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Investigation of the Interactions of Cationic Guar with Human Hair by Electrokinetic Analysis

Authors:
  • Ashland Specialty Ingredients
  • L'Oréal, USA

Abstract

Objective Cationic guar is an important polysaccharide used as a hair conditioning agent in personal care products. In this article, we report streaming potential data demonstrating its behavior as it interacts electrostatically with hair. Several cationic guar variants with different molecular weights (MWs) and charge densities (CDs) were examined. Methods All experiments were carried out with a custom‐designed streaming potential instrument so that in situ, real‐time data were monitored during the treatment of a hair plug with aqueous solutions of cationic guar and subsequent treatment with anionic surfactants—sodium laureth sulfate (SLES) and cocamidopropyl betaine (CAPB)—commonly found in contemporary shampoo formulations. Results The MW of the cationic guar variants plays an integral role in determining the thickness of the adsorbed polymer layer on the hair surface while CD influences the zeta potential. Data were also generated for the treatment of hair with a cationic flexible polymer (polyquaternium‐28) and cationic conditioning surfactant (behentrimonium chloride) to provide a frame of reference. The deposition behavior on hair of high MW cationic guar variants is distinct from these conventional molecules in terms of its electrokinetic properties. We also examined the electrokinetic behavior of cationic guar on hair types from different racial backgrounds. While the cationic guar treatment yielded similar results for the different hair types, anionic surfactant treatment resulted in quicker sorption and desorption from African, European 65% gray, and Mulatto hair as compared to Chinese, European dark brown, and Indian hair. Conclusion We introduce an in‐situ technique for measuring the dynamic sorption/desorption of charged molecules on the surface of human hair. Evaluation of a series of cationic guar species revealed varying behavior depending on the MW and CD of the polysaccharide. Our data also demonstrate differences in the desorption properties of typical shampoo surfactants for hair from diverse racial backgrounds.
Int J Cosmet Sci. 2021;00:1–16.
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1
wileyonlinelibrary.com/journal/ics
Received: 5 February 2021
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Accepted: 31 March 2021
DOI: 10.1111/ics.12704
ORIGINAL ARTICLE
Investigation of the interactions of cationic guar with human hair
by electrokinetic analysis
Roger L.McMullen
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DonnaLaura
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GuojinZhang
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BertKroon
This is an open access article under the terms of the Creative Commons Attribution- NonCommercial License, which permits use, distribution and reproduction in any medium,
provided the original work is properly cited and is not used for commercial purposes.
© 2021 Society of Cosmetic Scientists and the Société Française de Cosmétologie
Ashland Specialty Ingredients, G.P,
Bridgewater, New Jersey, USA
Correspondence
Roger L. McMullen, Ashland Specialty
Ingredients, G.P., 1005 US HWY
202/206, Bridgewater, New Jersey 08807,
USA.
Email: rmcmullen@ashland.com
Abstract
Objective: Cationic guar is an important polysaccharide used as a hair conditioning
agent in personal care products. In this article, we report streaming potential data
demonstrating its behaviour as it interacts electrostatically with hair. Several cationic
guar variants with different molecular weights (MWs) and charge densities (CDs)
were examined.
Methods: All experiments were carried out with a custom- designed streaming po-
tential instrument so that in situ, real- time data were monitored during the treatment
of a hair plug with aqueous solutions of cationic guar and subsequent treatment with
anionic surfactants— sodium laureth sulfate (SLES) and cocamidopropyl betaine
(CAPB)— commonly found in contemporary shampoo formulations.
Results: The MW of the cationic guar variants plays an integral role in determining
the thickness of the adsorbed polymer layer on the hair surface while CD influences
the zeta potential. Data were also generated for the treatment of hair with a cationic
flexible polymer (polyquaternium- 28) and cationic conditioning surfactant (behentri-
monium chloride) to provide a frame of reference. The deposition behaviour on hair
of high MW cationic guar variants is distinct from these conventional molecules in
terms of its electrokinetic properties. We also examined the electrokinetic behaviour
of cationic guar on hair types from different racial backgrounds. While the cationic
guar treatment yielded similar results for the different hair types, anionic surfactant
treatment resulted in quicker sorption and desorption from African, European 65%
grey, and Mulatto hair as compared to Chinese, European dark brown, and Indian hair.
