Thermodynamic study on the adsorption of sodium chloride at the water/hexane interface

The negative adsorption of sodium chloride at the water/hexane interface has been investigated by measuring interfacial tension as a function of pressure and concentration. By analyzing the results thermodynamically, the interfacial densities and the distance between the two dividing planes introduced so as to make the excess numbers of moles of water and hexane zero were evaluated. The results lead us to the view that the ions and hexane molecules distribute further apart from each other in the interfacial region. Furthermore, the volume of interface formation, Δυ, was evaluated; Δυ was found to be positive and independent of the concentration. In addition, the entropy, Δυ, and energy, Δυ, of interface formation were evaluated by measuring interfacial tension as a function of temperature. It was seen that Δυ, like Δυ, is independent of concentration, whereas Δs decreases slightly with increasing molality. This discrepancy was explained as resulting from the contribution of the entropy of mixing.