the chemistry of hair - journal of cosmetic science

WHAT DETERMINES EMULSION TYPE ? 207

Microscopy would probably be too slow, some breaking of the emulsion having possible occurred before the examination was carried out.

MR. J. PICKTHALL.' When talking of H.L.B. values, the speaker has referred to the oil solubility of calcium soaps. Are these soaps in fact oil soluble in the sense that we are considering ? In discussing the emulsifying action of the calcium soaps, the speaker has compared them with glyceryl distearate and aluminium stearate. Glyceryl distearate is indeed oil soluble but commercial aluminium stearate cannot be regarded as a true metal soap.

THE LECTURER: The question of the state of calcium soaps in the oil phase, which arose during the discussion, is clearly an important and interesting one. In my experiments I have not used calcium soaps because of the many unknown factors, and certainly a detailed physical-chemical investigation of the state of these materials in different oils, with and without traces of water present, would be most enlightening.

THE CHEMISTRY OF HAIR

C. S. WHEWELL, Ph.D., F.R.I.C., F.T.I.*

A lecture delivered before the Society on Llth December 1960.

The morphology and chemical constitution o• keratins are briefly discussed together with modern views on permanent setting and supercontraction. Special attention is given to the setting characteristics o[ chemically modified fibres. The results o[ studies on the setting o[ fibres in solutions o[ tetrakis (hydroxymethyl) phosphonium chloride and o• thiourea dioxide are reported.

DURING THE past thirty years a great deal of progress has been made in the understanding of the structure and properties of hair and related fibres. Although many aspects are still controversial, the general features of the structure of hair are well established and many of the important properties of the fibre can be explained in scientific terms. Much of the fundamental work has been carried out on wool rather than on hair, but it has long been realised that the basic structures and reactions of these two types of keratin are essentially the same, even though they differ in detail. The object of the present paper is to present some of the more recent work against a background of current thought on the chemistry of keratins.

General Considerations

Hair and other animal fibres usually consist of three distinct histological regions--cuticle, cortex and medulla. The cuticle or outer layer, consists

*Department of Textile Industries, The University, Leeds 2, Yorks.

208 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS

of scales laid down like tiles on a roof. It represents a comparatively small part of the fibre in wool and hair, but in other fibres such as those from the skunk or kolinsky, the cuticle is much larger. The scales, which have no fibrillar structure, can be divided into two parts, (i) the inner endocuticle which has a pitted honeycombed structure with ridges and is resistant to trypti• digestion, and (ii) the outer exocuticle which is smooth and featureless and is digested by trypsin. In addition to these two layers, a thin membrane-the epicuticle--is located over or around the scales. This cell membrane is characterised by its inertness to chemical reagents.

The cortex is the main constituent of hair and consists of cigar-shaped cells, the dimensions of which vary according to the particular type of keratin. The cortical cells are built up of fibrils and these are associated in sheets and embedded in more amorphous material. Fibrils are themselves composed of sub-fibrils which according to Mercer and Rees • may be responsible for the characteristic X-ray pattern given by the protein. Cortical cells are not, however, all identical in composition, and it has been demonstrated by Horio and Kondo •' that a fibre may be divided longitudinally into two hemicylinders which have been termed the paracortex and the orthocortex. The orthocortex is the more reactive and takes up dye more readily than the paracortex. The distribution of the orthocortex and paracortex varies with the type of fibre. In crimped wools, for instance, the orthocortex is always on the outside of the crimp, but in coarser wools the arrangement of the orthocortex and paracortex is annular. Immediately under the scales and surrounding the cortex is another continuous membrane, the sub cuticle which is more resistant to attack by chemical reagents than the rest of the fibre.

Many keratin fibres have medulla which is a honeycombed structure of protein containing air pockets running continuously or intermittently through the cortex. Little is known about its composition although it is believed to be deficient in sulphur.

