SOLUBiLiZATION OF THREE-COMPONENT SVSTEM 209 EO Molecule less than the the optimum number more than the Number optimum number optimum number Figure 6. Configuration of the S region which has an HLB close to that of POE (18 mols) dioleate. The results (Table III) show that POE (7 tools) monooleate very often gives a solubiliza- tion system separating into two layers similarly to the POE nonylphenyl ethcr type, and the stable homogeneous solubilization region is not very wide. Table III shows that, even at approximately the same HLB, the solubiliza- tion region becomes extremeiv wide as the molecular weight becomes larger, such as in the POE dinonylphenyl either type and the POE dioleate type. This seems to bc related to the steric structure of the surfactant molecule. 'Fable III Comparison of S Regions at ()ptirnum HLB Percenta•c Surfactants HLB of S Region POE (5 tools) nonylphenyl ether 7.8 POE (10 mols) dinonylphcnyl ether 8.7 POE (7 mols) monooleate 9.0 POE (18 mols) dioleate 8.9 26 59 29 57 The W/O type solubilization system is obtained when POE (3 mols) octylphenyl ether, POE (3 mols) nonylphenyl ether, POE (5 tools) dinonylphenyl ether, or POE (9 tools) dioleate is used. On the other hand, the W/O type solubilization system is obtained only in the oil-rich region when POE (5 mols) octylphenyl ether, POE (4 mols, 5 mols) nonylphenyl ether, POE (5 mols) oleyl ether, POE (10 tools) dinonylphenyl ether, or POE ( 18 tools) dioleate is used. In the case of the solubilization system, however, there was not much difference between W/O type and O/W type on several occasions and W/O type changes to O/W type gradually as the quantity of oil decreases.

JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The W/O type solubilization system is not very stable in general, and it often happens that, even though the system is of the W/O type immediately after preparation, it separates into two layers by aging, the bottom layer being the O/W type solubilization system and the top layer the W/O type. The stable homogeneous W/O type solubilization system is obtained mainly in the region containing a large quantity of the surfactant and starts to separate as the quantity of the surfactant decreases, but the stable O/W type solubilization system is obtained even in the region containing a small amount of the surfactant. The W/O type solubilization is chiefly obtained by the surfactants having a smaller number of EO molecules and the O/W type by those having a greater number of EO molecules. The surfactants with the optimum number of EO molecules produce both the W/O type and O/W type solubilizations. For example, in the case of the POE nonylphenyl ether type surfactants, 3-molecule ethoxylates produce the W/O type solubilization system, 4- and 5-molecule ethoxylates the W/O and O/W types, and 7- molecule and higher ethoxylates the O/W type. The optimum EO mole- cule number for solubilization is 4 and 5, and in the case of these surfactants, the W/O type appears in the system containing a large quantity of oil and the O/W type in the system containing a small quantity of oil. Thus, the S region obtained by the surfactant having the optimum EO molecule number (as in Fig. 6) is a mixture of the regions of the W/O type and the O/W type solubilization. For the purpose of studying the relationship between the optimum number of EO molecules for solubilization and that for emulsification, further experiments were carried out. 2-Octyl dodecanol and squalane, in addition to liquid paraffin, were used as the oils with the POE oleyl ether type and POE dinonylphenyl ether type surfactants for which the optimum number of EO molecules had been comparatively easily recognized in the solubilization experiments of liquid paraffin. The results are shown in Fig. 7. The S regions at two conditions (137 and 1-RT) are shown in this figure. The optimum EO molecule number for solubilization of 2-octyl dodecanol and squalane was 15 and 7 for POE oleyl ether and 15 and 10 for POE dinonylphenyl ether, respectively. Observations made after 7 days showed the same results. The optimum EO molecule number for emulsification means the: number of EO molecules in the surfactant giving the mo t stable emulsion when the exFeriments are carried out on the o•l-water-surfactant system by varying the water volume ratio from 30 to 70% and the concen-