True molar surface energy and alignment of surface molecules.

By use of data for surface tension, for the first time a method is presented for calculating true molar surface properties of liquids: free energy, entropy, and enthalpy. These new data allow full comparison with other molar quantities, such as enthalpy and entropy of vaporization. All data are at the normal boiling point. There are differences in behavior between various classes of nonpolar compounds. Rare gases and tetrahydrides of Group 14 form a separate category. The results agree with the experimental findings that water and alcohol molecules are aligned with the -OH groups attached to the surface. The data indicate that hydrogen peroxide and 1,2-ethanediol also have one -OH group directed toward the surface and one directed out toward the vapor phase. Small straight-chain amines have some structure at the surface, but the larger ones behave like the corresponding alkanes. Very polar compounds, such as nitriles, nitro compounds, and aldehydes, have little or no increased degree of order of molecules at the surface. Except for hydrogen-bonded compounds, molecules at the surface have surroundings similar to those in the bulk liquid.