Monte Carlo simulations of amphiphile self assembly

Self assembled amphiphiles have applications in numerous industries. Recent developments in synthetic organic chemistry have made possible the manufacture of a wide range of amphiphiles and functionalized nanoparticles that behave in a manner quite similar to their molecular analogs. Computer simulations can provide a useful and inexpensive tool to assist with their rational design. In this thesis, a minimal lattice model is used to examine the effect of molecular architecture on the properties of these systems. Monte Carlo simulations are performed in the grand canonical ensemble and combined via histogram reweighting to determine phase equilibrium and the critical micelle concentration (cmc) at which the first aggregates appear.;Isotropic rigid objects with nearest neighbor attraction and purely athermal interactions are studied to evaluate the effect of the underlying lattice on phase transitions. In comparison to continuum hard core fluids, the range of densities over which fluid-solid equilibrium occurs for coarse spheres and sphereocylinders is exaggerated, whereas the transition for hard cubes is suppressed. Amphiphilic lattice molecules composed of a rigid head group with one or more flexible chains are also examined. The phase transition type (macro-phase transitions that result in bulk phase separation and micro-phase transitions that result in the formation of micelles) depends on the relative size of the solvophobic and neutral portions of the amphiphiles, and none of the systems studied here exhibit both types of transition. Molecular geometry plays a key role in determining whether the micelles that form are spheres or flat bilayers. In a binary mixture of amphiphiles with identical interactions but dissimilar tail group architecture, the mixture concentration at which micelles first appear closely matches theoretical values obtained by assuming that the components form an ideal mixture. A model that includes orientational bonding on the solvophobic monomers is shown to reproduce the non-monotonic changes in cmc observed for many nonionic surfactants in aqueous solutions caused by the hydrophobic effect. This work may prove useful to experimentalists interested in the fabrication of amphiphiles that assemble in a predefined manner, since an exhaustive search of all possible molecular architectures is impractical in the laboratory, and many structures can be quickly characterized with these simulations.