The critical gelling point in silica gels containing lithium, sodium and potassium

TLDR: In this paper, the critical gelation concentration (CGC) for the nitrate salts of lithium, sodium and potassium was examined and the pH of the suspensions was held constant at its point of maximum stability (pH = 5.85).

Abstract: Silica suspensions (CabOSil) can be made to gel without loss of water by adjusting the ionic strength of the suspension with inert electrolyte ions. In this paper we examine this critical gelation concentration (CGC) for the nitrate salts of lithium, sodium and potassium. The pH of the suspensions is held constant at its point of maximum stability (pH = 5.85) and the ionic strength was varied (0.01–0.263 M) to determine the CGC. All of the systems gelled through water evaporation without loosing more than 10% of their volume. While this small volume change results in less sensitivity in determining the CGC, it can be said that lithium, sodium and potassium ions show a lyotropic trend. When lithium, which is the largest hydrated ion tested, is the counter-ion, the sol loses more water before gelling than in the presence of sodium which in turn has to lose more water than when potassium is the counter-ion. A similar lyotropic trend was found in the cleaning of these sols before gelation and in the ability of the gels to retain their salts upon drying and calcination to 200°C without evidence of salt segregation.