Breaking is a term applied to the irreversible coalescence of the individual emulsion or microemulsion droplets to form a continuous phase. To break an emulsion or microemulsion, the stabilizing forces must be weakened or eliminated.
The addition of electrolytes, i.e., salts for example, often causes an emulsion or microemulsion to break, since the electric charge of a boundary layer is very sensitive to the addition of foreign ions. The effectiveness of the added electrolyte increases with its valence. Thus, aluminum sulfate or iron sulfate are more effective than sodium sulfate, and sodium sulfate is in turn more effective than sodium chloride, i.e., see Ullmann's Encyclopedia of Industrial Chemistry, Volume A9, Page 311, VCH, Weinheim, Germany (1987).
In view of the above, to provide a microemulsion that was stable in the presence of salts would be unexpected, and contrary to conventional wisdom.
The need and usefulness of such a microemulsion in textile applications, for example, is evidenced by U.S. Pat. No. 5,466,746 (Nov. 14, 1995), in which an alkyl polyglycoside, which is a nonionic surfactant, is used to prepare mechanical microemulsions possessing some degree of stability with respect to the presence of foreign electrolytes such as sodium chloride. This is not totally unexpected, however, as nonionic surfactants carry no charge.
What is unexpected, and what has been discovered according to the present invention, is that microemulsions stable to the presence of foreign electrolytes can be prepared using cationic surfactants, which contain positively charged polar groups, especially if the microemulsion is prepared using an emulsion polymerization technique, rather than prepared mechanically as in the '746 patent.