The interfacial tension y between water and hexane solution of 1-octadecanol has been measured as functions of temperature T, pressure p, and mole fraction of 1-octadecanol in hexane x\ by the pendant drop method. The curves of y vs. T, y vs. p, and y vs. x\ show noticeable break points. Calculated thermodynamic quantities such as energy changes, entropy changes, and volume changes associated with the adsorption show the points of discontinuity at a certain temperature, pressure, and composition, corresponding to the above breaking points. It has been clarified that the first order phase transition between the expanded and condensed states takes place at the interface like that of monomolecular films spread on water surfaces. The expanded state is characterized by a relatively high energy, high entropy, and large volume, while the condensed state by low energy, low entropy, and small volume.
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