On the Thermodynamics of Reverse Micelles: Effect of Water on Micellization
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he Thermodynamics of Reverse Micelles: Effect of Water on Micellization A. I. Rusanov* Mendeleev Center, St. Petersburg State University, St. Petersburg, 199034 Russia *e-mail: [email protected] Received February 18, 2020; revised February 28, 2020; accepted March 4, 2020
Abstract—Water may be involved in reverse micelle formation either in the form of a hydration shell or as a solubilisate. Both cases have been analyzed within the framework of a theory based on the mass action law and the definition of the critical micelle concentration (CMC) via the constant of this law. In the first case, it is supposed that a surfactant is primordially hydrated, and water is a priori incorporated into the surfactant. Its amount is determined from the volume of the hydrated core of a micelle, while the aggregation number of the surfactant is calculated from the packing conditions for spherical, cylindrical, and platelike micelles. The CMC is, in turn, found from its dependence on the aggregation number, with this dependence being determined by the general theory. In the second case, water is taken into account as a separate component of a micelle, but in the form of a solubilisate (hydration water is, as before, introduced into the chemical formula of the surfactant). Here, the CMC appears to depend on not only the overall concentration of water (with no regard to its hydration form), but also on the value of the critical degree of water micellization. The effect of water on micellization has been analyzed at both its preset chemical potential and its constant amount. All versions of the theory lead to the conclusion that CMC decreases with an increase in water content, thereby confirming the prediction put forward by Eicke and Christen (Eicke, H-F. and Christen, H., Helv. Chim. Acta, 1978, vol. 61, p. 2258). DOI: 10.1134/S1061933X20050130
INTRODUCTION The difference between reverse and direct micelles is essential in many respects (see, e.g., review [1]). First of all, the micellization mechanisms are different for them. While surfactant molecules in reverse micelles are attracted to each other, in water, they are aggregated into direct micelles, because water pushes their nonpolar moieties out of itself. The term “hydrophobic effect” (in the case of other polar solvents, “lyophobic effect”) follows from this phenomenon. This name by itself witnesses the great role of water in the formation of normal micelles, when it serves as a dispersion medium. However, water appears to play a peculiar role in the micellization in nonpolar media as well. As early as in [2], it was noted on the basis of experimental data that water is an indispensable participant in the formation of reverse micelles, at least because it cannot be liquidated. Water is always present in surfactants irrespective of their drying (they cannot be calcined because of the danger of their decomposition). Water is always present in the environment (humidity is known to be an important characteristic of air). Hence, trace amounts of water can always be found in reverse
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