Colloid Chemistry of Silica: An Overview

Abstract

Silicon dioxide is the main component of the crust of the earth. Combined with the oxides of magnesium, aluminum, calcium, and iron, it forms the silicate minerals in our rocks and soil. Over millions of years silicon dioxide, or silica, has been separated from the original silicate rocks by the action of water to appear as quartz. In a few places it was deposited in the amorphous form as opal. Our English word silica has a very broad connotation: it includes silicon dioxide in all its crystalline, amorphous, soluble, or chemically combined forms in which the silicon atom is surrounded by four or six oxygen atoms. This definitely excludes all the organosilicon compounds made by man in which carbon atoms have been linked directly to silicon atoms—commonly referred to as "silicones", which do not occur in nature. Silica is soluble enough in water to play important roles in many forms of life. It forms the skeletons of diatoms, the earliest form of life that absorbed sunlight and began to release oxygen into the atmosphere. Many plants use silica to stiffen stems and form needles on the surface for protection. As animals developed, the role of silica became less obvious. But each one of us contains about half a gram of silica, without which our bones could not have been formed, and probably also not our brains. Silica has played a key role since the beginning of civilization, first in flint for tools and weapons and in clay and sand for pottery. The high strength and durability of Roman cement 2000 years ago is now known to be due to the use of a special volcanic ash that is an almost pure form of amorphous colloidal silica. Today there is active research on the use of the somewhat similar silica fume from electric furnaces to make a super-strong Portland cement. Our present technology would be very different without the silica for the catalysts of our oil refineries, for the molds for casting the superalloys in our jet engines, for modern glass and ceramics, electronic microcircuits, quartz crystals, and fiber optics. — Ralph K. Iler, Alexander Memorial Lecture, Australia, 1989. Reprinted with permission from Chemistry in Australia (October 1986, p 355).