j. Soc.Cosmet.Chem.,28, 231-241 (May 1977)
Applicationof the theoryof hydrophobic
bonds
to hair treatments
KATHLEEN E. HALL and LESZEKJ. WOLFRAM The Gillette
Company,PersonalCareDivision, Boston,MA 02I 06.
Received
July 16, 1976. Presented
Ninth IFSCC Congress,
June1976, Boston,MA.
Synopsis
A novel technique of HAIR TREATMENT via introduction of nonpolarresiduesinto hair structure in
HYDROALCOHOLIC MEDIA is described.HaJr modified in this manner exhibitsgreatly ENHANCED
SETTABILITY and HIGH SET RETENTION, even at high levels of ambient humidity. The setting be-.
haviorcan be manipulatedby uulizingthe differentialswellingresponseof treatedhair to water and aqueous
alcohols.
I. INTRODUCTION
Conformational stability of a protein, and thus, its responseto external mechanicalor
chemicalforces,dependson the type and number of stabilizingbondspresentwithin
the protein structure.In the caseof o•-keratin,this stabilityis primarily derived from
covalentcrosslinkingby cystineand intrachainhydrogenbonding. Some contribution
also comes from the electrostatic interaction of basic and acidic sidechains, as well as
from the hydrophobicbonding of nonpolarresiduessuchas proline, leucine, and
valine (1). However, the contribution of the latter is small, and in the intact fiber, the
covalentand polar interactionsgreatly overshadowthe nonpolar ones.
In an earlier work reported by Harris (2), it was shown that the wet mechanical
propertiesof reducedkeratin fiberscould be restoredwithout crosslinkingby alkylating the fiberswith high molecularweightmonohalides.Alkylationwith alkyl halidesof
low molecularweightproducedpermanentlyweakenedfibers.Successful
mechanical
recovery of the alkylated fibers was ascribed to the interaction of secondary
forces,arisingfrom the highmolecularweightresiduesincorporated
into the fiber
structureduring alkylation. In this respect, it is of particular interest to note that the
introductionof apolarresiduescreatesan environmentfavoringthe formationof hydrophobic
bonds,andthatthe strengthof thesebondsdependson the sizeandshape
of theintroduced
alkylgroups.
232
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
The term "hydrophobic
interaction"describes
the tendencyof nonpolargroupsto
associate
in aqueoussolution.This interactionresultsin an increased
orderingof the
watermolecules
intoa quasi-crystalline
structure
in whichthereisimproved
hydrogen.
bondingsurrounding
the nonpolargroups.Thesehydrophobic
regionsaredisrupted
in
nonpolar
solvents
because
stronger
solute-solvent
interactions
arethermodynamically
favored.
Thus,
aunique
property
ofthe
hydrophobic
bond
isitsdependence
onwater
foritsexistence.
Theimportant
practical
pointraised
bythese
considerations
istheutility
ofthistype
of
bonding
forsetting
hair.If thehydrophobic
bonds
couldresist
theswelling
pressure
generatedwithin the keratinwhichis exposedto high humidity, then the set-conforma-
tionwouldbemaintained
andanovelprocess
forhairmanipulation
wouldbefeasible.
Aninvestigation
oftheproperties
ofS-alkylated
keratin
was
therefore
undertaken,
and
thisreportisanaccount
of suchstudy.
II.
EXPERIMENTAL
,:
a. MATERIALS
ANDMETHODS
1. Reagents.'
The chemicals
utilizedin this studywere commercially
available:•ii
American
Chemical
Society
grade
reagents
andwereusedwithout
furtherpurification.
2. Cauc;sianhair.' Brown Caucasianhair as supplied
• was used w•thout
cleansing.
'
•:-:
.:.
3. Mechanical
properties.'
The mechanical
propertiesof hair were determined
on
table
model
Instron.*
Thefibers
were
mounted
onplastic
tabs
at2in.guage
length,•:55•:.
equilibrated
underthedesired
conditions,
andstretched
tobreakatarateof 1in./min.•
•':.•
The broken ends were conditionedat 65 per cent RH, cut off the tabs,weighed,and
the denier of the tested fibers calculated.
