Determination of Surface Free Energy of Kaolinite

-The adsorption of n-octane and water vapor on natural kaolinite was measured. From adsorption isotherms film pressures were determined which were then used to calculate the dispersion and nondispersion components of the kaolinite surface free energy. In addition, thermodesorption of water from the kaolinite surface was determined. These results suggest that physically adsorbed water remained on the kaolinite surface, even at temperatures as great as 125"(2. Therefore, experimentally determined dispersion and nondispersion components appear to relate to the surface precovered with a film of water. These values are: ~gw) = 34.4 mJ/m 2 for dispersion interactions and ~gw) = 60.2 mJ/m 2 for nondispersion interactions. Assuming a kaolinite surface precovered with a film of water, which decreased the free energy by the work of spreading, the following components of the energy for the bare surface were calculated: ~s = 67.6 mJ/m 2 and 3'~ = 103.4 mJ/m 2, for dispersion and nondispersion components, respectively. Key Words--Adsorption, Film pressure, Free energy, Kaolinite, n-Octane, Surface, Water. Abstrakt --Zmierzono adsorpcje pary wodnej i n-oktanu na naturalnymkaolinicie. Z uzyskanych izoterm adsorpcji wyznaczono cignienia film6w i nastgpnie skladow~ dyspersyjn~ i niedyspersyjn~ swobodnej energii powierzchniowej kaolinitu. Pr6cz tego, zbadano tak2e termodesorpcj~ wody z powierzchni kaolinitu. Z tych do~wiadczefi wynika, ~e woda zaadsorbowana fizycznie pozostaje na powierzchni kaolinitu nawet do 125~ Dlatego, sugeruje si~, ~e wyznaczona do~wiadczalnie skladowa dyspersyjna i niedyspersyjna dotyczy powierzchni pokrytej filmem wody. Wyznaczone warto~ci wynosz~: ~gw) = 34.4 mJ/m 2 dla oddzialywafi dyspersyjnych i "Y~w) = 60.2 rnJ/m 2 dla oddziatywafi niedyspersyjnych. Przyjmuj0c model, w kt6rym film wody na powierzchni kaolinitu obni~a jego swobodm~ energi~ o pracg rozptywania, wyliczono skfadowe tej energii dla czystej powierzchni kaolinitu. Sk~adowe te wynosz~: ~ = 67.6 rnJ/m 2 i "r~ = 103.4 mJ/m 2. I N T R O D U C T I O N Despite the fact that clays have been studied for many years, several important questions concerning their adsorption properties remain, especially with respect to water and the structure of adsorbed water (Low, 1979, 1982; Jackson et al., 1971; Jurinak, 1961; Murray and Quirk, 1980; Siracusa and Sumasandaran, 1986; MuUa and Low, 1983; Oliphant and Low, 1983). In addition to adsorption properties, wetting processes (Dekany et al., 1983; Szfint6 et al., 1986; Dekany et al., 1986), swelling properties (Low, 1987; Viani et al., 1983, 1985; Lagaly et al., 1983; Oliphant and Low, 1982), and electrokinetic properties are also of considerable interest (Delgado et al., 1986a, 1986b). At least four different, nonspecific mechanisms of water adsorption can be distinguished; i.e., hydrogen bond formation, hydration of surface cations, permanent dipoles of the surface-water dipoles interaction, and London dispersion forces interactions are present in all systems (Low, 1961; Jackson et al., 1971; SwartzenAllen and Matijevi6, 1974). The above mechanisms of water interaction account for the amount of adsorbed water and reflect the surface free energy components of the particular mineral (Fowkes, 1964, 1968; Girifalco and Good, 1957; TaCopyright 9 1988, The Clay Minerals Society mai et al., 1967; Schultz et al., 1977a, 1977b). The determination of these components is a problem from both the experimental and theoretical points of view (Zetflemoyer, 1969; Mittal, 1976; Wu, 1978; Boruvka et al., 1985). Several approaches can be used (Zettlemoyer, 1969; Wu, 1980), but no direct method for the energy determination exists. One method relies on the determination of the adsorption isotherm of a liquid or vapor. From this isotherm the film pressure, 7r, can be calculated. Based on the r value, the surface free energy component of the mineral can be estimated (Zettlemoyer, 1969; Jaficzuk et al., 1983; Chibowski and Ho/ysz, 1986; Staszczuk, 1984). This approach seems to be less uncertain for determining the dispersion component from the adsorption isotherms ofa nonpolar liquid, e.g., n-alkanes, in which, for all practical purposes, only dispersion interactions are present. Once the dispersion component is determined using a polar liquid (water or alcohol), "averaged" nondispersion interactions (dipole-dipole, hydrogen bond) can be estimated. The interpretation of the calculated component, however, is less unambigous than that for the dispersion component. A problem also exists concerning which method is used to determine the specific surface of clay minerals (SwartzenAllen and Matijevi6, 1974). The specific surface of a