(174g) Interface of Ion-Containing Aqueous and Organic Phases within and out of Confinement

Through a series of comparative molecular dynamics studies, we examined the relative behavior of organic and inorganic ions at the interface of cyclohexane with different aqueous solutions, within and out of confinement. In this regard, the organic phase was separately brought in contact with 1.0 M solutions of NaCl, NaOH, CaCl₂ and Ca(OH)₂. In the absence of solid surfaces, inorganic ions’ exclusion from the aqueous-organic interface was found to be proportional to their hydration strength, such that calcium and hydroxide were more intensely depleted from the interface as compared to sodium and chloride. This was reflected in larger interfacial tension increments for solutions composed of calcium as the cation or hydroxide as the anion. When sodium decanoate was added to the cyclohexane phase, they partly migrated to the interface and the rest of them formed micelles within the organic bulk region. Consequently, a fraction of the inorganic cations was drawn to the interface, however, their interface-propensity was still controlled by their hydration strength. In order to study the effect of confinement on such relative behavior, these two-phase solutions were inserted into a kaolinite nano-pore, with an octahedral and a tetrahedral type surface exposed to the liquid phase. In general, the more weakly-hydarted ions - sodium and chloride - exhibited stronger affinity to the solid surfaces. However, calcium and hydroxide were rather scattered within the water multilayers or at the aqueous-organic interface. In the absence of any aqueous solution, decanoates were found to be susceptible to strong adsorption to kaolinite surfaces, particularly to the hydrophilic octahedral type. However, in the presence of calcium chloride, they were all detached from the surface and adsorbed to the aqueous-organic interface. This was caused by the release of double amount of inorganic anions by this 1:2 salt, inner-sphere accumulation of chloride on the octahedral surface, and interface-propensity of calcium due to its higher charge density.