Ion Exchange in Reverse Micelles

Abstract

The distribution and dynamics of alkali cations inside Na-AOT reverse micelles have been investigated using Monte Carlo and molecular dynamics simulations. Water is modeled using the extended simple point charge (SPC/E) model. Simulations were carried out for alkali salts of Li+, Na+, K+, and Cs+ placed into the aqueous core of the reverse micelle, for situations corresponding to one and three molecules of added salt. In all cases, we observe that the larger K+ and Cs+ ions exchange with the Na+ counterion; however, the smaller Li+ ion prefers to remains solvated within the core of the reverse micelle. Our study reveals that the oil-water interface of the Na-AOT reverse micelle has the greatest selectivity toward Cs+ followed by K+ and Li+. A model based on enthalpic contributions illustrates that the solvation energies of the different cations in water control the ion-exchange process. The hydration number of the first water shell for Li+ situated in the aqueous core of the reverse micelle with radius R = 14.1 A was similar to that observed at infinite dilution in bulk water.

Charging Mechanism for Polymer Particles in Nonpolar Surfactant Solutions: Influence of Polymer Type and Surface Functionality

“…A likely factor promoting the ionization of surface functionality is excess moisture, , which might directly partition to the particle surface and behave as an “aqueous bulk” , rather than molecules confined in inverse micelle cores. , In such cases, the specific surface functional groups would be solvated more efficiently by water molecules and the locally high dielectric environments would promote their ionization. We measured the water content in nonpolar solutions of OLOA11000 using Karl Fischer titration and found that these solutions, prepared with the product as received (without further purification or drying), included a significantly larger amount of water than the PIBS-N solutions (Figure ).…”

Computer simulation of reverse micelles and water-in-oil microemulsions

“…Additional information on water dynamics in a polar core of a reverse micelle can be found in review [37]. In the review, works were described devoted to investigating the dynamics of water solubilized in the cores of reverse micelles and w/o microemulsions by the numerical simulation method.…”

Effect of cations on the structure of sodium bis(2-ethylhexyl)sulfosuccinate water-in-oil microemulsion

“…The salt concentrations in the water pools were changed as in Fig. 3. simulations has shown two different ion-exchange processes around the headgroups in the AOT reversed micelle (w 0 4) which depend on the species of alkali-metal salts (Pal et al, 2005), indicating that the larger K + ions are preferentially localized around the headgroup region due to the difference in solvation energies of different ions. In addition, the boundary between positive and negative hydrations is in between Na + and K + ions, and K + ions work as water-structure breakers (Mazitov, 1981;Uedaira & Tatara, 1998).…”

“…Recently, theoretical studies on the structure of reversed micelles (w/o microemulsion at low water content) occluding small amounts of water and ions have been carried out (Faeder & Ladanyi, 2000;Bulavchenko et al, 2002;Abel et al, 2004;Pal et al, 2005). Experimentally, the effect of salts on the AOT w/o microemulsion have been studied intensively by using various spectroscopic methods such as infrared, dielectric, conductivity, luminescence, electron paramagnetic resonance and nuclear magnetic resonance (Eastoe et al, 1992;Fioretto et al, 1999;Alvarez et al, 1999;Pitzalis et al, 2000;Burrows & Tapia, 2002).…”