238 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS TABLE V Effect of Apolar Residues on the Swelling Properties of Hair Per Cent Swelling • in: Per Cent Alkylating 50 Per Cent 50 Per Cent 20 Per Cent 50 Per Cent Reduction Agent Water Methanol Ethanol Propanol Propanol 0 None 31.4 n.d. b n.d. b 31.6 32.2 45 None 39.9 35.8 37.4 42.6 45.2 45 Methyl iodide 47.2 42.2 46.2 47.8 49.2 45 N-ethyl maleimide 45.3 41.0 45.5 48.6 51.1 45 N-hexyl maleimide 33.9 33.7 37.8 39.5 46.3 45 N-heptyl maleimide 34.8 33.6 37.7 40.9 46.4 82 N-hexyl maleimide 31.0 n.d. • n.d. • 44.9 56.5 82 N-dode cyl maleimide 28.2 n.d]' 31.9 36.4 47.5 82 N-benzyl maleimide 31.5 n.d. b 34.1 35.4 46.1 aCalculated on the treated weight of the fiber. "Not determined. substantial conformational stability to the reduced keratin structure and successfully.•:ii7 resist the swelling pressure. This conformational stability is, of course, lost in aqueous alcohols, where the phobic interactions between the apolar residues present in hair are prevented. Such a differential response to aqueous solvents offers a unique way of hair manipulation, par- ticularly with regard to setting. 7. Mechanicalproperties: Swelling data have shown that the imbibition of water by ratin can be restricted by hydrophobic modification of the fiber. Although, the precise nature of the intermolecular arrangement remains a subject of controversy (10, 1 the stabilization of hydrophobic bonds by water is not disputed. The strength of the hydrophobic bond is represented by the tendency of nonpolar groups to adhere to one another. The free energy of this process has been assessed (12), and in the case of the.. ( interaction of 2 methyl groups, was found to be -0.73 kcal/mole for alkyl sidechains.'"?? The increment in the free energy of binding was in the order of -0.37 kcal/mole per•:is? CHz group. The overall contribution of these hydrophobic crosslinks to the tion of the keratin structure will depend on the size, shape, and number of the in-.7:. troduced apolar residues. : Some further insight on these hydrophobic interactions was obtained from a study o the mechanical properties. It is well known that the wet strength of intact hair bears a linear relationship to the cystine content over a wide range of reduction levels (13). Us- ing this linear relationship as a guide, a preliminary assessment of the stabilization ef -'?' fect arising from hydrophobic interactions was obtained from the mechanical behavior

HYDROPHOBIC BONDS AND HAIR TREATMENTS Table VI Yield Stress of Hair Alkylated with N-Hexyl Maleimide 239 Yield Stress, g/denier in: Water 20 Per Cent Propanol 50 Per Cent Propanol Reduction Level, Per Cent Calculated a Observation Calculated a Observation Calculated a Observation 0 0.42 0.42 0.36 0.36 0.36 0.36 31 0.28 0.40 0.25 0.30 0.25 0.27 42 0.22 0.32 0,21 0.17 0.21 0.14 82 0.08 0.27 0.06 0.11 0.06 0.10 aCalculated yield stress = (intact hair yield stress) (100-per cent reduction/100). of alkylated fibers (Table VI). The alkylation was performed at 3 reduction levels using N-hexyl maleimide as the alkylating agent. If one relies on the fact that the percent reduction in work to stretch a reduced fiber is directly proportional to the extent of reduction, then it is evident that the alkylated fibers do not exhibit such a loss of strength. In water, the formation of hydrophobic bonds, via interaction of the hexyl residues, results in significant stabilization of the keratin structure. Although, some repairing effect was anticipated, the extent of the stabilization and particularly the resistance of the treated fibers to the external stresses was unexpected. Even with the maximum overlap of the apolar sidechains in- troduced in the alkylation step, the average strength of the newly formed bonds would not exceed 5 kcal. This is only a fraction of the energy loss which accompanies the breakdown of cystine crosslinks (-50 kcal/mol). It is apparent that the hydrophobic interactions which accompany the blocking of cysteine residues are very intensive, al- though, a possibility of cooperative multichain hydrogen bonding in a hydrophobic en- vironment cannot be excluded. 8. Setting properties ofalkylated hair.' The alkylation of reduced hair limits the swell- ing of hair in water. On the other hand, such hair can be readily deformed in alcoholic media. An attempt was made to utilize this change in swelling characteristic for the set- ting purposes. Thus, the hair was swollen in 50 per cent propanol and set on rollers. In the presence of alcohol, the alkylated hair is very pliable and moldable and conforms readily to the desired configuration. After setting the hair, the alcohol was removed by rinsing with water. Removal of the alcohol leads to the formation of hydrophobic crosslinks, which stabilize the new (set) configuration. Hair alkylated with N-heptyl maleimide retained the set in liquid HzO, while a tress alkylated with N-ethyl maleimide straightened within 1 to 2 min. This demonstrates that a water-resistant set can be attained by the introduction of apolar residues, providing that a sufficiently long sidechain is used. A 7-carbon alkyl chain appears to fulfill this requirement. The set stability of tresses was also assessed by the conventional manner. This involves the exposure of set tresses to controlled conditions of humidity and temperature and measuring the extent of relaxation with respect to time. Following the reduction, the tresses were rinsed with water and then alkylated with N-heptyl maleimide for 2 h. The setting was performed with 50 per cent propanol after thorough rinsing with water the