Effect of ω-hydrogenation on the adsorption of fluorononanols at the hexane/water interface: Miscibility in the adsorbed film of fluorononanols

The interfacial tension of the hexane solution of 1H,1H-perfluorononanol (FDFC₉OH) and its ω-hydrogenated analogue, 1H,1H,9H-perfluorononanol (HDFC₉OH), against water was measured as a function of the total molality and composition of the mixture at 298.15 K under atmospheric pressure. The existence of ω-dipole in HDFC₉OH makes the interfacial density larger in the gaseous and expanded states and smaller in the condensed state compared to FDFC₉OH. The phase diagram of adsorption (PDA) was constructed, and the excess Gibbs energy of adsorption (g^H,E) was calculated at each state in order to discuss quantitatively the miscibility of FDFC₉OH and HDFC₉OH in the adsorbed film. We found that the g^H,E value is negative in the gaseous state, while it is positive and increases with decreasing interfacial tension in the condensed state, These results are explained mainly by the balance of two effects induced by mixing of two alcohols: (1) Reduction of repulsive interaction between ω-dipoles aligning parallel in the adsorbed film because of the increase in mean distance between HDFC₉OH molecules. (2) The loss of effective dispersion interaction between hydrophobic chains due to the fact that the oblique orientation of HDFC₉OH molecules at the interface is mixed with the perpendicular one of FDFC ₉OH. We concluded that the factor (2) is negligible compared to the factor (1) in the gaseous and expanded films and exceeds the factor (1) in the condensed film, in which molecules are closely packed.