Colloidal gelation can be useful to prevent sedimentation of complex fluid products such as oral medicines or paints. Colloidal particles remain suspended in a liquid when interparticle repulsive forces are stronger than the attractive forces, especially at low volume fractions. When attractive forces dominate, colloidal particles may form an “attractive gelâ€. For aqueous suspensions of charged colloidal particles, electrostatic repulsive forces can be weakened by counterions. Similarly, agents can be added to strengthen attractive forces. For ionic surfactants at concentrations below the critical micelle concentration (CMC), there are no micelles, and they act as simple electrolytes. It is hypothesized that above the CMC of sodium dodecyl sulfate (SDS) surfactants (CMC = 8 mM), micelles will introduce attractive depletion forces between particles, altering gelation. Consequently, when compared to the baseline system of a simple salt, one may expect colloidal suspensions to gel at lower concentrations of SDS than of NaCl. Using negatively charged silica in water as a model colloidal system, the goal is to understand how gelation phase behavior differs in the presence of increasing concentrations of simple NaCl electrolyte versus SDS. The phase behavior of Ludox colloidal silica was characterized according to transitions from a fluid to an attractive gel phase. Purified suspensions were prepared at a variety of silica concentrations. Suspensions were monitored over time to record the onset gelation time, which was characterized qualitatively based on flow characteristics of the sample.
Thus far, a phase diagram mapping the effects of added salt and SDS on the silica suspensions has been prepared. The phase boundary between gel and fluid phases for SDS samples was observed at two to four times higher concentrations as compared to the phase boundary for NaCl samples for all the silica concentrations studied. In addition, gelation was faster for samples containing NaCl as compared to those containing equivalent concentrations of SDS. Whereas all NaCl added to solution contributes to screening of electrostatic repulsions, SDS micelles do not contribute to screening. Thus, only a fraction of sodium counterions for SDS added above the CMC contribute to screening. The results indicate that attractive depletion forces induced by SDS micelles were not sufficient to compensate for the weakened ionic strength enhancement caused by SDS addition compared to NaCl. The ionic strength effects dominate the gelation process.