Conclusion: We introduce an in situ technique for measuring the dynamic sorption/
desorption of charged molecules on the surface of human hair. Evaluation of a series
of cationic guar species revealed varying behaviour depending on the MW and CD of
the polysaccharide. Our data also demonstrate differences in the desorption properties
of typical shampoo surfactants for hair from diverse racial backgrounds.
KEYWORDS
cationic guar, cocamidopropyl betaine, human hair, sodium laureth sulphate, streaming potential,
zeta potential
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INTERACTIONS OF CATIONIC GUAR WITH HAIR
Abstrait
Objectif: Le guar cationique est un polysaccharide important utilisé comme condi-
tionneur capillaire dans les produits cosmétiques. Dans ce rapport, nous démontrons
l'utilisation de la technique du potentiel de streaming pour étudier comment le guar
cationique interagit électrostatiquement avec les cheveux. Plusieurs variantes del guar
cationique avec différents poids moléculaires et densités de charge ont été examinés.
Méthodes: Nous avons utilisé un instrument de potentiel de streaming pour les ex-
périences. Les études ont été réalisées en temps réel pour surveiller le traitement de
cheveu avec des solutions aqueuses de guar cationique suivi d'un traitement ultérieur
avec tensioactifs anioniques comment le sulfate de laureth de sodium et le cocami-
dopropyle bétaïne, des ingrédients généralement trouvés dans les formulations de
shampooing.
Résultats: Le poids moléculaire des variants du guar cationique joue un rôle intégral
dans la détermination l'épaisseur de la couche de polymère adsorbée sur la surface
des cheveux tandis que le densité de charge influence le potentiel zêta. Des données
ont également été générées pour le traitement des cheveux avec un polymère flexible
(polyquaternium- 28) et tensioactif de conditionnement cationique (behentrimonium
chlorure) pour fournir un cadre de référence. Le comportement de dépôt sur les ch-
eveux des variants de guar cationiques à poids moléculaire élevé est distinct de ces
molécules conventionnelles en termes de ses propriétés électrocinétiques. Nous avons
également examiné le comportement électrocinétique de guar cationique sur des types
de cheveux de différents milieux raciaux. Le traitement avec le guar cationique a
donné des résultats similaires pour les différents types de cheveux. En contraste avec
ceci, le traitement avec le tensioactif anionique a entraîné une sorption et une désorp-
tion plus rapides de cheveux africains, de cheveux européens (65% gris) et de cheveux
mulâtres en comparaison à les cheveux chinois, européens et indiens.
Conclusion: Nous introduisons une technique in situ pour mesurer la sorption et
la désorption dynamique de molécules chargées à la surface des cheveux humains.
L’évaluation d'une série des espèces de guar cationiques ont révélé un comportement
variable en fonction du poids moléculaires et densités de charge de le polysaccharide.
Nos données démontrent également des différences dans les propriétés de désorption
de tensioactifs de shampooing typiques pour les cheveux de diverses origines raciales.
MOTS CLÉS
guar cationique, cocamidopropyle bétaïne, cheveux humains, le sulfate de laureth de sodium,
potentiel de streaming, potentiel zêta
INTRODUCTION
Guar gum is a galactomannan polysaccharide from the
endosperm of the seeds of the guar plant (Cyamopsis
tetragonolobus). It is cultivated mostly in India and
Pakistan and is used extensively in food products as a tex-
ture and rheology modifier. It is characterized by a wide
range of high molecular weights and contains a straight
chain polysaccharide of D- mannose units connected by β
(1→4) glycosidic linkages with a pendant galactose unit
on alternating mannose units attached by an α (1→6) gly-
cosidic bond. Derivatives of guar gum have become in-
creasingly important in various industries, such as oil field
(fracking agents), textiles and paper. Cationic guar gum
includes a pendant hydroxypropyl group on the galactose
unit with quaternary amine functionality (see molecu-
lar structure in Figure 1). Two common synthetic routes
for producing cationic guar gum consist of reacting guar
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MCMULLEN Et aL.
