated with water (Ben Rha iem et al. 1986).

Vol. 45, No�9 1, 1997 Water vapor adsorption-desorption by

Table 1. Nature of the Cation Exchange Capacity after exchange.

Nature of exchangeable cations Mg 2+ Ca 2+ Sr 2+ Ba 2+

CEC obtained from am- monium acetate exchange (meq/100 g) 110.5 102.9 95.3 - -

CEC obtained from lanthane chloride exchange (meq/100 g) - - - - - - 102

Other ions Na + (meq/100 g) 1 0 0.7 0.9

Other ions K + (meq/100 g) 1.1 0.4 0 0

Other ions Ca 2§ (meq/100 g) 4.7 0 0.8 0.4

Other ions Mg 2§ (meq/100 g) - - 1 0.6 0�9

% of homoionicity 94.2 98.7 97.8 97.5 Theoretical CEC 102.1 101.3 98.9 96.5

Studies o f the dielectric relaxation of water adsorbed by montmori l loni te reveal, at least at very low water contents, a broad variety o f molecular environ- ments, as compared to the bulk solid and liquid phases (Sposito and Prost 1982). For montmori l loni te saturated with monovalen t cations located near the tetra- hedral sheet, P6zerat and M6ring (1967) and M a m y (1968) have suggested that the l - layer hydrate consists of water molecules arranged in a strained ice-l ike configuration. For the l - layer and 2-layer hydrates o f Na- beidellite, Ben Brah im et al. (1986) also proposed a model in which the pattern o f water molecules associated with the exchangeable cations is close to the configuration of hexagonal ice. For the l - l ayer hydrate of montmori l loni te saturated with divalent cations, the same structure may not exist because of the strong solvation energy of the exchangeable cations located in the interlamellar space, at least in the case o f Ca 2+ (Mamy 1968). The water molecules may be organized in a manner consistent with the hydration shell of the cation and the nearby presence o f parallel silicate layers.

Much is known about the adsorption and desorption mechanisms of water molecules on homoion ic montmoril lonites in the undersaturation domain, but it is still a controversial subject (Del Pennino et al. 1981; Moore and Hoove r 1986). Some of the issues that re- main unsolved are: 1) the true distribution and the real organizat ion o f water adsorbed on the external surface and in the interlamellar space of montmori l loni te in homogeneous or mainly interstratified hydrated states; and 2) the possible change in textural properties, such as the external surface area o f quasi-crystals, upon changes in relative humidity. The influence of monovalent cations Li +, Na +, K +, Rb + and Cs + on montmori l loni te hydration mechanisms was studied by Cases et al. (1992) and Br rend et al. (1995). Interstra-

divalent homoionic montmorillonites

rifled hydrated states encountered during adsorption and desorption cycles were quantified using the method described by Ben Brahim et al. (1986). In order to determine the structural changes upon hydration and dehydration, the exper imental X-ray powder patterns o f the 001 reflections were compared with theoretical simulations. During water adsorption and desorption, some textural changes were observed. Compar ison of the filling with water molecules o f the interlamellar space with the model proposed by Ben Brahim et al. (1986) has made it possible to characterize an increase o f the filling with relat ive humidi ty and a decrease with the size o f the exchangeable cation. In the present paper, the same approach is used to study the hydration and dehydrat ion mechanisms of montmori l loni te saturated with Mg 2+, Ca 2+, Sr 2+ and Ba 2+ cations.

E X P E R I M E N T A L

Materials

The invest igation was conducted using a Wyoming mon tmor i l l on i t e ( < 2 ixm fract ions) , suppl ied by C E C A S.A, Paris, France, previously dispersed, centr ifuged and Na-exchanged. Homoion ic Mg-, Ca-, Sr- and Ba-montmori l loni tes were prepared using the following procedure: the exchange was obtained by dia- lyzing 12 g L -1 montmori l loni te suspensions with nor- mal chloride solutions o f the suitable cation. The material was washed by dialysis until a chloride residual concentrat ion o f 10 -3 N was obtained. The samples were then centr i fuged and freeze-dried. After treatment, the purity of the dried material was higher than 99%. The structural formula was calculated f rom the chemical and electron microprobe analysis o f the Na form according to a method proposed by Yvon et al. (1990). It was assumed to be unaffected by the exchange with a particular divalent cation X:

�9 IV II H VI ($1 A ) A1 M Fe Fe i O 3.883 10.117 ( 1.530 g0.257 0.006 0.207 1 0 ) -

( O H ) 2 X o . 1 9 o

The cation exchange capacity (CEC) was determined by treating the suspensions with a m m o n i u m acetate or lanthane chloride (for Ba form). The results are presented in Table 1.

Methods

The texture in the dry state was characterized using: 1) convent ional step-by-step volumetr ic adsorption equipment in order to determine complete ni trogen gas adsorption-desorption cycles; 2) a continuous nitrogen adsorption procedure designed to work at ve ry low relative pressures f rom 10 5 to 0.3 in order to study the first stages of adsorption and to determine the specific surface area (Michot et al. 1990)�9 With this apparatus, the adsorbate is introduced through a calibrated microleak at a f low rate slow enough to ensure

10 Cases et al. Clays and Clay Minerals

Table 2. Distribution of water content after dehydration.

Nature o f exchangeable cations M g 2+ Ca 2+ Sr 2+ Ba 2§

Water bound to exchangeable cationst 4.2 4.4 3.2 1.8

H20 per cation:~ 4.4 4.6 3.4 2.0 Structural waterw 5.3 5.2 5.1 5.0

t Amount of water bound, in % of the final mass, to exchangeable cation at 100 ~

:~ Corresponding number of water molecules per exchangeable cation.

w Structural water in % of the final mass.

quasi-equilibrium conditions all along the adsorption isotherm. Prior to the experiment, approximately 500 mg (1000 mg for continuous procedure) of sample was outgassed at 100 ~ under a residual pressure of 1.3 10 -2 Pa.

In order to define the initial water content in the reference state (that is, after pretreatment of the sample), Controlled Transformation-Rate Thermal Analy- sis (CTRTA) was used following the procedure described in previous papers (Grillet et al. 1988; Brrend et al. 1995). The conditions selected were a sample mass of approximately 300 mg and a dehydration rate of 2.4 mg/h. The pressure was fixed in the cell between 1-2 Pa.

Water vapor adsorption-desorption isotherms were recorded using a continuous gravimetric apparatus built around a Setaram MTB 10-8 symmetrical balance (Poirier et al. 1987; Cases et al. 1992). Prior to each experiment, a sample of approximately 115 mg was outgassed at 100 ~ under vacuum.