Chemical Structure

The fundamental structural units in hair, and related keratins, are long peptide chains derived from the condensation of •-amino acids. These are linked together in one plane by covalent cystine linkages, and by electro- valent salt linkages, between charged polar side chains, and in the other by hydrogen bonding between suitably situated peptide groups and by less important Van der Waals forces. Fibres are not, however, homogeneous and two regions--crystalline and amorphous--have been differentiated. Because the crystalline region, comprising some 20% of the fibre, is responsible for the X-ray photographs of the keratin, much more is known about its structure. The first X-ray studies on the structure of keratins

THE CHEMISTRY OF HAIR 209

by Astbury a showed that, when a keratin fibresis stretched, the X-ray pattern given by the unstretched fibre (•-keratin) is gradually changed to a new form which is similar to that given by stretched or unstretched silk, in which the peptide chains are fully extended. It was, therefore, concluded that the peptide chains in the unstretched •-keratin are folded and that when the fibre is stretched to give the •-form of keratin, these folded chains are made straight. A great deal of work has been carried out on the nature of the •-fold, and now it is generally agreed that in the •-form each peptide chain is in the form of an •-helix.

Although there are still many unsolved problems, such as the precise nature of the •-fold and the compositions of the crystalline and amorphous regions, the above picture of a hair forms a useful, if perhaps oversimplified, basis on which to explain many of the important properties of the fibre.

The Pigmentation of Animal Fibres Although the importance of pigmentation in animal hairs has been fully

appreciated socially, the amount of serious academic investigation on the actual pigments in the fibre has been rather limited. Greater attention has been given to experiments in vitrio rather than to studies on hairs themselves. The colouring matter mainly responsible for the many shades of black and brown hairs is termed "melanin" and is found as a rule in the

form of granules. The granules are, however, seldom uniformly distributed in the fibre. Of particular interest is the finding 4 that in crimpy fibres the pigment is asymmetrically distributed across the fibre. The distribution bears a simple relation to the crimp in the relaxed fibre, since the more heavily pigmented portion of the cortex is always found on the same side of the fibre axis as its centre of curvature in parts of the fibre which appear curved, and crosses over where the fibre axis appears to have a point of inflection. In fibres which are not crimped, there is also an asymmetrical distribution of pigment. Detailed examination of many pigmented fibres has shown that more pigment is present in the paracortex of the fibre than in the orthocortex. It was also shown 5 that this asymmetry of fibre pigmentation originated in an unsymmetrical distribution of melanocytes in the follicle bulb from which the fibre is produced.

The pigment granules in black or brown wools have usually been isolated by boiling the fibre in 6N hydrochloric acid to dissolve the surrounding keratin. More recently greatly improved techniques have been developed which enable granules to be isolated without destroying their shape. One method • consists in boiling the hair in a mixture of phenol hydrate and thioglycollic acid, and in the other an alcoholic solution of hydrazine is used 7. Electron-microscopic examination of granules isolated by either of these methods reveals that the granules are elongated egg-shaped bodies,


the ratio of major and minor axes varying with the origin of the granules. The granules are probably complex in structure, being built up from layers of protein on which layers of pigment are deposited.

Electronmicrographs of pigmented cortical cells, which have been broken up by mechanical grinding and subjection of ultrasonic radiation, have shown 6 that the major axis of the granule tends to lie parallel to the main axis of the cortical cells, and that the granules are arranged end-to-end in rows, surrounded by the fibrillar keratin.

The chemical inertness of the natural melanins suggests that they are polymers of high molecular weight, and although the precise structure of the material is not known, it is generally accepted that they are formed by the action of tyrosinase to yield indole-5-6-quinone which then polymer- ises 8. There is also much evidence which suggests that the natural melanins are closely associated with heavy metals, especially iron 6.9, and with proteins.

The presence of pigment in the fibres appears to affect their physical properties. Black-brown pigmented fibres are more resistant than white fibres to attack by solutions of strong alkalis x0, reducing agents n, and pro- teolytic enzymes •'-. They are also more difficult to stretch and contract less when boiled in 5øfo sodium bisulphite solution 5 (see Table 1). This suggests that the pigment contributes to the stability of the fibres, perhaps by introducing new cross linkings.

Table 1

Mechanical Properties of Pigmented and Unpigmented Wool

Fibre Supercontraction Work required type % to Stretch 30 %

Pigmented 21.1 1.49 White 25.5 1.34

Pigmented fibres often require to be bleached and this is usually done by mordanting the material with an iron salt, and then treating it with a solution of hydrogen peroxide. Few data are available on this process. It was found, however, that when pigmented fibres are treated with iron salts they become darker. This suggests a union between the fibres and

Table 2

Absorption of Iron by Pigmented and Unpigmented Hair.

Fibre Type

White human hair ..

