Sphere-to-rod transitions of micelles in model nonionic surfactant solutions

The transition of spherical to rod-like micelles is studied in a lattice model for amphiphiles using the single chain mean-field (SCMF) theory and grand-canonical Monte Carlo (GCMC) simulations. A linear combination of the free energy of spherical and infinite cylindrical geometries is assumed in the SCMF theory to estimate the free energy of rod-like micelles. The SCMF theory finds that the symmetric H4T4 amphiphile at a dimensionless temperature scale, T-*, of T-*=6.5 favors the formation of spherical micelles at all investigated overall amphiphile concentrations. On the other hand, the asymmetric H3T6 amphiphile at T-* between 8.5 and 12.5 starts by forming spherical micelles (first cmc) at low overall amphiphile concentrations and then forms rod-like micelles (second cmc) as the overall amphiphile concentration increases. The GCMC simulations also find that the symmetric H4T4 amphiphile forms spherical micelles while the asymmetric H3T6 amphiphile tends to form rod-like micelles. The second cmc is found to increase with increasing T-* whereas it decreases with increasing tail length. Our results are in good qualitative agreement with experimental observations. (C) 2003 American Institute of Physics.