Zeta potential and particle size in dispersions of alumina in 50-50 w/w ethylene glycol-water mixture

Hypothesis: Dispersions of solid particles in glycols and in their mixtures in water have been proposed as promising heat transfer fluids, and the zeta potentials in such systems are often reported. We challenge a popular approach in which high absolute value of zeta potential implies excellent stability in such systems. Experiments: Nanoparticles of alumina were dispersed in 50-50 ethylene glycol-water mixture, and zeta potential and particle size were studied as a function of concentration of various solutes. Findings: Addition of HCl and CTMABr induced positive zeta potentials. A maximum value of about 120 mV was obtained at CTMABr concentration of about 10-5 M, and further increase in the concentration of the surfactant had moderate effect on the zeta potential. A maximum value of about 120 mV was obtained at HCl concen-trations of 10-5 -10-4 M, and further increase in the HCl concentration led to depression of the zeta potential. Addition of NaOH and KOH, and of anionic surfactants induced negative zeta potentials. With NaOH and KOH the maximum negative zeta potential of -60 mV was obtained at base concentration of about 10-3 M, and increase in the base concentration to 10-2 M had moderate effect on the zeta potential. Negatively charged particles showed substantial aggregation in spite of high negative zeta potentials.