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

Abstract

The selective adsorption isotherms of the n-C₆, C₈, C₁₂, C₁₄ and C₁₆ fatty acids at the silica/benzene and silica/n-hexane interface have been determined at 27°C. The adsorbents used were a fully hydroxylated silica carrying a small concentration of micropores on its surface (R.A.3) and an annealed but still fully hydroxylated silica (R.R.A.3/700). With the exception of the C₆ and C₈ acids the adsorption isotherms with the benzene solutions were independent of the chain length of the fatty acid. Adsorption from the n-hexane solutions showed that lengthening the hydrocarbon chain of the adsorbate decreased the surface coverage at fixed equilibrium solution concentration. The adsorption isotherms obtained from benzene solutions showed a limiting surface coverage equivalent to about 0.5 molecule of fatty acid per 100 Ų of adsorbent surface for the C₈-C₁₆ acids. From the n-hexane solutions the limiting adsorption ranged from about 1.85 solute molecules per 100 Ų of solid surface for the C₆ acid down to 1.40 for the C₁₆ species. Partly dehydroxylating the adsorbent so as to yield a silica surface carrying only single hydroxyls (R.A.3/700) resulted in this material showing virtually no selectivity for fatty acid adsorption from benzene solutions.