OIL-IN-WATER EMULSIONS 5 In contrast, synthetic surfactant emulsifiers do not form bilayer gel phases, although liquid crystalline phases are common. The chemical structures of synthetic surfactants in general are much simpler than those of the natural lipids, as most have only one hydrocarbon chain, containing 12-18 carbon atoms. In water, as the surfactant con- centration is increased, a variety of structures, including the bilayer neat phase, can form (Figure 2a). In the neat phase, the hydrocarbon chains are in the disordered or liquid crystalline state, similar to that described above for lipids above the phase transi- tion temperature. The thickness of the water layers is limited because excess water induces a phase transition to a miceliar solution. On cooling the neat phase to below the transition temperature, the surfactant crystallises out (Figure 2a). Fatty amphiphiles. Fatty amphiphiles such as long-chain alcohols, acids, and monoglyc- erides and pure saturated synthesised lecithins are too lipophilic to form bilayer phases in water, although they do exhibit marked crystalline polymorphism. For example, pure long-chain alcohols show at leat three solid modifications. The high-temperature o•-form separates first from the melt and is stable over a narrow temperature range. In this form the' hexagonally packed hydrocarbon chains are fully extended in the trans- conformation and there is rotational motion about the long axis of the molecule (cf. L• phase described above). At lower temperatures the [3 and 'y forms, where the hydro- carbon chains are non-rotating ([3-form) or tilted ('y-form), can co-exist, although the [3-form is usually in excess. The o•-[3 (or 'y-) transition temperature is lowered in the homologue admixtures such as cetearyl alcohol and in the presence of water, where the crystals often show limited swelling (9-11). These hydrated crystals (Figure 2b) are not usually referred to as gel phase, for their swelling is limited by the considerable strength of the van der Waals attractive forces between the lipid layers that balance osmotic repulsions. They are sometimes called "coagel"phase when dispersed as micro- crystals in water. MIXTURES OF SURFACTANTS AND AMPHIPHILES It is emphasised above that gel phases do not form when either surfactants or fatty amphiphiles alone are dispersed in water. However, under certain conditions (for ex- ample, in the presence of charged groups), the limited swelling of the fatty amphiphiles described above can be increased markedly to give gel phases. When heated to above the hydrocarbon chain melting temperature, the gel phase transforms to swollen la- meliar liquid crystalline phase (Figure 2c). The charge may arise from ionisable polar groups in the amphiphile itself. This is the case with neutralised monoglyceride emulsifiers, where neutralisation of free fatty acids normally present in the crude source material introduces small quantities of ionic sur- factant (12-14). Alternatively, the charge can arise from the addition of an ionic sur- factant, as in some fatty alcohol emulsifying waxes and the glyceryl monoester self- emulsifying waxes. Gel phases also form with fatty alcohols in the presence of small quantities of nonionic surfactant. Both the gel and liquid crystalline phases formed from these mixtures can swell to incorporate significant quantities of water in the interlamellar space. This distinguishes them from the "neat" liquid crystalline phases described above, where excess water will induce a phase transition. The swelling that occurs in the presence of charged groups is electrostatic and in some systems is so extensive at high water concentrations that it is

6 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS (a) liquid crystal i Above T c Tc Below T c Soap crystal (b) molten state Above T c I,c Below Tc hydrated crystal (c) Water swollen liquid crystal 'IT• bøvcTcT Below Water swollen gel phase Figure 2. Summary of the aqueous phases that form below and above the hydrocarbon chain-melting temperature for mixed emulsifiers and their components. (a) hydrophilic surfactant: soap crystals below and liquid crystals above the transition temperature. (b) fatty amphiphile: hydrated crystals below and the melt above the transition temperature. (c) fatty amphiphile combined with small quantities of surfactant: swollen gel phases below and swollen liquid crystalline phases above the transition temperature. described as "infinite." Gel phases that form in the presence of nonionic surfactants are a result of hydration mechanisms. The significance of lamellar phase swelling to emul- sion stability will be discussed in more detail with specific examples below.