Spontaneous repeptization of colloidal dispersions

While it is widely reported that electrocratic colloids de-stabilized through the addition of monovalent electrolyte produce aggregates that may be substantially repepetized upon sufficient subsequent dilution, the behavior of such colloids de-stabilized using polyvalent counter-ions appears to have been much less studied and may often be one of irreversible coagulation. The present study examines alumina and silica colloids de-stabilized with both mono- and polyvalent coagulants in the presence and absence of an adsorbing nonionic surfactant, Triton X-100 (TX-100). Destabilization using monovalent electrolyte was found to be reversible upon dilution, in the presence or absence of the TX-100, but coagulation using a variety of polyvalent counter-ions was found to be irreversible in the absence of TX-100. When the surfactant was present at sufficient concentration, however, the coagulation event was found to reverse itself spontaneously without dilution, to an extent and at a rate dependent on the coagulant concentration. At the end of the repeptization process, the aggregates were sometimes found to be only a few times larger than the original particles, but primary particles were not recovered. The adsorbing surfactant appears to act as a lubricating layer reducing the direct particle contact needed for sintering while the coagulating ions desorb from the surface via complexation with TX-100 molecules, resulting in recovery of electrostatic repulsion and repeptization following the coagulation event.