Black human hair .. Medium brown human hair

Light brown human hair

Iron absorbed (mole x 10-•/g)

0.28 1.87 1-48 1.21


the iron. More iron is taken up by pigmented fibres than by unpigmented material (see Table 2) and this suggests that the iron is adsorbed by the melanin--a view which has also been expressed by Stoves 9. More important, however, is the finding that the iron absorbed by pigmented fibres is more firmly held that that absorbed by white fibres (see Table 3). This has an

Table 3

Firmness of Union between Absorbed Iron and Pigmented and White Keratin.

Fibre Type Total Iron Iron removed after Absorbed (mg./g) Acidic Wash (mg./g)

W'hite .. 0.93 0.52

Pigmented .. 1.73 O. 17

important practical consequence, for it indicates the desirability of rinsing mordanted fibres before they are bleached. If this is done, most of the remaining iron will be where it is most required, i.e. absorbed on the pigment granules. When the material is subsequently immersed in hydrogen peroxide, the granules will be preferentially attacked because of the presence of the catalyst and the reaction will be concentrated on the granules. This will minimise the degradation which occurs when keratin is treated with hydrogen peroxide in the presence of iron salts. The effectiveness of the peroxide is, of course, dependent on the concentration and the pH; the higher the pH, the greater is the amount of bleaching but this is often at the cost of fibre degradation 5.

Surface Characteristics

Several very important properties of hair are associated with its peculiar surface characteristics. Of particular importance is the ability to felt or mat--a property which is responsible for the attractive appearance of many wool cloths and for their tendency to shrink and mat together unless chemically treated. The factors which affect the amount of felting which takes place have been discussed by many workers •a. The importance of the surface of the hair in determining the ease with which oils and grease are removed by detergent solutions has, however, received much less attention. It is well known that mineral oils cannot be removed from wool or hair by conventional methods. If, however, the fibres are treated with chlorine water or with many of the reagents which are used to make wool non-felting, the adhesion between the fibre and mineral oil is very much reduced. This is illustrated by the data given in Table 4 which also includes corresponding values for wool treated with silicones. These data suggest that the surface


Table 4

Removal of Oils from Chemically Modified Wools.

Treatment Residual Oil %

Silicone .......... 73 Untreated ........ 57

Hydrogen per•);•ide ........ 55 Hydrochloric acid ........ 42 Sulphuric acid ........ 41 Bleaching powder ........ 32 Permonosulphuric acid ...... 12 Caustic potash in alcohol ...... 4 Mixture of potassium permanganate and

sodium hypochlorite .. . 2 Acidified sodium hypochlori•; .. i. 1 Sodium hypochlorite ...... 1 Sulphuryl chloride ........ 1

of the wool fibre is essentially non-polar, for the adhesion between the surface of the fibre and mineral oil is high. The surface of the fibre is composed mainly of epicuticle and it has been suggested, therefore, that the epicuticle is not keratinous. Whether or not this is so, there is no doubt that it is not polar. Removal of the epicuticle by the reagents mentioned will reveal the true keratin underneath, and this is, as would be expected, hydrophilic and the mineral oil is, therefore, held less tena- ciously. It is interesting to note that when the fibre is treated with a silicone the affinity for mineral oil is greater than that of the untreated fibre.

Chemical Reactivity of Keratins

Although keratins are considered to be stable as compared with many other proteins, they do in fact show considerable reactivity towards chemical reagents. The main points of attack are as follows .'

(i) The peptide links (-CO-NH-) of the main chains.

(ii) The tyrosine and serine residues attached to the main chains.

(iii) The -S-S- links and the salt links joining peptide chains.

(iv) The hydrogen bonds between adjacent peptide chains.

Various aspects of the reactivity of wool have been reviewed by several workers •4 and it is unnecessary to discuss these here. Attention will be limited to those reactions which are important in the consideration of some of the physico-chemical properties of hair. It should be stressed, however, that all reactions between hair and chemical reagents are invariably complex. Seldom, if ever, is reaction limited to one type of link in the fibre. This


often makes interpretation of experimental results difficult, for there is still a great need for the elucidation of the mechanism of the reactions involving wool and hair. In spite of these limitations, however, it has been possible by emphasising the main features of a reaction, and neglecting side issues, to explain many of the interesting and unique properties of hair, and in the light of this information to develop new and commercially valuable processes. This approach is illustrated by considering the phenomenon of permanent set in hair.

