108 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS .J • ß 15 gO 45 60 75 TIME IN MIN. Figure g. Water rise as a function of time decrease in its flow properties (66%), while corn and wheat starches are less affected (28.6 and 17.6%, respectively). This can be explained on the basis of particle aggregation. The degree of aggregate formation in these experiments is in the order wheat corn potato. Since size enlargement by aggregation is known to improve the free running proper- ties of powders, it can be assumed that, in the presence of humidity, the stickiness produced by water vapor will be partially counteracted by the degree of aggregation. This finding is of practical importance in select- ing a starch as an ingredient in dusting powders. Wheat starch is pre- ferred to corn and potato in this respect. Diffusion Rate and Suction Potential Five grams of starch was packed in a glass tube (9 mm. diameter) closed at one end with filter paper and open at the other end. The

WATER VAPOR SORPTION OF STARCHES 109 powder bed was mechanically tapped, using a standard sieve shaker* until the minimum bulk volume was reached. The tube was immersed in a beaker containing distilled water so that the surface of the water just touched the filter paper. The water rise in the powder bed was • 60 // ' /- i i i i 15 30 45 60 75 TIME IN MIN. Figure 3. Hydraulic pressure as a function of time measured at successive time intervals. The hydraulic pressure was also measured at the same time intervals by means of a water manometer connected with the free end of the tube. The results are given in Table II and Figs. 2 and 3. Figure 2 represents the water rise or the linear diffusion in the powder bed as a function of time, while Fig. 3 denotes the hydraulic pressure produced as a function of time. These figures indicate that the linear diffusion rate and the pressure exerted are in the order potato corn wheat. * Friedrich Greyer K.G., Ilmenau (Germany).