Influence of the structure of boundary layers and the nature of counterions on the position of the isoelectric point of silica surfaces

Dependences of electrokinetic potentials of different silica materials (nano-and ultraporous glasses, a quartz glass plane-parallel capillary, and monodisperse spherical particles of silicon oxide) on the pH of solutions containing single-, double-, and triple-charged cations have been compared. It has been shown that the degree of hydration of a single-charged cation and the structure of an interface substantially affect the position of the isoelectric point (IEP). The most hydrated Na⁺ ions have virtually no effect on the position of the IEP up to their concentration of 0.1 M irrespective of the thickness of an ion-permeable layer at the surface of a solid phase. A reduction in the radius of a hydrated cation (K⁺, Cs⁺) enables its penetration into an ion-permeable layer and, as a consequence, causes the IEP to shift toward larger pH values depending on the parameters of this layer. Two IEPs are observed in LaCl₃ solutions: one at a pH value close to pH_IEP in NaCl solutions and another at a higher pH value corresponding to the charge reversal of the Stern layer.