In some
cases,
thecalibration
technique
(3) wasusedto followthechange
in thefiber:.:•:
performance.
Intactfibersweremountedasabove,equilibrated
in thedesiredsolvent,
andthen stretchedto 30 per centextensionat a rateof 1 in./min,usingthe tablemodel.•
Instron. After a 24-h relaxationperiodin water, the calibratedfiberswere giventhe
proposedchemicaltreatment.The ratioof the energyrequiredto stretchthefibers(30
per cent extension)the secondtime to that requiredinitially,wasexpressedasthe 30 .-:
per cent index.
4. AminoacidanaOsis.'Hai;/woolsamples
(•10 rag)were hydrolyzedat 105øCfor :•
24hin6NHC1
followed
bylyophilization
forremoval
ofHCI.
The
hydrolyzates
were
:•
analyzedfor cystineon a Phoenix$model M-7800 Micro Analyzer.
5. Liquid
retention
measurements.'
Theswelling
ofhairwas
determined
bytheliquid
retention techniqueas describedby Valco and Barnett (4). This involved measuring.
*De Meo Brothers, New York, N.Y.
]'Instron Corp., Canton, MA.
•Phoenix Instrument Co.
ß
HYDROPHOBIC
BONDS
AND
HAIR
TREATMENTS
233
the liquid retainedby the hair after a 30 min equilibrationin water or other specified
solvent.
6. Setting.' One gram (7 in.) tresseswere set on one-half in. rollers with water or
aqueousalcoholasspecifiedin the text. The settresses
were allowedto dry overnight
at ambient temperatureand humidity. After removing the tressfrom the roller, the
hairwascombed,beingcarefulto maintainthe alignmentof the hair fibers.
The set stability of treated hair was assessed
by measuringthe hanginglength of the
tressesafter variousrelaxationtimes,while exposingthem to maintainedconditionsof
humidityand temperature(85 per cent RH, 85øF).
B. RESULTS
AND
DISCUSSION
1. The reactionof reduced
hair with alkyI iodides.'
Earlier investigationsconductedby
Harris (2) on wool suggestedthat the wet mechanical
propertiesof reducedwool could
be restoredfollowing alkylation with long chain alkyl halides. Both the magnitudeof
the restorativeeffect and the simplicityof the alkylationstep suggestedthis approach
asbeingparticularlyattractivefor applicationto hair.An attemptwas,therefore,made
to evaluatethe efficacyof the alkylationreaction.
Calibratedhair fiberswere treatedwith 0.25 N potassiumthioglycolateat pH 5 (3 h at
50øC,25:1 bath ratio) to cleaveapproximately50 per cent of the disulfidebonds.Samplesof the reducedfiberswere then alkylatedwith 0.02 M alkylatingagentsuspended
in 0.1 M pH 8 phosphatebuffer utilizing 100:]_bath ratio. The alkyl halidesused as
blockingagentswere methyl, hexyl, and decyl iodides,respectively.After 20 h, at
35øC,the fiberswere thoroughlyrinsedwith runningtap water; and dried. Bulk sampleswere treated simultaneouslyin order to determine the weight changesfollowing
alkylation.A smallweightincrease(1.9 per cent)wasobservedonly in the caseof the
sampletreated with decyl iodide. This weight increasecorrespondedto lessthan 20
per centyield of the alkylation reaction.
The alkylationtreatmentalsohad a negligibleeffecton the mechanicalpropertiesof
the reducedhair (Table I). These resultswere in sharpcontrastwith the datareported
for woolby Harris(2). To ascertainwhetherthe reactivityof the substrate
(hairversus
wool) contributesto theselarge differencesin behavior,it wasdecidedto re-examine
the reactionsystemusingwool fibers.The reduction-alkylationcyclewasrun under
conditionsidenticalto thosedescribedby Harris.
