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 pK(a)* 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 pK(a)* 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 similar to7 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 similar to+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 pK(a)*. Examination of the individual contributions shows that the drastically lower basicity of propylene carbonate compared to water is mainly responsible for the increase in pK(a) upon transfer of the acid-base equilibrium of aliphatic ammonium/amine from the aqueous to the organic solvent.