2004 ANNUAL SCIENTIFIC SEMINAR 405 bicontinuous cubic liquid crystal. Bicontinuous cubic liquid crystal phase comprises intertwined precisely curved cylinders that pervade the entire material and provide a continuous percolation network. The cubic phase Bicontinuous Cubic Phase occurs in lipid/water systems as a discontinuous cubic, and a bicontinuous cubic phase. Bicontinuous cubic phase lyotropic liquid crystals are precise periodic, three-dimensional structures in which lipids and water bilayers are twisted into honeycombed nanostructures with continuous domains of water and lipid - like an organic zeolite with uniform aqueous channels having diameters that range from a few nanometers to tens of nanometers. The bicontinuous phase is a thermodynamically stable phase from the condition that the Laplace pressure can be balanced by the positive and negative saddle curvatures of the curved layers of cubic phase. This structure is a member of the family of"infinite periodical minimal surfaces" (IPMS). There are three known types ofIPMSs, diamond type (D-surface), primitive type (P­ surface) and gyro id type (G-surface) correspond to the cubic phases of CD, CP and CG, and are associated with the space groups Pn3m, Im3m and Ia3d, respectively. The ability of the monoolein/ water system to form cubic liquid crystalline phase has been well documented and modifications to the phase behavior have been defined. 2 Cubic liquid crystalline phase exhibits advantages over lamellar phase insofar as it shows good temperature stability, high internal surface area, a gel-like viscosity, relative insensitivity to salt and solvent compositions and it can be made from low-cost raw materials that are practical for commercial applications. Cubic Liquid crystal phase has been patented by Unilever, SC Johnson Wax, L'Oreal, Clorox and UCB and has been marketed as glyceryl monooleate and dioleate. Recent L'Oreal patents reveal compositions in which cubosomes are components of skin lotions and are used as emulsifiers to protect the skin against the effects of pollutants. These compositions are being touted as 'surfactant free'. This technology forms part of a trend that was begun in the late l 980's by the introduction of polymeric emulsifiers and is now being pursued via liposomes and Pickering Emulsions stabilized by particles. P&G technology revealed in recent patent applications offers the advantage of easily processed, precise nanostructured cubic phase particles. Rererences 1 Israel ichvili, J. N.: --111rer1110/ecular and Surface Forces", Academic Press, New York. 1992, p 380 2 Qiu. H., Caffrey, M .. ·The Phase Behavior of the monoolein/water system", Biomaterials, 1999, 21(3), 223-23-l.

406 JOURNAL OF COSMETIC SCIENCE PRINCIPLES OF EMULSION STABILIZATION WITH SPECIAL REFERENCE TO POLYMERIC SURFACTANTS Tharwat Tadros, Ph.D. Consultant: 89 Nash Grove Lane, Wokingham, Berkshire, RG40 4HE, UK tharwat@tadros. fsnet. co. uk This overview summarises the basic principles of emulsion stabilization, with special ref erence to polymeric surfactants. The main breakdown processes in emulsions are briefly described : (I) Creaming or sedimentation that is due to the gravity force . (ii) Flocculation that occurs due to van der Waals attraction, when there is not sufficient repulsion between the droplets. (iii) Ostwald ripening that arises from the higher solubility of the smaller droplets when compared to the larger ones. (iv) Coalescence that arises from the thinning and disruption of the liquid film between the droplets. (v) Phase inversion that may occur as a result of the increase in the volume fraction of the disperse phase or due to change in the conditions, e.g. increase of temperature. Many of these breakdown processes, e.g. flocculation, Ostwald ripening and coalescence can be overcome by the use of polymeric surfactants. Particular attention is given to two polymeric surfactants for stabilization of oil-in-water (0/W) and water-in-oil (W/0) emulsions. For 0/W emulsions, a hydrophobically modified inulin (RMI) was obtained by grafting several alkyl groups on the backbone of the inulin (polyfructose) chain. This is a graft copolymer AB0, where A is the stabilizing chain ( consisting of strongly hydrated linear polyfructose chain) and B is the "anchor" chain (that is strongly adsorbed at the 0/W interface or soluble in the oil droplets). For W/0 emulsions an A-B-A block copolymer consisting of oil soluble A chains ofpolyhydroxystearic acid (PHS) and B chain ofpoly(ethyleneoxide) (PEO), i.e. PHS-PEO-PHS, is the most suitable. The conformation of both polymeric surfactants at the 0/W interface is described. With HMI, the alkyl groups form "multi-point" anchors, leaving "loops" of polyfructose dangling in solution. This configuration produces enhanced steric stabilization. With PHS-PEO­ PHS, the PEO forms the "anchor" part (being soluble in the water droplets and the two PHS chains that are soluble in the oil form the stabilizing chains. A section is devoted to the interaction between emulsion droplets containing these adsorbed polymer surfactants. This interaction is referred to as steric stabilization and it is the combination of two main effects: (I) Unfavourable mixing of the A chains when these are in good solvent conditions this is referred to as the mixing interaction, Gmix• (ii) Loss of configurational entropy on significant overlap of the A chains this is referred to as the entropic or elastic interaction, Ge1- Combination of Gmix and Ge1 with the van der Waals interaction, GA gives the total energy distance curve for these sterically stabilized emulsions. This energy-distance curve shows only one shallow minimum at a distance of separation comparable to twice the adsorbed layer thickness, after which the total energy increases very sharply with further decrease of the separation distance. The main criteria for effective steric stabilization are summarized : (I) Complete coverage of the droplets by the adsorbed polymer. (ii) Strong "anchor" of the polymer chains to the droplet surfaces to prevent any displacement of the polymer on close approach. (iii) The A chains should