Immersion microcalorimetry at 30 ~ was used to determine the enthalpy of immersion in water of the montmorillonite sample (approximately 100 mg) versus the pre-equilibrium water vapor relative pressure, following a method described by Cases et al. (1992). The enthalpy curve was then used to derive the external specific surface area of tactoids immersed in water (final state) by applying a modified Harkins and Jura procedure (Harkins 1952; Cases and Franqois 1982).

The XRD patterns of oriented samples, placed in- side a controlled humidity and temperature chamber, were recorded on a specially built apparatus according to a procedure previously described (Brrend et al. 1995). The diffraction patterns were obtained using the CoKoL radiation with a CGR goniometer and an Inel CPS 120 curve detector. Data were collected simulta- neously over 60 ~ and were processed using a mul- tichannel Varrox analyzer (2048 channels for 60~ During the experiments, the sample temperature was controlled by circulating water in the double wall of the chamber. The chamber was connected to a water vapor source and given relative humidity values were obtained by increasing the temperature of the source. Diffraction patterns in adsorption were recorded at dif-

ferent times (30 min, 1 h, 2.30 h and >4 h, depending on the time required to reach equilibrium). The desorption data were obtained as previously described (Brrend et al. 1995). Using the DiffractInel software implanted in a Compaq Deskpro 286 computer, data were transferred from the analyzer and the peaks were automatically searched and calibrated using the d(001) and d(002) peaks of standard kaolinite added in the oriented films.

The XRD powder patterns were compared with simulated ones. The software used was developed in the Laboratory of Crystallography of Orlrans, France, using the formalism proposed by Drits and Tchoubar (1990) and adapted (Kerm 1988; Brrend 1991) to the study of the intensity diffracted along the 00 rod of the reciprocal space of poorly crystallized phyllosili- cates.

RESULTS

Characterization of the Dry or Initial State

The CTRTA curves illustrate the dehydration mechanisms of saturated homoionic montmorillonites heated to 1000 ~ For Ca-, Sr- and Ba-montmorillonites, the sharp weight loss in the low-temperature region corresponds to the desorption of physically adsorbed water. It is followed between 100 ~ and approximately 500 ~ by a gradual decrease corresponding to the desorption of water molecules linked to the exchangeable cations. At higher temperatures, the weight loss of approximately 5% represents the dehydroxy- lation of the structure (Table 2). In the central region and for a given temperature, the water content decreases in the order Ca 2+ > Sr 2+ > Ba 2+ following the decreasing solvation energy of the cations. The water content of Mg-montmorillonite is less than expected and the different kinds of water overlap.

In the initial state chosen for hydration studies (i.e., heating at 100 ~ the water content corresponding to water molecules bound to exchangeable cations (Table 2) is still high (>3% of the final mass). The one exception is Ba-montmoriUonite, which exhibits a be- havior similar to K-, Rb- and Cs-montmorillonites (water content approximately 2% of the mass of cal- cined samples) (BErend et al. 1995).

The continuous decrease of the CTRTA curve for Mg-montmorillonite agrees with the thermogravime- tric analysis of of Be- and Mg-montmorillonites reported by Poinsignon and Cases (1978). When homoionic montmorillonites were heated for 1 h at each 100 ~ interval between 100 and 900 ~ and cooled in a desiccator for 1 h, the d(001) spacing decreased from 15 to 10 ,~ at 300 ~ for Mg- and 400 ~ for Ca-montmorillonite. The irreversible dehydration temperatures of the Mg- and Ca-saturated specimens were found to be 400 and 600 ~ respectively (Kawano and Tomita 1991). The Mg interlayer cations appear to mi-

Vol. 45, No. 1, 1997 Water vapor adsorption-desorption by divalent homoionic montmorillonites 11

Table 3. Textural properties of samples obtained from nitrogen adsorption-desorption and immersion microcalorimetry.

Nature of exchangeable cations

Methods M g 2+ Ca 2+ Sr 2+ Ba z§

Point B SB? (mVg) First run 132.8 17.5 40.2 29.7

Second run 105.8

BET SBETt (m2/g) First run 112.5 16.7 34.3 25.4

Second run 90.0 CBEr$ First run 183.4 147 495 411

Second run 142.0 MBErw First run 7

69 24 32 Second run 9

t-plot S,ot~t (mVg) First run 116.1 16.9 37.2 28.4 Second run 89.6

Sext.lq[ (m2/g) First run 23.3 13.6 16.2 12.8 Second run 18.4

Sox~,2q[ (m2/g) First run 10.8 5.8 7.4 7.1

Second run 10.9

Continuous volumetry Sc# (mVg) 33.4 First run 38.8 First run 39.7 Second mn 40.1 Second run 38.9

C:~ 284 First run 485 First run 252 Second run 422 Second run 250

Mw 26 20-19 20-19

Harkins and Jura proce- Sext~ ~ (mVg) 50.2 63 63 56 d u r e

Meaw 18 12-13 12-13 14

Nitrogen desorption vol- D (,~)$:~ First run 67 37 41 33 umetry Second run 25

Point by point gas adsorption-desorption volumetry: t Total specific surface area calculated from the point B method (SB), the standard BET procedure (SBBT), the t-method

(Stot~), respectively. Continuous gas adsorption volnmetry: $ Energetic constant (C) of the BET theory related exponentially to the enthalpy of adsorption in the first adsorbed layer. w Mean number of layers per quasi-crystal (M) (average thickness of the elementary stacks) calculated from the correspond-

ing specific surface area and assuming a model of parallelepiped. q[ Respectively, external surface area exclusive of micro (Sext,1) and mesoporosity (Sext,2) calculated from the t-plot method. # Total specific surface area (Sc) calculated from the standard BET procedure. ~~ External specific surface area (Sext) deduced from the Harkins and Jura method. ~::~ Mean width (D) of parallel-sided slit-shaped pores corresponding to the lower closure point of the desorption branch of

the isotherms.

grate into the hexagona l holes o f the S i O 4 ne twork upon the rmal dehydra t ion . In the p resen t study, heating unde r v a c u u m was used. Despi te the d i f ferences in exper imen ta l condi t ions , the same exp lana t ion could be proposed: mig ra t ion o f M g 2+ cat ions into the hexagonal holes faci l i tates the dehydra t ion o f the inter la- mel la r spaces and m a y also faci l i ta te the dehydroxy- la t ion o f the structure.

