Surface and electrokinetic potentials of interfaces containing two types of ionising group

A general treatment is presented of the variation of the “wall” potential ψ0 with pH for an interface which derives its surface charge from the ionisation of two separate types (e.g., amino and carboxyl) of ionisable surface group. Simple forms of the derived equation are obtained under limiting conditions. The results are compared and contrasted with a published model which considers a single type of ionisable surface site. The present model gives a satisfactory description of electrokinetic measurements on a polyamide dispersion.