The orientation of solvent-dipoles at the surface of the pure solvent

Abstract

A method is described for the assessment of the preferential orientation of solvent-dipoles at the surface of the solvent from the surface potential χ and its temperature coefficient, dχ/dT. The method is based on the model of Levine et al. of the Stern inner region at the mercury-water interface in the absence of ionic adsorption. In this model two possible orientation states are assigned to the solvent-dipoles (in the “up” and “down” directions, respectively), with the magnitudes of the components of the dipole moment normal to the surface differing in the two states.

According to the described method a range of possible combinations of the orientation states and of the magnitudes of the fractions of the solvent-dipoles in each state can be outlined.

For the three solvents studied, i.e. water, methanol and ethanol, a range of possible combinations is found in which one orientation state, corresponding with the larger fraction, is quite well fixed while simultaneously in the other orientation state the angle between the dipole vector and the normal to the surface may vary over a large range. The fractions of the solvent dipoles in either state are relatively constant. In water, 60–80% of the surface molecules are oriented with their dipoles pointing to the water phase. The angle between the dipole vector and the normal to the surface is 60–90°. Probably, one of the hydrogen atoms is pointing to the air phase.

In methanol and ethanol, 90% of the surface molecules are oriented with their dipoles pointing to the air phase. The angle between the dipole vector and the normal to the surface is 70–80°. Probably, the alkyl groups are pointing to the air phase.

It appears that the orientation of the solvent-dipoles at the surface of the solvent is more strongly reflected in dχ/dT than in χ.