JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS aeration can be measured by such a means. Consistency is not so easy because temperature has a profound influence on any measurement involving viscosity. Nevertheless, measurements can be obtained with some reason- able accuracy and reproducibility if a rotating cup viscometer be employed. These tests are all looking tests put on to a numerical scale. As regards a general assay of composition, inspection of the formula will normally indicate one ingredient which is large in amount and which can be positively determined. For example, a brushless shaving cream will contain 70 or more per cent of water. Drying the water off is not satisfactory. In the first place desiccation takes time and involves at least two weighings (three if the dish is not already tared). Cooling time affects the speed of the method and, as it is a difference method, losses other than that of water may affect the result. It is better to deternfine the acid value. Since the acid value of the fatty acids is known or certainly ought to be known, varia- tions in the amounts of the other ingredients will affect the titrimetric deter- mination. Limits of pernfissible variations can indeed easily be set up. Looking now at the "when," everybody knows, that in making a cos- metical product there is generally some sequence in adding the ingredients. Firstly, there are the instances where some chemical reaction is to occur. For example, in making an emulsion using a soap to disperse a fatty material. If a soap is to be employed the question will arise as to whether the soap is to be dissolved in the water, or in the oil or made in situ. Every cosmetic chemist knows that these three methods, although presumably ending up as an emulsion of identical constitution, give different emulsions. Before a chemist can, therefore, decide at what stage, or when, the soap has to be present, it is necessary for him to look very closely in order to discern why these differences should be. If he can form some picture of the mechanism of emulsification then he can decide which is probably the better way of doing the job. It is true that at the present moment the view-point on emulsification is in the state of flux, especially as the latest work is indi- cating that all interfaces are fundamentally mosaic in character. Neverthe- less, for the purpose of this talk, a simple view mxy be taken. It is easy to imagine that if oil containing stearic acid is brought into contact with water, then at the interface the stearic acid will orientate itself so that the carboxyl group will push out of the oil into the water, but the long length of hydrocarbon chain will want to remain in the oil since it is oleophilic. The extent to which the chain will be pulled into the water will depend on the hydrophilic character of the polar ending, and the carboxyl group is not so strongly hydrophilic as one associated with a sodium or mono-valent ion. This means that if the interface is allowed to form and the projecting carboxyl groups are then changed into soap, the nett results will be that the polar end will become more hydrophilic and will tend to draw •the hydrocarbon chain further into the water phase. At the same time, 154

LOOKING AT THINGS however, if the amount of alkali is carefully controlled, as it should be, then if one, for the purpose of painting the picture, assumes that the amount of alkali is equivalent to half the total stearic acid, then, statistically, the inter- facial film will be composed of alternate bridges of sodium soap and of stearic acid. This interface will, therefore, be more likely to be uniform in texture so that it will tend to be strong and probably have few flaws in its "crystalline structure." Another method would be to dissolve the soap together with the free stearic acid in the oil phase and then add the water. In such a system as this, the soap molecules will be pushing out further from the surface of the oil than the unneutralised stearic acid, so that when water is added there will be a preferential superabundance of soap (sodium stearate) at the inter- face tending to squeeze out the unneutralised stearic acid from the interface. Such an interface will be unlikely to be as strong as one produced when the acid soap is formed in situ. Finally, if the soap is dissolved in water together with the free stearic acid, then because soap is a colloidal electrolyte and tends to associate with stearic acid forming micelies, the added oil may not easily break-up the micelles so that the free stearic acid is bound in the form of an acid soap and will have great difficulty in reaching the interface to help form a mosaic. In fact, the oil would slowly penetrate the micelle and associate with the oleophilic chains within. The chances are, therefore, that the first method is the best the third procedure will probably produce the most unstable emulsion. Looking at the problem in this way one can begin to appreciate that a soap produced by half-neutralising a fatty acid may not be quite the right proportion, and many instances are known where the most stable cream of this type is produced when the fatty acid is only about quarter-neutralised. A good example of this is to disperse 10 per cent of oil in water by means of stearic acid, sodium hydroxide and cetyl alcohol, the cetyl alcohol being employed as the stabiliser of the emulsion. If the molecular ratio of sodium hydroxide to free stearic acid is greater than one to three, although the emulsion may form quite readily, it will not be stable and will readily cream. Furthermore, the maximum body of the emulsion will be obtained when the amount of base neutralises about one-quarter of the stearic acid. Another instance of the way "good looking" can assist is in the simple case of dissolving a hydrophilic colloid, such as gelatin. If one takes a sheet of dry gelatin and puts it into boiling water the solution or dispersion of the gelatin may not be complete for several hours. Consideration of the pro- blem will indicate that when the dry gelatin meets hot water the gelatin tries to dissolve, but because the water cannot penetrate the dry material the dissolution takes place purely on the outside and takes time, in so far as the dissolving gelatin blocks the intermolecular spaces. On the other 155