PARAMETERS OF EMULSION STABILITY* BY ROBERT D. VOLD, PH.D., and ROBERT C. GROOT, DRs.t Presented September 20, 1962, Seminar, New York City ABSTRACT In this report a clear distinction is made between the various usages of the term "stability," and consideration is given to the fundamental properties of the system which are likely to be important according to each definition. The ultracentrifugal method is then described briefly and its use illustrated for determining the effect of the concentration of sodium dodecyl sulfate and of added sodium chloride on the ultracentrifugal stability of NujoLwater emulsions. Possible chemical changes are not considered in this presentation, and the emulsions treated are all of the oil-in• water type, although many of the same considerations would apply to water-in-oil emulsions. INTRODUCTION There is a continuing need for a reliable, rapid method for the prediction of emulsion stability and for a better understanding of those factors Which are of greatest importance in affecting the stability. This has led to a resurgence of interest in centrifugal methods as a possible tool for the rapid characterization of emulsions (1-5). However, many empirical correlations of the results of such experiments with long term observations of the emulsions under shelf conditions will be required before it can be established whether the results of the accelerated ultracentrifugal experi- ments can be used to predict the stability under ordinary conditions. Nevertheless, the ultracentrifugal method affords a convenient method for identification of the important variables involved and for describing them in terms of an objective quantitative measure. KINDS OF STABILITY AND THE FACTORS INFLUENCING EACH Shelf Life IN COM•4ON usage stability is often equated with long shelf life, i.e., little or no change in properties or appearance on undisturbed standing * This is a partial report of work done under contract with the U.S. Department of Agri- culture and authorized by the Research and Marketing Act. The contract was supervised by the Northern Utilization Research and Development Division of the Agricultural Research Service. • Univ. of Southern California, Dept. of Chemistry, Los Angeles 7, Calif. 233

234 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS under natural conditions for long periods of time. It is certainly a worth- while endeavor to attempt empirical correlation of various laboratory tests with such observations since this may be the only definition of stability having commercial validity. But one must resist the temptation to gen- eralize and remember that what holds for one type of system may not hold for another. Creaming or Clearing Creaming, or clearing, is a separation into layers of concentrated emul- sion and dispersion medium, resulting from upward sedimentation of the drops of dispersed phase. Consequently, its rate will be strongly influenced by the viscosity of the medium, the density difference between the phases, and shear gradients resulting from either thermal or mechanical convection. While an emulsion showing such separation into layers might well be classi- fied as unstable in terms of a shelf life criterion, it is not genuinely unstable in a colloid chemical sense since the oil phase remains dispersed as drops even though these may be concentrated or fiocculated. Generally the creamed emulsion can be restored to its initial state by simple inversion. Simultaneously with sedimentation there may also be a fiocculation process occurring, aggregation of two or more drops to form a single kinetic unit but with each drop maintaining its individuality. Since single drops and aggregates will sediment at different rates according to Stoke's law, the rate of this fiocculation process becomes important in any study of creaming. The fiocculation rate can be rationalized in terms of the repulsive forces tending to keep the drops apart and the attractive forces tending to bring them into contact. Among the former are electrostatic repulsion, which is governed by preferential adsorption of ions at the interface, and the ionic strength in the solution between the drops. Large drops, or aggregates of smaller drops, may also be broken up by shear gradients. In the case of water-in-oil emulsions steric hindrance of the adsorbed chains of emulsifier molecules can result in an entropic repulsion effective only at small dis- tances of separation (6, 7). Van der Waals attraction constitutes a general cause of flocculation of suspensions and emulsions and is likely to be important up to distances of ,separation of the order of the drop diameters. Its magnitude is deter- mined largely by the polarizability and may also be strongly influenced by the nature of the adsorbed layer surrounding the drop (8). If the drops are sufficiently small, Brownian motion may tend to bring them into contact, but for larger drops this effect will be minor only the convection currents due either to thermal gradients or mechanical agitation will be important.