Plots of mobility versus the square root of ionic strength (I(1/2)) do not show the linear behavior predicted by Kohlrausch's law. Classical electrolyte theory states that such deviations are to be expected due to the finite size of the ions. This paper uses the Pitts equation to account for the effect of ionic size on the ionic strength dependence of mobilities in CZE. Experimental mobilities for carboxylates, phenols, and sulfonates of -1 to -6 charge in aqueous buffers ranging from 0.001 to 0.1 M ionic strength were described by μ(-) = μ(0) - Az (I(1/2)/(1 + 2.4I(1/2))), where the constant in the denominator is empirically determined. Infinite dilution mobilities (μ(0)) determined by extrapolation of mobility data to zero ionic strength based on this expression yielded excellent agreement (100.3 ± 3.3%) with literature values for 14 compounds in a variety of buffers. The Pitts equation provides a reasonable estimate of the constant A for solutes up to a charge of -5. However, this constant also depends on temperature and the nature of the buffer counterion, presumably due to ion association. Thus it is most appropriate to determine the constant A empirically for a given buffer system.

The mobility of an ion is of fundamental importance in capillary electrophoresis. The size, shape, and other physicochemical parameters of monoamines are determined using molecular modeling. These parameters are used to generate regression expressions to predict absolute (infinite dilution) mobilities. Molecular volume or mass is the strongest determinant of electrophoretic mobility. However, molecular volumes calculated via molecular modeling varied systematically depending on the software used, and so molecular mass is the favored descriptor. Neither the classical spherical (Hückel) nor ellipsoidal (Perrin) models were reasonable predictors of mobility. In accord with empirical expressions, such as the Wilke-Chang equation for diffusion, the absolute mobilities correlate with mass (or volume) to a much greater power than predicted by Stokes's law. Incorporation of the effects of hydration using the McGowan waters of hydration increments further improved the predictions. The best equation for predicting absolute mobilities of monoamines is μ(0) = [(5.55 ± 0.73) × 10(-)(3)]/[W((0.579)(±)(0.026)) + (0.171 ± 0.054)H] where W is the molecular weight and H is the mean waters of hydration calculated using the McGowan increments. The uncertainties are the standard deviations of the parameters. This equation yielded an average prediction error of 4.1% for the data set used to generate the expression (literature absolute mobilities for 34 monoamines possessing no other functional groups), 7.2% for an independent data set from the literature (absolute mobilities for seven monoamines possessing other functional groups), and 3.3% for an experimentally determined data set (13 monoamines determined using capillary electrophoresis).

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