The Charge of Glass and Silica Surfaces

Ionization processes in aqueous solutions have long been a point of central interest to physical chemistry. Much progress has been made in recent years in understanding the charging properties of such different entities as small molecules, polyelectrolytes and various kinds of interfaces [1]. In this context, silica and silicate glass surfaces immersed in water are known to acquire a negative surface charge density, primarily through the dissociation of terminal silanol groups. The degree of dissociation and thus the surface charge density results from an equilibrium between counterions at the glass surface and free ions in the bulk electrolyte. Experimentally, this type of equilibrium and its dependence on the solution conditions can be studied by potentiometric acid-base titrations on colloidal dispersions of non-porous silica particles [2]. This technique actually measures the volume concentration of protons transferred between the surfaces and the solution. In order for the surfaces to accommodate a sufficient amount of charge, the electrostatic interaction between the surface sites must be screened at least partially by added salt ions, and/or the available surface area must be large. These constraints can be relaxed to some degree by resorting to alternative techniques like microelectrophoresis [3, 4], streaming potential measurements [5], conductometry [6] and electroacoustic methods [7, 8]. All of these methods, however, rely heavily on approximate models for electrostatic or hydrodynamic processes in the interfacial region, introducing uncertainties that are difficult to estimate. We are not aware of any way to measure directly the surface charge of silica in a solution of very low ionic strength. Theoretical studies of low ionic strength solutions typically deal with dense colloidal and macroionic systems and consider the regime of “no salt”, “low salt”, or “counterions only”. Here the defining assumption is that the overall ionic strength is due predominantly to the particles or macroions and the compensating counterions in solution, whereas the concentration of any additional ions is negligible. For colloidal dispersions this assumption is again legitimate if the specific surface area carrying the colloidal charge is large.