gum with 3- chloro- 2- hydroxypropyl trimethylammonium
chloride or 2,3- epoxypropyl trimethylammonium chlo-
ride, although there are a number of different synthetic
approaches that have been explored [1- 3].
An important application of cationic guar gum in the
personal care industry is in shampoos and body washes as
a coacervation and conditioning agent [4- 8]. Its quaternary
pendant amine group bears a positive charge, making the
polymer substantive to human hair, which has a negatively
charged surface at pH levels above its isoelectric point—
values ranging from 2.45 to 4.5 have been reported in the
literature and may be attributed to the chemistry of the
surface, which can change as a result of environmental ex-
posure (eg UV radiation, thermal exposure) or chemical cos-
metic treatments (eg bleaching, dyeing, etc.) [9]. In general,
cationic polymer adsorption on hair occurs due to electro-
static interactions between the polymer and the hair surface.
A particular challenge in the scientific community has been
to elucidate the process by which cationic polymers deposit
onto hair from solutions containing anionic surfactants [10,
11]. This is especially true for cationic guar, which forms
coacervates during its dilution from solutions of anionic sur-
factants, and effectively deposits on the hair surface [12- 14].
There are a number of methods that can be used to deter-
mine the deposition of a cationic polymer onto hair. Some
of these methods include fluorescent labelling in combina-
tion with optical microscopy, X- ray photoelectron spectros-
copy, radiotracer techniques and atomic force microscopy.
While these techniques have utility, they do not provide
quantitative dynamic data describing the sorption process.
FIGURE 1 Molecular structure of cationic guar (IUPAC: guar gum, 2- hydroxy- 3- (trimethylammonio)propyl ether, chloride; INCI: guar
hydroxypropyltrimonium chloride). The m subunit represents the number of native guar gum residues while n refers to the cationic- modified
portion of the molecule. The polymer charge density depends on the number of n units in the molecule
O
O
HO
OH
O
O
O
HO
OH
OH
O
O
HO
OH
O
O
O
HO
OH
OH
O
OH
HO
OH
OH
O
OH
HO
OH
O
mn
HO
NCl
Guar variant Trade name CD
Viscosity
(cps)
High CD- high viscosity N- Hance 3215 High 4200b
High CD- very low viscosity AquaCat PF 618 High <100a
Medium CD- high viscosity N- Hance 3196 Medium 4500b
Medium CD- low viscosity N- Hance CCG 45 Medium 40b
Low CD- high viscosity N- Hance 3000 Low 2700c
aA 10% (w/w) solution was tested.
bBrookfield RV (20rpm).
cBrookfield LV (6rpm).
TABLE 1 Viscosity, CD, trade name
and designation of the cationic guar variants
investigated in this study. Information
obtained from Reference [23]. Unless
otherwise indicated, viscosity measurements
were conducted on 1% (w/w) solutions of
the polymers
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INTERACTIONS OF CATIONIC GUAR WITH HAIR
Streaming potential is a very effective technique to mon-
itor the deposition of charged molecules onto human hair
and wool [15- 22]. The earliest studies on hair examined the
influence of salts on the adsorption behaviour of cationic
cellulose ether [15]. While simple anions were shown to
have little effect on the sorption of cationic cellulose ether,
cations inhibited their sorption on hair in the following
order: La3+>Al3+>Fe3+>Ca2+>Fe2+>Cs+>Na+>
Li+. These data suggest that cations interact with sur-
face charges on the fibre and the degree of the charge and
size of the ion play a key role in regulating this interaction.
