A Tale of Two\nTails: Tail Ordering of Stoichiometric\n1:1 DTAB:SDS Pairs Adsorbed at the Oil–Water Interface

Cationic:anionic surfactant mixtures\nadsorbed at an oil–water\ninterface stabilize foams in the presence of oil, making them essential\nto the oil, gas, and firefighting industries. The oil tolerance of\nfoams stabilized by surfactant mixtures, relative to pure (unmixed)\ncationic and anionic surfactants, results from the mixtures’\nenhanced flexibility in tailoring the physicochemical properties of\nthe interface. To judiciously employ these mixtures, it is necessary\nto characterize the structure–function property relationship\nof their surfactant monolayers that lend to oil-tolerant/intolerant\nfoams. In this work, we employ interfacial tensiometry and vibrational\nsum frequency spectroscopy to determine the composition (surfactant\npopulation and cationic:anionic ratio) and the structure (surfactant\nalkyl tail conformation) of monolayers prepared at the oil–water\ninterface by 1:1 DTAB:SDS (dodecyltrimethylammonium bromide:sodium\ndodecyl sulfate) mixtures. We show that the interfacial surfactant\ndensity of 1:1 DTAB:SDS mixtures greatly exceeds that of pure DTAB\nand SDS at similar concentrations up to and beyond their respective\ncritical micelle concentration. The enhanced interfacial adsorption\nof these mixtures is due to the adsorption of stoichiometric 1:1 DTAB:SDS\nsurfactant pairs that form through the attractive electrostatic interactions\nbetween surfactant headgroups. We find that these paired surfactants\npreferentially adsorb at the interface, causing the interfacial DTAB:SDS\nratio to be nearly 1:1. Additionally, we find that the SDS tail is\nmore conformationally ordered than the DTAB tail, even though they\nare expected to be conformationally identical along the entire tail,\nsince they are likely conjoined through van der Waals interactions.\nThis leads to the conclusion that the surfactant pairs are in a staggered\narrangement at the interface. These findings help to uncover molecular\nfactors that contribute to the enhanced oil tolerance, and in some\ncases oil intolerance, of foams stabilized by cationic:anionic surfactant\nmixtures.