2. AIkylationof woolwith alkyl iodides.'New Zealandwool sampleswere reduced
with 0.2 N potassium
thioglycolateat ph 4.5 for 2]_h at 35øC(25:]_bathratio).The
subsequent
alkylationwasperformedusing0.02 M alkylatingagent,methyl,hexyl,or
decyliodides
suspended
in ]_M, pH 8 phosphate
buffer,35øCat 100:1bathratio.The
alkylationproceededvery slowlyin the presenceof the longerchainhalides,aswas
evidentby the persistence
of thioglycolic
acidafter]_8h reactiontime.To insurethat
an excessof alkylationagent was present,fresh solutionsof the hexyl and decyl io-
dides,respectively,
wereaddedto thewoolsamples,
andthealkylation
continued
for
an additional7 h. Thesereactionsweremonitoredby measuring
the weightchanges,
aswell asexaminingthe propertiesof the treatedwool.
234
JOURNAL
OF THE
SOCIETY
OF COSMETIC
CHEMISTS
Table I
30 Per Cent Indicesof Hair SamplesFollowingTreatment wtth Alkyl Iodides
Treatment
30 Per Cent Index
Intact
Reduced
0.98
0.54
0.66
0.66
0.64
Reduced--CHal
Reduced--C6H•aI
Reduced--C•0H2•I
Table II
30 Per Cent Indicesof Wool Samples
FollowingTreatmentwith Alkyl Iodides
Treatment
30 Per Cent Index
Intact
1.02
Reduced--CHaI
0.63
Reduced--C•HtaI
Reduced--C•0H2•I
0.85
0.92
O.16
/•EC•, TGA
I
REMAINING
I[
0.08•
CHal
0
20
40
REACTION lIME,
min.
Figure 1. Reactivityof thioglycolicacidwith methyl,
ethyl and butyl iodidesin 40 per cent ethanol, pH 9
with respect to time
The alkylation of the reducedwool with hexyl and decyl iodidesresulted in weight
gainsof 1.7 and 2.5 per cent,respectively.It wasobviousthat the extent of alkylation
wasagainlow.On the basisof theweightgain,only0.18 to 0.20 m molesat most,of
SH per gram of wool had been alkylated.Yet, the mechanicalperformanceof the
alkylatedwool (Table II) conformedwith the earlier datapublishedby Harris. There
wasa steadyimprovementin the mechanicalrecoveryof the fiberswith the increasing
HYDROPHOBIC
BONDS
AND
HAIR
TREATMENTS
235
chainlengthof the alkylatingagent.However, this unexpecteddiscrepancy
between
the weightgainvaluesand the 30 per centwork indiceswasresolvedsatisfactorily
by
the aminoacidanalyses
of the treatedwools.The cystinecontentsof both hexyliodide
and dec¾1iodide treated sampleswere almostidenticalwith thoseof the untreated
unreducedwool (860 /xmol/g). The methyl iodide alkylated samplehad a cystine
content of 470 /xmol/g. It is, thus, obvious that the mechanicalrecovery of the
alkylatedwool fibershadbeenbroughtaboutby reformationof the keratindisulfide
andnot bythe residuereinforcementeffect.The mechanism
of thedisulfiderebuilding
isnotyetknown.
Mostlikely,thealkyliodides
undergo
some
secondary
reactions
involvingformationof iodine whichactsasan oxidantfor the proteinsulfhydryl.This
secondary
reaction
isunimportant
in thecase
of methyliodide,
whichreacts
withmercaptans
veryrapidly.An increase
in chainlengthof thealkylgroupcauses
a precipitous
drop in the rate of the alkylationreaction(Fig. 1), andthus,mayset a stagefor the
secondary
process.
3. Synthesis
ofN-alkyl maleimides:A moredependable
methodfor introducingapolar
residues
intothekeratinwasclearlyrequired.
N-ethylmaleimide
is oftenusedasa
standardblocking agent for protein sulfhydryl,and it was thought that its higher
homologues
mightbeofvaluein thisrespect.
