18-MEA DEPOSITION ON HAIR 41 is about 1 nm, and since the alkyl chain length of 18-MEA is 2.39 nm, the 18-MEA/ SPDA layer orients at an angle of around 25° to the air interface. Thus, the following model (Figure 12) is suggested, based on the results obtained from the present investigation: 18-MEA/SPDA forms a layer 1 nm in thickness, with both 18-MEA and SPDA bound tightly to the surface by the carbonyl and amide groups, orienting the hydrophobic part to the air interface at an angle of around 25°. Another question was whether SPDA is special for providing persistent hydrophobicity to alkaline-color-treated weathered hair surfaces. Additional studies, on the dynamic contact angles and the amount of 18-MEA sorption measurements, were conducted to elucidate the issue with the complexes of 18-MEA and a variety of long-chain tertiary amines. Table III shows the dynamic contact angles and the amount of 18-MEA sorption of alkaline-color-treated weathered hair treated with 18-MEA/long-chain tertiary amines. The chemical structures of these tertiary amines and the logP value, which is calculated by the ClogP method, are also listed in Table III. The combination of 18-MEA with SPDA or DSDA made the alkaline-color-treated weathered hair surfaces hydropho- bic, and their hydrophobicity was maintained even after one instance of shampooing with a plain shampoo [15 wt% of sodium polyoxyethylene lauryl ether sulfate (2.5 E.O.) with 2 wt% N,N-bis(2-hydroxyethyl)-dodecanamide solution adjusted to pH7 with phosphoric acid], while the combination of 18-MEA with SHDA and DAPS could not. Figure 11. Molecular orientation analysis of 18-MEA/SPDA membrane by angle-resolved X-ray photoelec- tron spectroscopy (ARXPS). θ represents photoelectron take-off angle. (a) Angle dependence of nitrogen/ carbon concentration derived from SPDA. (b) Angle dependence of COO/C-C concentration derived from 18-MEA. Figure 12. Schematic diagram of the cuticle surface of alkaline-color-treated weathered hair treated with 18-MEA/SPDA. 18-MEA/SPDA forms a layer with high wear resistance, orienting the alkyl chain (hydro- phobic moiety) at an angle of around 25° to the air interface.

JOURNAL OF COSMETIC SCIENCE 42 The amount of 18-MEA absorbed on the surface of alkaline-color-treated weathered hair treated with 18-MEA/DSDA or18-MEA/SPDA complexes was much higher than that of hair treated with the other complexes (18-MEA/SHDA or 18-MEA/DAPS). The logP values of DSDA, SPDA, SHDA, and DAPS are 11.127, 9.496, 8.393 and 8.255, respec- tively. DSDA and SPDA are more hydrophobic tertiary amines than SHDA and DAPS. The combination of 18-MEA with the relatively hydrophobic tertiary amines (DSDA and SPDA) could make the alkaline-color-treated weathered hair surfaces hydrophobic, and their hydrophobicity was maintained even after one instance of shampooing with a plain shampoo [15 wt% of sodium polyoxyethylene lauryl ether sulfate (2.5 E.O.) with 2 wt% N,N-bis(2-hydroxyethyl)-dodecanamide solution adjusted to pH 7 with phosphoric acid]. It seems that the results relate to how the alkyl chain of 18-MEA and the alkyl chain of the tertiary amines align with each other in the complexes in molecular order, that is, the alkyl chain of the hydrophobic tertiary amines, DSDA and SPDA, could make packed alignments with the alkyl chain of 18-MEA, while the alkyl chain of the hydrophilic tertiary amines, SHDA and DAPS, could not make packed alignments with the alkyl chain of 18-MEA because the hydrophilic moiety of the hydrophilic tertiary amines, the hydroxypropyl group in SHDA and the amide group in DAPS, would hinder closer pack- ing between the alkyl chain of the tertiary amines and the alkyl chain of 18-MEA. Regarding the 18-MEA lipid layer on the untreated healthy hair, two models have been proposed, although there is still some contradiction regarding the thickness of the 18- MEA layer. The fi rst model is the one in which 18-MEA orients to the air interface straight, based on the length of a 20-carbon chain of 18-MEA (18). The second model is the one where 18-MEA is folded back in the direction of the surface (19), based on the result that the thickness of the surface lipid layer is 0.9 nm, as obtained by Ward et al. using XPS (20). Here, we would like to compare the surface model of hair treated with 18-MEA/ SPDA and the natural 18-MEA layer on untreated healthy hair. The surface models of hair treated with 18-MEA/SPDA and the 18-MEA layer on untreated healthy hair seem to have some characteristics in common, including being about 1 nm in thickness, having the hydrophilic area of molecules binding tightly to the surface, and orienting the Table III Effects of a Long-Chain Tertiary Amine Structure in 18-MEA/Tertially Amine Complexes for a Persistent Hydrophobicity Chemical structure Docosyldimethyl- amine (DSDA) Stearoxypropyl- dimethylamine (SPDA) Stearoxyhydroxypropyl- dimethylamine (SHDA) Dimethylaminopropyl- stearamide (DAPS) logP 11.127 9.496 8.393 8.255 Dynamic contact angles* 87.2 ± 2.5° 87.6 ± 3.7° 71.1 ± 4.1° 70.6 ± 5.6° Amount of 18-MEA sorption** 18.9 ± 0.6 18.7 ± 6.0 2.7 ± 0.1 4.0 ± 2.3 *Dynamic contact angels of alkaline-color-treated weathered hair treated with 18-MEA/tertiary amine com- plexes after shampooing were measured (n=7). **Measured by LC/MS (μg/g-hair) (n=3).