Structure and properties of dicarboxylic acids at hexane/water interface: A molecular dynamics study

Abstract

Reduction of interfacial tension caused by surfactants at oil/water interfaces plays a crucial role in many technological processes and industrial applications such as detergency, food processing, cosmetics and personal care, wastewater treatment, pharmaceutical and oil industries. The properties and behavior of dispersions, foams, and emulsions are determined by the interfacial properties stabilized by the surfactants. We carried out molecular dynamics simulations to study the interfacial properties, structure and dynamics of dodecanedioic dicarboxylic acids at hexane/water interface over a wide range of surface coverage (area per molecule). Unlike monocarboxylic acid whose hydrocarbon tail points to the oil phase normal to the oil/water interface, the two carboxyl groups of the dicarboxylic acid are attached to the aqueous phase while its slightly curved carbon backbone lies parallel to the interface. The dodecanedioic dicarboxylic acid adopts a slightly curved linear conformation in the interfacial region. The adsorbed acids undergo a first-order gas-to-liquid phase transition as the density of acid increases. The characteristic thickness of the acid layer remains practically constant at large area per molecule (or, low surface density) until surface coverage reaches a “critical” value above which the thickness of the acid layer increases markedly deviating from its low density value. This layering transition has a significant influence on the spreading pressure, interfacial structure, and lateral diffusion coefficient of the acids. Additionally, we modeled a system of fully deprotonated negatively charged acids balanced by positively charged sodium counterions. We found that the interfacial tension of the charged acid interface is consistently lower than uncharged acid at moderate to higher surface density; however, at low densities, we observe little difference in interfacial tension between these two systems within the accuracy of the MD results. Additionally, we found that the rotational relaxation of the charged acids is significantly slower than that of uncharged acids owing to strong electrostatic interactions between the carboxyl groups COO−and sodium Na+cations which restrain the rotational motion of the acid. This molecular dynamics study has provided information and insights at the molecular level on the structure, equilibrium, and dynamic properties of dicarboxylic dodecanedioic acids at hexane/water interface.