WATER VAPOR SORPTION OF STARCHES 107 Cumulative distribution curves shown in Fig. 1 (cf. also Table I) indicate that the swelling tendency of potato starch is much more pro- nounced than that of corn and wheat, the latter exhibiting the least swelling capacity. In potato starch, the apparent increase in volume is 94%, and the absolute increase in particle volume is 124% the dif- ference is due to particle adhesion. In the case of cornstarch, the absolute increase in particle volume by swelling is approximately equal to the apparent increase in bulk volume. Therefore, the effect of 1o 20 3o 40 •o •o •a • • PARTICLE SIZE IN • Figure 1. Particle size distribution of starches in the dry and wet condition adhesion is nullified by aggregate formation. The absolute increase in particle volume of wheat starch (17%) would not be the only explana- tion for the doubling of bulk volume. There should be another factor initiating the increase in bulk volume this factor is probably the forma- tion of aggregates with high apparent volume. Regarding the variation in frictional properties, it is known that the interparticle friction depends on the particle shape, size, size distribu- tion, and surface roughness. At high humidity, a condensed water film deposits on the individual particles and causes the following changes: i, adhesion and decreased surface energy of the system, and ii, resistance to flow and sliding induced by higher interparticle friction. A comparison of the relative changes produced by humidity in the investigated starches indicates that potato starch suffers a significant

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