Adsorption of alkyl trimethylammonium bromides at the water/air and water/hexane interfaces

Surface and interfacial tension measurements have been performed at the water/air and water/hexane interface by drop profile analysis tensiometry for a series of alkyl trimethylammonium bromides with different chain lengths (C10, C12, C14 and C16). The effect of neutral phosphate buffer (10 mM, pH 7) has been investigated and the results have been compared with literature data obtained for the same surfactants in the absence of salt. The use of a modified Frumkin isotherm (Ionic Compressibility) taking into account the mean activity of all ions in solution leads to a real improvement of the data interpretation, while the classical Frumkin Compressibility model yields similar results but overestimates the adsorption parameters. At the water/hexane interface, the hexane molecules are incorporated in the surfactant layer and it results a kind of competitive adsorption for the shortest chain surfactant (10 carbons). For the longest chains, the attractive interactions between the hydrophobic chains of the adsorbed surfactants are strong enough to replace the solvent molecules from the interface.