However, it should be noted these differences are not ob-
served at all salt concentrations.
Streaming potential studies by Jachowicz and coworkers
were carried out with a custom- designed instrument capa-
ble of monitoring electrokinetic data (streaming potential
and conductivity) and flow rate of the streaming potential
solution [16- 19]. The latter parameter provides the perme-
ability of the hair fibre assembly (porous plug) and allows
for the calculation of the thickness of the deposited poly-
mer layer. The work by Jachowicz et al. enabled the in situ
analysis of a wide variety of molecules, such as those found
in shampoos and conditioners (cationic surfactants, anionic
surfactants, cationic polymers, silicone oils, etc.), provid-
ing information about their substantivity and removability
from hair.
In this article, we present novel data on the deposition
of cationic guar derivatives onto the surface of human hair.
Using a custom- designed streaming potential instrument,
we monitor streaming potential (zeta potential at the hair
fibre surface), thickness of the deposited polymeric layer,
and pH and conductivity of the streaming potential solu-
tion. We report the influence of MW and CD of cationic
guar on its hair deposition properties. In addition, we ex-
amine binding behaviour of cationic guar on various ethnic
types of hair. Additional experiments were carried out using
a miniature tensile tester to determine the force required to
comb through an assembly of hair fibres in the wet state.
Typically, cationic polymers decrease the wet combing
forces of hair. The motivation for conducting combing mea-
surements stemmed from the desire to elucidate any pos-
sible correlation between zeta potential and practical use
parameters related to the everyday application of cationic
guar to hair.
MATERIALS AND METHODS
The majority of the work reported in this article entails
streaming potential and permeability measurements lead-
ing to the calculation of zeta potential and thickness of the
adsorbed polymer layer. In addition, mechanical measure-
ments of the force required to comb through a wet hair
fibre assembly were carried out with a miniature tensile
tester. These data permitted a correlation between the
two techniques providing information related to the rel-
evance of zeta potential in consumer- perceivable hair care
applications.
Hair samples
All hair was purchased from International Hair Importers
and Products, Inc. (Glendale, New York, USA). The major-
ity of the tests comparing different variants of cationic guar
were carried out with European dark brown hair. Studies
were also carried out to compare the differences between
various hair types including Indian, Chinese, European
dark brown, European light brown, European 65% grey,
Mulatto and African. All hair was shampooed twice with 3%
(w/w) SLES:CAPB (12:2) prior to conducting experiments.
SLES (Steol CS- 130) was obtained from Jeen International
FIGURE 2 Illustration of the
customed- designed streaming potential
instrument used in this study
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5
MCMULLEN Et aL.
(Fairfield, New Jersey, USA), while the source of CAPB
(Amphosol CA) was Stepan (Northfield, Illinois, USA).
Cationic guar variants
Variants of cationic guar were obtained from Ashland, Inc.
(Wilmington, Delaware, USA) and are characterized by their
solution viscosity (MW) and CD. Table 1 contains a list of
the various cationic guar species examined in this work.
For comparison, we also conducted streaming potential
measurements of hair treated with a typical conditioning
surfactant and polymer containing cationic functionality.
Behentrimonium chloride (tradename: Jeequat BTMC- 85%)
was obtained from Jeen International. The cationic poly-
mer, polyquaternium- 28, contains vinyl pyrrolidone (VP)
and methacrylamidopropyl trimethylammoniumchloride
(MAPTAC) monomers, and was obtained from Ashland, Inc.
(tradename: Conditioneze NT- 20).
