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.

WATER VAPOR SORPTION OF STARCHES 105 was filled carefully in a fine stream into a tOO mi. glass measuring cylin- der, avoiding any movement which could promote close packing. After observing the initial volume, the cylinder was mechanically tapped, using the standard sieve shaker,* until no change in volume was observed. The final volume represents the volume at the close packing arrange- ment (bulk volume in cc./25 g. of starch). Angle of repose: The method was mentioned by Neumann (1), Train (t0), Grosby (tl) and Tawashi (12) and consists of pouring the powder through a funnel onto a horizontal sheet of graph paper until the top of the heap so formed reached the tip of the funnel. The diameter of the base of the heap was read and the angle of repose calculated from: H tan 0 - R where: tan 0 = coefficient of interparticle friction H = height of the funnel tip from the graph paper (20 min.) R = radius of the circular base in mm. 0 = angle of repose Each experiment was repeated three times, and the average was calcu- lated. Microscopical counting technique: The method was described in de- tail by Tawashi (13). A projection microscope was used for these de- terminations. The particle size range was divided into intervals. The number of particles in each size interval was counted in ten microscopical fields. The average number was calculated, and from the number frequency the volume distribution was determined. The mean volume diameter was computed on the basis of the volume distribution. The experimental results are shown in Table I and Fig. 1. Table I summarizes the changes which occur when dry starch is kept at 35øC and 95% relative humidity until the equilibrium moisture content is reached. These changes are increase in bulk volume and variable in- creases in frictional and swelling properties. Although the increase in bulk volume in the three different starches is nearly equal (almost 100%), there is a remarkable difference in the relative changes in the frictional and swelling properties. Generally speaking, a thin layer of water condenses on the outside of the particle in a moist atmosphere. This layer favors adhesion between the in- * Friedrich Greyer K.G., Ilmenau (Germany),