Minimum inner layer capacitance values at negative charge densities have been collected and analysed for 23 solvents. The solvent effect on the value at this capacitance minimum is shown to be related primarily to the solvent molecular size. The influence of the solvent dipole moment is considered in relation to the molecular size. The ion-dipole and dipole-dipole coupling constants are the most suitable parameters for characterizing the solvent effect for a large group of solvents. In the relations analysed, water is found to be a zero-order homologue of simple aliphatic alcohols.