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

The interfacial tension γ 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 pressure and concentration at 298.15 K in order to clarify the effect of ω-dipole on the orientation of fluorononanol molecules from the viewpoint of volume. The adsorbed films of both alcohols exhibit two kinds of phase transitions among three different states: the gaseous, expanded, and condensed states. The partial molar volume changes of adsorption v₁^-H - v₁^O in the expanded and condensed states were evaluated and compared between the two systems. The v₁^-H - v₁^O values of both alcohols are negative, and thus the alcohol molecules have smaller volume in the adsorbed film than in the bulk solution. Furthermore, the v ₁^-H value was obtained through the evaluation of v ₁^O by the density measurement of the bulk hexane solution. It was found that the v₁^-H value of HDFC₉OH is smaller than that of FDFC₉OH in the condensed state. On the basis of three matters concerning the molecular structure of alcohols, the occupied area at the interface, and the orientation of FDFC₉OH in the adsorbed film deduced from the earlier results of X-ray reflectivity measurement, the mean tilt angle of HDFC₉OH from the interface normal in the condensed film was estimated to be 15°. The thermodynamic estimation demonstrated here is highly valuable one to provide structure information on an adsorbed film.