]E3 IULS•[ON• ANB Tile COSMETIC CltEMI$i 370.) Above "300" the poly glycol oleates tend to become too water- soluble to be of interest as w/o emulsifiers, but diethylene glycyl mono oleate is a very good w/o emulisfier. The grade described as "200" which approximates to tetra ethylene glycyl oleate is a truly amazing substance. When five parts of this material are dissolved in 95 parts of petroleum jelly, it is possible to stir in 100 parts of water with ease, to produce an attractive and stable w/o cream. It is of grea• interest that the grade of petroleum jelly used does not affect the subse- quent emulsion in the same way as it does when used in conjunction with wool wax. If petroleum jelly is re- placed by mineral oil, then the addition of 100 parts of water results in a thin stable o/w emulsion. Quite apart from the interest that this par- ticular emulsifier arouses, this experi- ment demonstrates the importance of the petroleum products used in cos- metical preparations. SUMMARY Many chemists will undoubtedly call to mind examples of emulsions which are not completely explained by the theories so far outlined. Nevertheless, it can be said that with these theories before them it is pos- sible to approach the formulation of new emulsified products with a better understanding of the funda- mental requirements of the emulsi- fier. The theories may also assist the chemist to overcome certain of his existing difficulties. These theories offer a reasonable explana- tion of the mode of action of certain established emulsifiers and open the way to further research. BATH ESSENCES AND PREPARATIONS OF THE "MISCIBLE OIL" TYPE The preparations to be discussed under this heading are those in which mixtures of oil and water are presented in clear dispersed form. They are sold in the form of attrac- tive transparent liquids which, when poured into water, disperse in an emulsified state with the liberation of fragrant odours. It is possible to produce similar preparations which remain clear when diluted with water, but the cloudy dispersions are more popular in the bath and for sprays and disinfectants, and fit more neatly into the subject of this paper. The "miscible oil" will consist of a four component system: water, oil, emulsifying agent and an auxili- ary solvent. Water is a necessary and desirable constituent of the system and for reasons of economy should be present in as high a con- centration as safety limits permit. It is generally essential to use dis- tilled water. The oil will consist of a mixture of essential oils, synthetic and natural chemicals blended into the form of a compound perfume, and the amount present in the pre- paration will be determined by the consideration of 'strength of odour and price. The proportions of emul- sifier and "auxiliary solvent" should be kept as low as is consist- ent with the clear dispersal of the 153

)OURNAL OF TtlE SocIETY OF COS3iET[C CHEMISTS oil and with the stability of the emulsion resulting from the addition to a large quantity of water. The emulsifying agents are usually alka- line soaps of the fatty acids, which produce emulsions on the subsequent dilution of the "miscible oil." In addition, many of the synthetic detergents function as emulsifiers. These include a number of the long chain sulphates, and to some ex- tent, the non-ionic emulsifiers. The "auxiliary solvent" will be a water- soluble substance such as alcohol, a poly glycol or a poly glycol ether. Most organic compounds are soluble to about 0.1ø/o in water, and many of the aromatics used in per- fumery appreciably more so. How- ever, a large amount of energy is required to bring about this solution, as the rate of solution is dependent upon the diffusion of the particles to. be solubilised. In other words, the rate of solution is proportional to the area exposed. Surface active agents aid the dispersion and therefore pro- mote favourable conditions for solu- tion. Self-emulsification requires zero surface tension, as in the ex- treme case of water and glycerine which are completely miscible. Cer- tain aqueous salt solutions, especi- ally neutral salts of organic acids, have the power to dissolve greater quantities of slightly soluble sub- stances than does pure water at the same temperature. The effect (hydrotropic solution) is illustrated by the fact that 60 grms. of nitro- benzene will dissolve in 100 ccs. of concentrated sodium xylene suphon- ate solution at 80øC. It is generally accepted that solu- tion of otherwise poorly soluble sub- stances only takes place in water when micelles are present. Micelles are aggregates of colloidal dimen- sions. The ionic micelles are believed to consist of loose spherical aggre- gates of the hydrocarbon chain ions, with the polar heads turned towards the aqueous phase. When polar substances are solubilised they orientate in the micellar lattice so that their polar group sits side by side with those of the solubiliser whereas their hydrophobic portion penetrates in between the paraffin chains of the solubiliser. This solu- bilisation is called solubilisation by penetration and is distinct from solu- bilisation of hydrocarbons which produce a latteral sw. elling of the micelle and no lengthening along the direction of the polar groups. Thus, paraffin chain salts have power to dissolve organic compounds which are otherwise only slightly soluble in water. In the case of a substance partially soluble in water it is suggested that the polar groups are absorbed at the surface of the micelle with the par- affin chain inside the micelle. Al- though the surfaces concerned differ from the oil and water of emulsions, the suggestion of adsorption at the surface of the micelle would place the partially soluble substance in the category of a surface active agent. To produce a bath essence which is clear in the bottle, but which turns cloudy on dilution, we must choose a surface active agent capable not only of inducing the solubility of a 154