1. Soc. Cosmet. Chem., 22, 815-824 (December 9, 1971) Conceptual Clarification of the Terms Used to Describe Emulsion Behavior K. L. MITTAL, Ph.D.* Synopsis--The "TERMS" used to depict EMULSION BEHAVIOR are critically examined and the analysis dictates that some of the terms are unwarranted and should be superseded by better justified vocabulary. An attempt has been made to present an account of the terms which should be employed to represent the STABILITY behavior of an emulsion. The underlying concepts and the implications of various manifestations of INSTABILITY phe- nomenon are illuminated. The whole gamut of instability pattern is divided into physical and chemical forms. Physical instability is further represented by processes such as creaming, flocculation, inver- sion, coalescence, and demulsification. It is suggested that the use of the term "coagulation" should be avoided in describing the behavior of emulsion systems. The concept of reversible and irreversible instability is emphasized. Both forms of in- stability are discussed in their various ramifications. The phenomenon of irreversible in- stability can be realized in practice in micro and macro forms. INTRODUCTION The purpose of this communication is to check the pervasive con- fusion in the conceptual basis of the various manifestations of emulsion behavior, as regards its stability. It is of critical importance that all the terms should have clearcut meaning the use of vague and ill-understood terminology serves only to compound confusion. The terms creaming, flocculation, coagulation, coalescence, and de- mulsification are frequently employed to describe the stability behavior of emulsions but, unfortunately, these have been used inadvertently and * Present address, Electrochemistry Laboratory, John Harrison Laboratory of Chemistry, University of Pennsylvania, Philadelphia, Pa. 19104. 815

816 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS indiscriminately. In the present co•nmunication, only a few conspicuous examples--which are the source of confusion--have been cited but the literature is replete with such instances. Ecanow et al. (1) applied in- cautiously the discussion, valid in the case of solid particle suspensions, to emulsions and foams. They mention, "The coagulated emulsion is represented by a creamed emulsion in which the films and protective colloids of adjacent particles are in contact. Flocculated emulsions are not recognized." Fortunately, they have pointed out that their defini- tions are in contrast to those represented in the literature but this does not warrant the perpetuation of unwarranted terminology. It will be shown later that the term "coagulation" should not be used to describe emulsion behavior and a creamed emulsion is not a coagulated emul- sion. Furthermore, ttocculation and coagulation have been used as synonyms and interchangeably (2, 3), whereas the terms coagulation and demulsification have been treated similarily (2, 4-6). One might con- strue from the above that the terms creaming, flocculation, coagulation, and demulsification all refer to the same process but this is far from the truth. These terms represent widely different phenomena, as shown later. It is important to point out that some workers "coin" their own terms such practice serves only to augment the problem of terminology. One such instance is the paper by Garrett (7), in which arbitrary terms and definitions are used to describe the behavior of emulsions. Such widespread confusion and discordance in the use of terminology in emul- sion systems has prompted the writing of the present communication. DEFINITION OF EMULSION STABILITY An emulsion is a heterogeneous system consisting of at least one im- miscible liquid intimately dispersed in another in the form of droplets whose diameters, in general, exceed 0.1• (2). The confusion stems after the emulsion has been formed. Ideally, an emulsion is stable when there is no change in certain parameters (i.e., number of particles of the dis- perse phase, particle size distribution, total interfacial area, mean droplet size, chemical composition of the components involved, and other related paraineters) while standing undisturbed under normal conditions. This idealization cannot be realized in practice. Thermodynamics dictates that all emulsion systems will try to decrease their surface free energy, with the concomitant decrease in total interfacial area. Broadly speak- ing, an emulsion can show instability in one of two ways: chemical instability or physical instability.