TERMINOLOGY OF EMULSION BEHAVIOR 821 of coalescence, which might lead to demulsification, as shown in Fig. 1. (c) All the riocs may or may not coalesce. (d) The rate of fiocculation will depend upon the mechanism of fiocculation. Irreversible Instab ility This represents the condition of the emulsion in which the identity of the original disperse droplets or particles is changed and the "irreversibly unstable" emulsion cannot be restreed to its original form. Various manifestations of irreversible instability are discussed below. Inversion This simply means the inversion of the disperse phase to dispersion medium and vice versa. Inversion is a special case of irreversible in- stability as there is a change in emulsion type (for example, O/W to W/O). In inversion the droplets lose their identity and the "inverted" emulsion cannot be brought back to its original form by simple treat- ment, e.g., hand shaking. It is needless to expatiate upon the subject of "inversion" as there is no confusion and discordance in the use of this term. Coalescence and Demulsification Once the droplets are in close contact (by fiocculation), the inter- mingling of the droplets is possible and the flocs transform into single drops. A similar process is not possible in the case of sols, and the terms flocculation and coagulation cannot be used to represent this behavior. The appropriate term for the process of transformation of flocs into single large drops is "coalescence." Coalescence comes from the Latin "coalesce" meaning to unite by growing together or mix. Broadly speaking, the coalesced emulsion can be classified as a de- mulsified or broken emulsion but this is not entirely satisfactory. Coalescence brings about internal changes in the emulsion and there is no conspicuous outward manifestation. As the coalesced drops grow larger, these constitute a separate phase and the process is termed "de- mulsification" or "breaking." The above distinction poses a p•oblem: When does a coalesced drop constitute a separate phase? This difficulty can be obviated by pre- scribing that the coalesced drop is regarded as a separate phase when it is visually observable. This definition may not be in strict conformity with the thermodynamic definition of a phase, but it is not unreasonable from the pragmatic point of view. Keeping the above definition of a

822 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS phase in mind, irreversible instability can be observed on micro and macro scales. Coalescence or Micro Instability As discussed earlier, in flocculation the droplets approach each other and form a floc, but the droplets do not lose their identity whereas in coalescence each floc combines to form a single large drop. The coalesced drop cannot be disintegrated into its constituent droplets. Some of the vital characteristics of the coalescence process are: (a) The process o/: coalescence cannot be followed visually as the coalesced drops are too small to be seen this is in conformity with the definition of a sepa- rate phase set forth earlier. (b) The rate of coalescence is controlled by the parameters which affect the properties of the intervening medium be- tween the droplets, and the absorbed films of the emulsifying agents (9, 18). (c) Progress of coalescence can be followed by microscopic ob- servation. (d) Coalescence is precursor to the separation of the clear dis- perse phase, i.e., demulsification. (e) Depending upon the conditions, coalescence may or may not be followed by demulsification. (f) The rate of coalescence may or may not represent the rate of demulsification, de- pending upon the site of coalescence. If the site of coalescence is such that the transport of the coalesced drops is not required, then the two rates may be identical. (g) Rate of creaming is accelerated if the densi- ties of the coalesced drops and the dispersion medium are appreciably different. In such "cream" the original particles of the disperse phase have lost their identity and the phenomenon is termed "irreversible creaming." Demulsification or Breaking or Macro Instability This represents the second and the final form of irreversible insta- bility. The rate of breaking can be followed visually. As pointed out earlier, the appearance of the separate clear disperse phase constitutes demulsification. The broken emulsion cannot be restored to its original state unless subjected to reemulsification by severe shear forces. The overall possible behavior of an emulsion system is depicted in Fig. 1. It is clear from Fig. 1 that the rate of demulsification may be con- tingent upon the rate of fiocculation, the rate of coalescence, or the rate of transport of the coalesced drops to constitute a visually observable phase. To illustrate this point, a specific example is in order. The rate of separation of clear disperse phase is measured on an emulsion (which is already flocculated) in the ultracentrifuge (9, 18) and this rate may or