The resul ts deduced f rom the analysis of the adsorp t ion-desorp t ion i so the rms o f n i t rogen at 77 K on f reeze-dr ied montmor i l lon i t e s are s t rongly inf luenced by the nature o f the exchangeab l e ca t ions and by the exper imen ta l adsorp t ion p rocedure used (Table 3). The i so the rms are all type IV, exh ib i t ing a hys teres is loop associa ted wi th capi l lary condensa t ion in mesopore s and a l imi t ing up take over a r ange o f h igh re la t ive pressure. The hys teres is loops are of type IV, of ten associa ted wi th s l i t - shaped pores. A low-pressure hys-

teresis may be o b s e r v e d on all of the sys tems presented, ex tend ing to the lowes t a t ta inable pressures . This p h e n o m e n o n is p robab ly associa ted wi th the swel l ing of a non- r ig id s t ructure ra ther than wi th an i r revers ib le up take o f n i t rogen molecu les in sys tems con ta in ing micropores . For this reason, the ex tens ion o f this low-pressure hys teres is controls the lack o f re- producibi l i ty , as o b s e r v e d wi th the M g - e x c h a n g e d form.

Total specific surface areas ob ta ined f rom conven- t ional vo lume t ry were ca lcu la ted f rom the po in t B m e t h o d ( E m m e t t and B r u n a u e r 1937), the B E T procedure and the t -curves (de Boe r et al. 1966) (Table 3). Al l of the resul ts are in acceptable agreement . The values are d i f ferent for the 4 cat ions. The use of a con t inuous p rocedure appl ied leads, for Ca-, Sr- and Ba -mon tmor i l l on i t e , to h igher surface areas (Table 3). This d i sc repancy is cer ta in ly due to the fact that this


method allows more nitrogen molecules to enter small pores at lower relative pressure than the step-by-step method.

The large surface area obtained for Mg-montmorillonite may be either the external surface area of particles about 8 layers thick, or may indicate a contri- bution of the interlamellar space to the surface avail- able for nitrogen adsorption. The first assumption can be discarded because of the low value obtained for the external surface area using the t-plot method (Table 3, line 7). The "real" surface area of Mg-montmorillonite would then be less than 90 m 2 g 1. The evolution of the mean thickness of quasi-crystals derived from a model (assuming the plates are perfectly stacked, as in a deck of cards, and calculated from surface areas) (Cases 1985, personal communication; Cases et al. 1992) shows that the Ca-saturated montmorillonite presents an average quasi-crystal size of about 69 elementary layers, as compared with 24 and 32 for Sr- and Ba-samples. These values represent only a rough approximation, as there is a chance that the layers could be stacked in a fractal-like fashion, as suggested by Ben Ohoud and Van Damme (1990).

The evolution of the external surface area of quasi- crystals calculated from the analysis of the t-curves (Table 3) and the computation of mesopore distribution from a model of parallel-sided slits (Delon et al. 1986) shows that all the samples present an important micro- and mesoporosity. This would confirm the model previously described with the plates stacked in a fractal-like fashion (Cases et al. 1992), the mean width of parallel-sided slit-shaped pores extending from 25 to 40 ,~.

Water Adsorption-Desorption Isotherms

Isotherms are illustrated in Figure 1. The shape of the isotherms is influenced by the nature of the saturating cation. Because of their irregular shapes, these isotherms cannot be assigned to any type known. Ad- sorption phenomena interfere on interfaces whose surface areas are not constant, but depend on the water vapor relative pressure (that is, on the accessibility of the interlamellar space to water molecules). The adsorption isotherms are reproducible for Ba-montmorillonite but not for Mg- and Sr-montmorillonite. How- ever, the general shape of the isotherm remains unchanged for a given cation, which reveals constant hydration or swelling phenomena. The nonreprodu- cibility could be due to changes in the texture of Mg- and Sr-montmorillonite occurring during storage or to changes in the preparation of successive samples of the same montmorillonite (induced by a variation of vacuum condition or of initial solid concentration during freeze-drying).

For all of the montmorillonites studied except the Ba form, the hysteresis loops do not close at low water vapor relative pressures. Desorption of water mole-

cules located in the interlamellar space and micropores does not occur for relative pressures as low as 0.01, corresponding to a water vapor pressure of 4.10 -2 Pa at 30 ~ An increase in temperature (up to 100 ~ as during outgassing) or a decrease of outgassing pressure, for several h, would be necessary to return to the initial state.

Obviously, at high relative pressure, a 2-layer hydrate state is formed for all divalent montmorillonites (amount adsorbed of 13-14 mmol g l). A l-layer state is formed on Ba- and Sr-montmorillonite, and probably on Ca-montmorillonite, as well. By comparing the isotherms measured on divalent and monovalent montmorillonites (Brrend et al. 1995), it is possible to observe a continuous evolution of the shape of the isotherm from l-layer state predominant to 2-layer state predominant for increasing hydration energy of the exchangeable cations (Cs + < Rb + < K + < Na + < Li + < Ba 2+ < Sr 2+ < Ca 2+ < Mg2+). In the case of divalent

saturated montmorillonites, inner crystalline swelling occurs at the lowest relative pressures investigated. It is, therefore, impossible to determine the external surface area of the samples or to observe changes in the size of the quasi-crystals during the first stages of water adsorption.

XRD Patterns

Typical X-ray powder diffraction tracings of oriented samples are given in Figures 2, 3 and 4 for samples in the initial state (zero-layer hydrate state), and for samples in quasi-equilibrium with relative pressures between 0.4-0.6 and > 0.92, respectively. Variations of the d(001) spacings as a function of water relative pressure, P/Po, are presented in Figure 5. The curves can easily be correlated with corresponding adsorption isotherms (Figure 1), except for Ba-montmorillonite. In this case, the basal spacing increases up to 20 ,~ for P/Po = 0.92 and appears higher during adsorption than desorption for P/Po > 0.5.

Swelling kinetic rates were rapid for most of the steps of relative pressure investigated, except at the beginning of the formation of the 2-layer hydrate state.

The initial state of Mg-montmorillonite is not an homogeneous dry state: the d(001) is equal to 10.6 ,~ and a broad shoulder is present at 16.5 ,~; harmonics are broad and of relatively low intensity, as shown in Figure 2. Ca-montmorillonite is in a more homogeneous state characterized by a basal spacing value d(001) of 9.8 ,~ and harmonics at 4.81 and 3.18 ~,. However, a more hydrated fraction could be hidden by high background noise and broadening of the peaks observed at low relative pressure. For Sr-montmorillonite and Ba-montmorillonite, d(001) spacings of 10.0 and 10.2 .~, respectively, are observed. The homogeneous series of harmonics and a low shoulder for small angles indicate that a dry initial state is predom-

20 M g

18

16

14

~ 8

4

Vol. 45, No. 1, 1997 Water vapor adsorption-desorption by divalent homoionic montmori l loni tes 13

0.2 0.4 0.6 0.8

R e l a t i v e pressure

20

18-

16-

1

Sr

~ 14- ~ i . . , '

~ 8

2 ~

0 0.2 0.4 0.6 0.8 I


20

18

16

14

12

10

8

6

4

2

0

20

18

16

~ 8

4

2

0

Ca

0 0.2 0.4 0.6 0.8 1


ga

- - - - - - - q f - - I I

0,2 0.4 0.6 0.8


Figure 1. Water adsorption-desorption isotherms onto Mg-, Ca-, Sr- and Ba-montmori l loni te and reproducibility.


