Gelation of colloidal silica - the role of ionic species

Abstract

The effect of three different salts (KCl, NaCl and LiCl) on the gelation of colloidal silica was studied. The macroscopic gelation time of each sample was taken as the time when there was no more flow. Increasing the salt concentration led to a decrease in gelation time, as predicted by DLVO theory. In addition, the type of salt was found to have a large effect on gelation time. KCl was found to be the most gel-inducing and LiCl the least, in agreement with the Hofmeister series.

Particle tracking microrheology was used to study the gelation process on the micro-scale. The mean square displacement (MSD) of polystyrene tracer particles was determined at different times during the gelation process. The resulting MSD curves were superimposed onto a master curve from which the gel point was identified. The microscopic gelation time followed the same trends as the macroscopic gelation time, although it was found to be somewhat shorter.

Information about the sample viscoelastic properties was obtain from the MSD curves. For each sample the critical exponent n and the elastic modulus G' was calculated. It was found that with decreasing volume fraction of silica colloids, n increased and G' decreased, consistent with the observation that gels became weaker and softer. At constant silica volume fraction and varying salt concentration, two different trends were observed. Gels with NaCl appeared to become softer with increasing salt concentration (n increased and G' decreased) while gels with KCl showed a decrease in n and thus became harder as the salt concentration was increased (no clear trend was observed for G' as a function of KCl concentration). This remarkable difference might also be due to difficulties in analysing the samples, which were found to gel very suddenly due to the high volume fraction of silica. More research is needed to determine whether the observed trends truly reflect a difference in gel properties between gels with KCl and NaCl.