A New Empirical Equation to Take Into Account the Influence of Ionic Strength on Electrophoretic Mobility and Determination of Binding Constants of Ester Betulin Derivatives With Sulfated β‐Cyclodextrin by Affinity Capillary Electrophoresis

Abstract

An empirical equation relating electrophoretic mobility and ionic strength was proposed. The equation includes a number of parameters that are found using the mobilities of reference ions: two coefficients in the numerator describing the linear relationship of the multiplier in front of the square root of the ionic strength with the product of the ion mobility in the background electrolyte (BGE) without additives by the modulus of the charge number, raised to a certain power, and also the multiplier in the denominator before the square root of the ionic strength. The proposed equation was tested using the mobilities measured in BGEs with the addition of sodium chloride to adjust ionic strength and sulfated β‐cyclodextrin (S‐β‐CD) for 11 anions with charge numbers from −1 to −4. Measuring anion electrophoretic mobility over a wide range of values, from −17 to −76 × 10−9 m2 V−1 s−1, was achieved through a combination of conventional separation and separation in a thermostatically controlled part of a capillary. The equation describes experimental data much more accurately (the discrepancy is no more than 1.1%) compared to the previously used simple empirical equation. For the S‐β‐CD complexes of betulin derivatives, the apparent binding constants determined are equal to 40–60 M−1.