LATHERING POTENTIAL OF SURFACTANTS 227 Values of t .... for Na lauryl sulfate, Na lauroyl sarcosinate, Na C14_t6 olefin sulfonate and Na laureth(3)sulfate are 57 s, 58 s, 50 s, and 14 s, respectively. The minimum surfactant concentration, Cmin, which achieves maximum drainage time is determined by estimating the slope intercept nearest the leveling off of drainage time increase with concentration. In this case again, Na lauryl sulfate, Na lauroyl sarcosinate, Na C14_16 olefin sulfonate and Na laureth(3)sulfate yield values of 0.30%, 0.•, 0.30% and 0.60% respectively. The standardization of the pH of the solutions under study is especially important. Lather drainage times do vary with pH and comparisons between surfactants should be at the same pH. In order to standardize at a practical compromise, the surfactant systems were all run at pH 6.75 unless noted. Formulated shampoos were run at their existing pH. Those interested in formulation work will find value in determining the pH range yielding the highest lather drainage values in order to help optimize the lathering of the finished product. The effect of additives and co-surfactants on lathering is of obvious value. Since many additives are employed in formulation work because of their traditional reputation as foam stabilizing agents as determined from Ross-Miles foam-decay data, we re- examined their utility as lather promoting agents under the more dynamic conditions of this test. Our initial lather trials using Na lauryl sulfate and Na laureth(3)sulfate with the traditional foam stabilizer, lauramide DEA, were disappointing. Hasty, qualitative trials seemed to indicate that lauramide DEA, contrary to its presumed reputation, was depressing the lathering tendency of anionic surfactants. This was borne out in actual 80 drain time, '""-- • 2:1 Na Laureth (3) Sulfate: I Laur•lmJde DEA I 6O 4O 2O 0 sec. 0.3% 0.6% 0.9% Surfactant Concentration Figure 3. Lather drainage time versus concentration. 1.2% active

228 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS lather drainage time measurements (Figure 3) and was readily apparent during the test. Na laureth(3)sulfate-lauramide DEA mixtures, for instance, formed freeflowing, "soupy" lathers which lacked strength and integrity. Clearly, if one assumes that the use of an additive which yields a prolonged Ross-Miles foam-decay time would also lead to the enrichment of lather, there would be the possibility of being misled. The Ross-Miles test is usually run at a low active concentration (ca. 0.1%) which, under the rather static conditions of the test, yields a loosely structured foam. Additives, such as lauramide DEA, which prolong the decay rate of simple foams are useful in bubble bath and hand dishwashing products. Under higher active concentrations and more dynamic conditions, however, they seem to have an adverse effect on lather quality. The relatively poor lathers obtained with Na laureth(3)sulfate, even under the rigorous conditions of the test, prompted us to examine the effect of the addition of sodium lauroyl sarcosinate. As the curves in Figure 4 indicate, this addition of sarcosinate substantially improved the lather drainage times. Lather drainage times versus concentration for other commonly used surface active materials and selected mixtures are presented in Figures 5 and 6. LATHER OF SURFACTANT MIXTURE RATIOS AT A FIXED ACTIVE CONCENTRATION Since modern shampoo formulations are composed of two or more principal surfactants and lather boosters, it is natural to examine mixtures of this type in order to determine interactions and find a ratio which produces best results. In attempting this, we have fixed the total surfactant concentration at 1% active as this concentration is drain time, sec. 80 2:1 Na Laureth (3) Sulfate: Na Lauroyl Sarcosinate Na Laureth (3) Sulfate I 60 40 20 0 0.3% 0.6% 0.9% Surfactant Concentration Figure 4. Lather drainage time versus concentration. 1.2% active