Propylene carbonate as a nonaqueous solvent for capillary electrophoresis: mobility and ionization constant of aliphatic amines.

The two properties of aliphatic amines were investigated in propylene carbonate as solvent that are decisive for capillary electrophoretic migration: the actual mobilities and the pKa* values. Solutes were eight primary, secondary, and tertiary amines. Roughly, the actual ionic mobilities of the ammonium ions are inversely proportional to the solvent viscosity, fairly obeying Walden's rule. The pKa* values of the cation acids, HB+ (the corresponding acids of the amines, B), were related to the conventional pH* scale of the buffers. Determined from the effective mobilities as a function of the pH*, they are increased by approximately 7 units compared to water. This increase was interpreted based on the concept of the standard free energy of transfer of the individual species in the acid-base equilibrium. The corresponding medium effect on the proton, log mgammaH+ (the logarithm of the transfer activity coefficient mgammaH+) is approximately +8. The medium effect on the free base, B, was obtained from solubility data; it is about -1 and smaller. Plausible values for the medium effect on the cation HB+ (-1 to -2) lead to a sum of the increments, which corresponds with the overall effect, expressed by the change in pKa*. Examination of the individual contributions shows that the drastically lower basicity of propylene carbonate compared to water is mainly responsible for the increase in pKa upon transfer of the acid-base equilibrium of aliphatic ammonium/amine from the aqueous to the organic solvent.