Although,the N-alkyl maleimidesare not commerciallyavailable,theywere easilypre-
pared
bypyrolysis
ofthecorresponding
N-alkyl
maleamic
acids
(5,6).N-hexyl,
N-
heptyl,
andN-dodecyl
maleamic
acids
wereprepared
byreacting
maleic
anhydride
with the appropriateaminein glacialaceticacid.The acids'
wereisolatedin goodyields
(ca.85 per cent)aswhite crystallinesolidsandpyrolyzedwithoutfurtherpurification.
The propertiesof the maleamicacidsand the corresponding
maleimidesare givenin
TableIII. The low yieldsof final product(26 per cent)were due to a concurrentpolymerizationreactionleadingto a resinousby product.In the courseof our work, a one
stepsynthesis
wasalsoutilizedfor the preparationof N-alkyl (aryl) maleimides.The
overallyield continued,however, to be low (-30 per cent). The overall reactionis
showh below
CH•H
O-•-C
C=O
N/
+ RNH•
--,-CH--CO•H
170-180 • C.
o
C H-•-C
CH--CONHR
H
I
O =C
C---•O
+
H20
N
I
R
4. Reductionof hair with dithiothreitol(DTT).' DTT was used as an alternate
reductantin our studies.This reagent(8) causes
a specificandsymmetricscission
of the
236
JOURNAL
OF THE SOCIETY
OF COSMETIC
CHEMISTS
Table III
Propertiesof N-AIk¾1MaleamicandN-Alk¾1Maleimides
Maleamic Acid •
Derivative
Hexyl
Heptyl
Dodecyl
Benzyl
Maleimide •
MeltingPoint,øC
Melting Point,øC
78ø
75-77 ø
-140-142 ø
125ø @ 5.5 mma
43-44 ø
56-59 ø
73-75 ø
aBoiling point.
bObserved
me[tingpointswerewithintherangereportedbyColeman
etal (7).
REDUCTION,
%
60
1'20
TIME, min.
Figure 2. Effect of time on reduction of hair by 0.1
M DTT at self-pH, 35 C
disulfidebondswithout producingany byproductssuchasmixed disulfides.Its efficac'r:
asa reductantallowedusto performthereductionswiftlyat neutralpH anda tempera,
i)
ture of 35øC.
ß
Hair sampleswere treatedwith unbufferedsolutionsof 0.1 M DTT (self-pH, 5.4)
25:1bathratio,35øCforvarious
times.
Levels
ofreduction
werecalculated
onthebasis':
of the SH contentdetermined
viamersalyl
acidtitration(9). Thesedataagreedwith
the reduction
levelsdetermined
fromresidual
disulfide
analyses
(viaaminoacid:i
analysis)after cyanoethylationof the free SH groups.A plot of the reductionlevels
against
timeisshown
inFig.2.
5. Alkylationof hair with N-alkyl maleimides.'N-hexyl maleimidewasusedto alkylate
reducedhair containing0.6 m molesof SH per gramof hair. The extentof reactionwas
monitored
bydetermining
theresidual
SHfollowing
thealkylation.
Under
thecondi-:
tionsemployed,the alkylationwascompletewithin 2 h. The alkylationwascarriedout
in 20 per cent n-propanol/0.04M phosphatebuffer, pI-I 7 at 35øC, 100:1 bath ratio;
undertheseconditionsthe extentof hydrolysisof the N-alkyl maleimidesis negligible.
