906 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS Figure 9. Spherical micelle Two general factors determine what the cmc will be for a specific surfactant in solution. The first is the structure of the surfactant mole- cule and the second is the nature and amount of additives in solution. Concerning the surfactant molecule, as molecule weight increases for a given ionic species the cmc decreases. As branching increases (for a given molecular weight) cmc increases (Table I). Concerning addi- tives, salts generally lower the cmc, probably increasing the size of the aggregates. Table I Effect of Anionic Surfactant Molecular Weight and Structure on Critical Micelie Concentration Surfactant Cmc (m/l.) Ref. Sodium decyl (1) sulfate 3.4 X 10 -2 Sodium dodecyl (1) sulfate 8.5 X 10-a Sodium tetradecyl (1) sulfate 2.2 X 10 -a Sodium hexadecyl (1) sulfate 5.2 X 10-4 Sodium decanoate 9.4 X 10 -2 Sodium dodecanoate 2.3 X 10 -2 Sodium tetradecanoate 6.9 X 10 -a Sodium hexadecanoate 2.1 X 10-a Sodium tetradecyl (1) sulfate 2.2 X 10 -3 Sodium tetradecyl (2)sulfate 3.3 X 10 -3 Sodium tetradecyl (3) sulfate 4.3 X 10-a Sodium tetradecyl (4) sulfate 5.2 X 10 -a Sodium tetradecyl (7) sulfate 9.7 X 10 -3 (9) (9) lo) lo) 11) 11) 11) 12) 10) 13) (13) (lo) (13)

PHYSICAL CHEMISTRY AND PRODUCT DEVELOPMENT 907 Figure 10. Elongated spherical micelle Figure 17. Log boom micelle Water-soluble solvents such as ethanol and ethylene glycol increase cmc (14) but with high concentrations of ethanol, micelle formulation is almost eliminated (15). For nonionic surfactants with a given hydrophobe, as the EO chain increases, cmc increases slightly. For a given EO chain, as the hydro- phobe mol wt increases, cmc decreases sharply (Table II). This general information can be applied to formulation problems in the following ways: First, in terms of solubilization, when a hydrocarbon is added to a surfactant solution below its cmc, the solubility of the hydro- carbon is almost the same as in surfactant-free water. Above the cmc, solubility increases dramatically.