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.

Viscoelastic solutions of ionic surfactants with an added salt exhibit a surprisingly strong dependence of their behavior on the nature of the added coion. We apply a recently proposed molecular-thermodynamic model to elucidate the effect of a coion's specificity on the aggregation of cationic and anionic surfactants. We show that micellar growth and branching are opposed by penetration of coions inside a micelle's corona leading to an increase of the aggregate's preferential curvature. These effects result from hydration/dehydration and dispersion attraction of coions and are only important at high salinity where electrostatic repulsion of coions from the micelle is screened and where branching of micelles and viscosity maxima are observed. At low and medium salinity, the coion plays a minor role; its effect on critical micelle concentration and sphere-to-rod transitions is insignificant. Our molecular-thermodynamic approach describes the specific effects of both counterions and coions and their different roles at different salinity levels based on a unified physical picture.

Specific chemistry of added salt has a strong effect on the solubility of chemicals. In this work we measure the effect of several inorganic 1:1 salts on the aqueous solubility of n-hexane. For KCl, NaCl, and NaBr, our data agree with previous measurements. The effect of NaNO3 is measured for the first time. Based on our data, we report the Setchenov salting-out constants and discuss the specific effect of added salt on the solubility of n-hexane. We apply the ePC-SAFT equation of the state to correlate our data and report parameters of this equation. Calculated results reproduce the experimental data quite well.

The specific chemistry of added salt has a strong effect on the aggregation of surfactants. Although the molecular mechanism of this effect is still debated in the literature and there is no generally accepted quantitative theory, substantial progress has recently been achieved in the molecular thermodynamic modeling of ion-specific effects for solutions of ionic surfactants. In this work, we extend our previous aggregation model of ionic surfactants to solutions of nonionic surfactants in the presence of salts. Within this model, the specificity of ions is reflected by the difference in ionic diameters, the dispersion interaction with the micelle, and an effective parameter δ± that takes into account the hydration/dehydration of an ion in the micellar corona and is specific to every ion–surfactant head pair. The effect of specific salt on the hydrophobic contribution to the aggregation free energy is described via the Setchenov salting-out constants. We apply the model for sugar-based surfactants: n-alkyl glucosides and N-acyl-N-methylglucosides. This choice is motivated by the importance of this family of surfactants in biotechnology. We report the set of model parameters, including the Setchenov constants, for the surfactants in this family in combination with a number of 1:1 salts and illustrate good performance of the model in the description of the specific effect of added salt on the critical micelle concentration (CMC) and the growth of micellar aggregates.

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