JOURNAL OF THE SOCIETy OF COSMETIC CH.EMISTS tions of substances which led to molecular interactions at the air/ water interface. For example, an aqueous solution of sodium cetyl sulphate gave a stable thin emulsion when shaken with a solution of cholesterol in •nineral oil. When cholesterol was replaced by cetyl-alcohol the result- ing emulsion was stable, but viscous. The importance of the close associa- tion of the non-polar portions of the interacting substances is shown by the fact that sodium cetyl sulphate gave stable emulsions with elaidyl alcohol but not with oleyl alcohol. Similarly, cholesterol and sodium elaidate gave stable emulsions, while cholesterol and sodium oleate did not. Elaidyl alcohol and elaidic acid are transisomers of oleyl alcohol and oleic acid respectively. The latter pair resist close adlineation on account of the cis configuration of the molecule. The manner in which this type of steric hindrance may adversely affect emulsion stability is further demon- strated by the fact that strong com- plexes are formed between long chain alcohols and long chain amines, but less strong complexes between the same alcohols and the quarternary amonium compound corresponding to the amine. Also, sodium lauryl sul- phate -- CH• (CH2)nOSO3Na in conjunction with cholesterol forms s t a b 1 e emulsions spontaneously, while the corresponding sodium s e c o n d a r_y lauryl sulphate-- CH, C•0H2• •CHOS0.•Na is considerably less effective. PRACTICAL APPLICATIONS OF THE ABOVE THEORIES WITH REFERENCE TO O/W EMULSIONS The theory of interfacial cmnplex film formation is of .the greatest significance to the cosmetic chemist. It explains why emulsions stabilised by alkali soaps reverse when poly- valent salts, e.g., calcium salts, are added. The polyvalent cations neutralisc the charge on the oil particles and allows them to coalesce. The film is strengthened by a cross linking of the long-chain fatty acids by the polyvalent ions. OI, L-IN-WATER EMULSIONS Stable low viscosity o/w emulsions are not easy to prepare, but as a starting point the following experi- ment is of interest. When 2 gms. of high grade wool wax are dis- solved in 50 ccs. of mineral oil and shaken with 50 ccs. of an aqueous solution containing 2 grns. of so.dium lauryl sulphate (905/0 ester), emul- sification is spontaneous. One sharp shake is sufficient, and the tempera- ture of the solutions is not significant. The resulting emulsion is very thin despite the high oil concentration, and extremely stable. Emulsifica- tion takes pla, ce at very low tempera- tures which is very often a distinct advantage, When wool wax is replaced by qetyl alcohol the resulting emulsion is very viscous and' stable. By using mixtures of wool wax and cetyl alcohol. it is possible to produce emulsions which vary in viscosity between the two extremes. This 146

EMULSIONS AND THE COSMETIC CHEMIST tcchniquc forms a basis for the pro- duction of very many o/w emulsions. it is of particular interest on accourn of the small amount of mechanical energy required to effect emulsifica- tion, and thc fact that low tempera- tures may be employed. The theory of molecular complcx formation offers an explanation for thc success ooe many established emulsifiers. It is of interest even in the common vanishing cream made essentially from stoatic acid, water aml potassium hydroxide. In this case, complex formatiou occurs between the water-soluble potassium stearate and the oil-soluble stearic acid, which is used in excess of the a•mount requircd to neutralise the alkali employed. The fact that potassium distearate can be crystal- lised from such an emulsion supports the complex formation theory. Incidentally, therc is a strong prob- ability that this crystalline distearate is responsible for the phenomenon of "sheen" in vanishing creams. It is well-known that excess of fatty acid increases the efficiency of a soap as an emulsifier, and once more the formation of a complex between the soap and the fatty acid would sug- gest itself. The continued success of certain •nixed or compound proprietary emulsifying agents may reflect to some extent upon the ability of thc cosmetic chemist, but there is no denying the efficiency of these pro- ducts. Consider the value to the chemist of the type of agent based upon a mixture of cetyl alcohol with a surface active agent such as the corresponding cetyl sodium sulphate or phosphate. This substance no doubt functions in the way illus- trated by Schulman and Cockbain, i.c., a complex formation between the polar heads of the two sub- stances with close adlineation of the oil-soluble residues. When this type of emursifier is heated with water, thc water extracts the sodium alcohol sulphate, which forms a complex at thc interface with cetyl alcohol. The excess cetyl alcohol forms the oil phase and is emulsified in the water. In some cases., emulsions made from cetyl alcohol/cetyl sulphate mixtures require reheating to pro- duce fine particle size and homo- geneous emulsions. This is especially true in acid media and in the presence of high concentrations of electrolytes. This failure to produce stable emulsions at the first attempt is explained by thc fact that it is not always possible to extract all the cetyl sulphate from the mixture into the aqueous phase in one operation. This is strikingly demonstrated by making a mechanical mixture com- posed of 2 parts sodium cetyl sul- phate and 2 parts cetyl alcohol. This base will make a fairly good cream if heated with 50 parts of water and 50 parts of mineral oil. If, on the other hand, the 2 parts of sodium cetyl sulphate are dissolved in the 50 parts of water and the 2 parts of cetyl alcohol in the mineral oil, the effect of mixing the two phases is to produce an emulsion wh_ich can only be equalled. by the application of a great deal of mechanical effort 147