Vibrational Probe Studies of Water Interactions with Montmorillonite

-Imeraction of water with montmorillonite exchanged with Na +, K +, Co 2+, and Cu 2+ cations as a function of water content was examined using an FTIR/gravimetric cell designed to collect spectroscopic and sorption data simultaneously. Correlation of water desorption isotherms with infrared spectra of the clay-water complex showed that the position of the HOH bending band of water decreased as a function of water content. The largest decreases in frequency were observed for Cu 2+ and Co 2 +; smaller decreases were found for Na + and K + . In addition, the molar absorptivity of sorbed water increased upon decreasing the water content. The decrease in frequency and the concomitant increase in molar absorptivity were attributed to polarization effects on the sorbed water molecules by exchangeable cations. The interference fringes of a self supporting clay film permitted d-spacings to be determined optically and, therefore, changes in frequency, molar absorptivity, and water sorption behavior to be related directly to changes in interlayer spacing. The d-spacings obtained from the interference fringes were consistently larger by approximately 0.5 ~ than those determined using powder XRD. Key Words-Cation exchange, Desorption isotherm, FTIR, Gravimetric, Montmorillonite, Physisorption, Water I N T R O D U C T I O N Since the pioneering work of Mooney et al. (1952a, 1952b), the behavior of water molecules on smectite surfaces has continued to attract considerable attention. When less than three molecular layers of water are present in the interlamellar region, the structure of sorbed water on smectites has been shown to be influenced predominantly by exchangeable cations (Mooney et al., 1952b; Clementz et al., 1973, 1974; Prost, 1975; McBride, 1982; Sposito and Prost, 1982; Sposito et al., 1983; Bidadi et al., 1988; Grandjean and Laszlo, 1989; Kogelbauer et al., 1989; Hall and Astill, 1989; Trillo et aL, 1990; Fu et aL, 1990; Delville et aL, 1991; Tinet et al., 1992). Spectroscopic investigations of smectite-water interactions (Prost, 1975; Sposito and Prost, 1982; Brown and Kevan, 1988; Weiss et al., 1990a; Bank et al., 1991; Tinet et al., 1992) suggest that two distinct environments of sorbed water are present: 1) water molecules coordinated directly to exchangeable metal cations, and 2) physisorbed water molecules occupying interstitial pores, interlameUar spaces between exchangeable metal cations, or polar sites on external surfaces. On the basis of NMR and ESR (McBride and Mortland, 1974; McBride et aL, 1975; McBride, 1982; Brown and Kevan, 1988; Grandjean and Laszlo, 1989; Kogelbauer et aL, 1989; Delville et al., 1991), complex dielectric permittivity and neutron scattering data (Sposito and Prost, 1982), and other dielectric measurements (Fripiat et al., 1965; Bidadi et al., 1988), water molecules coordinated to metal cations on a clay mineral surface are considered to be in less mobile, Copyright 9 1992, The Clay Minerals Society restricted environments relative m those in bulk water. This molecular picture of coordinated water in the interlamellar region is consistent with vibrational spectroscopic studies of water sorbed on smectites (Farmer, 1978), Russell and Farmer (1964), for example, found that the OH stretching and bending bands in the infrared (IR) spectra of water sorbed on montmori l loni te and saponite were perturbed on going from higher to lower hydration states of these smectites. In the case of the HOH bending band (v2 mode) in the 1610 to 1640 cm -~ region, the frequency decreased and the molar absorptivity increased upon dehydration of the clay mineral. A similar result was reported by Poinsignori et al. (1978), who observed that the molar absorptivities of the OH stretching and bending bands in the IR spectrum of sorbed water depended strongly on the hydration energy of the exchangeable metal cation and on both the water content and surface charge density of the clay mineral. In the present study, the use of the v2 band of sorbed water is examined as a molecular probe of smectitewater interactions by establishing a relationship between vibrational spectroscopic data and 1) the surface coverage of water and 2) the observed basal spacing. Although previous IR studies of water-smectite systems (Russell and Farmer, 1964; Poinsignon et al., 1978; Farmer, 1978) have shown that water coordinated to exchangeable metal cations is distinct from physisorbed or bulk water, the correlation of IR data with the amount of water sorbed has not been determined. Moreover, it is not known how changes in the basal spacing of a smectite influence the vibrational properties of sorbed water molecules. Recently, Fu et