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j. Soc. Cosmet. Chem., 42, 369-378 (November/December 1991) Not all cosolubilizers are "coupling agents" CHRISTOPHER D. VAUGHAN and FREDERICK A. VAN ASSCHE, Ultimate Contract Packaging, Pompano Beach, FL 33069-4816. Received December 20, 1990. Presented at the mid-year meeting of the Society of Cosmetic Chemists, Boston, 1990. Synopsis Additives that induce miscibility have become known as "coupling agents." In this study, interaction energies between castor oil/mineral oil and 25 cosolubilizers suggest that "coupling" is not the mechanism producing solution. The effectiveness of each cosolubilizer was related (r = 0.9725) to matching the polarity of only the most polar component. The cosolubilizers we used did not exhibit significant "bridging" or "coupling" between the immiscible components, as is generally thought. Instead, the cosolubilizer appears to reduce the self-attraction of the most cohesive, or polar, component, thus allowing the weaker, non-polar material to penetrate the polar domain. This new, entropy driven mechanism is termed "cohesive blocking." Several practical examples and a solution strategy are provided. INTRODUCTION Cosolubilizers are used in a broad range of industrial technologies, yet techniques to induce homogeneity remain more art than science. Order of addition is often a critical but uncertain factor, indicating that the "cosolubilizing" mechanics remain complex and structure-dependent. Indeed, there are two greatly different means by which co- solubilization takes place (1), i.e., microemulsification and true molecular solubilization (cosolvency). In both cases an additive induces the apparent dissolution of the "insol- uble" phase. But cosolvent solubilization produces a more intimate molecular mixture of components, while microemulsification yields submicroscopic (invisible) aggregates of the immiscible component (2). In both cases an additive makes the previously immiscible mixture clear, yet the resulting "clear" solutions differ greatly. One contains microemulsified aggregates and one does not. This difference is hardly pedantic, since optimum order of addition of materials is critical for microemulsions (3) but not significant in the formation of true solutions. Chemists seeking to clarify insoluble, "cloudy" mixtures must know which mechanism is at work to assess the strategy of trying different orders of addition. Our study examines the thermodynamics of true molecular solubilization in which order of addition is not a significant factor but choice of cosolubilizer polarity is. HISTORICAL The understanding of solution mechanics has progressed dramatically since 1950 when 369