In using the next higher homolog, i.e., N-heptyl maleimide, the alkylation reaction
couldalsobe completedwithin 2 h. Followingthe alkylationwith N-ethyl maleimide,
HYDROPHOBIC
BONDS
AND
HAIR
T1LEATMENTS
237
Table IV
Effect of Time on the Alkylation of Hair with N-Ethyl and N-Hexyl Maleimide'•
Degree of Alkylarion, Per Cent
Time, Minutes
N-Ethyl Maleimide
N-Hexyl Maleimide
30
60
75
81
82
94
120
86
100
.i •[Maleimide]
= 0.01 M; Solvent= 20 per centn-propanog0.04
M phosphate
(pH 7) buffer;temperature
½ 35øC;bathRatio = 100:1.
approximately
15 per cent of the SH remainedunreactedafter 2 h, while a corresponding
sampleof hair treatedwith the N-hexyl maleimidecontainedno residual
SH (Table IV). This findingwasin accordwith the datapresentedby Heitz (6), who
determinedthe second-orderrate constants
for the bindingof N-heptyl maleimideto
i::i.i
yeast
alcohol
dehydrogenase.
There,it wasshownthatthereaction
ratefor theNß
heptylmaleimidereactionwas approximately8.4 times that observedwith N-ethyl
:'•':maleimide.
Thiswassomewhat
unexpected,
sincea chainlength
effectwasnotob-
:::served
inthereactions
ofthese
maleimides
withcysteine
and
glutathione
(6).Weob-
servedsimilar enhancement of the rate of alkylation in the case of reduced hair.
.:'.:':::
Followingreductionwith thioglycolicacid,the samplewasrinsedonly brieflyprior to
thealkylation,andthus,residualthioglycolicacidremainedin the fiber. At the end of
a 24-h treatment, the cysteinyl residueswere completely blocked by N-heptyl
maleimide,while free thioglycolicacidwasstill detectedin the alkylatingsolution.This
observation
andthepreviousresultson the alkylationof reducedkeratinindicatethat
•: anincrease
in thealkylsidechain
of themaleimide
leadsto fasterratesof alkylation
in
spite
ofthe.unfavorable
diffusion
factor.
Such
anenhancement
inthereactivity
maybe
'•i tentatively
ascribed
to theinteraction
between
thealkylsidechain
of thereactant
and
the nonpolarresiduesof the keratin,which apparentlyprovidean effectivehydro-
phobic
environment
forthecombined
cystine.
..
•., 6. Swelling
properties
of thealkylated
hair.' The extentof internalmodification
of ke,:.•:..ratinoftencanbereadilyassessed
fromthechangein theswellingcharacteristics
of this
:
protein.Thus, fissionof the disulfide bonds is accompaniedby an increasein water
•,
imbibition,
whichis almostdirectlyproportionalto the numberof crosslinks
severed.
According
to our hypothesis
of hydrophobic
modification
of hair,the introduction
of
apolarresiduesshouldcompensate
for at leastsomeof the disulfidebondbreakdown.
,
A strong
support
forthisviewwasobtained
fromtheliquidretention
measurements
of
reduced
andreduced-alkylated
hair(TableV). The reductiontreatmentalonecauses
a
large
increase
in swelling
in bothwaterandaqueous
alcohol.
Alkylation
ofreduced
hair
with methyliodideor N-ethyl maleimideslightlyintensifiesthe swelling.This incre-
mentin hydrationis probablycausedby the eliminationof weakhydrogenbonding
involving
thesulfurhydrogen
andtheapparent
inability
ofthemethylorN-ethylderivatives
to establish
anyspecific
interactions
with the environment.
On theotherhand,
alkylation
with either N-hexyl or N-heptylmaleimides
bringsaboutsignificant
decrease
inhydration.
Obviously,
theintroduction
ofhydrophobic
residues
canimpart
238
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
TABLE
V
Effect of Apolar Residueson the SwellingPropertiesof Hair
Per Cent Swelling• in:
Per Cent
Reduction
Alkylating
Agent
50 Per Cent
50 Per Cent
20 Per Cent
50 Per Cent
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
47.2
42.2
46.2
47.8
49.2
45
N-ethyl
45.3
41.0
45.5
48.6
51.1
45
N-hexyl
33.9
33.7
37.8
39.5
46.3
iodide
maleimide
maleimide
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-dodecyl
maleimide
N-benzyl
28.2
n.d]'
31.9
36.4
47.5
31.5
n.d.b
34.1
35.4
46.1
82
maleimide
aCalculated
onthetreated
weight
ofthefiber.
