142 JOURNAL OF COSMETIC SCIENCE THE IMPORTANCE OF MESOMORPHIC (LAMELLAR) PHASES IN EMULSION STABILITY Gillian M. Eccleston, Ph.D. Department of Pharmaceutical Sciences, University of Strathclyde, Glascow, Scotland, G40NR Dermatological lotions and creams are often complex multiphase emulsions with stability and physical chemical properties related to the mesomorphic phase behaviour of the emulsifiers rather than to droplet interactions described by classical emulsion theories. A knowledge of the relationship between the properties of such formulations and their microstructure is essential to optimise manufacture, provide cosmetic elegance and to control the delivery of a cosmetic agent to the skin. In this presentation, the microstructure and properties of a range of fluid and semisolid liquid paraffin-in-water emulsions (aqueous creams) stabilised by either (i) combinations of surfactants (ionic cetrimonium bromide or non- ionic ceteth 20) and pure or mixed homolog fatty alcohols or (ii) partially neutralised fatty acids will be discussed. In particular, tl•e important relationships between emulsion microstructure, stability and rheology. The rclevance of oil free ternary systems, formed by dispersing mixed emulsifiers in controlled percentages of water, to investigations into emulsion microstructure will be emphasised. The stabilities and theological properties of emulsions containing fhtty alcohol/surfactant mixed emulsifiers (emulsifying waxes) are controlled by the swelling properties of an a-crystalline lameliar gel network formed when mixed emulsifier interacts with continuous phase water. Despite the complexity of these systems, combinations of low and high angle X-ray diffraction measurements (Daresbury Laboratory Synchrotron Radiation Source, SRS) provide direct and accurate measurements of the interlamellar water spacings (1). These spacings affect the theological properties of the creams. The influence of batch variations of the surfactants or changes in homologue composition of the alcohol on emulsion stability and rheology can also be correlated directly to extent of swelling of the lameliar gel network. The cationic emulsifying wax shows phenomenal swelling in water. The reflection that incorporates interlamellar water increases continuously to over 500A at 93% water. The trend is not influenced by the inethod of incorporation of the components and swollen lameliar phase is also identified in the corresponding emulsion (Figure 1). sssA I st Order Ternary Gel 280A • I -•-- 3rd Order 0.010 0.020 CrEam lst Order '•x 2rid Order '•- 3rd Order 0.• 0.• 0.• 0.• 0.010 0.014 0.018 S (1 / A) Fig. 1 Comparison of small angle X-ray diffraction patterns of cetrimonium brolnide emulsion and ternary system The extensive swelling is electrostatic in nature. The surthctant molecules interposition among the fhtty alcohol molecules and electrical double layers arise t¾om the dissociation and diffusion of counterions from the surfactant headgroups at the surface of the bilayers into the surrounding water. Electrostatic repulsion between a4jacent

2000 ANNUAL SCIENTIFIC MEETING 143 bilayers arise from the overlap of the electrical double layers. The thickness of the double layers is inversely proportional to the square route of the concentration of electrolyte in the surrounding water. Added salt reduces swelling between the bilayers by a non-specific compression of the double layers (Figure 2). Fig. 2 The influence of sodium chloride on the interlamellar spacings of cationic emulsion and ternary system. Decreases in apparent viscosities on addition of salt (from 107 mPas at zero salt to l0 mPas at 2% salt) are correlated with reductions in interlamellar swelling which result in decreases in the phase volume of the lameliar structures. On storage, creams become thicker as water layer distance increases (from -460 A after three weeks storage to -600 A after six months) probably due to surfactant rearrangement in the bilayers. High angle diffraction data confirm that the hydrocarbon bilayers are in the hexagonal {x-crystalline mode of packing. Ternary systems and creams prepared from pure alcohols, although initially semisolid, are rheologically unstable and break down on storage to form mobile crystalline fluids. A preliminary low angle X-ray study into the kinetics of structure break down shows that the swollen lameliar gel phase formed initially swells even further on storage betbre separating into unswollen monoclinic crystals. The comparatively limited swelling that occurs in emulsions containing non-ionic surfactants is due to hydration of the polyoxyethylene groups (interlamellar distances of around 110A). Initial consistency increases on storage occur as the POE groups slowly hydrate. The influence of batch variations of POE surfactant on theological properties can be related to variations in POE chain length, fbr there is a linear relationship between POE chain length, interlamellar thickness and apparent viscosity. Emulsions containing partially neutralised fatty acids show more complicated phase behaviour. Whilst swollen bilayer phases form in systems neutralised by triethanolamine, such phases are not apparent in emulsions containing sodium or potassium hydroxide as neutralising agent. The possible structures of these non-lamellar phases that also incorporate large volumes of water will be discussed. 1. Eccleston, G.M., Bchan-Martin, M.K., Jones,G.R. and Towns-Andrews, E. lnt. ,l. Pharmaceutics 203 127 (2000).