Permanent and Temporary Set

Astbury and Woods •5 while studying the elastic properties of wool observed that when a moist wool fibre is stretched and held under tension

for an appropriate time and allowed to dry, it remains elongated when the tension is released. The increase in length or the "set" imparted in this way is not, however, permanent, but is partially reduced if the relative humidity of the air surrounding the fibre is increased and completely reversed when the fibre is immersed in cold water. This particular type of set is called "cohesive set" and is attributed to the formation of weak

bonds, probably of the hydrogen type, when the fibre is in the stretched state and which are broken when the fibre is placed in water. When, however, the stretched fibre is heated in water at less than 100øC the set produced is not released in cold water but is released by water at the same temperature as that of the setting medium. This type of set is known as "temporary set". If, however, the fibres are boiled in water for one hour while being maintained stretched, they do not recover their original length when the extending force is released and they are subsequently boiled freely in water for one hour. They have a final length greater than the original length, and the set is therefore described as "permanent set".

The explanation of permanent set followed from the observation that if a stretched fibre is boiled in water and is then released and boiled freely in water for one hour, it does not take a set but contracts to a length less than that of the original unstretched fibre. This phenomenon is known as "supercontraction". If the time of boiling the fibre in the stretched state is plotted against the amount of set, or supercontraction, obtained after the treated fibre has been boiled freely in water for one hour, a curve similar to that shown in Figure I is obtained. Treatment in the stretched state for two minutes induced supercontraction, while treatment for one hour or more sets the fibre. Intermediate setting times will give rise to set or supercontraction. This observation together with others, based on the corresponding changes which take place in the X-ray photograph of the stretched fibre, suggest that the setting process involves a breakdown of

6

9,14 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS

side chains, between peptide chains, followed by the rebuilding of the same or new stabilising linkages in the extended configuration. Although there is considerable difference of opinion about the nature of these two processes of fission and rebuilding, the basic underlying principle is now firmly established.

•o

Io

Io 30. Lo 70 •'0 qO Ioo ,:o •3.0

Figure 1

On the basis of a great deal of experimental evidence Speakman and his colleagues x6 have stressed the importance of the covalent disulphide links in permanent setting. The reactions involved are illustrated by the following equations:

Fission:

R.CH2.S.S.CH2. R--•R.CH•.S.OHq-R.CH•.SH R. CH2 . S . OH --• R . CH •-- O q- H•S

Rebuilding: R.CH2.S.OHq-H•N-R--• R.CH•.S.NH.R R. CH = O q- NH• . R --• R. CH ----- N. R


The more important observations on which this view is based are summarised below:

(i) Removal of sulphur from hair prevents setting.

(ii) Complete deamination by nitrous acid or blocking of amino groups with 1-fluoro-2: 4-dinitrobenzene prevents or reduces set.

(iii) Fibres dyed with dyes of high affinity cannot be set.

(iv) Fibres cannot be set in strongly acid media, since "under these conditions the basic side chains are fully combined with acid and cannot therefore take part in reaction with the broken sulphur linkages".

Whatever detailed criticism may have been put forward against this view, there is little doubt about the important role of the disulphides in setting, for it has been shown that permanent set can be obtained by treatments at low temperatures which bring about fission first of -S-S- links and then the formation of new links. Of particular importance in this connection is the discovery that setting is greatly facilitated by the presence of alkalis or reducing agents, such as sodium bisulphite or thioglycollates. Alkalis facilitate setting by promoting hydrolysis and bond rebuilding which may involve -S-NH- or C-S-C- links, while reducing agents break the -S-S- links to yield thiol groups, and these eventually re-form when the fibre is in the stretched state or during subsequent rinsing and oxidation. The possibility of forming -S-NH- links is not now regarded as the major cause of the setting. A dear demonstration of setting, involving covalent links, is provided by the treatment first with ammonium thioglycollate and then with trimethylene dibromide to form new links as in the equation--

•S-S-q --• •-SH HS•

•-SH HS-q q- By (CH2)3 By --• k-S-(CH•,)a-S-q Much of the evidence which has been brought forward in support of

the above mechanism of setting is of an indirect nature and attempts to isolate compounds which contain the postulated new links have so far been unsuccessful. Furthermore, Alexander •* showed that concentrated solutions of lithium bromide, a hydrogen bond breaker, were capable of reversing the set completely in permanently set fibres. Lithium bromide is not, however, able to break the covalent bonds first postulated by Speakman. In view of this and other observations, increasing emphasis is being placed on the role of hydrogen bonds in permanent setting, and it has been suggested that when a keratin fibre is stretched the peptide chains are uncoiled and converted to the p-form. When the fibre is boiled in water or steamed, the hydrogen bonds between the peptide chains are broken, probably by


