A Tale of Two Tails: Tail Ordering of Stoichiometric 1:1 DTAB:SDS Pairs Adsorbed at the Oil-Water Interface

Cationic:anionic surfactant mixtures adsorbed at an oil-water interface stabilize foams in the presence of oil, making them essential to the oil, gas, and firefighting industries. The oil tolerance of foams stabilized by surfactant mixtures, relative to pure (unmixed) cationic and anionic surfactants, results from the mixtures' enhanced flexibility in tailoring the physicochemical properties of the interface. To judiciously employ these mixtures, it is necessary to characterize the structure-function property relationship of their surfactant monolayers that lend to oil-tolerant/intolerant foams. In this work, we employ interfacial tensiometry and vibrational sum frequency spectroscopy to determine the composition (surfactant population and cationic:anionic ratio) and the structure (surfactant alkyl tail conformation) of monolayers prepared at the oil-water interface by 1:1 DTAB:SDS (dodecyltrimethylammonium bromide:sodium dodecyl sulfate) mixtures. We show that the interfacial surfactant density of 1:1 DTAB:SDS mixtures greatly exceeds that of pure DTAB and SDS at similar concentrations up to and beyond their respective critical micelle concentration. The enhanced interfacial adsorption of these mixtures is due to the adsorption of stoichiometric 1:1 DTAB:SDS surfactant pairs that form through the attractive electrostatic interactions between surfactant headgroups. We find that these paired surfactants preferentially adsorb at the interface, causing the interfacial DTAB:SDS ratio to be nearly 1:1. Additionally, we find that the SDS tail is more conformationally ordered than the DTAB tail, even though they are expected to be conformationally identical along the entire tail, since they are likely conjoined through van der Waals interactions. This leads to the conclusion that the surfactant pairs are in a staggered arrangement at the interface. These findings help to uncover molecular factors that contribute to the enhanced oil tolerance, and in some cases oil intolerance, of foams stabilized by cationic:anionic surfactant mixtures.