SOLVENTS, HU•IECTANTS AND BLENDING AGENTS unless cumulative in action and the U.S. Food & Drug Administration find quantities less than 5% accept- able even when the humectant is ethylene glycol. Recently, however, triethylene glycol, polyethylene glycol and buty- lene glycol have been acknowledged as skin irritants and should not be used indiscriminantly as glycerol substitutes •3. The main difficulty encountered i• the use of alternative humectants arises from the fact that their relative performance appears to vary accord- ing to the particular formulation employed. Practical tests are thus the only safe means of selecting an alternative humectant for a particu,.- lar application but there is evidence to show that propylene glycol and sorbitol alone or in combination can be successfully applied as alter- natives. •REFERENCES See for example N. S. Peel, Soap Perfumery • Cosmetics, 1950, 23, 1013, 1130. See for example Parsil & Whiteacrc, Drug • Cosmetic Industry, 1950, 66, 516. 3. Hall, Proc. Sci. Sect. T.G.A. (through S.P.C., 194't, 17, 513). 4. Schimmel Briefs Feb., 1950 (through Drug & Cosmetic Ind., 1950, 66, 576). 5. B.P., 548, 329. O. A m e r, Manuiacturing Chemist, 1949, 20, 544. 7. E. S. Lower, American Per[umer, 19•t9, 53, 121. 8. B.P., 629, 102. 9. E. Chadwick & G. Pears, Soap, Per[umery C• Cosmetics, 1950, 23, 257. 10. R. W. Moncrieff, "Les Fixateurs en Parfumerie ". La Parfumerie Modernc. P.M. No. 14, 1949. 11. R. G. Harry, "Modern Cosmetico- logy" Leonard Hill, 2nd Edn. 1944. 12. Soap, Per/umery & Cosmetics, 1946, 19, 465. 13. Fette u. Seifen, 1950, 52, 423. The a, uthors would like to acknowledge' •he facilities provided for the prepara- tion of this paper by British Industrial Solvents Ltd. 121

MOLECULAR INTERACTION IN BULK SOLUTION OF SODIUM DODECYL SULPHATE (SDS) AND LONG CHAIN ALCOHOLS By R. MATALON, PH.D.* -- THE MOLECULAR interaction be- tween long chain alkyl sulphates and long chain alcohols has been investi- gated at the air-water interface using the monolayer technique. The alcohol is spread on the water surface and the detergent is injected under- neath '. The interaction is then studied using various means such as Iollow•ng the pressure rise of the rnono.layer .kept at constant area = or the expansion of the monolayer kept at constant psessure. This study is limited to those alcohols which lead to a stable monolayer on the water surface. When the alcohol is water-soluble, other methods may be used. The precipitation generally observed be- tween anionic (SDS) and cationic (CTAB) compounds has been shown '• to be inhibited by the presence of polar compounds, i.e. long chain alcohols such as octyl alcohol and nonyl alcohol present in equimolecu- lar ratio with the artionic detergent. Under these conditions the long chain alcohol adlineates with the anionic detergent, producing a micelle struc- *Department of Colloid Science. University of Cambridge. 122 ture of greater fluidity and tending to inhibit the formation of a solid precipitate between the artionic and cationic compounds. The precipita- tion is also inhibited with the shorter chain alcohols C7, C6, C5, etc., provided that these alcohols are present in greater atnounts. These observati{ms suggest that the association ionic detergent-- polar compound is governed by the hydrophobic -- hydrophilic balance in the alcohol. The present work brings further evidence confirming this hypothesis. The formation of a mixed structure SDS -- alcohol should decrease the electrical repul- sion forces usually acting between the ionised polar group (--SO 4 ) and this should result in a greater stability of the structure in the adsorbed layer. Figures 1 and 2 show that the •' SDS is foa•n volume of Icc :•o• increased by the presence of alcohols. A marked stabilisation of the foam is also observed with CsOH and C,OH. This is easily demonstrated by the' stability of the laminae to drainage in the rectangular wire platinum frame 4. The results ob-