Adsorption Behavior of the Cetyltrimethylammonium Chloride Reverse Micelles on Porous Silica Gels

Abstract

The adsorption behavior of reverse micelles of cetyltrimethylammonium chloride (CTAC) onto porous silica gels of various pore sizes has been investigated. A decrease in the amount of CTAC in the reversed micellar bulk solution in chloroform/water added to the silica gel in a column was observed, indicating the CTAC adsorption onto the silica surface. The adsorption profile obtained showed a maximum of the adsorbed surfactant amount, where the point of surface saturation may be reached, beyond which the adsorption amount remained unchanged. The adsorption of CTAC was also dependent on the specific surface area of the silica gel used. These results suggest that a CTAC monolayer may be formed and completed on the silica surface. The adsorbed CTAC amount slightly decreased with increasing the water to surfactant molar ratio, R, in the reversed micellar bulk solution, although at R < 2 the CTAC adsorption remained constant with silica gels having larger pore diameters. Even in the presence of traces of water moisture in the bulk solution used for the CTAC adsorption, the adsorption of water on silica gel was also observed and the molar ratios, R_Γ, of the water to surfactant adsorbed on different silica gels were almost unity, implying that the silica gel may tend to hold some of water molecules originally adsorbed on its surface. The adsorption of CTAC molecules appears to occur through the adsorbed water molecules, probably polarized at the silica surface, and a CTAC-water-silica nanointerface seems to be formed. The adsorbed water amount or R_Γ also increased with an increase in R and on silica gels with larger pore diameters and a plateau was observed in the R range of 3 - 4, beyond which an increase in R_Γ occurred again. The water molecules present in the vicinity of the adsorbed CTAC and the silica surface may possess different properties at different R and R_Γ values.