SURFACTANT SELF-ASSEMBLY AT A HYDROPHILIC SURFACE - A MONTE-CARLO SIMULATION STUDY

We present a coarse-grained lattice model for (nonionic) surfactant ad sorption, which is solved by performing simulations using the Monte Ca rlo method. We focus our attention on the behavior of surfactant molec ules whose head groups have a relatively weak attractive interaction w ith a solid (hydrophilic) surface. The simulations show that the surfa ctant molecules start to self-assemble on such a surface at a solution concentration which lies below the cmc. The surfactant forms discrete aggregates on the surface, which are very similar to the almost spher ical micellar aggregates in solution, Increasing the surfacant concent ration leads to a strong increase of the adsorbed amount. These findin gs agree well with a wide variety of experimental data for nonionic su rfactant molecules adsorbing onto hydrophilic surfaces such as SiO2. W e have also studied the effect of the adsorption energy and the head/t ail ratio on the adsorption behavior. When the attraction between the surfactant head group and the surface increases, the shape of the adso rbed aggregates changes to a prolatelike structure, and the aggregates orient themselves parallel to the surface, A reduction of the head gr oup size leads to the formation of larger aggregates. The main contrib ution of this paper lies in the attempt to understand the basic featur es of a rather broad range of surfactant systems from a simple model w hich (hopefully) contains the essential characteristics that dominate the adsorption behavior of nonionic surfactants.