ORIENTATION OF ORGANIC ADSORBATES FROM THERMODYNAMIC PARAMETERS - A CASE-STUDY

Values of Γs, the surface excess at saturation, and of ΔEN, the adsorption potential shift at saturation, for propargyl alcohol adsorbed at the Hg/solution interface and at the free surface of water, have been used to make two independent estimates of the orientation of the adsorbate molecules at the two interfaces. Calculations have involved the discussion of the molecular conformation, of the position of the molecular dipole, of the value of the permittivity in the Helmholtz equation and of the contribution to ΔEN of the displaced solvent dipoles. Results have shown that while the picture is qualitatively reasonably understood, it is quantitatively seriously inconsistent. It is pointed out that ignorance of too many interfacial molecular parameters makes these calculations unavoidably ambiguous so that, when performed, results should be taken with great caution and awareness of their limitations. © 1990.