Electrokinetic permeability analysis
Electrokinetic and permeability data were collected using
a custom- designed streaming potential instrument. The
streaming potential instrument employed in this study is
based on the same operating principles as that reported in
Jachowicz et al. [18]. As shown in Figure 2, it consists of
a central control unit connected to the streaming potential
cell, Consort C861 multiparameter analyser (Consort nv,
Turnhout, Belgium), Pennsylvania model 7300 bench scale
(Pennsylvania Scale Company, Lancaster, Pennsylvania,
USA), 10 L low density poly(ethylene) carboy (VWR
Scientific, Radnor, Pennsylvania, USA), and two 1 gal low
density poly(ethylene) bottles (VWR Scientific). Tygon
tubing R- 3603 (Norton Performance Plastic Corporation,
Akron, Ohio, USA) connects the various components and
allows streaming potential solution to flow from one part of
the instrument to another. All water used in the streaming
potential experiments was purified with a Milli- Q Integral
5 (MilliporeSigma, Merck KGaA, Darmstadt, Germany)
water purification system at a resistivity of 18.0 MΩ⋅cm
at 25°C.
An electrolyte solution (5×10−5 M KCl) flows from
the electrolyte solution deposit tank (10 L carboy) through
the streaming potential cell to the waste collection cham-
ber where the quantity of solution is monitored as a func-
tion of time to determine the flow rate. The direction of
solution of flow in the streaming potential instrument is
illustrated by the small arrows. ACS reagent grade (99.0-
100.5%) KCl was obtained from Sigma- Aldrich (St. Louis,
Missouri, USA). Two treatment containers are connected
to the same flow loop as the electrolyte solution. In all of
the examples provided in this report, Treatment 1 (1 gal
bottle) consist of a 0.01% (w/w) solution of a cationic guar
variant while Treatment 2 (1 gal bottle) was a 0.3% (w/w)
solution of SLES:CAPB (12:2) intended to represent a
typical surfactant composition found in most commercial
shampoos. Please note that much lower concentrations of
polymers and surfactants were employed in this work than
normal application concentrations due to the tightly packed
nature of the hair fibre plug in the streaming potential cell.
The streaming potential instrument was pressurized to 6.8
- 7.0bar using compressed air, allowing the streaming po-
tential solution to flow properly during the course of an
experiment. The streaming potential cell was constructed
with poly (methylmethacrylate) and two perforated Ag/
AgCl electrodes that contain six holes that are 1 mm in
diameter, which evenly distributes the flow of solution
through the hair plug between the two electrodes. Each
electrode is 0.5mm thick and has a diameter of 11mm. For
each experiment, 2 g hair swatches are shampooed with
3% (w/w) SLES. The hair is extensively rinsed to remove
shampoo followed by drying. Then, 0.5g of hair was cut
FIGURE 3 (a) Photograph of a
hair tress after bleaching with a window
treatment frame. Note that only one part
of the window treatment frame (one of the
acrylic portions) is shown in the photograph.
(b) Wet combing curve of hair after the
bleaching treatment
(a)
(b)
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INTERACTIONS OF CATIONIC GUAR WITH HAIR
into 4- 6mm pieces after wetting with the KCl electrolyte
solution. The hair snippets were then soaked in KCl solu-
tion for 1hour. The wet snippets were then placed between
the streaming potential electrodes to form a hair plug. The
gap between the electrodes was 7.5mm.
The anode and cathode are connected to the instrument
control unit by alligator clips and electrical wiring, allowing
the measured potential difference to be converted to a digi-
tal signal, which is relayed to a standard desktop computer
operating Windows 7 (Microsoft Corporation, Redmond,
Washington, USA) and containing the custom software de-
signed for the instrument. The Consort C861 multiparameter
analyser is connected to a pH and conductivity electrode posi-
tioned along the flow route of the streaming potential solution.
The zeta potential (
𝜁
) was calculated using the
Smoluchowski equation:
where
𝜂
and
𝜀
are the viscosity and dielectric constant, respec-
tively, of the KCl electrolyte solution,
𝜅
is the conductivity of
the streaming potential solution, E is the streaming potential
and P is the pressure driving the flow of the solution.
We also estimated the thickness (
𝛿a
) of the adsorbed layer
on the hair surface using Equation 2:
where
is the average pore radius (21.4μm) of the fibre plug,
Q
is the flow rate before deposition, and
Qa
is the flow rate after
deposition [18].
