EMULSIONS AND THE COSMETIC CHEMIST of the formula, it is possible to allow all the oil to be rinsed away while still in the emulsified condition. WATER-IN-OIL EMULSION,•q The w/o creams are important, even though these comprise but a small percentage of the cosmetical preparations. Unfortunately, the amount of work so far carried out on w/o emulsions is considerably less than that on the opposite type and, consequently, the specific require- ments of w/o emulsifiers are less clear. Most w/o emulsifiers contain a non-ionised polar' group in the molecule. A combination of a weak polar group such as hydroxyl and a long chain hydrocorbon results in an oil-soluble substance which will stabilise w/o emulsions. However, such a substance has relatively little power to lower surface tension and it is, therefore, necessary to introduce mechanical energy to bring about small particle size and force the emulsifier to the interface. For this reason, it is necessary to dissolve the emulsifier in the oil phase and slowly add the water with continual stir- ring and, in some cases, subsequen• homogenising. To fulfil the condi- tions of a rigid uncharged filrn at the interface and to form this film with a minimum of mechanical energy would appear to call for an oil- soluble body used in conjunction with a neutral water-soluble body. Thus, Drs. Shulman and Cockbain have shown that cholesterol and digitonin form water-in-oil emul- sions. This surely offer the chernist a line of investigation. Absorption bases made from mix- tures of woolwax and petroleum bodies are well-known to the indus- try. They absorb large quantities of water and work best at tempera- ture._• between 35øC and 40øC. In making creams from these bases it is essential to add the water to the stirred oil phase in small quantities, making sure that emulsification is complete before further addition is made. Important as the purity of the wool wax is, it is doubtful whether it is generally realised that the petro- leum bodies have a great effect upon the efficiency of the base. Prior to the last war it was possible to buy a petroleum jelly which, on the addi- tion of from 5-10% of wool wax, would absorb large quantities of water with relatively small effort. The grades of jelly now available are far less accommodating, and re-- course has often to be given to supplementary emulsifying agents. It is possible to rectify the position to some extent by fortifying the jelly with mixtures of petroleum wax and oil. While little information on the constitution of the various petroleum jellies is available, it is possible that the products which are most satis~ factory are mainly branch-chain hydrocarbons, and that those now available are mainly of a straight- chain structure. An emulsion made from the correct type of petroleum jelly will tolerate relatively' large amounts of acids, alkali and electro- lytes, but this is not true of all enmlsions made from present-day 151

JOURNAL OF THE SOCIETY absorption bases. Polyvalent salts (magnesium sulphate, for instance) actually tend to improve the emul- sion made from wool wax absorption bases. In cases where the wool wax possesses a fairly high acid value, there may be a danger of phase in- version when alkali is added, owing to the fortnation of an interfacial complex between the resulting soap and the woo] Wax alcohols. One of the most serious factors affecting the stability of cosmetic creams made from absorption bases is the nature of the added perfume. Many per- fumery ingredients, notably the ter- pene alcohols and in particular ter- pineol, have a strongly adverse affect upon water-in-oil emulsions stabil- ised by wool wax. An emulsion pro- duced by adding 50 parts of water to 50 parts of a really efficient absorption base will tolerate up to 2• parts of terpineol. It is surpris-. ing how little of this alcohol can be incorporated in creams made from most of the bases now offered to the industry. In fact, the technique of assessing the amount of terpineol which a cream of this type will toler- ate may be used as a yard-stick whereby the stability of the emulsion can be measured. The answer lies in the use of the correct type of jelly, but very great care must be exercised in choosing a perfume for the cream the chemist intends plac- ing on the market. It is misleading to read that soaps of polyvalent metals are water in- soluble but oil-soluble. Metallic soaps such as calcium and mag- nesium, stearates are practically in- 152 OF COSInETIC CHEMISTS soluble in mineral oil when in the anhydrous state. Aluminium stear- ate, although soluble in mineral oil at elevated temperatures, is not a trne soap but rather an absorption compound. Traces of water no doubt aid the dispersion of metallic soaps, but it is really necessary to prepare w/o emulsions employing polyvalent metal stearates by either an inversion process or by the use of a water-soluble polyvalent ion. An example of this is the common combination of mineral oil, beeswax, and lime water, which produces emulsions far more efficiently than is possible directly from a calcium •oap. Certain non-ionic bodies are most efficient w/o emulsifiers, and special mention should be made of the pro- prietary Sorbitan Sesquioleate. The use of oil-soluble non-ionic agents in conjunction with the water-soluble type to produce w/o emulsions i.q of interest. A study of the oleates of the poly glycols is strongly recommended. These esters are available in several types, graded according to the mole- cular weight of the poly glycol. The reference number given to these oleates usually reveals their nature. Thus, a figure of 400 would suggest that the ester has been made from a poly glycol with an average molecu- lar weight of approximately 400. (The condensation of 9 molecules of ethylene glycol would give a product possessing a molecular weight of 414, while 4 molecules of diethylene gly- col would condense to yield a pro- duct with a molecular weight of