a hydration process, and the cohesion between the peptide chains is reduced. If the steaming is prolonged, the extended structure is stabilised by the reformation of the side chains or more stable hydrogen bonds, to such an extent that the action of steam on the slackened fibre cannot then reduce

the main chain adhesion sufficiently for folding of the chains to occur. Farnworth •8 regards both hydrogen bond fission and breaking of covalent

bonds as important, for he has shown that treatment with a mixture of a hydrogen bond breaker and a reducing agent is more effective than treatment with either reagent alone. This author also stresses the observation that with fibres set in water for one hour, at various temperatures, and then released in water, the set will be stable to a temperature of 10øC above the setting temperature but is lost at a temperature which exceeds the setting temperature by 20øC.

It is probable that permanent set results from the breakdown and reformation of hydrogen bonds following the splitting of -S-S- links. The more rapidly this occurs, the more rapidly is set attained. Conse- quently, reducing agents such as bisulphites and thioglycollates, are effective setting agents. Any change in the molecular environment of the -S-S- links may also affect its reactivity. For example, the introduction of dinitrophenyl groups when hair is treated with dinitrofluorobenzene will alter profoundly the reactivity of the -S-S- links in addition to eliminating the amino groups. The formation of covalent cross4inks will, of course, add to the efficiency of the setting process. In each method of setting, either covalent bond or hydrogen bond formation may be the predominating feature, and it is unlikely that a single mechanism can be used to explain the effectiveness of all methods of inducing permanent set. It must be stressed, however, that the general principle of bond fission followed by bond rebuilding in the deformed state is still acceptable.

It is clear from the above discussion that the amount of permanent set obtained will be dependent on the previous history of the hair, and the conditions under which the setting is carried out. The magnitude of these effects can be quite large as illustrated by the following results of recent work carried out in collaboration with L. S. Bajpai.

Setting Properties of various types of Keratins

According to Mitchell and Feughelmann •9, finer fibres take on a greater amount of set than coarser ones and Stoves "ø has concluded that the set

attained under various conditions is affected by the cortex/cuticle/medulla ratios and by the fundamental composition and the crystalline/amorphous ratios of the particular fibre. The differences of response to setting in boiling water are illustrated by the figures given in Table $.


Table 5

% set (q-ve) or supercontraction Type of Animal Fibre (--ve) obtained after boiling

in stretched state for:

2 min. 2 hours

Mohair ...... --26.9 26.6

Alpaca ...... -- 5-9 16-2 Human hair .... --2-4 7-9 Horse hair .... --0.5 11-9

Horse hair (descal•(i) .. -- l'S l 1-S Pig bristle ...... --0'6 12.7

It is interesting to note that descaling horsehair does not affect its ability to take a set.

Setting of Chemically Modified Hair Treatment with hydrolysing agents. Hair which has been treated with

0.1 N hydrochloric acid at room temperature, or at the boil, has the same set/supercontraction characteristics as untreated fibres, but those treated with dilute solutions of caustic soda or potassium cyanide, supercontract and set less readily. This is no doubt due to the conversion of -S-S- links as follows:

R.CH2S.S.CH2.R9-KCN-+R.CH•SK 9- NCS.CH•.R -+ R. CHq. S. CH• . R 9- KCNS

/ / CH.CH•.S.S.CH2. CH + HOH--+ CH. CHq. S. OH 9- CH.CH•. SH

/ /

CH . CH• . S . OH --• C = CH• /

/ / C = CH• 9- SH . CH2. CH -+ CH.CH•. S. CH• . CH

/ / Deaminated Hair. Deamination with nitrous acid reduces both the set

and the supercontraction obtained in boiling water, as shown in Tabl• 6.

Table 6

Treatment % Set or Supercontraction obtained after:

2 min. 2 hours

None -- 7.9 14.5 Deaminated with nitrous acid -- 1.0 -- 1.1


This may be due to the elimination of amino groups or to the stabilisation of the -S-S- links by oxidation to -S(O2)-S- or -S(O)-S-

Nitric Acid. Hair treated with 40% nitric acid • solution for 18 hours does not supercontract under the conditions used and does not take a set {Table 7). This is no doubt due to the oxidation and stabilisation of the -S-S- links, but may also be associated with steric hindrance resulting from

Table 7

Setting of Fibres treated with Nitric Acid.

