SOLUBILITY PARAMETERS IN COSEMTIC FORMULATING 323 solvents. Co-solubilizers may be identified by the medial location of their solubility parameters between two relatively immiscible materials. In this way isopropyl myristate (8 = 8.02) functions as a well known coupling agent in wax systems, like lipsticks, which incorporate normally immiscible mineral oil/castor oil (8 = 7.09/8.90) com- binations. Truly, some creative combinations of non-solvents have been produced which together exhibit good solvency, while being ineffectual alone. One common example of this "synergism" is the ethyl ether/ethanol (8 = 7.37/12.55) solvent combination for nitrocellulose (8 = 11.25). Individually these two solvents exhibit poor solvency for nitrocellulose. On the other hand, reverse synergism is also recognized where solvent mixtures interact with each other. The solubility of benzalphthalide was evaluated as an example of using solubility pa- rameters to determine the best choice of solvent and to demonstrate both the value and limitations of the solubility parameter for this purpose. Benzalphthalide is a new UV absorber of interest as a potential sunscreen. It is polar enough to demonstrate the separate solubility effects of the solubility parameter and the field deformations caused by inductive polarization and/or hydrogen bonding. It was the test material of choice because its manufacturer (VanDyk & Co.) describes it as a material with "limited solubility." The majority of cosmetic materials are less polar than benzalphthalide and their solubilities may be determined primarily from the solubility parameter. As ma- terials become more polar or more hydrogen bonded, the polar and hydrogen bonding forces expectedly contribute more to the solubility of that material. Two methods were used to determine the levels of solubility in the subject solvents. For volatile solvents, excess benzalphthalide was stirred and then allowed to stand isothermally at 25 degrees C. overnight before the supernatent was dried to constant weight. For solvents of lower volatility, incremental additions of solute were stirred isothermally until saturation was observed. Figure 2 represents the results of the solubility study plotted against solubility param- eter. Figure 3 shows the same results plotted with respect to solubility parameter and di- electric constant. Hydrogen bonding is believed to be responsible for any anomalous results in this plot. The dependence of solubility on the value of the solubility parameter is apparent from both graphs. However, the contribution of the polarity provides increased precision to estimates of solubility in other solvents and to evaluation of the solvating forces surrounding benzalphthalide. One may also determine the solubility parameter of benzalphthalide to be about 10.9 from these results. Solubility studies like this are the ultimate empirical method for determining a solubility parameter. Among the non-polar majority of cosmetic materials, the overwhelming contibutor to effects of solubility is the solubility parameter. APPLICATIONS OF THE SOLUBILITY PARAMETER The solubility parameter has been applied successfully in fields other than cosmetics and toiletries, yielding practical solutions to many problems. These solutions have, in turn, been used to develop new products and processes and to improve on old tech- nology. All of the methods reviewed below may be applied to cosmetic systems without modification.

324 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ETHYL ALCOHOL OCTANE 6 7 8 9 10 11' 12 15 SOLUBILITY PARAMBTBR SOLUBILITY OF BENZALPHTHALIDE Figure 2. Results of the solubility study plotted against solubility parameter. PROPYLENE CARBONATE 14 15 The petroleum lubricant industry, for example, discovered the root causes of "slip" and "oilyness" through use of the solubility parameter (27). This new understanding led directly to the development of novel, vastly improved lubricants. The concepts of "slip" and "oilyness" or "lubricity" are clearly important to cosmetic technology. Solubility is of major importance in biochemical processes. In 1964 Hansch (28)showed that drug activity depended equally on the lipid solubility of the drug molecule as well as its chemical reactivity. No drug can be effective if it cannot reach the location where it must work. From this rationale, correlations with solubility parameters have proved useful for designing insecticides, formulating liquid dosage pharmaceuticals, and an- alyzing transport of molecules through biological tissues (29). Wetting and dispersion of pigments by solvents was analyzed by solubility parameter (30) to yield the surprising conclusion that although dispersability was related to cohesion parameter, wettability was not. Instead, wettability was found to be inde- pendent of pigment type due to moisture normally found adsorbed on the surface of pigment particles. Plasticizers and solvents for resins are now routinely chosen by their solubility parameter (31), and new effective solvent mixtures (some constructed exclusively from non-sol- vents) are easily constrt•cted by use of the parameter (32). Foam control can be understood using cohesion parameters. The activity of a surfactant