390 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS make varifiable or refutable predictions. The techniques discussed assist in predicting short-term emulsion stability, but the practical cosmetic chemist is vitally interested in long-term stability, not minutes or hours, but weeks or months. At present he cannot find out empirically if prophetic judg- ment can be made this would be most valuable. It is appreciated, of course, that such long-term stability may be affected in complex systems by migra- tion of trace impurities to the interface, but could these techniques be of assistance ? THE LECTURER: The techniques I have described could give you the information you require provided the change taking place occurs at the interface. It is my experience that there is no substitute for time and one must therefore be prepared to set up the experiment to last for the required period. It may be that surface chemical techniques will detect changes much earlier than those detected by storage tests on the product. For instance, migration of divalent ions to a surface stabilised by a sodium soap will be manifest as a change in rigidity of fhe interface. TECHNIQUES OF FOAM MEASUREMENT G. E. NEU, B.Sc.* Delivered at the Summer Confere•ce of the Society on 25th August 1960. The physical properties of shampoo and toothpaste foams are character- ised using seven different measurements. These measurements are easy to perform and yield reproducible results. A subjective assessment of the foam is correlated with the measurements described. INTRODUCTION FOAMINC IS of major importance in shampoos and toothpastes, and the nature of the foam may be critical in determining the acceptability of these products. With toothpaste, the volume must be sufficient but controlled, and one of the chief qualities required is easy rinsing away the foam is also important to the flavour which it disperses throughout the mouth to allow maximum contact with the taste buds. The actual requirements of these two products are very different. A shampoo is normally expected to give a thick, creamy, voluminous foam, which is associated by the user with cleansing power and emolliency. Even if a shampoo has excellent cleansing properties and leaves the hair in very good condition afterwards but only provides a small amount of a thick, *Unilever Ltd., Isleworth, Middlesex.

TECHNIQUES OF FOAM MEASUREMENT 391 weak foam, it is unlikely to be accepted as a good shampoo. Rinsing is expected to be easy. The domestic user assesses the foam removed by what is seen in the wash basin, so that it is important that the col/apse should be rapid and complete on dilution, and the foam should not simply wash off the hair. The detergent concentration in the solution applied to the soiled head, even allowing for the amount of water in the wetted hair, is high (1-5%-2.0%), which is well above the critical micelle concentration for the detergents studied. The amount of soil removed from the hair exhausts only about 5% of the detergency of the shampoo. Toothpaste foams are generated under more diverse conditions than shampoos. The volume and composition of the saliva, the amount and type of debris in the mouth, the vigour and time of brushing the teeth, vary considerably. Thus a toothpaste has to be more versatile in pro- ducing foam under these differing conditions. The volume must be suffi- cient to fill, but not overfill, the mouth it must feel solid, creamy and viscous and not elastic, thin or light. Finally, it must rinse out easily, but it is not important if it does not collapse. The concentration of detergent in toothpaste/saliva mixture from the mouth is about 0.25-0.40% which in some cases is very near the critical micelie concentration level. What are the important factors controlling the properties of foams ? We would like to be able to express the subjective properties in objective parameters, and a better understanding achieved by empirical methods will point to further research work in this field. Our aim has been to generate a foam under conditions which simulate actual usage, and to develop techniques for measuring all of the properties of the bulk foams which are possibly important in the subjective assessment. Previous work in this field has been almost exclusively devoted to a consideration of foam volume and drainage. These are relatively simple parameters which can easily be assessed, but we have extended the work to cover other, more nebulous, factors. These factors are, beside the foam volume and drainage, the viscosity, the change in viscosity with age of the foam (here called viscosity differential), light transmission, bubble size and distribution, and breakdown during rinsing. Results on the last technique are not yet sufficiently complete for presentation. When these properties are known it may be possible to forecast the characteristics and expected behaviour of the foam. The methods described are applicable to both toothpastes and shampoos and to other products. EXPERIMENTAL Generation of Foam Our aim was to make large amounts of foam under strictly controlled 5