258 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS observations on our findings and your co•nments as to whether our mixing procedures might be at fault. V•re made up four gels:- (i) Distilled water - 2% Lapo•ite. This took about two weeks to form any struc- ture. (ii) Tap water - 2% Laponite. Gelled overnight. (iii) 50:50 Sorbitol syrup-water-2% Laponite. Gelled overnight. (iv) 50:50 Glycerol-water-2% Laponite. Took approximately two weeks to form a gel, but this made up only the lower half of the mixture. The upper half remained fluid. The gels were made using a high-speed stirrer, and in the case of (iv) by prc- dispersing the Laponite in the glycerol. DR. S•4so•: (i) The gel does not set up in distilled water, because a definite concentration of electrolyte and/or solvent is required to reduce the thickness of the electrical double layer on the surfaces of the dispersed particles. When the donble layer is thick, the particles cannot approach each other sufficiently to form a gel. A structure does, however, slowly form as a result of thermal collisions. Your observation of gel forming in two weeks sounds reasonable. (ii) Tap water contains a low concentration of electrolyte, so the gel is fortned more readily than in distilled water. At higher electrolyte concentrations, the gel forms even more quickly than your observed 16-24 h. (iii) With the sorbitol-water-Laponite system, the same comments as given under (ii) apply. (iv) The difficulty you experienced with the gelling of glycerol-water-Laponite system is new to us. ls it possible that the Laponite settled to the bottom before it was fully dispersed and started swelling from there? A homogeneous gel is obtained if the initial mixing produces a strong enough gel to hold up the rest of the undispersed clay until that, too, disperses. We have a mixture here of 60% glycerol-40% water-2% Laponite which is quite homogeneous, clear and stable. M•ss A. E. You•½: Do you have any experience of the incorporation of inorganic pigments, such as titanium dioxide and dyestuffs in these gels? Da. S•NSO•: The largest market we have at present for Laponite is in the paint industry, where it is used in the presence of high concentrations of pigments and sometimes dyestuffs. The paints containing Laponite are found to be very stable. M•. J. W. Rm•m)so•: Would the ion exchange reaction which normally affects the stability of Laponite gels in the presence of cations take place less readily with the organic modified Laponite briefly referred to? Da. •N•u•: In an organic medium there is, of course, very lnuch less chance of cation exchange, not only because there are fewer cations present, but also because the large organic cations on the clay are almost irreversibly adsorbed. Gel formation in these systems is not dependent on the effect of electrolytes on the electrical double layer, and therefore the stability is also independent of the factors that normally control the stability of inorganic systems.

J. Soc. Cosmetic Chemists 21 259-269 (1970) ¸ 197o Society of Cosmetic Chemists of Great Britain Sampling and assessment of mixtures for cosmetics and pharmaceuticals powder j. A. HERSEY* Presented at the sy•nposium on "Powders", organised by the Pharmaceutical Society of Ireland and the Society of Cosmetic Chemists of Great Britain, at Dublin, on 18th April 1969. Synopsis--This paper discusses a planned approach to the SAMPLING of PHARMA- CEUTICAL TABLETS in which samples are taken from the MACHINE to ensure satis- factory sampling performance from each station throughout the tabletting run. The purpose of sampling must be considered before deciding on the sampling plan and sample size. An INDEX of HOMOGENEITY has been applied to results previously described by an index of MIXING using, as a basis, the randomized state. •he degree of homog•neity is of the utmost importance to the mixture consumer, where the completely randomized mixture may not be sufficiently homogeneous for his purpose. Where possible, a parallel has been drawn between the COSMETIC and pharmaceutical POWDER fields. INTRODUCTION Powder mixing is an important unit operation in both pharma- ceutical and cosmetic industries. In pharmacy the preparation of unit solid dosage forms, such as tablets and capsules, requires a considerable understanding of the powder mixing process in order that a high degree of homogeneity between products may be attained, especially when highly potent drugs have to be dispensed in a large bulk of inert diluent. The formulation of powder cosmetics is a no less arduous task where pigments must be so dispersed that on application to the skin, a layer of single particles must present a homogenous colour reaction. *Department of Pharmaceutics, The School of Pharmacy, University of London. Now at Pharmaceutical Division, Sandoz A.G., CH-4002, Basle, Switzerland. 259