Modeling of the Effects of Ion Specificity on the Onset and Growth of Ionic Micelles in a Solution of Simple Salts

Abstract

A new version of the molecular thermodynamic model has been developed that takes into account the effect of ion specificity on the free energy of aggregation. The specificity of salt is reflected by differences in the bare ionic sizes and polarizabilities leading to the difference in the dispersion interaction of ions with the aggregate. The model also contains parameters that characterize the compactness of ionic pairs formed between a mobile ion and surfactant's headgroup. The values of these parameters show that more chaotropic heads form tighter pairs with chaotropic ions whereas more cosmotropic heads form more compact pairs with cosmotropic ions. The formation of compact pairs in the micelle corona diminishes the preferable curvature of the aggregates and promotes their growth. The model has been applied to aqueous solutions of cationic (alkyltrimethylammonium, alkyldimethylammonium, and alkylpyridinium) and anionic (alkylsulfate and alkylcarboxylate) surfactants in the presence of simple 1:1 salts. With a single set of parameter values, the model reproduces the critical micelle concentration-salinity curves and the sphere-to-rod transitions or the absence of thereof and describes the aggregate growth for different simple salts, in good agreement with experiment.

Nonclassical Hydrophobic Effect in Micellization: Molecular Arrangement of Non‐Amphiphilic Structures

“…In contrast, for classical ionic surfactants, the counterions decrease both CAC and micellar size according to the Hofmeister series. [30][31][32] Nevertheless, highly hydrated small cations (Li + and H 3 O + ) are more prone to condense onto COSAN aggregates as compared to Na + and K + , which is in line with the Hofmeister series.…”

Molecular thermodynamics of soft self-assembling structures for engineering applications

“…These hydration effects have been lumped in an additional model parameter that takes an individual value for every surfactant head–mobile ion pair (although this value does not depend on the length of surfactant tail). Such models have been thoroughly tested for aqueous solutions of cationic (alkyltrimethylammonium, alkyldimethylammonium, and alkylpyridinium) and anionic (alkylsulfate and alkylcarboxylate) surfactants in the presence of simple salts . With a single set of parameter values, the model reproduces the critical micelle concentration − salinity curves and the sphere‐to‐rod transitions or the absence thereof and describes the aggregate growth for different salts, in good agreement with experiment.…”

“…One of the major problems with the classical micellization models is that they have only limited success in reflecting the effect of specific ions on formation and the subsequent growth of aggregates upon the addition of salt . In recent versions of models the specific chemistry of ions is reflected by differences in the bare ionic sizes and in ionic polarizabilities leading to the difference in the dispersion interaction of ions with the aggregate. The dispersion interactions are quantified via the force constants.…”

Viscoelastic properties of new mixed wormlike micelles formed by a fatty acid salt and alkylpyridinium surfactant

“…Long entangled wormlike micelles of surfactants can impart to aqueous solutions pronounced viscoelastic properties [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15], which are quite sensitive to external stimuli: ionic strength [2,8], type of salt [11,15], temperature [10], various additives including polymers [6,7] and hydrocarbons [12][13][14]. Such responsiveness is provided by the self-assembled character of micellar chains formed by weak non-covalent interactions [1,9].…”

“…Wormlike micelles can be formed by many common ionic surfactants upon addition of salt [2,8,11,15], which shields the repulsion between similarly charged surfactant heads thus making the cylindrical packing more favorable in comparison with spherical one. Since a wormlike micelle consists of a cylindrical central section and semi-spherical end-caps, the increase of the fraction of cylindrical parts at the expense of the spherical ones suggests the growth of wormlike micelles in length.…”