Conditions to produce so-called microemulsions: Factors to increase the mutual solubility of oil and water by solubilizer

Abstract

It was confirmed from the studies of phase diagrams that Schulman's so-called micromulsion is not an emulsion, but a solubilized solution. Hence, a microemulsion is not an adequate term for such system, but a swollen micellar solution may be adequate. A dispersed system containing microdroplets, which is not thermodynamically stable, may be called a microemulsion. It is desirable to find conditions to produce so-called microemulsions in which the solution is stable and solubilization is so large that oil and water mix over wide composition range. As much as about 20–25 wt% of surfactant was necessary to produce Schulman's so-called microemulsions. The following conditions were found in order to produce microemulsions with a far less amount, about 5–10 wt%, of solubilizer: (a) Optimum hydrophile-lipophile balance (HLB) or phase inversion temperature (PIT) of a surfactant. (b) Optimum mixing ratio of surfactants (solubilizer), i.e., optimum HLB (or PIT) of the mixture. (c) Optimum temperature for a given nonionic solubilizer. (d) The closer the HLBs of two surfactants, the larger the solubilization. (e) The larger the size of solubilizer the more efficient the solubilization. (f) Mixtures of ionic and nonionic surfactants which are durable to temperature change.