Setting Time % Set or Supercontraction (min.)

1 -0.3 (-4.5) S --0-2 (--18.9) 5 --0.2 (--14-3)

10 --0.4 (--12.3) 15 --1.7 (--8-4) 20 --2-0 (--0.6) 60 --3.6 (13.0)

(Figures in brackets denote values for untreated fibres

the introduction of -NO• groups in the tyrosine residues. These nitro groups may also affect the strength of the hydrogen bonds, involving the --OH groups in the tyrosine residues, and this may contribute towards the stabilisation of the fibre.

Iodination of hair also reduces the set and supercontraction as can be seen from Table 8.

Table 8

Time of % Set or Supercontraction obtained Setting with: (min.) Iodinated hair Untreated hair

ß _

1 0.6 0.8 2 --1.3 --7-9 5 --0.5 --12.0

10 --1-2 --10.0 15 --1.4 --6.5 30 -- 1.5 --2.2 60 -- 1-3 5.6 90 -- 1' 1 11-7

120 --0.9 14-5

This stabilisation arises from the introduction of iodine into the tyrosine nucleus, and this substantially increases the bulkiness of the side chains which no doubt adds to the difficulty of main chain coiling. In addition,


the introduction of iodine may affect the hydrogen bonding involving the -OH residues, and further, the -S-S- links may be stabilised either by virtue of alterations in the molecular environment or by oxidation.

Reducing agents. Pretreatment of hair with certain reducing agents increases the set they take when boiled in the stretched state (see Table 9).

Table 9

Set/Supercontraction Characteristics of Reduced Hair.

Treatment % Set or Supercontraction after ß

2 min. 2 hours

Sodium hydrosulphite --9.7 (--7.9) 20.5 (14-5) Sodium bisulphite .. --9.7 16.5 Sulphur dioxide in

methanol .. --7-2 21-7

This is a consequence of the fission of the -S-S- links which increases the effectiveness of the setting. The products of the fission are such that new links can subsequently be formed.

Oxidising agents. In general, treatment of hair with oxidising agents affects the set/supercontraction characteristics, but the changes which take place are very dependent on the conditions of treatment. For example, mild treatment with peracetic acid makes the fibres unable to take a set, due no doubt to the stabilisation of the -S-S- links by oxidation to -S(O2). S- or -SO. S-. As the severity of the reaction is increased the fibres set more readily. Thus while untreated human hair gives 14.5% set, hair treated with 2% peracetic acid for 45 min. gives a corresponding value of 30-1ø/o .

Table 10

Set/Supercontraction Characteristics of Oxidised Hair.

Treatment % Set or Supercontraction after:

2 min. 2 hours

2% peracetic acid (5 min.) .... --0.9 (--7.9) 6-1 (14.5) .... (60 min.) .. --7.1 22.5

,, (240 min.) .. --4.7 30.1 10 vols. l•drogen peroxide .... --0.1 1.6 Acidified KMnO4 ...... --5.5 8.6 Acid K•,CraO7 .. -- 11.6 11.4 1% KIOa, 1% HC1; '2'0øC: •'hours --0-5 --0.6

.... 40øC; 4 hours .. 20.9 23-5

Hydrogen peroxide and acid or alkaline potassium permanganate or dichromate in general destroy the ability of hair to take a set in water. Treatment of hair with acidified potassium iodate under certain conditions, on the other hand, improves the set (see Table 10).


Setting in Various Media

It is well known that hair can be more effectively set in solutions of borax or bisulphites than in water at the same temperature. The mechanism involved is probably different from that involved in water setting. In alkali setting the formation of -C. S. C- links is an important feature of the reaction, while setting in bisulphites is largely a process of reduction followed by reoxidation of the broken -S-S- links, while the fibre is in the stretched state. Thioglycollates are perhaps the best setting agents at low temperatures, their effectiveness being due to their ability to reduce the -S-S- links at comparatively low temperatures. Subsequent oxidation of the broken bonds, and hydrogen bond formation are responsible for the permanent set. In the last few months two other reagents have been found to assist setting. These are tetrakis (hydroxymethyl) phosphonium chloride (THPC), and thiourea dioxide.

