Surface potential of a particle in an electrolyte solution containing rodlike zwitterions

Abstract

The surface potential of charged colloidal particles is calculated for the case where they are suspended in an electrolyte solution containing zwitterions of concentration C z as well as univalent pointlike ions of concentration C 1. A zwitterion is regarded as a rod of length a carrying two charges + e and − e ( e is the elementary electric charge) at its ends. Equations determining the surface potential are derived which are based on the theory of Carnie and McLaughlin ( Biophys. J. 44 (1983) 325) on the effects of large divalent cations on the particle surface potential. It is found that when κ a « 1, zwitterions behave as neutral species and when κ a å 1, they behave like univalent electrolytes of concentration C z/2 (not C z), where κ is the Debye-Hückel parameter at zero zwitterionic concentration. Approximate expressions for the surface potential as a function of σ (the surface charge density of the particles), C 1, C z, and a are derived.