The operation of drying is one of dominating importance in many branches of industry, among which may be mentioned the drying and seasoning of timber and textiles, the curing of tea and tobacco, the manufacture of photographic films, glue and gelatin, pottery, paper, paints and varnishes, toffees, macaroni, milk and other dried foods and fruits; and “air-processing,” as it is called in America, is, in itself, an industry of considerable magnitude. It is not surprising, therefore, that much attention has been given by chemical engineers to the design of drying-plant, and a considerable literature on this subject exists. The theoretical side of the subject, however, has not been so thoroughly developed, and there are many gaps in our knowledge of the factors underlying the process of the evaporation of water from colloid materials. There are two main groups of factors governing the rate of evaporation of water from any kind of material. The first group is fairly well understood, and comprises all those factors that are external to the material concerned, such as the temperature and humidity of the drying atmosphere, and the effective velocity of the air over the surface of the stock. These factors are known generally as the “drying conditions.” The second group comprises what may be termed the internal factors, such as the chemical and physical properties of the material being dried and the changes that occur in these as the drying proceeds. These factors have not been so thoroughly investigated as the drying conditions, possibly on account of the difficulty of treating them from a purely physical and thermo-dynamic standpoint. They are, however, of no less importance industrially, while their study as a purely scientific problem is of much interest and may throw some light on the physical and physico-chemical nature of colloid materials. Moisture may exist in materials in at least two distinct forms: as free moisture adhering to the material and with a vapour pressure equal to that of water in bulk, and as “sorbed” moisture, the vapour pressure of which is always less than that of water in bulk. The rate of evaporation of water, under constant drying conditions, at any instant per unit of surface is proportional to the difference between the vapour pressure of the evaporating water and the pressure of the water-vapour in the adjacent atmosphere,
i. e.
Exploring the difference in xerogels and organogels through
in situ
observation
