2081
BINDING OF PHENOLS BY HAIR
The Binding of Phenols by Hair. 111. Volume Changes Accompanying Binding of Phenols by Hair
by M. M. Breuerl Gillette Research Laboratories, Reading, Berkshire, England
(Receiued December 9, 1969)
The volume (changes occurring during the binding of four phenolic compounds to “De Mea" hair have been measured. The results suggest that the binding of the phenols disorganizes the hydration layer of the hair. The nature of the binding forces between hair and the phenolic compounds is also discussed.
Introduction The binding of phenols to hair is accompanied by a simultaneous release of hydration water and by positive enthalpy and entropy changes. These results indicate that the entropy increase, which occurs as a consequence of changes in the hydration layers of the keratin, and of the phenols, is the driving force in these sorption processes. The measurement of thie volunie changes accompanying these interactions promises to be a useful method for studying the hydration changes.a
Experimental The purification of De Meo hair, the preparation of acid-treated hair, and the experimental procedures have been described p r e v i o u ~ l y . ~ ~ ~
Results The volunie changes accompanying the binding processes, AVS were calculated from AVT, the total measured volume change, and from AVD, the known value of the volume change which occurs during the dilution of the aqueous reagent solution.
The values of AVs are plotted as a function of f , the concentration of the phenolic compound in equilibrium, in Fig. 1 4 . The latter quantity was determined spectrophotometrically from solution samples which were withdrawn from the dilatometers after completion of the experiments. In the case of phenol both the data obtained with virgin and acid-treated hair can be represented on a single curve, having a broad plateau as a maximum and dropping suddenly to negative values (Fig. 1).
The behavior of resorcinol is exceptional, insofar as it is the only phenol studied which causes volume contraction when adsorbed by hair. The binding curves for virgin and acid-treated hair are similar in shape (Fig. 2), but the sorption processes on acid-treated hair are accompanied by larger volume contractions. The results obtained with catechol and pyrogallol are qualitatively very similar (Fig. 3 and 4); in both cases the sorption processes are acconipaiiied by volume expansions. In the case of acid-treated hair AVs increases snioothly with f, but for virgin hair the curves exhibit maxima and minima. The volume changes accompanying the binding of the various phenols can also be represented as a function of r , the amount of phenol bound to hair in moles/g. The values of r were computed froni the appropriate binding constantse2 The apparent molal volumes of the various phenols, 4, are functions of their concentrations. In order to obtain a more uniform picture of the volume changes which occur during the sorption of the various phenols, it appears more useful to compute the value of AVs’, the voluine change which occurs when phenols in a standard hydration state, chosen arbitrarily as the hydration state at infinite dilution, are transferred from solutions into hair. The value of AV,’ was calculated from the known dependence4of 4 on f by means of the equation ( I ) TJnilever Research Laboratory, Isleworth, Middlesex, England. (2) See part I, M . M. Beuer, J . Phys. Chem., 6 8 , 2067 (1964). (3) J. Rasper and &M.Kauamann, J . Am. Chem. Soc., 84, 1771 (1962). (4) See part 11, M. M. Breuer, J . Phys. Chem., 6 8 , 2074 (1964).
Volume 68,Number 8
August, 1964
2082
A I . M. BREUER
AVs' == AVs - (& - q 5 ) ~
(2)
o virgin .acid
where denotes the apparent molal volume of the phenol in question a t infinite dilution. In Fig. 5 , 6, 7, and 8 the values AVS' are represented as a function of r for phenol, resorcinol, catechol, and pyrogallol, respectively. It is evident from Fig. 5-8 that virgin hair and acid-treated hair differ in their behavior; whereas in the case of the former, plots of AVs' us. r exhibit maxima which are followed, a t critical values of T , by an abrupt fall to a nzinimuni (volume contraction), the AVS' against r plots for the latter are generally smooth curves coinposed of two fairly linear sections with moderately different slopes. Phe-
r
0 virgin hair 0 acid treated hair
0
I 3
ovirgin hair .acid treated hair
e
-t
5 4
3
-2
2
\
t
I 0
I
f
mole. 1;'
no1 seems to exhibit an exceptional behavior, as ft gives identical curves for virgin and acid-treated hair.
treated hair
Discussion
-3
n
0
1 mole. I
.-'
Figure 2. The volume changes accompanying the binding of resorcinol as a function of the concentration.
The Journal of Phgsical Chemistry
3
mole. I-'
Figure 4. The volume changes accompanying the binding of pyrogallol as a function of the concentration.
0' virgin hair .acid
2
I
f
0'5
Figure 1. The volume change accompanying binding of phenol as a function of the concentration.
G
mole.^?
Figure 3. The volume changes accompanying the binding of catechol as a function of the concentration.
-4 -51
-'I-
I
2
I
f
+5r
mzY
I
0
hair treated hair
The quantity AVS' can be regarded as being composed of two terms: (a) the change occurring in the apparent niolal volume of the phenol as a consequence of its transfer from the aqueous solution into its new environment and its attachment to the hair, and (b) the volume change reflecting on the structural changes which occur in keratin as a consequence of the phenolkeratin interaction (e.g., changes in tertiary structure, hydration layer, etc.). For a given compound, it can be assumed a priori, that the magnitude of AVs' will be the same, irrespective of the value of r (assuming
2083
HAIR
BINDING O F P H E N O L S BY
o v i r g i n hair .acid treated hair
r x
lo3 m o l e .
o v i r d n hair *acid treated hair
9-1 r x
Figure 5. Plot of AVs’ against r for phenol.
lo3
mole. 9-1
Figure 7 . Plot of AVB’ against r for catechol. ovirgin hair oacid t r e a t e d hair
o virgin hair 0
acid treated hair
‘2
-
-3-
I ’ O7
-4-
(.I.