Mechanical measurements of combing force
Combing analysis was achieved using a miniature ten-
sile tester (Model 170) manufactured by Dia- Stron, Ltd.
(Hampshire, UK). The combing measurements were carried
out on wet hair with the following instrumental parameters:
range, 2000 G; gauge, 2 G; size, 50mm; phase 1 (extension),
350%; phase 2, 0%; phase 3, 0%; and phase 4, 0%. In all ex-
periments, hair tresses were combed several times to remove
entanglements before performing combing measurements.
To better differentiate between treatments, studies were
conducted on European light brown hair that was bleached
in two regions of the tress using an acrylic (Acme Plastics,
Woodland Park, New Jersey, USA) frame (two pieces) contain-
ing two sheets of silicone rubber (McMaster- Carr, Elmhurst,
Illinois, USA) material sandwiched together by the frame.
A hair tress (2g with dimensions of 3.175×20.32cm) was
placed between the two silicone rubber sheets and bleaching
treatment was carried out by placing the bleaching paste in the
windows region of the frame and thoroughly saturating the
fibres [24]. Hair was subjected to a one- hour bleaching cycle
with 120 g of Clairol Professional BW 2 Powder Lightner
(The Wella Corporation, Woodland Hills, California, USA)
and 147 mL of Salon Care Professional 20 Volume Clear
Developer (Arcadia Beauty Labs LLC, Reno, Nevada, USA).
The resulting mixture was applied to damp hair.
A photograph of one piece of the acrylic frame alongside a
hair tress bleached with the device is provided in Figure 3. Also
included in the figure is a representative figure of a combing curve
obtained for a bleached hair tress. After bleaching, the entire hair
tress was treated with a 1% (w/w) solution of the cationic guar
variant. Wet combing measurements were obtained after bleach-
ing and after treatment with cationic guar. The combing work in
(1)
𝜁
=
4𝜋𝜂𝜅E
P𝜀
(2)
𝛿
a
R=1
(
Qa
Q
)14
FIGURE 4 Illustration of the ion
concentration surrounding the hair fibre
surface when hair is immersed in an aqueous
electrolyte solution
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7
MCMULLEN Et aL.
the window treatment regions of the combing curve was calcu-
lated using the Dia- Stron, Ltd. Windows application software.
RESULTS AND DISCUSSION
The adsorption of thin films of macromolecules on the surface
of hair is an important area of research, especially in the applica-
tion of shampoos and conditioners, which can modify its surface
characteristics and tactile properties. Adsorption is driven by
entropy and the interactions between the macromolecules and
the surface [25]. In this study, we monitor the interactions of a
cationic surfactant (behentrimonium chloride), flexible polymer
(polyquaternium- 28) and polysaccharide (cationic guar) with
the surface of human hair using streaming potential. We also
monitored the effects of an anionic shampoo system (SLES-
CAPB) on the binding properties of the conditioning treatments.
The surfactants and polymers can be absorbed or adsorbed to
hair. Additional studies of cationic guar variants allowed us
to determine the influence of CD and MW on their electroki-
netic behaviour. The adsorption behaviour of cationic guar and
SLES- CAPB was examined on hair types from various ethnic
origins allowing us to identify two types of behaviour. Finally,
mechanical wet combing forces were measured for hair fibres
assembled in a tress. Using a specialized technique that con-
sisted of bleaching the hair tress in selected regions, we were
able to identify a relationship between cationic guar charge den-
sity and combing force reduction.
