Thermal Stability of Dispersions of Amino-Functionalized Silica in Glycol and in 50-50 Aqueous Glycol

Dispersions of amino-functionalized silica in ethylene glycol (EG) and in aqueous glycol show excellent stability at room temperature. Stability at elevated temperatures would be much desired with respect to their potential application as heat-transfer fluids. Amino-functionalized silica was dispersed in EG and in 50-50 aqueous EG by mass. HCl and acetic acid were added to enhance the positive zeta potential. The dispersions were stored at 40, 60, 80, and 100 degrees C for up to 28 days, and zeta potential and apparent particle radius were studied as a function of elapsed time. The particles showed a positive zeta potential in excess of 40 mV (Smoluchowski), which remained unchanged for 28 days. Such a high absolute value of zeta potential is sufficient to stabilize the dispersion against flocculation and sedimentation. The apparent particle radius in acidified dispersions was about 70 nm, and it was stable for 28 days. The particles were larger in pH-neutral dispersions. The apparent particle radius was about 80 nm in fresh dispersions and it increased on long storage at 80 and 100 degrees C.