JOURNAL OF THE SOCIETY sample to conditions more rigorous than those it will ever meet in the field. If the product remains un- affected for a time by these drastic tests, it is fairly certain that it will be stable for a longer time under the milder conditions of actual use. But when the mixture under test shows evidence of instability, might it not be a case of breakdown under severe conditions where under ordinary cir- cumstances the product would be- perfectly stable ? If several different kinds of accelerated tests show several types of instability in the product, it is probably justifiable to reject the formulation. But if the degree of breakdown is slight, and there is a possible doubt about the conclusion based on speeded-up tests, it is usually worth while to re- peat the test under conditions less drastic and more nearly approach- ing actual use, even though such a test will necessarily be of longer duration. Generally, the milder test of longer duration is the more re- liable. A commonly applied laboratory test is the centrifugation of the sample. This procedure accelerates creaming changes, and in some cases will also speed up coalescence of the dispersed phase. Creaming can also be accelerated by dilution of the emulsion so as to reduce the vis- cosity of the external' phase. Separation of the two phases of the emulsion occurs where dispersed droplets coalesce unduly, frequently upon contact. In any case only a small percentage of collisions result in coalescence, or the emulsion could OF COSMETIC CHEMISTS not have been formed at all. Study- ing the stability of an emulsion with regard to this type of breakdown involves increasing the frequency and force of meetings between drop- lets. This can be accomplished to some extent by centrifuging, which may cause a closer packing of drop- lets and thus result in more frequent contacts, although it should be em- phasised that centrifuging is usually of small value in predicting phase separation (its main use being the acceleration of creaming). Raising the temperature of the emulsion causes an increase in the general thermal agitation, and also produces a lower viscosity in the external phase. Both these effects tend to increase the rate of droplet contacts and also their force, so that any in- stability due to droplet coalescence will be magnified. Accordingly, storage of samples at elevated tem- peratures, in the range of say 110 ø to 160øF., is essential in judging product .stability. Breakdown by phase separation can occasionally be directly observed microscopically. If during the course of a few minutes' examination, several instances of droplet coal- escence are seen, the emulsion will no doubt separate relatively quickly. More valuable information is ob- tainable by microscopic examination if one makes a particle size distribu- tion count. About 1,000 to 2,000 particles are measured and classified into particle size groups, say 0 to 1 micron, 1 to 2 microns, and so forth. It is generally found that this distri• bution changes wi•t.h time in such a 196

STABILITY OF COSMETIC E3iUL$iON$ manner that the size of the classifica, tion containing the largest numbel of particles remains constant, but the percentages of particles in the various size groups change in the direction of a larger average par- ticle size. After a time the rate of change decreases and perhaps stops, after which the emulsion will almost always be found to have been stabi- lised and undergo no further change. It is therefore of value to follow the progress of stored samples, which show no visual change, by means of microscopic determination of droplet size distribution. A halt in progressive changes in the size dis- tribution is thus an indication that the emulsion has reached a stable state as regards phase separation. It goes almost without saying that the product should be put through several freezing and thawing cycles. Some com•nercial products arc sub- ject to freezing during shipment, and should be able to withstand this treatment. It seems probable that attaining stability to freezing is re- lated to the problem of obtaining a stable emulsion where the internal phase is solid, as in a wax emulsion. This problem is often met by the selection of an emulsifier of high solubility. The gelling of an emulsion or col- loid is a result of the formation of an internal structure within the liquid. "Viscosity" of the gel de- creases as the emulsion is agitated, and the viscosity therefore depends on the degree of agitation and period of standing before viscosity is measured. It is sometimes possible to know when gelling is probable, as the liquid will show a departure from ordinary true solutions in its viscosity behaviour. This is measur- able with a rotating cylinder type of viscosimeter, where Newtonian liquids show constant viscosity in- dependent of the rate of rotation, while an emulsion subject to gelation will often show decreasing viscosity with increasing rotational speed of the viscosimeter. It seems paradoxical, but it is nevertheless true, that phase separ- ation sometimes occurs in emulsions when they are agitated. It is well known that one kind of mechanical work may produce a good emulsion, while the wrong kind of stirring re- sults in a poor product from the same formula. Milk may be taken as an example- rapid shearing motion for a short time results in homogen- isation, while prolonged slow churn- ing causes the separation of the butterfat. This phenomenon can perhaps be in part explained on the basis of one kind of mixing causing rapid motion of the dispersed par- ticles so that forceful collisions occur frequently, while a different motion may break droplets into smaller ones and give greater stab- ility. Many other factors involved have been postulated, but no com- pletely satisfactory explanation has been advanced. As a cosmetic pro- duct may be subjected to long periods of vibration during ship -• ment, tests with different types of agitation or vibration should be made. It is quite common actually to ship samples over long distances (Continued on Page 208) 197