Zeta potential of hair
The electrical charges on the hair surface determine the zeta
potential at the hair fibre liquid interface when hair is immersed
in an aqueous solution. These charges arise due to the amino
acid functionalities present on the surface of the hair fibre and
are affected by the pH of the electrolyte solution. The low iso-
electric point of hair suggests that its surface is populated with
a considerable quantity of acidic groups. Zeta potential is also
influenced by ions and molecules in the solution phase that can
be adsorbed on the surface of hair [26]. Figure 4 contains an
illustration of a negatively charged hair fibre containing oppo-
sitely charged ions in the Stern layer and a mixture of posi-
tive and negative ions in the diffuse layer. The slipping plane
is located at the boundary between the Stern and diffuse layer,
and is defined as the interface separating mobile and immobile
fluid. The zeta potential (typically presented in units of mV)
is the potential difference in this region. The KCl electrolyte
solution used in the streaming potential experiment essentially
provides the necessary ions for this phenomenon to take place.
Hair treated with a conventional
cationic surfactant
Long chain alkyl quaternary surfactants and cationic poly-
mers are typically employed as conditioning agents for hair
due to their electrostatic attraction to the negatively charged
hair surface. The in situ streaming potential instrument used
in this work is ideally suited to dynamically monitor the
sorption and desorption of molecules from hair. To estab-
lish a baseline, we first present data obtained for one of the
most commonly used cationic surfactants in hair conditioner
formulations, behentrimonium chloride. Structurally, it has
a quaternary amine polar group attached to a twenty- two
carbon aliphatic chain. Unfortunately, there is little infor-
mation available in the literature about the mechanism of
interaction of cationic surfactants with the hair surface [27].
FIGURE 5 Zeta potential data of hair
as a function of flow time before, during
and after treatment with behentrimonium
chloride and SLES- CAPB. Trial 1 and
Trial 2 refer to two different trials that were
carried out on different hair plugs
8
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INTERACTIONS OF CATIONIC GUAR WITH HAIR
In addition to being cationic, the surfactants must have a
substantial aliphatic component (at least C12 and up to C22)
in order to adsorb and remain on the hair surface, even after
rinsing. This implies that in addition to a charge driven
mechanism, there is also a hydrophobic element, which re-
lies on van der Waals interactions between the hydrophobic
moieties of the cationic surfactants. Typically, long chain
alcohols are formulated with the cationic surfactants in
conditioning preparations. It is believed that the surfactants
form aggregates with the fatty alcohols, allowing them to
also bind to the hair surface [9].
Figure 5 contains a zeta potential profile for hair subjected
to two treatment cycles with behentrimonium chloride, each
subsequently followed by treatment with SLES- CAPB. At
the bottom far left of the plot, the zeta potential of untreated
hair is plotted over a timeframe of 15min demonstrating its
negative surface charge characteristics. During this period, the
KCl solution flows through the hair plug. This is followed by
a 5- min treatment with behentrimonium chloride (indicated as
cationic surfactant in the figure) and then a 15- min rinsing step
(20- 35min) with the electrolyte solution. During the rinsing
stage, there is a downward slope that corresponds to excess be-
hentrimonium chloride being removed from the hair surface.
Between 35 and 40min, a solution of SLES- CAPB passes
through the hair plug. Immediately, the zeta potential drops
significantly due to some removal of behentrimonium chlo-
ride as well as the agglomeration of adsorbed SLES bearing
a negative charge on the surface of the hair. More than likely,
negatively charged SLES probably interacts with the cationic
groups of behentrimonium chloride. As the hair plug is rinsed
(40- 55min), SLES is removed from the hair and the surface
becomes less negatively charged. We should note that in the
case of behentrimonium chloride treatment of hair not all of
the SLES is removed from the fibre plug. This is reflected by
the slope of the plot, which does not reach a plateau.
A second treatment with behentrimonium chloride is ad-
ministered between 55 and 60min followed by a rinse step
from 60 to 75min. Note the difference in the slope of the
data after the first treatment (20- 35 min) versus the second
treatment (60- 75min). After the first treatment, excess be-
hentrimonium chloride that is not electrostatically bound to
the hair is gradually rinsed off. However, after the second
treatment a plateau is reached immediately. At the onset of
the first treatment cycle, the hair surface is free of cationic
and anionic ingredients. However, during the second treat-
ment cycle, behentrimonium chloride could interact with re-
sidual SLES- CAPB on the surface by electrostatic and van
der Waals interactions, which would result in the formation
of a complex. The removal of this complex would not result
in a significant change in the zeta potential.