"Notdetermined.
substantial
conformational
stability
to thereduced
keratinstructure
andsuccessfully.•:ii7
resist
theswelling
pressure.
Thisconformational
stability
is,ofcourse,
lostinaqueous
alcohols,
where
the
phobicinteractions
between
the apolarresidues
presentin hairareprevented.
Sucha
differential
response
toaqueous
solvents
offersaunique
wayofhairmanipulation,
particularlywith regardto setting.
7. Mechanicalproperties:Swellingdatahaveshownthat the imbibitionof water by
ratincanberestricted
byhydrophobic
modification
of thefiber.Although,
theprecise
natureof theintermolecular
arrangement
remains
a subject
of controversy
(10, 1
the stabilizationof hydrophobicbondsby wateris not disputed.The strengthof the
hydrophobicbond is representedby the tendencyof nonpolargroupsto adhereto one
another.The free energyof thisprocesshasbeenassessed
(12), andin the caseof the..(
interaction
of2 methyl
groups,
was
found
tobe-0.73kcal/mole
foralkyl
sidechains.'"??
The incrementin the free energyof bindingwasin the order of -0.37 kcal/moleper•:is?
CHzgroup.
Theoverall
contribution
ofthese
hydrophobic
crosslinks
tothe
tion of the keratinstructure
will dependon the size,shape,andnumberof thein-.7:.
troducedapolarresidues.
:
Some
further
insight
onthese
hydrophobic
interactions
was
obtained
from
astudy
o
the mechanicalproperties.It is well knownthat the wet strengthof intacthair bearsa
linearrelationship
to thecystinecontentovera widerangeof reductionlevels(13). Us-
ingthislinear
relationship
asaguide,
apreliminary
assessment
ofthestabilization
ef-'?'
fectarisingfromhydrophobic
interactions
wasobtainedfromthemechanical
behavior
HYDROPHOBIC
BONDS
AND
HAIR
TREATMENTS
239
Table VI
Yield Stressof Hair Alkylated with N-Hexyl Maleimide
Yield Stress,g/denier in:
Water
Reduction Level,
Per Cent
Calculated a
20 Per Cent Propanol
Observation
50 Per Cent Propanol
Calculated a
Observation
Calculated a
Observation
0
0.42
0.42
0.36
0.36
0.36
0.36
31
42
0.28
0.22
0.40
0.32
0.25
0,21
0.30
0.17
0.25
0.21
0.27
0.14
82
0.08
0.27
0.06
0.11
0.06
0.10
aCalculated
yieldstress= (intacthairyieldstress)(100-percentreduction/100).
of alkylatedfibers(Table VI). The alkylationwasperformedat 3 reductionlevelsusing
N-hexyl maleimide asthe alkylatingagent.
If one relieson the fact that the percentreductionin work to stretcha reducedfiber is
directly proportional to the extent of reduction, then it is evident that the alkylated
fibersdo not exhibit sucha lossof strength.In water, the formation of hydrophobic
bonds,via interaction of the hexyl residues, results in significant stabilization of the
keratinstructure.Although, somerepairingeffect wasanticipated,the extentof the
stabilizationand particularly the resistanceof the treated fibers to the external
stresses
wasunexpected.Even with the maximumoverlapof the apolarsidechains
introducedin the alkylationstep,the averagestrengthof the newlyformedbondswould
not exceed 5 kcal. This is only a fraction of the energy loss which accompaniesthe
breakdownof cystinecrosslinks
(-50 kcal/mol).It is apparentthat the hydrophobic
interactions
which accompanythe blockingof cysteineresiduesare very intensive,although,a possibilityof cooperative
multichainhydrogenbondingin a hydrophobic
environment
cannot be excluded.
