112 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS same final pressure as corn starch, which has 7.5 cc. entrapped air/5 g. of powder. Wheat starch has a final pressure which is less than that of corn and potato. This means that the final pressure obtained in the case of wheat and corn is mainly due to the entrapped air, while the swelling capacity is nearly compensated by shrinkage of the powder bed. In potato starch the final pressure obtained is due to the evident swelling capacity of the grains, which is greater than the negative force of shrinkage and the relatively small amount of entrapped air. Influence of A erosil on the Rate of Linear Diffusion The diffusion rate experiment has been repeated on each starch after the addition of gradually increasing amounts of Aerosil (from 0.005 to 100%). Intimate mixing of Aerosil with starch was a•hieved in a Turbula shaking mixer.* The results, given in Figs. 4, 5, and 6, repre- 8 Figure 6. 165 min. 135 min. ! 0.01 0.1 1 10 100% AEROSIL CONC. (LOG SCALE) Influence of Acrosil on suction rate of wheat starch sent the linear diffusion rate plotted against the Aerosil concentration at different time intervals. The graphical representation indicates that the addition of finely divided silica effects a remarkable change in the linear flow rate in the powder bed. The action of Aerosil on the linear diffusion rate is demonstrated by two peaks or two critical concentra- tions. The first concentration represents a minimum diffusion rate, and the second represents a maximum diffusion rate. This behavior can be explained on the following basis: * W. A. Bachofen, Basel.

WATER VAPOR SORPTION OF STARCHES 113 The addition of small amounts of Aerosil produces rearrangement and new orientation of the starch grains. The very fine Aerosil particles separate the starch grains from one another, and this results in relatively wide voids. Since the size of the voids is partially responsible for the suction potential, the smaller the void the higher the suction potential. Therefore, the suction will be suppressed, and the first critical concen- tration will correspond to the amount of Aerosil producing the smallest suction potential for the packed column. The addition of more Aerosil starts to fill the voids between the particles in the powder bed. The effective pore size gradually di- minishes, favoring an increase in suction potential. The maximum rate of linear flow is obtained at the second critical concentration where the minimum pore size is reached. Further addition of Aerosil above the second critical value produces an expansion in the powder bed. The system is then assumed to consist of a matrix of Aerosil in which the starch grains are dispersed. The effective pore size increases greatly with the expansion in the powder bed. Thus suction potential and rate of linear diffusion will be seriously affected. On the basis of these basic experiments it is possible to modify the water permeability and capillary suction in a powder bed by appropriate addition of Aerosil. This finding is of practical importance in tablet disintegration since the suction potential and rate of permeability, coupled with the swelling capacity, are the main factors responsible for the disintegration time. The proper choice of a starch with suitable swelling capacity and the correct content of Aerosil in tablet formula- tions might explain the short disintegration time obtained with such tablets. REFERENCES (1) Neumann, B. S., Flow ]•roperties of Disperse Systems, Ed. Herroans, J. J., North-Holland Publishing Co., Amsterdam (1953). (2) Gregg, S. J., and Behrens, A. J., J. Appl. Chem. (London), 1, 139 (1951). (3) Whistler, R. L., Methods in Carbohydrate Chemistry, Vol. III, Academic Press, Inc,, London, 1963, p. 120. (4) Craik, D. J., and Miller, B. F., J. Pharm. ]•harmacol., 10, 136 (1958). (5) Millet, J., and Parisot, J., ]•roc. U. N. Intern. Conf. ]•eaceful Uses At. Energy, 2nd. Geneva, after CM. 56, 68596 (1962). (6) Ulmann, M., Ern•hrungs-forschung, 1, 96 (1956). (7) Dumon,,•kii, A. V., and Nekrgach, E. F., after C.A. 56, 9636 (1962). (8) Czetsch Lindenwald, H. V., E1-Khawas, F., and Tawashi, R., J. Soc. Cosmetic Chemists, 16, 251 (1965). (9) Tawashi, R., Pharm. Ind., 25, 655 (1963).