346 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS quantitatively predictive about emulsion instability? Before we look at the problems of instability, however, let us first develop a better picture of what we want in the way of a stable emulsion. PHYSICAL CHEMICAL STATE OF AN EMULSION The classical picture of an emulsion, whether it is oil-in-water or water-in-oil, is that of a liquid system with discreet droplets of one phase dispersed throughout another, showing little or no tendency to cream/sediment or coalesce. Using classical colloidal science principles we know that the state of free energy for two phases dispersed together leads eventually to coalescence of the droplets and a corresponding reduction in the interfacial area of contact between the phases. Such coalescence arises through I Distance Figure 1. Typical potential energy diagram for emulsion drops: A) Primary Minimum B) Repulsive Barrier C) Secondary Minimum.

STABILITY ASSESSMENT OF EMULSIONS 347 the attractive forces represented by the primary minimum of the typical potential energy diagram in Figure 1. It, therefore, is necessary to provide a barrier to coalescence at the droplet surface using appropriate surfactants, polymers, or other agents which accumulate at the interface. Our attention, therefore, is strongly directed toward the development of repulsive barriers, electrostatic and/or steric, as seen in Figure 1, to keep the droplets as far apart as possible. In reality we know that this picture of discreet droplets separated from one another is highly oversimplified. Rather, we recognize that at just about any level of phase volume ratio discreet droplets are in a state of flocculation with an energy of interaction associated with the secondary minimum in Figure 1. It is this state of flocculation which is responsible for the rheological properties of the emulsion, giving it a higher apparent viscosity than would occur with completely separated particles. It is this flocculation, and the level of rheological structure produced, which also gives emulsions their desirable characteristics as cosmetic products and which assists in resisting instability. Of considerable importance in determining the extent to which such structuring occurs without coalescence are phase volume ratio the nature of the interfacial barrier and the physical state of the system making up the external phase. The greater the coverage of internal phase droplets, the better the chance that highly flocculted emulsions will not coalesce on standing. Important considerations here are surface charges (zeta potential) polymer molecular weight and level of hydration. Also of considerable importance is the surface rheology of the interfacial barrier and its ability to respond to stresses at the interface which bring about coalescence. The state of the bulk phase external to the droplets is also of considerable importance in stabilizing an emulsion. Higher bulk viscosity occurs because of excess polymer and surfactant dissolved in the external phase. In the case of concentrated surfactant and polymer solutions the possibility of stabilization because of liquid-crystalline phases must also be considered (1). Such complex highly ordered phases produce a high bulk viscosity, as well as influencing the state of flocculation. To this we must also add situations where solid materials are present to improve the overall rheological character of the system. Whether these represent highly hydrated networks of hydrophilic solids, such as clays, or crystalline waxes and other fatty substances, the result is an enhanced level of particle-particle interaction and an overall increase in apparent viscosity and stability. The picture presented for a typical liquid or semi-solid cosmetic emulsion, thus, is one in which the system is in a high state of flocculation. This flocculation literally "freezes" the emulsion particles into a network which resists creaming/sedimentation or coalescence, and produces significant mechanical structure leading to high apparent viscosities in the absence of significant external stresses. Based upon this picture, therefore, I would argue that any approach to the assessment of physical stability must deal with the total emulsion as it sets up after preparation, and must take into account all ingredients, their physical state and their location, i.e., in the bulk or interfacial phases. Studying isolated parts of the system or diluting the emulsion to look at individual drops is of limited significance in stability assessment. Subjecting emulsions to any set of conditions during evaluation which alters the physical or chemical state of ingredients, or their location, likewise will lead to assessments which may not reflect the situation which the evaluation is supposed to indicate. This idea will be developed more fully later in the discussion.