2 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS There is now substantial evidence that the formation of specific lamellar phases that are capable of incorporating large quantities of water is an essential requirement for the stability of many commercial emulsions. Such phases can also impart the required rheo- logical properties (for example, ranging from mobile lotions to thick semisolid creams) to some products (1-3). However, there is much confusion as to the type of lameliar phase that forms in a specific emulsion. Most of the literature concentrates on the formation of liquid crystalline phases and fails to identify the equally important gel phases, even though each phase imparts totally different properties to emulsions con- taining them. It is not always appreciated that the lameliar liquid crystalline state is rarely dominant in cosmetic emulsions containing long-chain alcohols, acids, or mon- oglycerides as co-emulsifiers. The commercial literature contains many articles that incorrectly discuss the presence of bilayer liquid crystalline phases in emulsions that are actually composed mainly of gel phases. In this paper, the microstructures and properties of multiple-phase oil-in-water emul- sions of cosmetic use (in particular dermatological) will be described. Particular atten- tion will be given to the various lameliar liquid crystalline and gel phases as well as to other crystalline phases that occur in such emulsions. It will be shown how the behav- iour of many real emulsions during manufacture, storage, and use (i.e., after applica- tion to the skin) can be related to their component phases. STRUCTURE OF LAMELLAR LIQUID CRYSTALLINE AND GEL PHASES Lamellar phases in which surfactant molecules are arranged in bilayers separated by layers of water are formed in water by a range of surface-active materials under specific conditions. The hydrocarbon chains of the bilayers can exist in a number of physical states (4,5), the most relevant to emulsions being the so-called ordered, or gel, and disordered, or liquid crystalline, states (Figure 1). In the gel state the hydrocarbon chains are packed in a hexagonal subcell with rotational motion about the long axes, whereas in the liquid crystalline state they are disordered and liquid-like. The order- disorder transition, Tc, is essentially the melting of the hydrocarbon chains without any loss of long-range stacked bilayer structure. The transition occurs at a characteristic temperature, influenced primarily by the characteristics of the hydrophobic portion of the surfactant. Transition temperatures increase with increasing acyl chain length and decrease when unsaturated or branched chains are present. Such bilayer states are of interest in several scientific areas, and consequently a con- fusing number of different nomenclatures are used to describe them. The lameliar liquid crystalline phases that occur above the phase transition temperature have been called neat, G, or L= phase. Bilayer gel phases that occur below the transition tempera- ture are also referred to as c•-crystalline gel or L• phase (4), and the transition tempera- ture as the chain melting temperature, CMT, or the penetration temperature, Tpen (1). The designation L, i.e., lameliar, with the subscript c•- for the disordered liquid crys- talline phase and the subscript [3- for the more rigid gel phase, is not entirely satisfac- tory. Confusion can arise because identical Greek letters c•- and [3- are used to describe crystalline polymorphs of some amphiphilic emulsifiers, including the fatty alcohols. In this paper the ordered and disordered states will be described simply as gel and liquid crystalline phases. Several other types of gel phase have been reported in the general

OIL-IN-WATER EMULSIONS 3 // WAT E R Below Above -- T½ • Tc WATER GEL LIQUID CRYSTAL Figure l. The order-disorder transition, T o and the lameliar gel and liquid crystalline phases that form spontaneously when a natural surfactant (polar lipid) is dispersed in water. literature about surfactants (for example, the tilted L•' gel phases or inderdigited monolayer phases (4-6)), but these have not been reported in emulsions. Both the gel and liquid crystalline states formed by bi-alkyl lipids (Figure 1) are well known to biological scientists, as they represent the fundamental structure of most animal cell membranes and are also an important structural element in the barrier function of the stratum corneum (7). Many polar lipids such as the lecithins are natural surfactants with a hydrophobic portion composed of two hydrocarbon chains of different lengths and degrees of unsaturation. In cell membranes these are finely balanced to give the required levels of order and disorder, and the transition temperature is close to physiological temperature. Small changes of temperature, pressure, or other biological stimuli can locally fluidise or crystallise the membrane to make it more or less perme- able. In the skin the long hydrocarbon chains and high transition temperatures of the stratum corneum lipids imply that the normal organisation of this barrier is the gel state. There is less information about these states in the emulsion literature, although both natural and synthetic emulsifiers often form liquid crystalline and gel phases in water