438 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS pouring of the material particularly as the rate of gel reformation is not too rapid. The rate of build up of Laponite static yield value is more rapid than with bentonite and little settling of suspended solids would take place. The low plastic viscosity of Laponite gels enables easy pouring in relation to a bentonite gel of similar yield value as the latter has a higher plastic viscosity. ACKNOWLEDGEMENTS The authors would like to thank J. Ocron and P. P. Georgakopoulos for certain experimental results. (Received: $Oth June 1969) REFERENCES (1) Nixon, J. R., Georgakopoulos, P. P. and Carless, J. E. J. Pharm. Pharmacol., 18 283 (1966). (2) Saunders, P. R. and Ward, A. G. Proc. 2nd Int. Conf. Rheol., Oxford p.284 {1954). (3) Chaw]a, B. P.S. Ph.D. Thesis, London (1967). (4) Blixon J. R., Georgakopoulos, P. P. and Carless, J. E. J. Pharm. Pharmacol., 20 521 (1908). ($) Cureper, C. W. Bl. and Alexander, A. E. Austral. J. Sci., A5 146 (1952). (6) Levy, G. J. Pharm. Sci., 51 947 (1962). (7) I-Iovwink, R. Elasticity, Plasticity and the Structure of Matter. 2nd. Ed. p.13 (1958) (Dover Publications, Blew York) (8) Blixon, J. R. and Chawla, B. P.S. J. Pharm. Pharmacol., gl 79 (1969). (9) De Butts, E. H., I-Iudy, J. A. and Elliott, J. H. Ind. Engng. Chem., 49 94 (1957). (10) Barry, B. W. and Shotton, E. J. Pharm. Pharmacol., 119 Suppl., 110S-120S (1967). (11) Neumann, B. S. Rheologica Acta, 4 250 (1965). (12) Cheng, D.C. I-I. Brit. J. Appl. Phys., 117 258 (1966). (15) Veis, A. The macromolecular chemistry of gelatin. 891 (1964) (Academic Press, New York) (14) Bungenberg de Jong, H. G. in Colloid Science. (Ed. Kruyt, H. R.). 2, 335 (1949) (Elsevier, Amsterdam). DISCUSSION MR. A. Mogs: Have you any idea about the rate of formation of the gels? Would you say that in Fig. 6 you have two rates of reaction, the fast reaction immediately you stress the gel, and the slow reaction following? This may not be due to two different structures but to one structure decaying rapidly at first and slowly subse- quently. DR. NIXON: The time for complete formation of the Laponite B gels was 16-20 h at 20 ø, after preliminary high speed stirring with a Silverson mixer. This process was not speeded by heating. With regard to your second point, it is quite possible that there is one structure xvhich undergoes rapid breakdoxvn followed by slower equilibrium and a single line

SOME APPLICATIONS OF RIGIDITY AND YIELD VALUES 439 is possible through the experimental data, as indicated in Fig. 6. It must also be remembered that the Ferranti-Shirley viscometer has a slow response and is being pushed to its limit by using a 10 s time axis. Bentonire gave a greater indication of being a double reaction than did Laponite B. MR. J. B. WILK•NSON: Have you made any studies of the effect on the theology of these gels caused by altering the water structure? There is always some question with protein gels whether the protein structure controls the water or vice versa. You will know that this is a matter of considerable interest in many circles. DR. NixoN: The structure of water has been investigated recently with regard to its association with bentonire gels and no correlation between water structure and these gels was found (15). As the yield value increased the nmr picture remained the same. Although it could be an important point we have not at the moment investi- gated this. DR. S. D. G•RSHON: The movement that takes place in the gel rigidometer can be either due to disruption of adhesive forces between the gel and the glass, or cohesive forces within the gel. If the cohesive forces are strong within the gel, it is conceivable that you would just simply be moving the whole gel along the surface. DR. NIXON: This is a standard method of measuring gel rigidity and with the small pressures used, slip of the whole gel along the surface of the glass is very unlikely. In any case we have measured the height of the meniscus at the side, both at rest and under applied pressure, and there has been no detectable movement, whereas the centre of the gel meniscus can be seen to move and return to its original position when the force is removed. DR. J. J. MAUSN•R: You have described the various factors which influence rigidity. Does pH play any role in these measurements and how critical is the control of pH? DR. NixoN: pH played a more important part in controlling the rigidity of gelatin gels. For this reason all the gelatin gels were adjusted to a definite pH. MR. D. F. ANST•AD: You mentioned that bentonire contained impurities which rendered its use not quite as acceptable as that of Laponite B. You used the pharma- ceutical grade of bentonire. Would I be correct in assuming that your impurities are not chemical? DR. NixoN: The bentonire used throughout this work has been one batch of B.P. grade and the characteristics of this material can vary considerably from batch to batch. The B.P. monograph gives limits for the alkalinity and arsenic content as well as for the amount of "gritty particles" present, but does not say whether these are silicacious. You would be correct in saying that our main bentonire impurity is not chemical. MR. D. F. ANST•D: It is possible to obtain completely grit-free bentonire to comply with pharmaceutical specifications. PROF. C•R•.ESS: The B.P. specification does not control the impurity limits or gel strength as closely as people would like. It tends to be rather a loose description. The calculation of yield values which we have used for Laponite B is rather a crude method utilising the determination of static yield values. Other people have attempted (15) J. Pharm. Sci. 511 714 (1969)