OIL-IN-WATER EMULSIONS 15 1671 Initial l'5hr 8hr -1 24hr •/•"• 32•80 I 1640 I 328O Shear Stress (dyne cm -2) Figure 8. Emulsion prepared with cetearyl alcohol and ceteth 20: (a) flow curves for the emulsions aged for the stated times (b) photomicrograph of the freshly prepared emulsion.

16 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS The situation is more complex when fatty acids are used as the amphiphile. Gel phases formed with stearic acid are metastable and convert to coagel phases (microcrystals and water) when mixed at high shear rates. Thus a semisolid product results if mixing is discontinued at the transition (i.e., setting) temperature and the emulsion is allowed to cool undisturbed. A more mobile emulsion is formed if it is mixed below the transition temperature, as considerable mobile coagel phase develops. BATCH VARIATIONS OF EMULSIFIERS Batch variations in the emulsifier components as well as in the mixed emulsifier com- position can influence the microstructure and properties of emulsions. Fatty alcohols. Homologue composition of fatty alcohols markedly influences the quality and properties of lotions and creams prepared with them and may be the cause of unexpected instabilities in established formulations (26-28). For example, emulsions prepared with pure alcohols and ionic surfactants, although semisolid initially and of a relatively high consistency, break down on storage to form mobile liquids. These changes occur because the gel phase formed by pure amphiphiles after the heating and cooling cycle of manufacture is metastable at low temperatures. On storage the interla- mellar water gradually reduces to more stable [3- and %forms, accompanied by poly- morphic phase transitions, possibly from the u-crystalline configuration. As the net- works entrapping the oil droplets disintegrate, emulsions thin, and the lamellar struc- tures visible microscopically in emulsions disappear and are replaced by crystals (cf. Figures 9b and 5b). Concurrently, in DSC a low-temperature endotherm representing network crystallization develops and increases in intensity (Figure 9). These differences between the abilities of pure and mixed homologue amphiphiles to form stable gel phases have caused much confusion in the past. In early work the gel network phase was called "frozen liquid crystalline" phase because it was thought to be a metastable state that formed on cooling lamellar liquid crystalline phases to below their chain-melting temperatures. More recent information indicated that this termi- nology was misleading, as it is now known that metastable gel phases form only with pure homologue amphiphiles. With mixed homologues such as cetearyl alcohol, ther- modynamically stable gel phases can form spontaneously at low temperature without a prior heating and cooling cycle. In fact, emulsions can be prepared at room temperature using solid micronised cetearyl alcohol, although the extensive mixing required and the resultant aeration of the product do not make this a commercially viable method of preparation. (2). Surfactants. The homologue composition of the ionic surfactant does not markedly influ- ence consistency, probably because it is present in small amounts and its function is essentially to provide charged groups. With nonionic surfactants, batch variations in the lengths of the POE chains can influence consistency. If longer POE chains are used, more water is trapped interlamellarly, and hence consistencies are higher (23). Ratio ofsurfactant to fatty alcohol. Traditionally, commercial emulsifying waxes and those of the various pharmacopoeas contain a large excess of alcohol. For example, the official waxes of the British Pharmacopoea contain ! part by weight of surfactant to 9 (ionic) or 4 (nonionic) parts of cetearyl alcohol, representing approximate molar ratios of 1:9 and 1:20, respectively. Although these ratios are not critical, for gel phase can form with as