Selective adsorption of n-fatty acids at the silica/benzene and silica/n-hexane interface. Part 2.—Heats of adsorption

Abstract

The heats of (selective) adsorption of the n-C6, C8, C12, C14 and C16 fatty acids at the silica/benzene interface have been determined from measurements of the heats of immersion of “fully” hydroxylated silicas in benzene + n-fatty acid solutions. Within experimental error, the molar heat of adsorption is independent both of the surface coverage of the fatty acids and of their hydrocarbon chain length. The heats of solution of the fatty acids in benzene at 27°C have also been measured and indicate that the acids exist as monomeric species only at low solution concentrations. At concentrations above about 10–2m the species tend to associate due to hydrogen bonding between the carboxylic head groups as shown by infra-red spectroscopic studies. The present results and those in the preceding paper indicate that the limiting adsorption is observed because only the monomeric fatty acid species is surface active, causing the surface coverage to tend to a limiting value as the concentration of the monomeric species in the equilibrium solution phase approaches an upper limit set by the aggregation process.