I N I T I A L S T A T E ( D r y S t a t e )

10.21 K

Ba ~ 4.98 K 3.28

10.03

Sr

Ca 3,18

10.59 K

K

, , , , J , , J , , , 0 20 15 10 9 8 7 6 5 4 3.5 3 d

Figure 2. Observed XRD tracings of samples in the initial state (dry state)�9

inant. It is still associated with some hydrated states (< 10%).

As previously discussed (Brrend et al. 1995), the use of a reflection procedure in diffraction decreases the angle resolution as the corresponding signal-to noise ratio decreases, specially in the case of Ca-montmorillonite at low relative pressure and Ba-montmorillonite at high relative pressure. This is due to the interference between the incident and reflected beams; however, the rest of the spectrum remains unchanged. Therefore, the possibility of superimposing calculated with experimental spectra is lost. This increases the difficulties of determining the average layer number, M. The experimental spectra are then interpreted by looking for the model that best fits the d(001) peak position as well as the harmonics positions. Under these conditions, the positions of the reflections alone have been taken into account to describe the proportions of the different types of layer (Figures 6 and 7). In the initial state, the mean number of layers per stack, M, was fixed arbitrarily for Mg-montmorillonite. For other cations, it was deduced from the position of the d(001) peak using Scherrer's determination. These last values characterize mainly the interferential coherence domain and can then be considered as default values (Table 4). For higher water relative pressures, M was increased for Mg- and decreased for Ba-mont- morilionite at P/Po = 0.92 in order to improve the cor-

0 . 4 < P / P o < 0 . 6

3.15

14.20 12.72 K

5.10 K 3.08

K

Ca .

15.05

K Mg

30 20 15 109 8 7 '6 5 4 3:5 3 d ~

Figure 3. Observed XRD tracings of samples under a relative pressure between 0.4 and 0.6 during adsorption and after reaching the quasi-equilibrium time: Mg-montmorillonite, P/Po = 0.6; t = 19 h; Ca-montmorillonite, P/Po = 0.5; t = 96 h; Sr-montmorillonite, P/Po = 0.5; t = 18 h; Ba-montmorillonite, P/Po = 0.55; t = 4 h.

respondence between experimental XRD patterns and theoretically simulated ones. The computation procedure has been described in detail previously (Brrend 1991).

Model basal spacings of dry and hydrated states were set, taking into account the harmonics of the quasi-homogeneous states or, in the absence of such states, the results reported in the literature (Suquet et al. 1977; Del Pennino et al. 1981; Kamel 1981; Or- merod and Newman 1983; Iwasaki and Watanabe 1988). The basal spacing in the dry state increases with the real size of the cation (Ca 2+ < S r 2+ < Ba 2+ < Mg2+), the Mg 2+ remaining partially hydrated (Table 4). In the l-layer hydrate state, basal spacing deter- minations are imprecise and seem to be modified by variations in the relative pressure. For relative pressures lower than 0.9, experimental patterns reveal a quasi-homogeneous 2-layer state. Indeed, homogeneous series of values of basal spacings are observed that do not correspond to 1-1ayer-2-1ayer hydrate state interstratifications. The patterns were then simulated using a set of 3 values of basal spacing for Mg-montmorillonite corresponding to substates 2a, 2b and 2c (Table 4), and of 2 values for Ca- and Sr-montmorillonite corresponding to substates 2a and 2b, according


P t P 0 -> 0 . 9

3 . 2 0 K

19.83 ~.~ ~ A

5 . 2 7 3 . 1 6

S r

C a

i ~ sa9 II h

M g 4 1 K K

A | 3 . 1 9

, , , R ) i ~ ~ ) t I , 30 20 ~ ~ 0 9 s 7 6 s 4 3 s 3 d ~

Figure 4. Observed XRD tracings of samples under a relative pressure higher or equal to 0.90 during adsorption and after reaching the quasi-equilibrium time: Mg-montmorillonite, P/Po = 0.92; t = 66 h; Ca-montmorillonite, P/Po = 0.90; t = 19 h; Sr-montmorillonite, P/Po = 0.92; t = 24 h; Ba- montmorillonite, P/Po = 0.92; t = 24 h.

to the work o f Suquet and P6zerat (1987). These authors describe 3 successive 2-layer hydrate substates for Mg-saponi te for increasing water vapor relative pressures. Such an evolut ion o f the inter lamellar space probably occurs as well for poorly crystal l ized smec- tires such as montmori l loni te .

Interstatification o f 0-, 1- and 2-layer hydrates are observed at low relative pressure for all o f the montmoril lonites. An homogeneous 2-layer hydrate state is formed during adsorption at g iven values o f relat ive pressures (0.4 for Mg-, 0.7 for Ca-, 0.9 for Sr- and > 0.9 for Ba-montmori l loni te) . In addition, Ba-mont-

mori l lonite presents an interstratification 2-1ayer-3-1ay- er hydrate, d(001) = 18.5 ,&.

During desorption, a predominant l - layer state is observed at different relat ive pressures, depending on the nature of the interlayer cation. It appears upon the final outgassing for Mg-montmori l loni te , at rlow rela- t ive pressures for Ca- and Sr-montmori l loni tes and for 0.05 < P/Po < 0.58 for Ba-montmori l loni te . Quasi- homogeneous 2-layer hydrate states are observed for higher relat ive pressures.

Enthalpy o f Immersion, or an At tempt to Characterize the Final Size of Quasi-Crystals

The enthalpy of immers ion in water steadily decreases with increasing precoverage relat ive pressure for the different saturated homoionic montmori l loni - tes, down to a constant value for relat ive pressures >0.75 that depends on the nature of the exchangeable cation. These values were used to derive the corresponding external specific surface area, Sext, fo l lowing the Harkins and Jura method, and calculated using the relationship:

AHp~ ..... = - UL.S~xt [2]

where UL represents the internal energy of the water- water vapor interface taken equal to 0.1195 J m - : at 30 ~ The values obtained are reported in Table 3 (line 11). They are determined without any assumption concerning the value of the cross-sectional area of the water molecule adsorbed on the solid. The mean thickness of the quasi-crystals in solution, M, is calculated f rom a model assuming flat layers, with the interla- mel lar space in a 2-layer hydrate state, perfect ly stacked as in a deck of cards. In a first approximation, these values (Table 3, l ine 12) can be considered as the mean size o f stacks for high water relat ive pressures.

