Effect of ω-hydrogenation on the adsorption of fluorononanols at the hexane/water interface: Temperature effect on the adsorption of fluorononanols

The interfacial tensions (γ) of the hexane solutions of 1H, 1H-perfluorononanol (FDFC₉OH) and its ω-hydrogenated analogue 1H, 1H, 9H-perfluorononanol (HDFC₉OH) against water were measured as a function of temperature and concentration under atmospheric pressure in order to know the effect of ω-dipoles on the adsorption behavior of fluorononanols. The interfacial pressure (π) versus mean area per adsorbed molecule (A) curves consist of two discontinuous changes among three different states: the gaseous, expanded, and condensed states. The A values at given π in the gaseous and expanded states are larger for HDFC₉OH than for FDFC₉OH. The changes in partial molar entropy (s₁ ^-H - s₁^O) and energy (u₁ ^-H - u₁^O) of adsorption were evaluated. Their values are negative, and therefore, the alcohols have a smaller entropy and energy at the interface than in the bulk solution. Furthermore, the u ₁^-H - u₁^O value is more negative for HDFC₉OH than for FDFC₉OH in the expanded state and also in the condensed film just above the expanded-condensed phase transition point. This seems to be due to the following: (1) HDFC₉OH may tilt from interface normal for ω-dipoles to interact effectively with water molecules in the interfacial region and to reduce their own repulsive interaction between neighbors arranging parallel in the adsorbed film. This leads to a lower u₁^-H value for HDFC₉OH than for FDFC₉OH. (2) The contact of ω-dipoles with hexane molecules in the bulk solution is energetically unfavorable, and thus, the u₁^O value of HDFC₉OH is expected to be larger than that of FDFC₉OH.