Analysis of Binding Equilibrium of Phenylalkylamines to Brij 58 Micelle through the Measurement of Electrophoretic Mobility by CZE

Binding equilibrium of nine kinds of phenylalkylamines (PAA) as non-charged species to nonionic Brij 58 micelle has been investigated through the measurements of the electrophoretic mobility by capillary zone electrophoresis. Analysis of the binding equilibrium involves: (1) acid dissociation constant of PAA in an aqueous solution, Ka, (2) conditional acid dissociation constant of PAA in Brij 58 micelle solution, Ka', and (3) binding constant of PAA+ as protonated species of PAA to the micelle, KB,HB. Binding constant of non-charged species of PAA to the micelle, KB,B, was then determined through the three equilibrium constants. Both binding constants of KB,HB and KB,B increased with increasing molecular volume of PAA, and linear free energy relationship was verified between the molecular volume and logarithmic value of the binding constants. Binding constants of KB,B are larger than KB,HB for every PAA. This result also suggests the hydrophobic binding that less charged species are more strongly bound to the hydrophobic surfactant micelle.