46 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS groups where crystalline forces are weakest. If the crystal forln, i.e., the crystal .phase, is unchanged, specific volume measurements should be an •ndication of the presence of voids in the crystal lattice. Thus, in Figure 11 the minimum in specific volume occurs in the range of commercial stearic acid. At this point, where the palmitic and stearic acids are present in about equal molar proportion and the opportunity for selective pairing is greatest, the end groups again fit together so there is a maximum in degree of fitting of chains into the crystal lattice. This results in less voids and lower specific volume. Specific volume, then, is another basic factor which shows that there is a difference between commercial stearic acid and combinations containing significantly higher or lower proportions of stearic in palmitic acid. 1.04 1.02 1.00 0.98 • i • • I I I I • , 0 10 20 30 40 50 60 70 80 90100 MOL % STEARIC ACID Figure 11. Mol % stearic acid rs. specific volume of solid. SOLUBILITY The solubilities of binary mixtures of fatty acids correlate well with melt- ing points. With a mixture of palmitic and stearic, maximum solubility occurs at about 70 per cent palmitic-30 per cent stearic, which is the mini- mum melting point or eutectic. The solubilities of all mixtures of palmitic- stearic acids are in relation to their melting points. Also, the solubility curves of many of the derivatives of high molecular weight fatty acids in any specific solvent are qualitatively similar (2). The solubility of various mixtures of fatty acids in different solvents varies considerably. It depends on the temperature, the proportions of fatty acids present, the structure of the molecules, presence of groups cap- able of hydrogen bonding or dipole attraction, and the number and posi- tion of such groups (1). From this it is obvious that the determination and prediction of solu- bilities of the components in some of the complex cosmetic formulations are extremely complicated.