The different curves reveal that: 1) The values o f the immers ion enthalpy without precoverage (that is, 148 J g-~ for Mg-, 130 J g ~ for Ca-, 105 J g- i for Sr- and 86 J g-~ for Ba-montmori l loni te) fo l low the hydration energy of the exchangeable cations; high hydration energies correspond to high immers ion enthal- pies. 2) The heat released upon immers ion decreases sharply for increasing precoverage relative pressures. 3) The surface field is totally screened for precoverage higher than 0.75. According to Hagymassy et al. (1969), this value corresponds to a statistical coverage on the external surface o f the quasi-crystals o f approximately 2.0 or 2.3 water-adsorbed layers, depending on the value of the energetic constant taken into considerat ion (C = 5.2 or 50 to 200, respectively). Therefore, water on the external surface of the quasi- crystal forms an approximate bilayer, whereas in the inter lamellar space, an homogeneous 2-layer hydrate state is formed for Mg-, Ca- and Sr-montmori l loni te and a predominant 2- (70%) and 3-layer hydrate interstratified is formed for Ba-montmori l loni te .

D I S C U S S I O N

Textural and Structural Evolu t ion During Water Adsorpt ion

Controversial results regarding the textural characterization of dried montmori l loni te are reported here and in the literature. Specific nitrogen B E T surface areas may vary f rom 15 to 130 m 2 g-l. They either increase or decrease after exchange of a monovalen t


i 5" 14.

"~ 13"

12.

11.

10"

Mg

+ t < 2.5 h r s

D t = 2 . 5 h r s

�9 t >> 2.5 h r s

Sr 19" 19 I 18 + t < 2.5 h r s

[] t = 2.5 h r s 17

�9 t >> 2.5 h r s

16

9 I I I I I 0.2 0.4 0.6 0.8 1

R e l a t i v e P r e s s u r e

"Va

+ t < 2.5 h r s

D t = 2.5 h r s

�9 t >> 2.5 h r s

18-

ga

t < 2.5 h r s +

[] t = 2.5 h r s / " / 17-

16-

* i 15- 14-

13-

I~1 12-

11-

10 ]

0.2 0.4 0.6 0.8


Ca

0.2 0,4 0.6 0.8 1 R e l a t i v e P r e s s u r e

9 I t I I I I 0 0.2 0.4 0.6 0.8 1


Figure 5, Evolution of the experimental 001 reflections with water relative pressure for Mg-, Ca-, Sr- and Ba-montmorillonites.

Vol. 45, No. 1, 1997 Water vapor adsorption-desorption by divalent homoionic montmorillonites

Table 4. Parameters used for the simulation of XRD data.

17

Nature o f the exchangeable cation

States M g 2§ Ca 2+ Sr 2§ Ba 2§

Initial state M from apparent 7 8-10 7 8 d00~t

M from Scherrer for- Dry state inhomoge- 8 6-7 7 mula neous

d00~ (-~)$ 10.0 9.55 9.64 9.85

One-layer hydrate M from apparent d0ol 12 8-10 7 8 state 0.05 < P/P0 < 0.6

doo~ (.&) 12.1 12.07 to 12.50 12.1 to 12.25 12.0 to 12.3

Two-layer hydrate M from apparent d00~ 18 8-10 15 8 state P/P0 > 0.6 P/P0 = 0.92

d0ol (/~) substate a 14.0 15.15 15.26 15.9 to 16.1 d001 (,~) substate b 14.75 15.6 15.79 - - d001 (,~) substate c 15.9 - - - - - -

t M is the mean number of layers per quasi-crystal. $ d00~ (,~) is the basal spacing for the considered hydrate state.

cation by a divalent one (Stul and Van Leemput 1982). Textural characteristics of dried materials seem diffi- cult to general ize and are strongly dependent on the starting material (nature o f exchangeable cation, texture in the natural occurrence, method used to prepare homoionic saturated samples) and on the drying conditions.

More consistent results have been reported f rom the study of gels or suspensions. Stacks o f 3 to 6 layers for Mg-, 3 to 7 for Ca- and 6 for Ba-montmori l loni te have been described by Schramm and Kwak (1982) in diluted clay suspensions. Particles o f 6 platelets with 3 monolayers o f water molecules be tween adjacent platelets were described by Pons et al. (1982) for a Ca-montmori l loni te .

The size distributions o f dilute bentonite suspensions as a function of the nature of the saturating cation, measured using centrifugal photosedimentat ion, indicate the existence of preferred size ranges for quasi-crystals. The Stokes ' diameters, which mainly represent in these condit ions the basal extension of the particles, are approximately 300 nm for Ca- and Mg- montmori l loni te , and 40 nm for K- and Na-montmoril lonite (Whal ley and Mull ins 1991).

Results obtained in the present work using ni trogen adsorption volumetry , X R D and immers ion microca- lor imetry indicate a splitting o f initial dried particles into smaller ones during hydration o f Ca-, Sr- and Ba- montmori l loni te (Tables 3 and 4). F rom nitrogen adsorption measurements and X R D patterns, it is clear that Mg-montmor i l lon i te is still hydrated after heating to 100 ~ under a pressure o f 1-2 Pa. In the first step of hydration, an increase of the order of the stacks is observed with X R D ; for example, the ratio signal upon background of Ca-montmori l loni te increases with water relat ive pressures, and the broadening of reflections

for Mg- and Sr-montmori l loni te decreases when the 2-layer hydrate state is formed.

The number o f layers per stack determined by modeling of the X-ray patterns in the 2-layer state agrees with the mean values obtained f rom the Harkins and Jura procedure. This means that, except for the Ba form, the interferential coherence domain corresponds to the final thickness of the particles. An evolut ion o f initial particles formed of more-or-less parallel layers to better-organized (and smaller) stacked quasi-crystals is then expected for montmori l loni tes saturated by divalent cations. This evolut ion is different f rom the decrease of particle size by splitting proposed for Li- and Na-montmori l loni te (B6rend et al. 1995; Cases et al. 1992).

