126 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figures. All of the suspensions, which were flocculated in the absence of salt, tended to become defiocculated when small amounts of sodium sulfate were added. This effect was most pronounced in the suspensions containing the surfactants with the longer hydrophilic chains. However, as the concentration of sodium sulfate was raised, a point was reached at which the suspensions once again became more flocculated. The increase in flocculation, deduced from a rise in sedimentation volume (Figure 1) and apparent viscosity (Figure 2) occurred when the sodium sulfate concentration was about 0.5 to 0.6 M and seemed to be independent of the hydrophilic chain length of the surfactant. The effect of sodium sulfate on surfactant adsorption on benzocaine is shown in Figure 3. Adsorption of polyoxyethylene nonylphenols with mean polyoxyethylene lOO 8o 40 2O 0.2 0.4 0.6 0.8 1.0 concentration of Na2SO 4 (iv i) Figure 3. Effect of sodium sulfate on adsorption of various polyoxyethylene nonylphenols onto benzocaine. ([]) n = 7.5 (A) n = 15 (I) n = 50. lengths of n=15 and n=50 increased gradually as the sodium sulfate concentration was raised. There was a precipitous rise in the percent adsorbed (and a correspondingly steep loss from solution) when the sodium sulfate concentration reached 0.5 to 0.6 M. The shorter chain surfactant exhibited a more pronounced increase in adsorption in the presence of low concentrations of sodium sulfate and a ste. ep rise when the sodium sulfate concentration reached 0.2 M. The decrease in sedimentation volume at relatively low sodium sulfate concentrations indicates that the sediment was more closely packed than in the absence of salt. The increase in surfactant adsorption suggests that the surfactant layer at the particle surface became somewhat more concentrated, leading to increased repulsion between the particles. It is also possible that there was improved wetting in the suspensions containing the surfactant with n = 50. The cloud points of these surfactant solutions in the presence of sodium sulfate are plotted in Figure 4. In each case, the addition of sodium sulfate resulted in a drop in

EFFECT OF SALTS ON FLOCCULATION 127 o o lOO 80 60- 4o 2o I 0.2 0.4 0.6 0.8 J • concentration of Na 2S04(M) Figure 4. Effect of sodium sulfate on cloud point of solutions of various polyoxyethylene nonylphenols and polyethylene glycol 4000. (0) n = 7.5 (•) n = 15 (©) n = 50 ([]) polyethylene glycol 4000. cloud point. The slopes of the curves for the surfactants with n=7.5 and n=15 are approximately parallel. There is a change in slope when n=50. Polyethylene glycol 4000, which is identical chemically to the hydrophilic portion of these surfactants, is included in Figure 4 for reference. The slope of the plot of cloud point against sodium sulfate concentration for polyethylene glycol 4000 (equivalent to a value of n of about 90) was parallel to the plot for polyoxyethylene (50) nonylphenol. The cloud point represents the temperature at which the surfactant micelies grow large enough to cause visible changes in the optical •operties of the solution. Further heating results in .phase separation or coacervation, in which a water-rich and a surfactant-rich phase coexist. The addition of sodium sulfate affects the surfactant solutions in much the same, way as an increase in temperature would. Thus, with an increase in sodium sulfate concentration, the cloud point could be reduced until it occurred at room temper •ature. Further addition of the salt resulted in phase separation. In the case of the surfadtant with n=7.5, phase separation occurred at a sodium sulfate concentration of about 0.4 M. The other surfactants coacervated at a concentration of about 0.8 M. These data help to explain the adsorption results in Figure 3. The precipitous loss of surfactant from solution that was attributed to adsorption actually indicated separation of a surfactant-rich phase from the rest of the system. This was difficult to observe visually. In the flocculated suspensions containing sodium chloride (to be described below), however, a thin layer of a separate liquid phase could be seen just above the sediment. It is interesting to note that, in the presence of benzocaine, phase separation took place at sodium sulfate concentrations that were lower than those required to induce coacervation in the absence of the solid.