HAIR COLORING--MODERN FORMULATION CONSIDERATIONS 295 heavy metals by forming cysteine mercaptides in the hair and nails. Thus, there are many routes by which hair dyes may eventually contact and be precipitated by metallic ions. Since this is never desirable, sequestrants have a definite place in most formulations. THE EFFECT OF pH in most cases where a substantive dye is used to color the hair, the effect of pH is crucial. One of the reasons why the aromatic amines produce such wash-fast shades on the hair is that they swell it, due not only to their own pH but also to added alkali. They are thus able to penetrate into the hair somewhat before being oxidized into a pigment which no longer will wash out. Henna and other vegetable dyes have been applied from both alkaline and acidic media. Once frequently used in color shampoos to give color high- lights, the optimum pH for henna substantivity according to Harry (7) is pH 5.5. When dealing with simple solutions of acid dyes, it seems at first as if pH alone controls the color yield on the hair. Such solutions give relatively little color above pH 5, but as this is lowered below pH 3, the color yield in- creases dramatically. It is for this reason that the capsule rinses often con- tain 99 per cent dry organic acid. Even one of the more modern color rinse solutions which appeared a few years ago contained about 30 per cent citric acid to give an effective in-use pH of about 2.5. In this connection, it should be noted that the hair, being amphoteric, reacts with such acidic solutions and raises their pH during the dyeing operation. Therefore, if pH is critical to the particular formulation and yet for other reasons it con- tains very little free acid, it is wise to buffer well. Such hair-acid reactions take place quite quickly, substantial acid being absorbed from the solution long before the dyes will be picked up. The nature of the particular acid used is important as well as the pH it gives. Alexander and Hudson (30) give an excellent and very detailed dis- cussion of this phenomenon. When keratin is dipped in dye-acid mixtures, the anion sorbed by the fiber is controlled not only by pH but by the relative affinities of the two. If a dye anion and a simple acid such as HC1 are mixed and applied to the hair, the dye will be sorbed, but at very low pH values the acid will not combine at all. In contrast, HC1 alone is picked up in stoichiometric quantities. Furthermore, substituting benzene sulfonic acid for the HCI in the above dye-acid mixture results in more dye being sorbed at any given pH. It can be demonstrated that at any given pH, sulfuric acid gives better color yields on the hair than acetic or hydroxyacetic acids, which in turn are better than citric or phosphoric acids. There are cases where the use of the acid form of a gum or sequestrant will do quite well, thus simplifying the

296 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS ß formulation by reducing the number of components. Studies of this sort are well worth making if your product will contain pH dependent dyes. The pH sensitivity of acid dyes is related directly to their basicity. Diserens (6) classifies them in increasing order of sensitivity as follows: 1. Triarylmethane dyes 2. Monosulfonates 3. Disulfonates 4. Trisulfonates Level colors are not generally used in hair dyes since good levellin'g is ob- tained only at the expense of exhaustion, while the paramount problem in hair coloring is to get maximum color from a cold dyebath in a short period of time. Generally speaking also, level dyes have poor wash fastness. The sensitivity of anion-cation complexes to pH is also critical. When such complexes are formed by reacting ionic dyes with surfactants of oppo- site charge, it will generally be found that the maximum color yield occurs at a rather specific pH for each combination of agents. The particular pH which will be crucial often cannot be predicted, apparently depending on the relative "ionic strengths" of the materials being reacted. This is par- ticularly true when amphoteric surfactants are used as part of the color complex. SOME D,z•.-HAIR REACTIONS The protein keratin consists of long polypeptide chains cross linked in several ways. One is a salt linkage between acidic and basic amino acids, giving the protein its amphoteric character. Another cross ]ink is the di- sulfide one, cystine, which is easily reduced to the mercaptan cysteine. Most of the major classes of dyes react readily with the amphoteric kera- tin fibers. Studies by Meyer (11) and others show that 1200 grams of wool react with one gram equivalent of acid. This, of course, includes acid dyes. It can be presumed that one equivalent of base will titrate about 1600 grams of wool since only 8 per cent of the keratin side chains are basic while ] ] per cent of them are acid. In any case, these acid-base equivalents ex- plain the stoichiometric limitation on total dye uptake by the hair. Keratin fibers swell excessively when the disulfide crosslinkage has been broken. Reducing the cystine to cysteine during permanent waving of hair, or oxidizing it to cysteic acid by peroxide bleaching, will result in such increased swelling. If the swelling is done with dye solutions instead of just water, the hair can obviously take up more color than usual. Furthermore, acidified dye solutions will harden such swollen hair, contracting it, and thus be less able to wash out again later. This accounts for some of the difficulty encountered in removing "temporary rinses" from damaged hair. These are some of the physical prope• ties of damaged hair which affect the dye uptake.