Adsorpt ion of Water Vapor on the External Surfaces o f Quasi-crystals

The total amount of water, Q, adsorbed onto montmori l lonite for a g iven relat ive pressure can be eval- uated by adding the adsorbed amount, Qads, and the initial water content after heating to 100 ~ determined by thermal analysis (Table 2). Part of this water is adsorbed on the external surface, which may be defined as the cumulated surface of pores larger than the critical slit-width of condensation. The sharp increase of water adsorbed for relat ive pressures higher than 0.9 corresponds to the condensat ion in interparticular macroporosity. The remaining water is adsorbed in the interlamellar space where water molecules are bound to exchangeable cations, or to water molecules of the cation hydration shells, or where they form weak hy- drogen bonds with clay surface oxygen atoms. The properties o f the system depend on the proportions o f the different kinds of sites. Several methods o f esti- mating the ratio o f the amount of water adsorbed on

i

lOO;

~ 80-

~ 60. 0

20.

i

ol ~ - ~ 7 , ,~ . ~ :"~.. , 0 0.2 o.4 0.6 0.8 1

M g

a T / / p [] ~ - e ,J" 3O

.... o /

X �9 ~, I x ; '~ '

~o \ / I~ll l k .: 71 I.I \ / ?'- " :~ i "

tO ; "

' x , "

,: i ' , g ~

,, .~ '0

X ," ", o ! ~, ~, ~" ~ = =

0 0,2 0.4 0.6 0.8


0.2 0.4 0.6 0.8 1

Relative Pressure

S r

,,/'/ , /

Relative P r e s s u r e

C a 100 q

/

A 8o- . .f ~ 6 o

/ ,

2o ,

0S --O---1S --4:]-- 2S cum - - I - - 2 S a " " ~ ' 2Sb - - 4 - - 2 S c - - / k - - 3S

0 0.2 0,4 0.6 0.8 1

Relative Pressure

loo B a

80 A

~ 6 o

~ 40

~ 2o


Figure 6. Evolution of the different hydrate states (0, l and 2) with relative pressure during water vapor adsorption for Mg-, Ca-, Sr- and Ba-montmorillonites.

88~ T/ ", i"~ r

l i / /' I : ",,,,,,,, #'

~ 60 i ; �9 .

7 i ,' ", .o,t i

0 0.2 0.4 0.6 0.8 1


100 -

,..,80 q

m 60 -

,~ 40 -

20~

0 [ 0

-" 0S ----o--- 1S ----13-- 2S cure - - I - - 2 S a --<~'" 2Sb - � 9 + 3 S

S r

X % ~ ' I ~J~ ,' # % ,"

,' \

I S , "~ ', ~o ~- 0.2 0.4 0.6 0.8


100

A 8O

I

60

~ ~~

20

100

A 80

60

o~

40

20

0 0.2 0.4 0.6 0.8 1


Ca

J l/',,, / %~ 2"

%,

,f %~1

..i w I .~ j I k.J I n

0 0.2 0.4 0.6 0.8


Ba


Figure 7. Evolution of the different hydrate states (0, 1 and 2) with relative pressure during water desorption for Mg-, Ca-, Sr- and Ba-montmorillonites.


Table 5. Evolution of the filling up of the interlayer water versus mean interlamellar swelling during adsorption of water vapor on Mg-, Ca-, Sr- and Ba-montrnorillonites.

Nature of Adsorpt ion: water vapor relative pressure montmori l -

lonite 0 0,04 0.05 0.08 0 A 0 0.20 0.25 0,30 0.40 0.50 0.55 0.60 0.70 0.80 0.92

Mg-Mont. Interlamellar swelling (%)

% of Filling

Ca-Mont. Interlamellar swelling (%)

% of Filling

Sr-Mont. Interlamellar swelling (%)

% of Filling

Ba-Mont. Interlamellar swelling (%)

% of Filling

30 110 170 180 200 200 200 200 200 200 80 52 42 47 54 60 67 73 79 84

28 53 130 175 200 200 133 121 82 86 88 98

0 24 34 49 70 100 130 180 200 0 49 68 87 86 90 86 78 84

50 50 55 70 100 140 220 29 46 80 88 78 93 79

external surfaces of quasi-crystals (including capillary condensation) to the amount adsorbed in the interlamellar space have been proposed in the literature (Sposito and Prost 1982). When capillary condensation can be neglected, the amount of external adsorbed water, Qext, (mol g-l) can be calculated by taking into account the external surface area, Sext, and the surface coverage 0 of a nonporous adsorbent with the same energy of adsorption (Hagymassy et al. 1969):

Qext = 0Sext/NA ~ [1]

where N A is the Avogadro number and tr the cross- sectional area of a water molecule adsorbed onto the external surface, taken equal to 14.8 ,~2 (Hagymassy et al. 1969).

The external surface areas of divalent homoionic motmorillonites in the hydrated state are equal to or lower than the surface determined from immersion enthalpy data (Table 3). The value for 0 was deduced from the reference isotherm presented by Hagymassy et al. (1969) corresponding to C constant values of the BET equation higher than 50. This value could be expected from the high energy of immersion of montmorillonites.

In the case of Ca- or Sr- montmorillonite, where the external surface area is equal to 63 m 2 g-~, Q~xt is about 1 mmol g-~ for P/Po = 0.3 and 2.5 mmol g-~ for P/Po = 0.9. Therefore, for a relative pressure of 0.30, water adsorbed onto the external surface represents approximately 17% and 25% of the total amount adsorbed for Ca-montmorillonite and Sr-montmorillonite, respectively. In the undersaturation domain, water is mainly located in the interlamellar space of divalent montmorillonites. The ratio of external water to internal water is less than that for monovalent montmorillonites (B6rend et al. 1995).

Filling of the Interlamellar Space

The irregular shapes of both the adsorption and desorption branches of water isotherms (Figure 1) and

the evolution of basal spacings (Figure 5) with water relative pressure suggest a complex water adsorption mechanism, due to the fact that water adsorbs on the external faces of quasi-crystals and in the interlamellar space. For example, particular evolution of the amount of interlamellar water with relative pressure is expected from the difference in shape between adsorption isotherms (the evolution is more or less continuous, Figure 1) and the corresponding variations of the basal spacing (a tendency to a step-by-step increase, Figure 5). This type of evolution reveals the formation of the hydration shell of the cation, followed by the filling of the remaining intedamellar space. It can be quan-

tified using the filling ratio Qinr/Qint th (Cases et al. 1992; B6rend et al. 1995). The experimental amount of water, Qi,t, present in the internal space, Qint = Q

- Qext, is compared to the amount, Qint th, required to form an homogeneous monolayer or bilayer in the interlamellar space. Referring to the model of Ben Bra- him et al. (1986) for Na-beidellite, the area occupied by the water molecule in the l- layer hydrate is 7.8 AZ, and that in the 2-layer hydrate is 8.67 ~2. This last value is also considered, if necessary, for the 3-layer hydrate. The comparison takes into account the extent of internal surface area and the proportion of hydrated interlamellar spaces determined by simulation of XRD patterns.

