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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


    )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 con- trolled 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 in- creasing 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 ex- changeable cations o f the dry clay. F rom the desorp- tion cycles of sorption isotherms, Na-montmori l loni te is known to form a l - layer hydrate and Ca-montmo- rillonite 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 lead- ing 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 predomi- nant 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 con- tent corresponds mainly to mult i layer adsorption and capil lary condensation.

    In water-saturated materials, large differences be- tween 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 va- por 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 inter- layer spacing decreases with increasing intensity of the initial drying (Cases et al. 1992). In Ca-montmori l lon- ite, stacks o f 8 to 11 layers (number o f layers increas- ing with pressure) separated by about 3 to 4 layers o f water form the walls o f a network o f large pores sat- urated with water (Ben Rha iem et al. 1986).

  • Vol. 45, No9 1, 1997 Water vapor adsorption-desorption by

    Table 1. Nature of the Cation Exchange Capacity after ex- change.

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

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

    CEC obtained from lan- thane chloride ex- change (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 09

    % 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 ad- sorbed 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 satu- rated 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 con- figuration. 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 asso- ciated 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 lay- ers.

    Much is known about the adsorption and desorption mechanisms of water molecules on homoion ic mont- moril 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 mono- valent cations Li +, Na +, K +, Rb + and Cs + on mont- mori 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 meth- od 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 hydra- tion 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


    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, cen- tr ifuged and Na-exchanged. Homoion ic Mg-, Ca-, Sr- and Ba-montmori l loni tes were prepared using the fol- lowing 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 ma- terial 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 treat- ment,


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