Dissipative Particle Dynamics for Surfactant Solutions

Abstract

Surfactants are present in many everyday products such as detergents, sham-poos, paints, and foods. Because of the amphiphilic nature of surfactant molecules, they self-assemble into lyotropic liquid crystal structures when in solution. There exists a wide range of possible solution phase structures, e.g. micellar, hexagonal, lamellar, etc, de-pending on the solution composition. The structure of these phases leads to distinct phase dependent rheologies. The rheology can be very difficult to predict numerically, and therefore is often measured experimentally for such systems. The specific surfactants to be studied in this work are alkyl ethoxysulfates (AES). These anionic surfactants are one of the most common components of personal care products. Small scale model-ling of the ‘clustering’ behaviour of surfactant molecules in solution helps us to under-stand the effects of the phase structure on the rheology of the material. Multiple simulation methods are possible for this type of investigation, but this talk will focus on the use of Dissipative Particle Dynamics (DPD). DPD is an off-lattice, mesoscopic simulation technique which involves a set of particles moving in continuous space. While similar, DPD has benefits over Molecular Dynamics (MD) techniques. In comparison, it has the potential for reaching longer length and time scales than MD. This makes DPD an ideal simulation method for such systems, as MD methods struggle to capture the self-assembly process of surfactant molecules, due to the long time scales involved. Most existing DPD research focuses on understanding equilibrium behaviour. However, the complex behaviour of surfactant solutions under shear flow is not well under-stood. Studying surfactant solutions via the DPD method allows us to investigate phase or structural changes that are induced in the fluid, as a result of applied shear. For example, when studying the micellar phase under increasing shear using DPD, we can show that micelles transform from spheres to worm like micelles forming in the direction of applied shear. How structural properties, such as the Radius of Gyration, are influenced by application of shear help us to understand these phase changes on a molecular level. This talk will also present how DPD can be used to calculate the shear viscosity of a fluid, along with the challenges in calculating such properties. The viscosities calculated can be compared with those found experimentally, in order to see if DPD is a viable method for predicting the viscosities of such systems.