For the different samples investigated, the evolution of the filling of the inteflamellar space as a function of the mean proportion of interlamellar swelling (defined as the sum of the proportions of hydrated interlamellar space balanced by the number of layers: homogeneous l-layer state = 100% and 2-layer -- 200%) is presented in Table 5 for adsorption and Table 6 for desorption, respectively. Only values corresponding to P/Po <-- 0.8 (that is, to X-ray patterns allowing an ac- curate modeling) have been reported. The filling near saturation is approximately 80-95%, rather compara- ble to Na-beidellite hydrate, but twofold less for lower

VoL 45. No. 1, 1997 Water vapor adsorption-desorption by divalent homoionic montmorillonites 21

Table 6. Evolution of the filling up of the interlayer water versus mean interlamellar swelling during desorption of water from Mg-, Ca-, Sr- and Ba-montmorillonites.

Nature of Desorptaon: water vapor relative pressure montmori l -

lonite 0 .80 0.75 0.60 0.58 0.46 0.40 0.35 0.30 0.26 0.20 0.15 0.13 0.10 0.05 0.04 0

Mg-Mont. lnterlamellar swelling (%) 200 200

% of Filling 84 75

Ca-Mont. Interlamellar swelling (%) 200

% o f Filling 96

Sr-MonL Interlarnellar swelling (%) 200

% of Filling 83

Ba-Mont. Interlamellar swelling (%) 150

% of Filling 110

200 200 195 190 100 64 48 40 31 26

190 180 78 73

186 112 107 103 0 82 94 85 62 0

130 95 90 20 63 53 39 40

105 100 90 85 80 0 105 87 75 58 38 0

relative pressures, as already not iced for monovalent cations (Cases et al. 1992: B t r e n d et al. 1995).

The predominant l - layer hydrate state of Ba-montmori l lonite during adsorption and of Ca-montmori l lonite during desorption also corresponds to a sharp evolution of the filling. During adsorption, filling versus mean interlamellar swell ing increases more or less gradually until it reaches a maximum. The mean number o f water molecules per cation required to observe a 2-layer hydrate state is about 10 _+ 1 for Mg-, 20 --- 3 for Ca-, 16 --- 2 for Sr- and 25 --- 2 for Ba-montmoril lonite. Results presented in the literature show that the gap be tween Mg- and Ca-clays may be smaller for other materials or other pretreatment condit ions (Tarasevitch and Ovcharenko 1975).

A value of 1 is often used for the mean density o f adsorbed water, and leads to l ikely results (Ormerod and N e w m a n 1983). The variat ion o f water interlayer filling and the successive formation o f 2 or 3 homogeneous states (Mg- and Ca-montmori l loni te , for in- stance) for a g iven hydrate show the complexi ty o f the interlamellar water structure hidden behind this approximation.

C O N C L U S I O N S

The study of the hydration o f a Wyoming montmori l lonite saturated by alkaline-earth cations at 30 ~ shows a gradual variat ion of water adsorption and swelling, depending on the solvation energy of the cation. Al though quantitative results are strongly dependent on the nature of the samples and preparations used, some general trends are noticeable:

1) During the first hydration stage, Ca-, Sr-and Ba- montmori l loni tes exhibit a splitting perpendicular to z of the initial dried quasi-crystals into smaller ones.

2) The hydration of homoionic montmori l loni tes corresponds to an increase in the order in the stacks, including the formation of the 2-layer hydrate state.

3) The classical succession 0-1ayer-l-layer-2-1ayer hydrates is wel l -observed during desorption. During adsorption, the l - layer hydrate is replaced by an interstratified 0-1ayer/1-1ayer/2-1ayer hydrate observed at low relative pressure for all the montmori l loni tes . The formation o f a 2-layer state f rom the 0-layer state part- ly occurs. For Mg- and Ca- montmori l loni te , the homogeneous 2-layer hydrate predominates over an ex- tensive range of relat ive humidity,

4) Water is mainly located in the interlamellar space. The filling near saturation is approximately 8 0 - 95%, rather similar to the Na-beidel l i te water vapor system.

5) The complet ion of the interlayer water is gradual for the larger cations due to their lower solvation energy and larger basal spacings. Montmori l loni tes with smaller divalent cations form a 2-layer hydrate at lower water content.

6) During desorption, the interlamellar space emp- ties but the basal spacing remains nearly constant because exchangeable cations keep their first solvation shell over a large domain of relat ive pressures. The lower is the final mean interlarnellar swell ing; the higher is the size o f the exchangeable cation.

7) A given hydrated state o f rnontmoril lonite may correspond to a few structural organizations o f the interlayer water, as is the case for better crystal l ized smectites (Suquet and P6zerat 1987; Pons et al. 1989). The 2-layer hydrate of Mg-montmor i l lon i te exhibits a set of 3 basal spacing values corresponding to 3 substates. In the case o f Ca- and Sr- montmori l loni te , 2 substates were determined.

A C K N O W L E D G M E N T S

The authors thank the French Ministry of Research and Technology (Phygys program), the PIRSEM-ARTEP program and the CNRS (Department SDU) for support of this work. They also thank G. Besson (Laboratoire de Mintralurgie et de Cristallographie-Orl6ans), Y. Grillet (Centre de Thermo-


dynamique et de Microcalorim6trie du CNRS - Marseille) and E Lhote (Centre de Recherches P6trographiques et G6ochi- miques-CNRS-Vandoeuvre) for their help and advice during the course o f this study.

R E F E R E N C E S

Ben Brah im J, Armagan N, Besson G, Tchoubar C. 1986. M6thode diffractom6trique de caract6risation des 6tats d 'hydrata t ion des smecti tes et stabilit6 relative des couches d' eau ins6r6es. Clay Miner 21: l 11 - 124.

Ben Ohoud M, Van D a m m e H. 1990. La texture fractale des argiles gonflantes. CR Acad Sci Paris, Set II 311:665-670.

Ben Rha iem H, Tessier D, Pons CH. 1986. Compor tement hydrique et 6volution structurale et texturale des montmorillonites au cours d ' u n cycle de dessicat ion-humectat ion: I. Cas des montmorillonites calciques. Clay Miner 21:9-29.

B6rend I. 1991. Les m6canismes d 'hydratat ion de montmorillonites homoiniques pour des pressions relatives inf6rieu- res h 0,95 [th6se de doctorat d 'universit6]. Nancy, France: Institut National Polytechnique de Lorraine. 330 p.

B6rend I, Cases JM, Frangois M, Uriot JP, Michot L, Mas ion A, Thomas E 1995. M e c h a n i s m of adsorption and desorption o f water vapor by homoionic montmori l loni tes : 2. The Li +, Na +, K*, Rb + and the Cs + exchanged form. Clays Clay Miner 43:324-336.

Cases JM, Francois M. 1982. Etude des propri6t6s de l ' eau au vois inage des interfaces. Agronomie 2:931-938.

