Selective adsorption of methyl esters of n-fatty acids at the silica/benzene and silica/carbon tetrachloride interface. Part 1.—Adsorption isotherms

Abstract

The adsorption isotherms of a series of n-fatty acid methyl esters at the silica/benzene and silica/carbon tetrachloride interfaces have been determined. For the benzene solutions both the selectivity and the limiting adsorption decrease with increasing chain length of the ester. Based on an analysis of literature data on the enthalpies and entropies of mixing of hydrocarbons in benzene, it is suggested that for the benzene solutions the adsorption limit is set by the presence at the interface of an adsorbed solvated layer. The limiting adsorption from benzene at 25°C corresponds, in terms of the interfacial molecular concentration of the ester, to approximately one third of the adsorbent's isolated surface silanol groups. Increasing the adsorption temperature from 25 to 50°C increases the limiting adsorption from the benzene solutions and it is suggested that this increase is due to a decrease in the solvation or “ordering” of the adsorbed layer as the temperature is raised. Removal of the hydrogen bonded silanol groups from the adsorbent's surface by prior heating produces no measurable change in the adsorption properties from either solvent and this is taken as support for the thesis that the adsorbent's free hydroxyl groups are the adsorption centres. The adsorption isotherms from carbon tetrachloride demonstrate a greater selectivity in the adsorption process than there is from the benzene solutions and also that the limiting adsorptions are close, in terms of the interfacial molecular concentration of the adsorbate, to the interfacial concentration of the adsorbent's free surface silanol groups.