SODIUM I.AURYL SULFATE-LAURYL ALCOHOL-WATER SYSTEMS 395 EXPERIMENTAL A description of the purity of materials used and the experimental techniques has been previously published (1). RESULTS AND DISCUSSION In Fig. 2, the experimental values of FDTT arc plotted against lauryl alcohol concentration. Each curve represents constant sodium lauryl sulfatc. It is apparent that an increase in the alcohol concentration leads 0.05 0.O4 •'0,0• 0.02 O.OI 24øC O.2 0.4 0.6 Na lauryl sulfate, g/100 g soln. Figure 4,--Isotherms of film drainage transition temperature. to an increase in the FDTT at constant detergent concentration up to the region where further addition of lauryl alcohol results in little or no in- crease of FDTT. Figure 3 is obtained by taking vertical slices (at constant alcohol concen- tration) through the curves of Fig. 2. Increasing the detergent concen-

396 jOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS tration while holding the alcohol constant results in a decrease of the FDTT, the effect leveling off at high detergent concentrations. Another method of sectioning Fig. 2 is presented in Fig. 4. Each curve represents compositions of the system which give rise to the same value of the FDTT. The sharp break in each of these isothermals occurs at 0.226 per cent sodium lauryl sulfate and is quite close to the CMC, 0.234 per cent, reported by Mysels (3) for pure sodium lauryl sulfate. The rapid $0 c_ I0-- __ o o I r I 80 85 90 95 I0-t- LoglOZ Figure 5.--Film drainage transition temperature vs. log Z. rise of the curves beyond the CMC illustrates that very much higher ratios of lauryl alcohol to sodium lauryl sulfate are required to give the same FDTT above the CMC than below it. The shape of these isothermals is similar to what is found when the solubility of a long chain alcohol in a detergent is obtained at constant temperature. Above the critical con- centration there is a sharp increase in the quantity of alcohol which dis- solves in the detergent solution. The nearly linear character of the curves