8. Settingpropertiesofalkylated hair.' The alkylation of reducedhair limits the swelling of hair in water. On the other hand, suchhair can be readily deformed in alcoholic
media.An attemptwasmade to utilizethischangein swellingcharacteristic
for the settingpurposes.Thus, the hair wasswollenin 50 per cent propanol and set on rollers. In
the presenceof alcohol, the alkylatedhair is very pliable and moldable and conforms
readilyto the desiredconfiguration.After settingthe hair, the alcoholwasremovedby
rinsingwith 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
maleimidestraightenedwithin 1 to 2 min. This demonstratesthat a water-resistantset
canbe attainedby the introductionof apolarresidues,providingthat a sufficientlylong
sidechain
isused.A 7-carbonalkylchainappearsto fulfillthisrequirement.
Thesetstabilityof tresses
wasalsoassessed
bytheconventional
manner.Thisinvolves
theexposureof set tressesto controlledconditionsof humidityandtemperatureand
measuringthe extent of relaxationwith respect to time. Following the reduction, the
tresses
were rinsedwith water and then alkylatedwith N-heptyl maleimidefor 2 h. The
settingwasperformedwith 50 per centpropanol;after thoroughrinsingwith waterthe
240
JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS
Table VII
Set Stability of Hair Following Alk¾1ationwith N-Hept¾1 Maleimide
Hanging Lengths (cm):
After Following Relaxation Time (min):
Sample
0
5
7.5
I0
15
30
150
Intact (H=O set)
2
7.5
11
12
13
13
13
Intact
2
5.0
7
8
I0
11
12
Reduced/hepryl
maleimide
2.5
3.5
4
4
4.5
5.0
6
•Measured at 84 per cent RH, 85øF.
Table
VIII
Effect of ReductionLevel on the Set Stabilityof Alkylated Hair
Hanging Lengths(cm) at
Relaxation Times (min)::
Treatment
3
6
9
12
20
I0
45
Intact (H20)
2
3
5.5
8
Intact
2.5
3
3.5
5.5
8.5
11
10.5
15' Reduction/hepty[
maleimide
30' Reduction/hepr¾1
maleimide
60' Reduction/heptylmaleimide
2.5
2
2
5.0
3
2.5
6
3.5
3.0
7.5
4
35
9
5
4
I0
6.5
5.0
tresseswere air dried overnighton the rollers (alternatively,the set hair wasdried with
a hand held dryer). The set stability is shown in Table VII.
The alkylation resultsin excellentset stability during a prolonged exposureto conditionsof high humidity.Additionalexperimentsindicatedthat increasingthesizeof the
alkyl substituentabove C7 offered no benefit.
Optimal interactionof the hydrophobicsidechainsor, indeed, the mobilityof such
chains,may not be possiblein the caseof the dodecylresiduesbecauseofstericfactors.
Thus, the maximalstabilizationof the keratin would not be attained.It is of interestto
note that the best results have been obtained with hair alkylated either with heptyl or
with benzyl maleimides.
The effectof alkylationon the setstability(measuredat 84 per centRH, 85øF)wasdeterminedat severalreductionlevels(TableVIII). It appearsthatalkylationof reduced
hair having less than 50 per cent cleavagedoes not greatly improve the setting
properties.
An importantconsideration
in thereduction-alkylation
process
is therespective
bond
energiesof the system.As discussed
earlier,disulfidebondsrepresentapproximately
50 kcal/molof stabilization
energy.The introduction
of two apolarresidues
(heptyl
sidechain)
for the blockingof a reduceddisulfidewill contributeonlyabout3 kcal/rnol
of bindingenergy.At low reductionlevels,the introduction
of a few hydrophobic
crosslinkscannotcompetewith the residualdisulfideswhichsignificantly
contributeto
the overall settingcharacteristics
of intacthair. At very high reductionlevels,only a
HYDROPHOBIC
BONDS
AND
HAIR
TREATMENTS
241
fewrestraining
disulfidecrosslinks
remain,thus,the stabilizing
influence
of the hydrophobiccrosslinks
becomes
moreprevalent
in maintaining
the nativestructure
of the
keratin.
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