Monte Carlo simulations of surfactant aggregation and adsorption on soft hydrophobic particles

Monte Carlo simulations with an explicit description of counterions are performed to investigate the adsorption of ionic surfactants at the interface between water and soft hydrophobic and penetrable particles. The surfactant molecules are represented at a coarse-grained level, their hydrophobic tails interact with each other through a Lennard-Jones potential, whereas their hydrophilic head and their counterions interact through a Coulombic potential. Two colloidal hydrophobic particles interact with the surfactant hydrophobic chains through a modified Lennard-Jones potential. By increasing the surfactant confinement between non-adsorbing colloidal particles, micellization is achieved and the micelle aggregation number is found to increase. Adsorption isotherms are determined for various interaction strengths between the surfactants and the particles. It is found that increasing this parameter increases the level of the adsorption plateau. The adsorbed surfactant molecules form conical aggregates which evolve into elongated structures by increasing the surfactant concentration and the strength of the interaction. The presence of micelles in solution is shown to be controlled by the level of adsorption and saturation of the hydrophobic particle surfaces. This study provides for the first time a comparison of surfactant micellization in solution and aggregate formation at one interface by considering hydrophobic and electrostatic interactions.