Calcium chloride adsorption at liquid-liquid interfaces: A molecular dynamics simulation study

We carried out molecular dynamics simulations (MD) to investigate the adsorption of calcium chloride (CaCl₂) at n-hexane-water interfaces. We also measured the interfacial tensions (IFT) of the selected systems making use of the pendant-drop method. The histograms of hexane, water, and the ions indicate an electrical double layer (EDL) near the interface. The trend of the EDL indicates that chloride anions intend to adsorb to the interface more intrinsically than calcium cations. The measured interfacial width of the n-hexane-water interfaces decreases with the salt concentration. The average densities of the interfacial and bulk aqueous solutions demonstrate density heterogeneity in the direction perpendicular to the interface. The calculated density profiles enabled us to measure the adsorption values of each component. It is found that the salt particles are repelled from the liquid-liquid interface, resulting in an enriched bulk solution with salt. We, therefore, report negative adsorptions of CaCl₂ and the consequent reproduced increasing IFT with salt concentration. Consistent with the EDL trends, the negative adsorption of calcium cations is higher than that of chloride anions.