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

The interfacial tensions (gamma) of the hexane solutions of 1H,1H-perfluorononanol (FDFC9OH) and its omega-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 omega-dipoles on the adsorption behavior of fluorononanols. The interfacial pressure (pi) 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 pi 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 u1(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 omega-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 value for HDFC9OH than for FDFC9OH. (2) The contact of omega-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.