Anionic:Cationic Surfactant Mixtures at the Oil-Water Interface

Abstract

Anionic:cationic surfactant mixtures are unique in that they synergistically enhance and largely govern the properties of the oil-water interface, even when one surfactant is present in trace amounts. These behaviors of the mixtures have significant implications for various industries. For instance, in oil-related industries wherein these mixtures are used to create oil-tolerant foams, small quantities of surfactants can be used to form these important foams. On the other hand, when such enhanced interfacial properties are undesirable, as in the case that foams need to be unstable in the presence of oil, trace amounts of ionic surfactant impurities can detrimentally affect a product's performance. In both scenarios, this oil tolerance is dependent on the mixture’s composition: the surfactant mixing ratio as well as the anionic and cationic surfactant themselves. Hence, it is challenging to predict which mixture compositions create oil-tolerant foams since the underlying behaviors that govern a foam’s oil tolerance is unclear.

In order to judiciously employ these anionic:cationic surfactant mixtures, it is necessary to characterize the structure-function property relationship of their monolayers that lend to oil-tolerant foams. This thesis helps elucidate this relationship by employing 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 monolayers prepared at the oil-water interface by SDS:DTAB (sodium dodecyl sulfate:dodecyltrimethylammonium bromide) mixtures across a range of surfactant mixing ratios and total surfactant concentrations. These measurements reveal that the interfacial surfactant density of SDS:DTAB mixtures greatly exceeds that of pure SDS and DTAB 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 SDS:DTAB surfactant pairs that form through the attractive electrostatic interactions between surfactant headgroups. We find that the mixed SDS:DTAB monolayers predominantly contain these paired surfactants and a diminutive number of unpaired surfactants. The unpaired surfactants likely act as point defects that disrupt the tight packing of the adsorbed surfactant monolayers. 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 also uncover molecular-level factors that may contribute to the enhanced oil tolerance of foams stabilized by anionic:cationic surfactant mixtures and its dependence on the mixture’s composition.