Molecular theory of surfactant micelles in aqueous solution

This paper presents an overview of recent theoretical work on the molecular theory of micelle formation. A primary emphasis is given to the role of computer simulation of condensed materials in understanding micelle structure and thermodynamics. Much of the detailed discussion focuses on recent Monte Carlo studies of a simple molecular model of micellar aggregates. For clarity of presentation, a compact, physical organization of micelle thermodynamic equilibrium ratios is advocated. This procedure provides a simple basis for physical reasoning about the molecular roles of attractive and repulsive forces in micellization thermodynamics. The molecularly coarse-grained micellar structural information available from current small angle neutron scattering (SANS) measurements is surveyed. The structural predictions of the reviewed Monte Carlo calculations are shown to be in good qualitative agreement with the SANS data. The Monte Carlo results indicate that micelles should be viewed as fluid aggregates with a low surface free energy relative to water-hydrocarbon interfaces. The computer experimental results suggest that dynamic surface and shape fluctuations should be considered in understanding micelle structure at a molecular level. Several instantaneous structures are graphically displayed to illustrate that these transitory structures could be qualitatively described as “dry” but irregularly shaped. Configurations drawn from Monte Carlo calculations on cylindrical and bilayer structures of infinite extent are used to illustrate the role of surface flexibility in these systems.