Quantifying extra interaction forces in charged colloidal dispersions from frontal ultrafiltration experiments

A method for determining extra non-Derjaguin–Landau–Verwey–Overbeek (DLVO) interaction forces for electrostatically stabilised colloidal systems from frontal ultrafiltration data has been developed. The method is based on a previously developed analysis of frontal ultrafiltration and osmotic pressure data. The extra non-DLVO interaction forces for silica were incorporated into a dynamic ultrafiltration model by way of a power law description to more accurately predict filtration rates at low ionic strengths (<0.03 M) where deviations from experimental filtration rates were observed. As the ionic strength was decreased a long-range attraction was observed. The osmotic pressure data indicated that the system seemingly jumped to an ordered structure with a volume fraction of ∼0.5, indicating that a phase transition had occurred. The method requires only simple apparatus, small sample quantities, short preparation times and allows quantification of extra interaction forces for a wide concentration range. The experimentally determined extra interaction forces have been compared with the predictions of two characteristic representatives of recent theoretical descriptions of non-DLVO interactions in concentrated colloidal systems. Neither of the theoretical descriptions could account for the experimental data.