  Hypotheses: The aggregation number and length of spherocylindrical (rodlike,
wormlike) micelles in solutions of an ionic surfactant and salt can be
predicted knowing the molecular parameters and the input concentrations of the
species. This can be achieved by upgrading the quantitative molecular
thermodynamic model from the previous parts of this series with an expression
for the electrostatic component of micelle scission energy that is the excess
free energy of the spherical endcaps with respect to the cylindrical part of
the micelle. Theory: The thermodynamics of micellization is extended to the
case of multicomponent system, which may contain several surfactants (both
ionic and nonionic) and salts, taking into account the effect of counterion
binding in the Stern layer on the micellar surface. Furthermore, the
considerations are focused on a system that consists of single ionic surfactant
plus salt. Findings: Excellent agreement was achieved between the theoretical
model and experimental data for wormlike micelles from anionic and cationic
surfactants at various concentrations of salt and temperatures. In accord with
the experimental observations, at high salt concentrations, the model predicts
loss of chemical equilibrium between the endcaps and cylindrical part of the
wormlike micelles, which implies transition to self-assemblies of other, e.g.
branched, morphology or the onset of crystallization and phase separation.

Soft Condensed Matter