LAPONITE CLAY--A SYNTHETIC INORGANIC GELLING AGENT 257 DR. NEUMANN: This is just a question of concentration. The higher the concentra- tion of Laponite, the higher the rigidity. We could not handle mixtures of higher concentration than 5% Laponite, because they become so stiff that our mixing equip- •nent could not cope. If we had used a colloid mill or a Zed blade mixer, we could have made some rigid gels. Ma. A. Moils: The disadvantage of clays is the slow rate of swelling of colloidal particles. Wood, Catacalos and Lieberman (5) found that it took six months to achieve complete swelling in a 5% Veegum dispersion. Have you studied the swelling rate of Laponite? DR. NF•UMAN•: If Laponite is mixed with a polar organic compound and water added, equilibrium is reached quickly. Even when the clay is added to the solvent- water mixtures, equilibrium conditions are reached in less than a day. One of the differences between natural clay and Laponite is that the latter has smaller crystallites and consequently larger surface area. Dispersion is more rapid if more surface is exposed to solration. MR. M. J. I(oo'r: We have used both heat and high stress for swelling Laponite- type clays. Could you comment as to which is the best method? DR. SANso,a: If the application of heat is possible, the dispersion is faster at higher temperature, as with all clays. However, Laponite is quite easy to disperse in cold water with high shear mixing. MR. R. H. McDowEI•I•: Presumably the Laponite gels have a true static yield value, below which an applied shear stress does not decay with time. Have you measured this, and if so, how does it compare with the extrapolated stress value? DR. N•u•I•N•: The true static yield value is, of course, lower than the extrapolated shear stress. We have not measured it accurately, but have estimated it by a simple bottle inversion test. The gels were stored in approx. 75mm diameter bottles, and inverted after 16-24 h. If they did not flow, they were given a "pass" tnark. DR. SaNsoM: We have recently acquired a Weissenberg rheogoniometer and so the yield value can be studied at very low shear rate. As shown by Carless and Nixon (3), it is difficult to estimate a static yield value with the Ferranti-Shirley viscometer. MR. J. M. B[a}c•wa¾: Reverting to Mr. Watson's question. I wonder whether the problem with toothpaste might not be calcium ions reacting with the clay? DR. SA•so•: It could be - it depends on the concentration. MR. J. W. CI•im, t•a•aI•F,: I should like to revert to Mr. Watson's contribution concerning the rate of formation of gel structure of the glycerol-water-Laponite system, and would like to supply some more detail. I should be grateful for your (5) Wood, J. H., Catacalos, G. and Lieberman, S. V. A rheological study of the aging of Veegum suspensions. f. Pharm. Sci. õ9. 354 (1963).

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.