Molecular Thermodynamic Modeling of the Specific Effect of Salt on the Aggregation of Nonionic Surfactants

Abstract

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.

Model for estimating activity coefficients in binary and ternary ionic surfactant solutions

“…There are several molecular thermodynamic theories to account for ion-specific effects on micellization (Warszyński et al 2002;Moreira and Firoozabadi 2010;Danov et al 2014;Lukanov and Firoozabadi 2014;Zhu and Free 2015;Hao et al 2016;Khoshnood et al 2016;Koroleva et al 2019), but their numerical implementation is complex and computationally prohibitive to atmospheric models. As a way forward, a simple approach to estimate the aggregation number n, thermodynamic potentials and equilibrium constant of the micellization reaction could be based e.g.…”

Spectroscopically Quantifying the Influence of Salts on Nonionic Surfactant Chemical Potentials and Micelle Formation

“…Although some prior experimental and theoretical studies have described the influence of salts on the critical micelle concentration of 12HD and other nonionic surfactants, − none of these studies have characterized the effect of salts on the hydration shells of the monomeric and aggregated species or quantitatively related the corresponding observations to the chemical potentials of both the free monomers and aggregates. The application of Raman-MCR to the elucidation of ionic micelle structures has previously been described, but did not extend to nonionic surfactants or the influence of salts on micelle formation.…”

“…The application of Raman-MCR to the elucidation of ionic micelle structures has previously been described, but did not extend to nonionic surfactants or the influence of salts on micelle formation. Prior theoretical treatments of micelle salt effects have focused primarily on changes in the solubility of the free surfactant monomers, effectively treating the micelle as a reference state whose free energy is not significantly influenced by salts. − Moreover, these prior theoretical analyses have not attempted to relate the influence of salts on micelle formation to Wyman-Tanford theory. ,− …”