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

370 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS this journal (4) published the first analysis of "blending agents" for the immiscible, mineral oil/castor oil system. Later, coupling agents for castor oil in lipstick were examined by Cadacamo and Cadacamo (5), and recently, coupling in general was ex- amined by Hertz (6). These papers describe a mechanism for cosolubilization suggesting that the cosolubilizer be a material of intermediate compatibility. The mechanics de- duced from our present investigation do not support the prior art, but instead we propose a new concept of solution cosolubilization based on cohesive energies of the system components. The importance of cohesive energy measurement was first recognized by Joel Hildebrand in 1950 (7). He initially measured cohesive energies of nonelectrolyte materials and assigned values in cal/cc. Because these energies correlated well with solubility proper- ties and the square roots were additive, he named the square roots of cohesive energies "solubility parameters." Solubility parameters form a "polarity" scale, effectively ranking all materials from oily (low cohesion) to waterlike (high cohesion). This system was readily adopted by the paint and coatings industry (8) and the plastics industry to provide rapid and accurate choices of plasticizers (9,10) and solvents, primarily due to the work of Burrell (11) and Hansen (12). In 1968 Burrell received the American Chemical Society's first award in the chemistry of plastics and coatings for his contribution to the practical understanding of solubility. More recently the system has spread to the pharmaceutical (13-15) cos- metics (16), adhesives (17), textile (18), and synthetic chemical (19) industries. In all, solubility parameters provide a convenient and quantitative means to evaluate and predict molecular interactions. THEORY Many measures of polarity have been proposed. Reichardt (20) compared over 40 em- pirical solvent polarity parameters, not including the Hansch Parameter (21) or the Kauri-Butanol Number (22) that have found wide practical success. Most empirical polarity measures cannot be expressed in energy values suitable to quantify molecular interactions. For this purpose one needs a polarity scale tied to the thermodynamic values of cohesive energy. The solubility parameter (7) fulfills this function and is historically quoted in calories. Some recent articles use joules (23) or the equivalent pressure (Mpa) (24). Cohesive energy was first derived from heat of vaporization (AHv) (25). Solubility parameter (8) = AHv where V M is molar volume. Later, boiling point (26), refractive index (27), surface tension (28), GLC retention volumes (29), and tables of molecular parts (30) provided more convenient derivations. Accuracy of the various methods depends on the specific applications to which they are applied. This study uses prior published values, primarily from the boiling point method. Coupling. In physics "coupling" refers to a pair of equal parallel forces acting in opposite directions (31). In regular solution chemistry, coupling has come to mean the linking of