Journal
ADVANCES IN COLLOID AND INTERFACE SCIENCE
Volume 206, Issue -, Pages 17-45Publisher
ELSEVIER
DOI: 10.1016/j.cis.2013.02.001
Keywords
Micelles of ionic surfactants; Counterion binding; Degree of micelle ionization; Critical micellization concentration; Micelle aggregation number; Conductivity of micellar solutions; Mixed surfactant micelles
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Funding
- National Science Fund of Bulgaria [DID-02-18/2009]
- Unilever R&D, Trumbull, USA
- FP7 project Beyond-Everest
- COST Action CM1101 of the ESF
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On the basis of a detailed physicochemical model, a complete system of equations is formulated that describes the equilibrium between micelles and monomers in solutions of ionic surfactants and their mixtures with nonionic surfactants. The equations of the system express mass balances, chemical and mechanical equilibria. Each nonionic surfactant is characterized by a single thermodynamic parameter - its micellization constant. Each ionic surfactant is characterized by three parameters, including the Stern constant that quantifies the counterion binding. In the case of mixed micelles, each pair of surfactants is characterized with an interaction parameter, beta, in terms of the regular solution theory. The comparison of the model with experimental data for surfactant binary mixtures shows that beta is constant - independent of the micelle composition and electrolyte concentration. The solution of the system of equations gives the concentrations of all monomeric species, the micelle composition, ionization degree, surface potential and mean area per head group. Upon additional assumptions for the micelle shape, the mean aggregation number can be also estimated. The model gives quantitative theoretical interpretation of the dependence of the critical micellization concentration (CMC) of ionic surfactants on the ionic strength; of the CMC of mixed surfactant solutions, and of the electrolytic conductivity of micellar solutions. It turns out, that in the absence of added salt the conductivity is completely dominated by the contribution of the small ions: monomers and counterions. The theoretical predictions are in good agreement with experimental data. (C) 2013 Elsevier B.V. All rights reserved.
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