PREDICTION OF ELECTROPHORETIC MOBILITIES - PART 2 - EFFECT OF ACID DISSOCIATION-CONSTANT ON THE INTRINSIC MOBILITIES OF ALIPHATIC CARBOXYLATES AND AMINES

Electrophoretic mobility is the most important parameter governing the separation of solutes in capillary zone electrophoresis (CZE). Classi cal theoretical models for the prediction of absolute mobilities consi der only the effect of hydrodynamic friction, Application of these mod els to literature mobility data demonstrates that they are not appropr iate for small organic solutes. Accurate prediction of absolute mobili ties of small ions requires an additional frictional term, the dielect ric friction, Dielectric friction results from the work necessary to o rientate the solvent dipoles in response to the solute charge. This st udy investigates whether the pK value of a charged functional group ca n be used as a relative measure for the dielectric friction, Using the pK(a) as a measure of dielectric friction of weak acids, the absolute mobility of aliphatic carboxylates is given by: mu<INF>0</INF> = 10<S UP>-3</SUP> (6.8 +/- 1.2)/V<SUP>(0.620+/-0.036</SUP>) + (0.6 +/- 0.24) pK<INF>a </INF>where V is the van der Waals molecular volume (Angstrom <SUP>3</SUP>) determined by molecular modeling, The uncertainties are the standard deviations of the parameters. The average, error between the predicted absolute mobility and literature values for 15 aliphatic carboxylates was 3.7%, Similarly, use of the pK<INF>b</INF> as a prox y for the dielectric friction for weak bases yields the following expr ession for the absolute mobilities of aliphatic monoamines:<INF></INF> mu<INF>0</INF> = 10<SUP>-3</SUP> (7.8 +/- 1.3)/V<SUP>(0.62+/-0.03</SU P>) + (0.66 +/- 0.23)pK<INF>b </INF>The average prediction error for 3 4 aliphatic monoamines was 4.5%.