Application of inverse gas chromatography to the study of the surface properties of slates

Inverse gas chromatography (IGC) at zero surface coverage was used for determining the surface characteristics of a slate sample. Previously this sample had been characterized by means of X-ray diffraction (XRD), chemical analysis, infrared (IR) spectroscopy and nitrogen adsorption. Quartz, muscovite and chlorite were the minerals foming the slate. A well-defined mesoporous structure with pores of 15 Angstrom of interlayer distance was observed. Chemical analysis and IR spectroscopy corroborated the X-ray results. The surface characteristics of the slate were defined in relation to nonspecific and specific interactions with organic molecules. In accordance with the nonspecific interactions, the London component (gamma(S)(D)) of the surface free energy gave an estimation of the surface energy of the slate sample. The obtained values for gamma(S)(D) were 140.0, 124.8, 108.2 and 96.8 mN m(-1) at 100, 110, 120 and 130 degrees C, respectively. These values were characteristic of an inorganic sample of high surface energy. The values of the thermodynamic variables - differential heat of adsorption, free energy and entropy of adsorption - were in accordance with these results. At the same time, specific interactions were characterized by the epsilon(pi) parameter and the acid-base (K-A, K-B) indices. The positive value of epsilon(pi) (0.09 kJ mol(-1)) indicated the presence of Lewis acidic sites on the slate surface. The value of K-A (acid index) of 1.17 was higher than the value of K-B (base index) of 0.37; this result confirmed the acidic nature of the studied slate's surface.