J. Soc. Cosmetic Chemists, 17, 103-114 (1966) Water Vapor Sorption and Potential of Starch Grains Suction F. EL-KHAWAS, M. Pharm., R. TAWASHI, Ph.D., and H. VON CZETSCH LINDENWALD, Ph.D.* Synopsis--The influence of water vapor sorption on the particle characteristics of potato, corn, and wheat starch is evaluated critically. The suction potential of the starch grains in a packed column is studied in both the presence and the absence of colloidal silica. The re- sults of these experiments lead to the conclusion that the suction potential and the rate of linear diffusion are closely related to the amount of silica dispersed in the powder bed. INTRODUCTION In different industries, particular attention has been given to vapor sorption and its influence on the properties of particulate ma- terials (1-4). Millet and Parisot (5) reported that the frictional and wear phenomena of graphite and carbon are related to their surface sorption properties. Ulmann (6) reported that the type of water present in potato starch, either in bound or adsorbed state, can be studied by the slow reduction in weight by means of infrared radiation. Dumonskii and Nekrgach (7) found a quantitative correlation between the amount of combined water and the heat of wetting evolved from starches. In a previous communication (8) it was shown that the bulk volume, frictional properties, and size distribution of cornstarch were markedly influenced by humidity. The intimate mixing of cornstarch with 0.5% hydrophobic silica reduced the interparticle friction to a definite value which remained constant at all degrees of humidity. The purpose of the present study is to evaluate the process of water vapor sorption and the suction potential in different starches, with the * Alexandria University, Department of Pharmaceutical Technology, Alexandria, Egypt. 103

104 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS aim of obtaining a basic understanding of the action of starch as a tablet disintegrant and to provide further information about the influence of Aerosil ©* as an additive. RESULTS AND DISCUSSION Materials Corn, wheat, and potato starches were selected for this study. These were dried at 100øC to constant weight. As an additive, Aerosil © (colloidal silicic acid or hydrophilic silica) was used experimentally. Water Vapor Sorption Accurately weighed samples (approximately 1 g. each) of dried starch were placed in a humidity ovenS, adjusted to 95% relative hu- midity and 35øC. The samples were quickly weighed every twenty- four hours, until the equilibrium moisture content was attained. The water sorption in each sample has been calculated in gram per cent. Larger quantities were treated in the same way in order to investigate the packing, frictional, and swelling properties of the moist material. (a) The packing characteristics were studied by mechanical tap- ping. The tapping experiment was carried out with 25 g. of the ma- terial. The bulk volume at the close packing arrangement was measured for both dry starch and starch at equilibrium moisture content (95% relative humidity and 35 øC). (b) The rheological properties were studied by the angle of repose. The coefficient of interparticle friction was taken as a measure of the frictional properties for the dried and wet material. (c) The swelling properties were studied by comparing particle size distribution in both dry and wet conditions. The particle size distribution was determined by the microscopic count technique. The relative change in the mean volume diameter was taken as a measure of the swelling capacity. The experimental details of the mechanical tapping, angle of repose measurement, and microscopic count technique are described in a previous communication (8) but are repeated below: Mechanical tapping: The tapping experiment (9) was carried out as follows: Twenty-five grams of starch, having a loose packed volume, * Aerosil is colloidal silica manufactured by Degussa, A.G., Frankfurt/M (Germany). t Gallenkamp, London (EnglandS.