Dissipative particle dynamics simulations of surfactant systems: phase diagrams, phases and self-assembly.

Abstract

Surfactants, as lyotropic liquid crystals, exhibit a whole host of ordered liquid phases, as a function of surfactant concentration, solvent identity, and any additives present. These ordered phases confer properties to the solution on a macroscopic level, and so understanding ordered phase formation is critical in predicting the physical behaviours of surfactant--containing mixtures.

Typically, household cleaning products contain sufficiently low surfactant concentrations that micellar solutions form, resulting in isotropic, low viscosity liquids. However, in recent years ``single unit dose'' (SUD) products have became increasing popular. The formulation of SUDs results in high concentrations of surfactant, which can result in undesirable properties such as excessively high viscosity, turbidity, and poor product dispersal. This work aims to elucidate the nature of the ordered structures that form in these products, and understand the molecular driving factors that result in their formation.

To gain an understanding of the molecular--level interactions involved in these liquid mixtures, we applied the Dissipative Particle Dynamics (DPD) simulation method. DPD is the ideal technique to study mesophase formation of these mixtures as it is exceptionally fast, allowing one to simulate on the mesoscale (the pertinent length scale for mesophase studies) while retaining detail on the order of molecular fragments. The main body of this work has been dedicated to investigating the parameterisation of DPD models in a tractable manner.

We have developed a highly transferable DPD model of the most common anionic surfactants: sodium dodecylsulphate, linear alkylbenzene sulphonate, and alkylether sulphate. Our model reproduces the phase behaviour of both individual isomers and isomeric mixtures, across entire phase diagrams (with respect to concentration). Although there are some difficulties in producing chemically tractable models with DPD, once developed these models are an incredibly powerful tool in studying phase behaviour from a molecular perspective.