ZINC PYRITHIONE PREPARATIONS 103 DISCUSSION AND APPLICATION The solubility of ZPTO in organic amines may be looked upon as the interaction of a Lewis acid and a Lewis base in which the ZPTO, acting as a Lewis acid, accepts a pair of electrons from an amino group thus forming a covalent bond. Zinc is a class a metal, that is, it is an electropositive metal that forms its most stable complexes with ligands in which the donor atom is nitrogen, oxygen, or fluorine (4). The sta- bili•ty of the resulting metal complex is a function of the nature of the ligand. With class a metals, the greater the base strength of the ligand the greater is the tendency to form stable complexes. However, this was found not to be the case in all instances. According to dissocia- tion constant tables, cyclohexylamine, with a pK• of 10.64, should be equal to if not a better solvent for ZPTO than dodecylamine (pK• 10.62) (5). It was shown that cyclohexylamine was a considerably poorer sol- vent than dodecylamine. Ethanolamine, with a relatively low pK• of 9.50, was capable of dissolving ZPTO to over 30%. The high solvency of ethanolamine can be explained through chelation. The formation of a chelate structure usually lends itself to stabilizing the complex, and the more extensive the chelation, the more stable the system. Fig- ure 2 shows a possible three-dimensional chelate structure of ethanol- amine. Observe the formation of a 5-membered chelate ring and note that the spatial arrangement of the coordination bonds allows zinc to be positioned in the center of a tetrahedral structure (6). However, 2-amino-2-methylpropanol, which can be thought of as a dimethyl deriv- ative of ethanolamine, NHsC (CHa)sCHsOH, is an extremely poor sol- vent. This is not surprising if the steric hindrance of the two methyl ETHANOLAMINE Figure 2. ZPTO-ethanolamine complex

104 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS groups on the carbon atom alpha to the amino function is taken into ac- count. Many of the solubility inconsistencies in this study can be ex- plained by steric hindrance. In general, large bulky ligands form less stable metal complexes than do analogous smaller ligands. Although the most stable metal chelates containing saturated ligands form 5-membered chelate rings, saturated 6-membered rings are fre- quently encountered (7). Figure 3 shows the 6-rnembered ring that 1,3-diaminopropane forms with ZPTO. It is a good solvent. Figure 4 shows the complex formed when ZPTO is dissolved in N-(3-aminopropyl)-diethanolamine. Observe that a zinc chelate struc- ture is formed with the tertiary amino nitrogen atom and an oxygen Figure 3. ZPTO-1,3-diaminopropane complex C H•C H• 0 H