5 m
-
0
-5’6-
*II)
a
-7-
-8-A-
I
2
I
r x
~
3
I
4
mole. ~ 3 g-1
rx
lo3
mole, g-,’
Figure 6. Plot of AVs‘ against r for resorcinol.
Figure 8. Plot of AVs’ against r for pyrogallol.
that no phenol-phenol interaction is present between the bound molecules). This seems to be a reasonable assumption in view of Langmuiric characteristic of the absorption isotherm.2 The causes for the deviation from linearity of the AVS’ us. r curves can be attributed therefore to changes which occur in the hair and its hydration structure. One of the most striking differences between virgiii and acid-treated hair is the sudden volume contraction which occurs in virgin hair after a certain amount of resorcinol, catechol, or pyrogallol is adsorbed. The most plausible explanation for this volume contraction In virgin hair is the occurrence of a disorganization process iii certain parts of the hair structure, involviiig an exposure to water of hitherto buried polypeptide side chains. The exposure of both polar and nonpolar side chains to contact with water involves volunie contraction as a consequence either of electrostriction
(polar group-water interactions) or an increase of the ice-like structure of water (nonpolar group- water interaction). On the basis of the available data, however, no distiiiction can be made between these two alternatives. The absence of similar volume coiitractioii phenomena with acid-treated hair indicates that these denaturation processes also occur during the soaking of hair in 0.1 N HCl. Further support for this view comes from results which show that acid-treated hair has a higher acid binding capacity (about 11% more) and lower water retention (at given humidity) than virgin hair. Also, the fact that larger positive volunie changes accompany thc binding of pyrogallol and, to a lesser extent, that of catechol to virgin hair than to acidtreated hair, is very interesting. This different binding behavior suggests that virgiii and acid-treated hair differ in their surface structure (hydration structure) Volume 68, -Turnher 8
August, 1564
2084
rather than in their inner structure. Both the X-ray diffraction patterns and the mechanical properties of virgin arid acid-treated hair are practically identical. (The 30% index, i e . , the work needed to stretch a hair fiber to 30y0 of its original length, is only 2% less for acid-treated hair than for virgin haira2) On the basis of the above dilatometric results, it seeins as if virgin hair possesses a different hydration layer (different in structure and possibly in extent) from that of acid-treated hair. Adsorption of phenols brings about a collapse of this “extra” hydration layer to varying extents, depending on the nature of the phenol in question. I t is also interesting to note in this connection that acid-treated hair has a lower water uptake, a t a given humidity, than virgin hair by about 4 X lop4mole/g. of hair.2 These findings seein to support Klotz’s hypothesis5 a t least to a limited extent, which states that proteins in their native state are surrounded by organized water layers (icebergs), which niay collapse during reactions with other compounds. The AVs’ us. r plots, especially in the case of acidtreated hair, also provide some qualitative information concerning the nature of the binding forces between the phenolic compounds and hair. Thus, after the initial adsorption of 1.4 X inole/g. of resorcinol, the binding of any additional resorcinol is accompanied by 2 ~ n i . ~ / m o lvolume e contraction. This negative volume change, and the fact that both the enthalpy and the entropy of resorcinol-keratin interaction are negative quantities,2strongly suggest an H-bond format>ioii between resorcinol and keratin. On the other hand, the positive slopes observed with catechol and pyrogallol on the A V S ’ against r plots indicate that, although H-bond forinatioii may exist, hydrophobic
The Journal of Physical Chemistry
M. M. BREUER
interactions are more These results are in good qualitative agreement with the solution properties of these three phenol^.^ The more positive initial slopes observed in the case of all the above-mentioned three phenols with acid-treated hair (Fig. 6, 8) might be due to the existence of “icebergs” or more limited extents. Finally, the results with phenol indicate that in this case an entirely different adsorption mechanism is at work. The close resemblance between the AVS’ us. r curve and the curve representing the change of apparent molal volume of phenol as a function of concentration in aqueous s ~ l u t i o nsuggest ,~ that phenolphenol interactions play an important role during the sorption process. Both the dilatometric results and the absorption isotherms (indicating strong nextneighbor interactions) point toward a mechanism in which only few phenol molecules bind directly to the polypeptide chains. These molecules seem to act as nuclei for a crystallization or condensation process for other phenol molecules, forming phenol crystallites or droplets in the pores of the hair. This hypothesis is also supported by the fact that hair adsorbs much larger quantities of phenol, given the same reagent concentration, than any of the other phenolic compounds studied.2
Acknowledgment. The author wishes to thank M r .
R. Lee for his skillful technical assistance and the Management of Gillette Industries Ltd. for permission to publish this paper. (5) I. Vi. Klotz, Science, 128, 815 (1958). ( 6 ) G. NBmethy and IT. A. Rcheraga, J . Chem. Phus., 36, 3701 (1962): J . P h y s . Chem., 6 6 , 1773 (1962).
