Control of the surface charge density of colloidal silica by sodium hydroxide in salt-freeand low-salt dispersions

Electrical conductivity measurements and conductometric titrations were performed on dilute salt-free aqueous dispersion of a colloidal silica (diameter: 0.11±{}0.01×{}10^-6nm) whose charge number can be varied with the quantity of coexisting sodium hydroxide. In the absence of sodium hydroxide, the silica particle had an effective (net) surface charge density σₑ of 8×{}10^-8nC cm^-2. Titrations by hydrochloric acid were performed in the presence of sodium hydroxide. When the NaOH concentration was sufficiently high, the titration curve could be divided into three regions with regard to slope values. These regions could be ascribed to titrations of excess sodium hydroxide, ionizable surface groups having Na ions as counterions, and excess hydrochloric acid. The analytical surface charge density σₐ, estimated from the titration curves, increased with increasing [NaOH]. It was found that the concentrations of Na and OH ions in excess were negligible when the [NaOH] was smaller than 2.5×{}10^-4nM and the volume fraction of the silica, {φ}, was larger than 2.6×{}10^-3. The value of σₐ at this threshold was 1.8×{}10^-5nC cm^-2. Under these conditions we could control the σₐ value by varying [NaOH]. The present system provides larger possibilities in studying the influence of charge density on the physico-chemical properties of ionic colloidal systems. Viscosity measurements were performed for salt-free and low-salt dispersions at [NaOH]'s where its excess concentration was found to be negligible. The σₐ dependence of the viscosity was in good agreement with previous results obtained from ionic latices having various σₐ's. A relationship between the effective charge density at an infinite dilution, σ_e|φ=0, and σₐ was examined for latex systems. An empirical relation, ln σ_e|φ=0=0.49 ln σₐ-1.0, was obtained by using σ_e|φ=0 values determined by the conductivity for latices with various σₐ's from 0.21×{}10^-6 to 5.6×{}10^-6nC cm^-2. By assuming that this relation holds also for the silica system, viscosity data were analyzed. Satisfactory agreement was observed between the experimental value and Booth theoretical value on the first-order electroviscous effect.