398 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS up and breaking down forces are obviously equal. An increase of speed shifts the equilibrium state in favour of the building up forces, until an equilibrium is again attained at a higher specific volume. For example, the specific volume of foam for M at which this equilibrium is attained is 7.9, 11.5, 16.6 and 21.4 ml/g of solution for speeds of 520, 620, 720 and 820 r.p.m. respectively. The subjective feel of a shampoo foam is also influenced by other char- acteristics not studied in this programme. For instance, the adsorption of detergent on the hands is important, so two foams of similar physical characteristics may still feel different. We studied the viscosity of the foam during building up and during its breaking down. The viscosity build up is similar to the foam volume generation, except that peak viscosity is reached after only 4-6 minutes' beating (Graph 2). M produces the most viscous foam, reaching a peak after 4 minutes' beating. H, C and N reach their peaks after 6 minutes' beating. The speed of beating has a great influence on the viscosity of the foam produced (Table 1). This is true with both toothpastes and with shampoos. Table 1 Effect of speed of Mixmaster on the foam viscosity of M and CD Viscosity in centistokes Speed in Shampoo r.p.m. detergent Toothpaste M CD 520 16.8 66.6 620 30.6 88.0 720 48.6 148.4 820 57.0 195.0 In practice, therefore, the manner in which the shampoo is used is a critical factor in the production of a foam. From the user's point of view, a shampoo which foams when very little work is done on it would be popular. Detergents H, N and C are not suitable for such a shampoo as no foam is produced below 400 r.p.m. M, however, produces foam at 250-300 r.p.m. and this possibly provides further justification of its use. The concentration of the shampoo detergent under usage conditions is about 5-10 times the critical micelle concentration. This means that the detergent concentration should not be very critical. In fact, the variations in foam viscosity are negligible between 1% and 2% active detergent concentration so that differences which are likely to be encountered in practice are unimportant.

TECHNIQUES OF FOAM MEASUREMENT 399 The effect of temperature on the viscosity of the foam was also investi- gated. The air was kept at constant temperature, but the detergent solution and the mixing bowl and beaters were heated or cooled to the required temperature, between 10 and 40øC. The 10øC solution of M produced a more viscous foam (19-2 centistokes at 1 minute beating at 720 r.p.m.) than the samples at 20øC, 30øC and 40øC. The foam viscosities of these samples were almost the same (13.0, 12.6 and 12-4 centistokes for the 20, 30 and 40øC samples respectively), which is surprising. The size of the aliquot in the mixing bowl does not influence the foam viscosity. A 25 cc aliquot is insufficient to produce any foam. 50 and 75 cc aliquots generate foams which have similar viscosity characteristics. G•APH 3 PLOT OF VISCOSITY DIFFERENTIAL FOR SOILED AND UNSOILED SHAMPOO DETERGENT SOLUTIONS SOILED UNSOILED AGE. OF FOAM IN MiNUT[$ Some detergents produce initially viscous foams which after a few minutes lose most of their viscous nature, whilst other detergents produce less viscous foams which are stable for 3-5 minutes. The viscosity differ- ential measures this rate of degradation of the foam. We have found the viscosity differential to be a distinct characteristic of each foam. The viscosity differential results of the shampoo detergent solutions are shown in Graph 3. The curves show two distinct sections, a steep initial 6