We discuss unexpected effects in surface complexation studies with very small particles at high solid concentrations and low ionic strength. While a high surface area concentration is beneficial to experiments like potentiometric titrations or even necessary as in zeta-potential measurements with very small particles in scattering techniques or with any kind of particles in accousto-sizer set-ups, the combination with low ionic strength may result in the peculiar situation that excess surface area does not result in the extent of reaction that would occur in the presence of sufficient number of counter-ions. In titration experiments such lack of counter-ions would be accounted for by the need of adding acid or base, which in turn can change the effective ionic strength. In zeta-potential measurements although the solid concentration will typically be lower than in titrations, the experimental data may also be affected by this effect. Overall the ionic strength will be ill defined and hamper comparison among data and methods. This may explain why authors report effects of solid concentration on zeta-potential. A concise way of fixing the experimental conditions would be to work at constant surface area concentration and ionic strength that would generate sufficient excess of counter-ions to keep a constant ionic strength within the experimental window investigated. Our calculations also show that a deficit in counter-ion can reverse the charge-size dependency, which may explain why different trends have been reported in the literature.

Size and solid concentration effects in surface complexation models

255th National Meeting (2018)

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