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.
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