MICROSCOPIC SHRINKAGE FREE SILICA-GELS BY ELECTRODIALYSIS

Silica sol-gels were prepared by electrodialysis of a 1.37M silica sol ution in water. Microscopic shrinkage of the few nanometer pores of th e gels during drying was investigated by thermoporometry. The colloida l silica sol was prepared by removal of sodium ions in the sodium sili cate solution through a Nafion membrane. A uniform colloidal silica so l resulted with silica particles of about 5 nm diameter as measured by transmission electron microscopy. Gelation of the sol occurred at pH from 2.7 to 7.8. The pore structure of the hydrogels was studied by th ermoporometry before and after drying. Air drying did not lead to shri nkage of the micro-pores, as measured by the equal ice crystal radius, R(n), and ice porc volume, V(n), of hydrogels and xerogels. Pore size was determined mainly by the gelling pH.