Comparison of the adsorption of linear alkanesulfonate and linear alkylbenzenesulfonate surfactants at liquid interfaces

Linear alkanesulfonates and linear alhylbenzeneSulfonates constitute a large fraction of the surfactants used in commercial detergents and cleansers. Despite the industrial significance and the possible environmental impact of these compounds, very little is known regarding the molecular properties of these compounds and how they relate to macroscopic properties desired in applications. This study employs vibrational sum frequency spectroscopy (VSFS) to examine and compare the molecular structure of surfactants in these two classes as they adsorb at organic/water and air/water interfaces. The linear alkane- and alkylbenzenesulfonates studied are, respectively, dodecanesulfonate and dodecylbenzenesulfonate. By measurement and comparison of the vibrational spectra of these adsorbed surfactants, changes in:the Orientation of the aromatic ring and the conformation of the alkyl chains are examined as a function of the number density of surfactant molecules at the interface. The change in aromatic ring orientation as a function of surface concentration is quite different for the dodecylbenzenesulfonate at the air/water interface relative to that at the organic/water interface (CCl4/ water). The alkyl chains of the dodecylbenzenesulfonate are highly disordered at both interfaces as a function of interfacial concentration, in stark contrast to what is observed for the dodecanesulfonate. These results are discussed in terms of the disruptive nature of the benzene ring and the higher degree of hydrophobicity of the alkyl chain relative to the benzene ring near the ionic: sulfonate group.