The effectiveness of the former is illustrated by the following data from which it is evident that hair sets much more readily in THPC than in water. For example, stretched human hair treated for 2 minutes in a 2% solution of THPC at pH 6.1 acquires a set of 18.5% whereas in water a supercontraction of 7-9% is obtained. After two hours' treatment, 33.2% set is recorded on fibres initially extended 40%. The efficiency of the setting solution is clearly dependent on the temperature, concentration and pH of the solution, but marked improvement is obtained with 0-1% solutions at the boil. Of particular interest is the observation that THPC in presence of 0.1N hydrochloric acid sets hair. In absence of THPC no set is obtained. Furthermore, although deaminated, iodinated or oxidised fibres cannot be set in water they do take a set in 2% THPC solution. On the other hand, fibres treated with potassium cyanide cannot be set in either water or THPC solution. This suggests that the effectiveness of THPC is con- nected with its ability to split the -S-S- links, presumably by reduction.

Setting is also facilitated by the presence of thiourea dioxide NHa. C: SOa. NHa, a reagent often used in the reduction of vat dyes. As with THPC, hair treated in the stretched state in 0.1% thiourea dioxide solution at pH 5 takes a permanent set after only two minutes' treatment.

Supercontraction

Supercontraction of hair was first observed in a series of experiments in which stretched fibres were steamed for various times, and then boiled

freely in water for one hour. If the time of steaming is limited to two minutes, the fibres contract. It was later observed aa that unstretched fibres also contract when boiled in solutions of bisulphites and other reagents, capable of breaking the -S-S- links. Subsequently Zahn et al aa demonstrated

THE CHEMISTRY OF HAIR 22I

that supercontraction could also be realised by boiling hair in phenol or formamide, both of which are capable of breaking hydrogen bonds. More recently 24, the phenomenon of reversible supercontraction was discovered during an investigation on the action of copper complexes on keratins. It was found that when wool or hair is treated with cuprammonium hydroxide solution there is a rapid reaction although the keratin is not dissolved. Copper is absorbed and the fibres contract in length. The absorbed copper cannot be removed simply by washing in water, but immersion in 5% sulphuric acid removes it completely. At the same time the contracted fibres revert to their original length. This contraction-and- recovery phenomenon has been termed "reversible supercontraction" and is clearly an important property of animal fibres. It is also interesting to note that the fibres containing copper do not give the characteristic • or [• X-ray photographs usually associated with keratins. In fact, when the fibres have absorbed the maximum amount of copper they do not yield an X-ray photograph. When, however, the copper is removed by acid, the characteristic X-ray photograph can be obtained.

The amount of copper absorbed depends on the concentration of copper in the solution, the time and temperature of the treatment and the previous history of the fibre. Reaction is rapid, and large amounts of copper are taken up, the maximum amount being of the order of 30%. It would appear that the cuprammonium hydroxide solution first swells the fibres, and then most or all of the co-ordinated ammonia groups are displaced from the co-ordination complex, various groups in the wool molecule taking their place. In order to account for the large amount of copper absorbed it is necessary to postulate that the -CO-NH- groups in the main peptide chains of the keratin are involved in the reaction. The rate at which hair

contracts when placed in solutions of cuprammonium hydroxide depends on the concentration of copper in the solution, on the temperature and on the previous history of the fibre. In most types of fibrous keratins, the maximum contraction obtained is 32%. Provided the treatment has not been too severe, the fibres recover their original length when subsequently immersed in 5% sulphuric acid, but the degree of reversibility decreases as the concentration of the copper in the solution, and the time and temperature of the treatment, increase.

The mechanism of the supercontraction is believed to be as follows: The cuprammonium hydroxide causes the fibre to swell and penetrates between the peptide chains in the keratin molecules. The union between the metal and the NHa groups in the complex [(Cu(NHa)4 + +.• is replaced by links between the metal and various groups in the peptide chains. As a consequence of the high swelling, and of copper absorption, there is a reduction in the less permanent interactions between chains, and this allows

'222 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS

them to assume a more stable state by irregular folding over and above the norma/ helical fold characteristic of fibres in the •-form. This folding is resisted by cross links such as the -S-S- links. Consequently if these have been broken in a previous treatment, the folding is facilitated. Unless excessive fission of the -S-S- links has occurred, the -S-S- links tend to pull the system back to its normal length but are unable to do so as long as the copper is present and co-ordinated with the main chains. Treatment with sulphuric acid breaks this union and the intact -S-S- links are then able to pull the fibre back to its original form.

