Computer simulation of self-assembling amphiphilic systems.

In this thesis, the results of a series of molecular computer simulation studies undertaken to investigate amphiphilic self-assembly are presented. Here, the aim has been to develop a coarse-grained model for amphiphilic behaviour, and examine its ability to exhibit free self-assembly of complex structures at moderate computational cost. Firstly, the development of a novel single-site model for an amphiphilic molecule is addressed. The model is based on mixtures of (rod-like) Gay-Berne and (spherical) Lennard-Jones particles, the rods being taken to be single-site models of amphiphilic molecules immersed in a solvent of spheres. The hydrophobic effect, believed to be the main driver of amphiphilic self-assembly, is incorporated by giving the rod-sphere interaction a dipolar symmetry. Results obtained indicate that free self-assembly of micellar, lamellar and inverse micellar arrangements can be readily achieved. Following on from these preliminary simulations, a refined rod-sphere potential has been used to study the micellar region in greater detail. The effects of both the amphiphilic strength and hydrophilic-lipophilic balance on micellar properties are examined. We find that these key molecular interaction parameters can be used to control the size, shape and internal structure of micelles. Interesting intermicellar phenomena can also be accessed within these simulations such as micelle fusion and exchange of long-lived monomers between micelles. Furthermore, a 'rattling' motion of short-lived monomers, leaving and re-entering micelles, can be observed. Finally, binary mixtures of amphiphiles have been studied as a function of their mutual degree of attraction and the mixture composition ratio. The amphiphile with longest 'hydrophobic' tail is found to dominate the monomer phase whereas the micelles showed very different structures. A two-layer radial shell structure in the well-mixed micelles is found for mutually attractive amphiphile types. As the mutual attraction is reduced, structurally segregated sphero-cylindrical micelles dominate. In these, amphiphiles with large head groups tend to form the end caps of a cylinder made of amphiphiles with short head groups. When the mutual interaction is reduced even further, two distinct coexistent micellar phases are then observed with most micelles containing only one type of amphiphile.