Cases JM, B6rend I, Besson G, Francois M, Uriot JP, Thomas E and Poirier JR 1992. Mechan i sm of adsorption-desorption o f water vapor by homoionic montmoril lonite. 1. The sod ium exchanged form. Langmui r 8 :2730-2739.

de Boer JH, Lippens BC, Linsen BG, Broeldaolf JCP, Van der Heuvel A, Osinga T. 1966. The t -curve o f mult imolecular N z adsorption. J Colloid Interface Sci. 21:405-414.

Delon JF, Lietard O, Cases JM, Yvon J. 1986. Determinat ion of porosity o f platy material using sli t-shaped pores and bevelled pores. Clay Miner 26:361-376.

Del Pennino U, Mazzega E, Valeri S, Alietti A, Brigatti ME Poppi L. 1981. Interlayer, water and swelling properties o f monoionic montmoril lonites. J Colloid Interface Sci 84: 301-309.

Drits VA, Tchoubar C. 1990. X-ray diffraction by disordered lamellar structures. Berlin: Springer-Verlag. 371 p.

Emmet t PH, Brunauer J. 1937. The use of low temperature van der Waals adsorption isotherms in determinating the surface area by ion synthetic ammon i a catalyst. J Amer Chern Soc 59:1553-1564.

Glaeser R, M6ring J. 1968. Domaines d'hydratation homog~ne des smectites. CR Acad Sci Paris, Ser D 267:463-466.

Grillet Y, Cases JM, Francois M, Rouquerol J, Poirier JE. 1988. Modification o f the porous structure and surface area of sepiolite under v a c u u m thermal treatment. Clays Clay Miner 36:233-242.

H a g y m a s s y J, Brunauer S, Mikhail RS. 1969. Pore structure analysis by water vapor adsorption: I. t -curves for water vapor. J Colloid Interface Sci 29:485-491.

Harkins WD. 1952. The physical chemis t ry of surface film. New York: Reinhold. 411 p.

Iwasaki T, Watanabe T. 1988. Distribution of Ca and Na ions in dioctahedral srnectites and interstratified dioctahedral mica-smecti tes . Clays Clay Miner 36:73-82.

Kamel MW. 1981. Etude de l ' imbibition, du gonflement et du dessbchement de quelques argiles [th~se de doctorat d 'uni- versit6]. Toulouse, France: Universit6 de Toulouse. 187 p.

Kawano M, Tomita K. 1991. X-ray powder reflection studies on the rehydration properties of beidellite. Clays Clay Min- er 39:77-83.

Ke rm AG. 1988. Etude et caract6risation des premiers stades d 'hydrata t ion d ' une nontronite [th~se de doctorat d 'un iv- ersit6]. Orl6ans, France: Universit6 d 'Orl6ans. 56 p.

M a m y J. 1968. Recherches sur l 'hydratat ion de la montmorillonite: Propri6t~s di61ectriques et structure du film d 'eau. Ann Agron 19(2-3):175-292.

Michot L, Franqois M, Cases JM. 1990. Cont inuous volu- metric procedure for gas adsorption: A means to study surface heterogeneity. Langmui r 6 :677-681.

Moore DM, Hoover J. 1986. Ordered interstratification o f dehydrated and hydrated Na-smecti tes . Clays Clay Miner 34:379-384.

N e w m a n ACD. 1987. The interaction of clays with clay mineral surfaces. In N e w m a n ACD, editor. Chemis t ry o f clays and clay minerals. Bi rmingham, AL: Longman. p 237-274.

Ormerod EC, N e w m a n ACD. 1983. Water sorption on Ca- saturated clays: II. Internal and external surfaces o f montmorillonite. Clay Miner 18:289-299.

P6zerat H, M6ring J. 1967. Recherches sur la posit ion des cations 6changeables et de l ' eau dans les montmoril lonites. CR. Acad Sci Paris, Ser D 265:529-532.

Poins ignon C, Cases JM. 1978. Etude de l ' eau d 'hydratat ion des cations compensateurs de smecti tes homoioniques. Bull Mineral 101:469-477.

Poirier JE, Franqois M, Cases JM, Rouquerol J. 1987. Study of water adsorption on Na-montmoril lonite: New data ow- ing to the use of a cont inuous procedure. In: Liapis AI, editor. Proc 2nd Eng Foundat ion Conference on fundamen- tals of adsorption. New York: AIChE. p 473-482 .

Pons CH, Rousseaux F, Tchoubar D. 1981. Utilisation du rayonnernent synchrotron en diffusion aux petits angles pour l '6 tude du gonf lement des smectites. I: Etude du sys- t~me eau-montmori l loni te-Na en fonction de la temp6ra- ture. Clay Miner 16:23-42.

Pons CH, Rousseaux E Tchoubar D. 1982. Util isation du rayonnement synchrotron en diffusion aux petits angles pour l '6 tude du gonf lement des smectites. II: Etude de dif- f6rents syst~mes eau-smecti tes en fonction de la temp6ra- ture. Clay Miner 17:327-338.

Pons CH, Pozzuoli A, Rausse l -Colom JA, De la Calle C. 1989. M6canisme de passage de l '6tat hydrate a une couche ~t l '~tat 'z6ro couche ' d ' une vermiculi te-Li de Santa- Olalla. Clay Miner 24 :479-493 .

Sch ramm LL, Kwak JCT. 1982. Influence of exchangeable cation composition on the size and shape of montanorillonite particles in dilute suspension. Clays Clay Miner 30 :40-48 .

Sposito G, Prost R. 1982. Structure o f water adsorbed on smectites. C h e m Rev 82:554-573.

Stul MS and Van Leemput L. 1982. The texture o f montmorillonites as influenced by the exchangeable inorganic cation and the drying method. I. External surface area related to the stacking units of the aggregates. Surface Tech- nol 16:89-100.

Suquet H, De la Calle C, P6zerat H. 1977. Structure de la saponite de Kozakbv hydrat6e ~ 2 couches et satur6e avec les cations Na, Ca et Mg. CR Acad Sci Paris, Ser D 284: 1489-1491.

Suquet H, P6zerat H. 1987. Parameters influencing layer stacking types in saponite and vermiculite: A review. Clays Clay Miner 35:353-362.

Tarasevitch JI, Ovcharenko FD. 1975. Adsorpt ion sur des min6raux argileux. Kiev, Russia: Naukova Dumka. 351 p.

Whal ley WR, Mull ins CE. 1991. Effect o f saturating cation on tactoid size distribution in bentonite suspensions. Clay Miner 26:11-17.

Yvon J, Baudracco J, Cases JM, Weiss J. 1990. E16ments de min6ralogie quantitative en microanalyse des argiles. In: Decarreau A, editor. Materiaux argileux, structure, propri6- t6s et applications. Paris: SFMCGFA. p 473-488 . (Received 1 March 1995; accepted 22 February 1996; Ms.

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