160 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS EXPERIMENTAL MATERIALS The material used as a model for human hair has already been described (1). 2-Hy- droxy-1,4-naphthoquinone (lawsone) and 2-naphthol were supplied by Sigma Chemical Co. Ltd., and 1,4-naphthoquinone was obtained from Fluka Chemical Co. Ltd. All were used without further purification. ABSORPTION SPECTRA Readings were obtained using a Kontron spectrophotometer (model UVIKON 860) and 10-mm matched quartz cells. COLOR MEASUREMENT L.a.b. values. Reflectance readings were obtained using a Phillips PU7908/24 integrat- ing spheroid connected to a PU8800 spectrophotometer. Readings were recorded at 10-nm intervals between 380 and 760 nm for the calculation of L.a.b. values. Black specular caps were used to measure diffuse reflectance. A computer program was pre- pared, using MINITAB to calculate L.a.b. values from the data. The reproducibility of the L. a.b. values was checked by carrying out replicate determinations on standard tiles. Identical results, agreeing with the standard values, were obtained. DYEING PROCEDURE The same procedure was employed as before (1), except that pH values were adjusted with either 0. ! M NaOH or 0.1 M HCI. DISSOLUTION Dyed felts were transferred to the rotating basket of a GB Calvena Ltd. ASCO disso- lution rate apparatus (model 6ST), 600 ml of water was used as dissolution medium, and the basket rotated at 100 rpm. Dissolution was carried out for 60 min at 50øC. SOLUBILITIES OF LAWSONE Solubilities were determined in 0.1 and 0.001 M HCI, water, and 0.001 and 0.1 M NaOH. An excess of lawsone was added to the test solution and shaken at an elevated temperature (55øC). The temperature was then reduced to 40øC, and samples of solution withdrawn through a syringe filter, diluted, and assayed spectrophotometrically at 290 nm. Samples were taken at intervals until constant results were obtained. Solubilities in water and the acid solutions were low and constant (0.02% w/v), but increased to 0.05% in 0.001 M NaOH and to 2.0% in 0.1 M NaOH.

DYEING WITH LAWSONE 161 DISCUSSION UPTAKE AND DISSOLUTION Results will be discussed in turn, in terms of hue, saturation, and substantivity. HUE Savranskii and Pilipenko (2) calculated the energies of alternative w electronic forms of lawsone using MO-LCAO-SCF methods, and concluded that the compound was tauto- meric. The UV spectra of lawsone solutions go through an isosbestic point (1), sug- gesting that the ionized and unionized species are involved in a pH-dependent equi- librium (Eq. 1). Absorption in the visible region by lawsone solutions increases with increasing pH the tautomer in excess in alkaline solutions is therefore more deeply colored than the species associated with low pH values. O O OH f.• 0- / x, N + H+ O) 0 0 (Eq. 1) 1,4-Naphthoquinones usually give yellow solutions, and 1,2-naphthoquinones orange or red solutions. The yellow color of 1,4-naphthoquinones, as exemplified by (I), in contrast to the orange of the 1,2-compounds, is considered to result from restriction of resonance arising from the stability of the 1,4-quinone group, which demands that the bond joining the two rings is unsaturated (3). This bond fixation prevents formation of canonical forms in which there is a single bond joining the rings. 1,2-Quinones have canonical forms involving both ring junctions. The repulsion between the partial neg- ative charge on the 1-ketone and the negative charge on the 2-oxygen of the lawsone anion could push the equilibrium shown in Eq. 2 to the right, giving a 1,2-naphtho- quinone structure (II) with a characteristic reddish-orange hue. This would explain why lawsone, although it is a 1,4-quinone, gives an orange-colored solution in aqueous alkali. There would be no repulsion between the 1-keto and 2-hydroxy groups in undissociated lawsone in fact, NMR evidence indicates that there is strong intramolecular hydrogen bonding between these groups in CDCI 3 solution (4). Our own infrared results (5) support this conclusion. The carbonyl-stretching region of lawsone in carbon tetrachlo- ride consists of a doublet at 1662 and 1672 cm-•. The carbonyl-stretching peak of 1,4-naphthoquinone appears at 1675 cm- • (6), so that the smaller 1672 peak must be due to free carbonyl and the larger peak at 1662 cm-• can be assigned to 1-carbonyl, which has been shifted to a lower frequency by intramolecuIar hydrogen bonding with the 2-hydroxyl group. Support for this suggestion came from hydroxyl-stretching fre- quencies (5). The band for 2-naphthol lies at 3610 cm-•, while that for lawsone is smaller and located at 3411 cm- 1. Shifts of this order occur when hydroxyl is intramo- lecularly hydrogen bonded to form a non-conjugated ring, which suggests that undis- sociated lawsone occurs partly in the form indicated by (III). Such complexation would