Interfacial Behavior of Non-Equimolar SDS/DTAB Mixtures Adsorbed at the Oil-Water Interface

Surfactant mixtures are employed in various industries, either through intentional mixing or the presence of impurities. These mixtures can synergistically enhance and largely govern the properties of an oil-water interface compared to their pure counterparts. These enhanced properties can help stabilize liquid foams in oily environments, making these surfactant mixtures invaluable to oil-related industries. Unfortunately, the underlying molecular-level mechanisms that govern a foam's oil tolerance are unclear. Here we employ interfacial tensiometry in tandem with vibrational sum frequency spectroscopy to determine the composition (surfactant population and anionic/cationic ratio) and structure (surfactant alkyl tail conformation) of surfactant monolayers adsorbed at the oil-water interface for SDS/DTAB (sodium dodecyl sulfate/dodecyltrimethylammonium bromide) mixtures across a range of surfactant mixing ratios and total surfactant concentrations. We show that, irrespective of the surfactant mixing ratio, monolayers formed by SDS/DTAB mixtures predominately contain paired surfactants. The interfacial activity of the individual surfactants, rather than their headgroup properties, determines their interfacial concentration. Moreover, the unpaired surfactants are concluded to negligibly affect the interfacial packing of surfactant monolayers. These measurements provide new insight into the molecular-level factors that drive the formation and composition of mixed anionic/cationic surfactant monolayers and possible insight into the mechanisms that engenders foam stability.