HAIR COLORING--MODERN FORMULATION CONSIDERATIONS 301 reason is that a film of high dye concentration "plates out" of the solution onto the surface of the hair. The film may contain 5 to 10 per cent dye compared to the 1 per cent in the solution as a whole. This can easily double the color yield for any given amount of dye. Due to their water solubility levels and hair substantivity, it will gener- ally be found that the less polar solvents are more effective for this use. They also present a cost advantage over the more common solvents such as ethanol which must (16) be used in amounts of forty to sixty per cent for similar effect. The degree of sulfonation of a dye generally controls its solubility in solvents. Shades made up for nonsolvent formulations there- fore sometimes change character radically when solvents are added. We have found, as would be expected, that not only the sparingly soluble alcohols and aldehydes give this effect, but also the following: l. Sparingly soluble nonionic surfactants, including some of the modi- fied lanolin products. As mentioned previously, Beste (1) patented the use of certain nonionics for exactly the reverse effect---the sotbed film of sur- factant forming a barrier to dyes insoluble in it, thus slowing down the dye- ing rate to give more control over the results. 2. Oil soluble dyes with just the right amount of solubilizing or disper- sion, such as DC Red 37 or Nigrosine SS. 3. Artion-cation surfactant complexes often combine low water solubility and high hair substantivity with the virtue of being excellent dye solvents because of their structure. It is occasionally necessary to solubilize such complexes somewhat, but this should not be done too well (17) or all effect is promptly lost and no additional color results on the hair. SWELLING AGENTS Materials such as urea, thiourea, urethane and formamide can be con- sidered either as dye solvents (18, 20, 2l) or hair swelling agents (19, 22). In the case of urea and thiourea at least, there has been considerable der- matological evidence that such agents stretch the hydrogen bonds and can disrupt the cystine of keratin, forming cysteine and thereby providing new dye sites for sulfonated dyes. This allows the hair to dye more deeply and more permanently. Therefore, it is unwise to add urea to a temporary hair coloring for two reasons: if the product contains color complexes, they may be solubilized to the point of reducing initial color, and that color which does take on the hair will be more difficult to wash out again later. It is beyond the scope of this general paper to go much into the contro- versy over why hair dyes take more deeply on newly permanent waved hair. Many feel that such a treatment merely makes the hair swell more than usual, therefore permitting it to take up more color than usual. It can be shown quite dramatically, however, that a chemical reaction also takes place, by using cystine as an analogue for hair. When it is mixed

302 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS with disulfonated acid dyes in an alkaline thioglycolate medium, reaction takes place fairly readily, resulting in "dyed" cystine which withstands several hours of refluxing with no change of color. The shades produced are the same as on thioglycolate damaged hair, even from mixtures of four or five colors. Effects of this sort, though much less intense, can also be produced by substitutin• urea for the thioglycolate, or by using cysteine with just a simple solution of the dyes. In contrast, none of these effects are obtained if monosulfonated dyes are substituted for the disulfonated ones. We thus have another insight into the mechanism by which damaged hair is dyed rather permanently by the "temporary" colors. ANION-CATION COMPLEXES The use of these color complexes constitutes the most striking new trend in hair dyeing in recent years. At first they were often introduced accidentally during attempts to add substantive "conditioners" to make a better product. The time has come, however, when their properties nmst be studied much more systematically. Through their deliberate use, it is possible to add a new dimension to hair dyeing---a product not truly substantive but which gives transparent, strongly adhering surface coloration to the hair. If prop- erly formulated, it does not rub off, is water insoluble and yet can be com- pletely removed with a simple shampoo. These complexes are the core of most of the formulations approaching the "full intensity rinse" ideal which are appearing on the market today. Useful color complexes may be made in many ways. Anionic (acid) dyes have been reacted with cationic surfactants either directly or by being applied to hair previously treated with the cationic (5, 17, 23, 24, 25, 26) surfactant. The reverse has also been done--reacting cationic dyes with anionic agents (25, 26, 27, 28). Dyes have been applied in the presence of both, that is, in the presence of the reac/ion produc/of anionic and cationic materials (2, 3, 4, 9). Finally, hair colorings have been sold which contain both acid and bakic dyes, thus forming a dye to dye complex which is later partially solubilized for use in the final product. The use of such combinations has been growing steadily over the past few decades in textile dyeing and finishing. However, their full realization did not become apparent until they were applied more recently to cosmetic use where they have not been restricted only to hair dyes, but are being put to work as film formers in such items as hand lotions and creams. A number of apparently overlapping patents have appeared recently, using the princi- ple for such uses and also for hair colorants. The proper balance of ionic agents and dyes will give insoluble or slightly soluble precipitates which are essentially nonpolar, adsorb phenomenally on the surface of the hair from a water base, and in general show many of the partiti(m ratio effects already discussed under "Solvents." They can be