It is interesting to note that introduction of copper into hair increases its ability to absorb moisture. This may be due to the presence of new sites associated with the copper or with a decrease in the crystalline portion of the fibre.

Repair of Damage In many treatments of hair and wool, there is considerable degradation

and consequently attempts have been made to repair this. The damage arises from the breaking of covalent bonds and consequently would be repaired by the formation of new links. Alternatively, the damaged fibre could be reinforced by depositing a polymer inside. In some cases, however, it is considered preferable to stabilise the hair before a particular treatment, so that the amount of damage which occurs is reduced. For example, in animal hair bleaching with iron-catalysed peroxide, it is advis- able to treat the fibre hair with a cross linking agent, e.g. formaldehyde, before it is bleached rather than to attempt to cross link the bleached fibre. Although there is some doubt as to whether the formaldehyde treatment does in fact reduce the amount of damage, as assessed by measurements

Table 11

Cross-linking Agents for Wool. Halogen compounds (difluoro-2: 4 dinitrobenzene,

4,4'-difluoro-3, 3'-dinitrophenylsulphone). Hexamethylene diazides. Polymethylene dihalides (on reduced wool). Glyoxal. Formaldehyde. Hexamethylene tetramine. Epoxy compounds. Epichlorhydrin. Dithioglycollide. Ethylene imines. Chloromethylethers. Glycol esters of methane sulphonic acid. Di-isocyanates. Tetraethyl pyrophosphite. Metal salts, especially mercuric acetate. Quinones. Dimaleimide.• (on reduced wool).


,of the elastic properties of the fibres, hair bleached in the presence of formaldehyde is preferred commercially.

It is clear from these considerations that cross linking of keratins is an important topic. Very few reagents are, however, commercially acceptable, but some of those which have been used in laboratory investigations {see Table 11) may point the way to tlie commercial reagents of the future.

(.Received: 9th January 1961)

REFERENCES

Mercer, E. H., and Rees, A. L. G. Australian 7[. Exptl. Biol. Med. Sci. 24 147 (1956) Horio, N., and Kondo, T. Textile Research J. 23 373 (1953) Astbury, W. T., and Street, A. Phil. Trans. Roy. Soc. London Ser. A 230 75 (1933) Laxer, G., Whewell, C. S., and Woods, H. J. J. Textile Inst. 45 T482 (1954) Laxer, G., and Whewell, C. S. Proc. International Wool Textile Research Conference

F~186 (1955) Laxer, G., Sikorski, J., Whewell, C. S., and Woods, H. J. Biochim. et Biophys. Acta

15 174 (1954) Filson, A., and Hope, J. Nature 179 211 (1957) Mason, H. S. Pigment Cell Growth (1953) (Academic Press, Inc., New York) Stoves, J. I,. Research (London) t5 298 (1953) Richter, R., and Stary, Z. Arch. Dermatol. u. Syphilis 178 373 (1939) Goddard, D. R., and Michaelis, L. J. Biol. Chem. 106 605 (1934) Stary, Z. Bulletin, Faculty of Medicine, University of Istanbul 12 329 (1946) Wool Science Review 3 (3) (1949); ibid 40 (14) (1955); ibid 16 (17) (1957); ibid 18

(18) (1960) Moncrieff, R.W. Wool Shrinkage and its Prevention (1953) (National Trade Press,

London) Howitt, F. O. J. Textile Inst. $1 120 (1960) Astbury, W. T., and Woods, H. J. Phil. Trans. Roy. Soc. London Ser. A 232 333

(1933) Speakman, J. B. Proc. International Wool Textile Research Conference C-302 (1955) Alexander, P., and Hudson, R.F. Wool--Its Chemistry and Physics (1954) (Chapman

& Hall, Ltd., London) Farnworth, A. Textile Research J. 27 632 (1957) Mitchell, T., and Feughelmann, M. Textile Research J. 28 453 (1958) Stoves, J. L. 2nd (?uinquenial Wool Textile Research Conference, Harrogate (1960) Whewell, C. S., and Da Silva, M. A. J. Textile Inst. 48 T98 (1957) Speakman, J. B. J. Soc. Chem. Ind. London $0 T1 (1931) E16d, E., and Zahn, H. Kolloid-Z. 108 94 (1944)

Melliand Texttiber 30 517 (1949) •Whewell, C. S., and Woods, H. J. Soc. Dyers • Colourists Symposium "Fibrous

Protein" 50 (1946)

the chemistry of hair - journal of cosmetic science

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