SURFACE HETEROGENEITY OF TRIMETHYLPHENYLAMMONIUM-SMECTITE AS REVEALEDBY ADSORPTION OF AROMATIC-HYDROCARBONS FROM WATER

Adsorption studies of aromatic hydrocarbons of various molecular sizes on organoclays in aqueous solution were carried out for characterizin g the surface heterogeneity of organo-clays. Benzene, toluene, p-xylen e, ethylbenzene and n-propylbenzene adsorption by a smectite with 5 di fferent exchange degrees of trimethylphenylammonium (TMPA) cations for Ca2+ was measured. The Langmuir isotherm equation did not adequately describe the experimental data, especially for small molecules, wherea s the Dubinin-Radushkevich (DR) equation combined with a gamma-type ad sorption energy distribution function described all experimental data well, suggesting the surface and structural heterogeneity of TMPA-smec tites. The calculated adsorption energy distributions indicated that t he apparent heterogeneity depends on the molecular size of adsorbates. Small adsorbate molecules such as benzene explore a highly heterogene ous surface of TMPA-smectites while large molecules such as n-propylbe nzene detect a relatively homogeneous surface. The surface fractal dim ension was dependent on the extent of TMPA exchange for Ca2+. When TMP A content is less than 75% of the cation exchange capacity (CEC) of th e smectite, the heterogeneity decreases as TMPA content increases; it increases with TMPA content thereafter: These results are related to t he size distribution of micropores in TMPA-smectites, which are define d by the 2 semi-infinite aluminosilicate sheets and the interlayer cat ions. The micropore size distributions and, hence, heterogeneity are c reated in part by the inhomogeneity of the charge density of clay surf aces and the tendency for cation segregation in these systems.