Dessicants properties

The porous samples used in this study were plugs made up from fine glass powder of a few microns in diameter so as to give a non adsorptive, simply-structured medium. They were formed by compressing the powder with a small amount of moisture in a brass cylinder and leaving for several days at room temperature, after which they were dried under mild conditions. Final drying and desorption is carried out in a vacuum dessicator. The appropriate physical properties of a porous sample are given in Table 1. 

For special purposes, it is manufactured separately as the so-called microporous glass, whose porosity and pore size can be regulated within certain limits (pore diameter in fractions to tens nm), by adjusting the glass composition and conditions of separation (temperature, time). The glass shows selective absorptive properties its specific surface area amounts to several hundred m g and the pore space takes a third to half the volume. Microporous glass is used as an adsorbent, dessicant and catalyst carrier. 

Pons et al. have also followed the dessication-rehydration cycle in the montmoril-lonite clays by the SAXS method. This is an interesting study because the swelling properties of clays play an important role in the structural organization of soils. Two main conclusions arise from this work ... 

SBS. The diffusion coefficient of cyclohexane at 20°C in an SBS block copolymer was measured as a function of film thickness and casting surface. Films of SBS TR-41-2443 (Shell Chemical Co., Houston, TX) were cast under identical conditions from benzene onto glass. Teflon, or previously cleaned mercury surfaces. The films were allowed to dry for a day by evaporation within a partial benzene atmosphere. The films were dried further in a vacuum dessicator for two days before being used in a sorption experiment. The properties of the copolymer are described in Table I. 

The disaccharide trehalose, shown next, is the major reason why some plants and animals can withstand prolonged periods of dessication (severe dehydration). It is widely used in cosmetics and commercial foods, because of its high water retention properties. Is it a reducing or nonreducing disaccharide ... 

Table 19.21. Performances and selected properties of drying agents and dessicants... 

To measure these properties, the test piece is dried to constant mass, weighed in air, and then transferred to an airtight dessicator, which is then evacuated until a minimum pressure is reached. After the vacuum is maintained for a set time period, the liquid is introduced until the specimens are completely covered and to ensure open pores are fiUed. In most cases distilled water is used. However, for materials that may hydrate or if assessing ex-service materials, which may contain water-soluble corrosion products, then parafiftn is preferable. 

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