Surfactant desorption and scission free energies for cylindrical and spherical micelles from umbrella-sampling molecular dynamics simulations

Hypothesis: The free energies associated with adsorption/desorption of individual surfactants from micelles and the fusion/scission of long micelles can be used to estimate the rate constants for micellar kinetics as functions of surfactant and salt concentration. Experiments: We compute the escape free energies Delta G(esc) of surfactant from micelles and the scission free energies Delta G(sciss) of long micelles from coarse-grained molecular dynamics simulations coupled with umbrella sampling, for micelles of both sodium dodecylsulfate (SDS) in sodium chloride (NaCl) and cetyltrimethylammonium chloride (CTAC) in sodium salicylate (NaSal). Findings: For spherical micelles, Delta G(esc) values have maxima at certain aggregation numbers, and at saltto-surfactant molar concentration ratios R near unity, consistent with experiments. For cylindrical micelles, SDS/NaCl shows a minimum, and CTAC/NaSal a maximum in Delta G(esc), both at R - 0.7, while Delta G(sciss) of CTAC micelles also peaks at around R similar to 0.7 and that of SDS micelles increases monotonically with R. We explain the non-monotonic dependence of escape and scission free energies on R by a combination of electrostatic screening and the decrease of micelle radius with increasing R. Transitions from predominantly spherical to cylindrical micelles, and between adsorption/desorption and fusion/scission kinetics with changing salt concentration can be inferred from the free energies for CTAC/NaSal. (C) 2021 Elsevier Inc. All rights reserved.

Automated summary

By calculating the free energies of different types of micelles at varying salt concentrations, transitions in micelle shapes and dynamic processes can be inferred.