Journal
JOURNAL OF COLLOID AND INTERFACE SCIENCE
Volume 599, Issue -, Pages 773-784Publisher
ACADEMIC PRESS INC ELSEVIER SCIENCE
DOI: 10.1016/j.jcis.2021.04.138
Keywords
Surfactant micelles; Free energies; Kinetics; Coarse-grained molecular dynamics; Umbrella sampling
Categories
Funding
- National Science Foundation [CBET 1907517]
- National Natural Science Foundation of China [51888103]
- China Scholarship Council
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By calculating the free energies of different types of micelles at varying salt concentrations, transitions in micelle shapes and dynamic processes can be inferred.
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.
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