A second treatment cycle with SLES- CAPB is carried
out (75- 80min) followed by a subsequent rinse cycle (80-
95 min). Overall, we find that this technique is extremely
reproducible as demonstrated by the data plotted for Trials 1
and 2 in Figure 5. Based on the flow rate data, we calculated
the deposited thickness of a layer of behentrimonium chlo-
ride on the surface of hair according to Equation 2 and found
the following values after each cycle: 0.03 ± 0.06μm (first
treatment); 0.02 ± 0.03μm (first SLES- CAPB treatment);
0.01 ± 0.02 μm (second treatment); and 0.04 ± 0.03 μm
(second SLES- CAPB treatment). Essentially, the thickness
of the layer is only a fraction of a micron— on the order of
tens of nanometers— and is extremely difficult to measure
with accuracy using this technique. In the sections below,
we discuss polymeric treatments, which yield more repro-
ducible data.
FIGURE 6 Zeta potential profile
of hair treated with polyquaternium- 28
as compared to behentrimonium
chloride
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9
MCMULLEN Et aL.
Hair treated with a conventional
cationic copolymer
The zeta potential plot for polyquaternium- 28 is plotted
alongside behentrimonium chloride in Figure 6. Often
times we observe differences in the zeta potential of the un-
treated hair plug. This could be attributed to the packing of
the fibres in the streaming potential cell. Even when there
are differences between untreated hair samples in the first
rinse cycle, comparable values of zeta potential are found
after the first treatment cycle with a cationic surfactant or
cationic polymer. In Figure 6, there is a clear distinction
after the first treatment cycle with the cationic species. The
zeta potential for behentrimonium chloride is greater in
magnitude than that measured for polyquaternium- 28. This
is not surprising since we would expect the cationic sur-
factant to have higher charge density over a given region of
the hair surface than the polymer, which has an 80:20 mon-
omer ratio (w/w) of VP to MAPTAC. Also notable in the
zeta potential plots is the difference between slopes during
the rinsing stage after the first treatment with the cationic
species. The slope is much greater for hair treated with be-
hentrimonium chloride than with polyquaternium- 28. This
seems to suggest that the cationic surfactant molecules form
multiple layers on the hair surface— many without forming
an electrostatic bond— and are probably removed during
the rinsing cycle. In the case of polyquaternium- 28, it ap-
pears that a greater percentage of the polymer that initially
deposits on the surface of hair remains there throughout the
rinse cycle.
FIGURE 7 Zeta potential profile
of hair treated with low CD- high
viscosity cationic guar as compared to
polyquaternium- 28
FIGURE 8 Zeta potential profile of
hair treated with low CD- high viscosity
and high CD- high viscosity cationic
guar
10
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INTERACTIONS OF CATIONIC GUAR WITH HAIR
There is a clear distinction between behentrimonium chlo-
ride and polyquaternium- 28 when the hair is subsequently
treated with SLES- CAPB (see the second rinse cycle in Figure
6 between 40 and 55min). The zeta potential of the polymer-
treated hair decreases, but not nearly as much as hair treated
with the cationic surfactant. Possibly, there is greater charge
density on the surface of behentrimonium chloride- treated
hair facilitating greater interaction with SLES. However, after
15 min of rinsing, the zeta potential of cationic surfactant-
treated hair almost reaches levels of polyquaternium- 28- treated
hair. A possible explanation could be that the cationic polymer
forms a coacervate (in situ) with the anionic surfactant, which
could prevent excessive SLES molecules from penetrating into
the slipping plan where the zeta potential is recorded.
After the second treatment cycle with the cationic species,
there is another rinsing step (60- 75 min) where the deposi-
tion behaviour of both ingredients changes relative to the first
treatment. We typically observe this behaviour with all types
of cationic species used to treat hair. It demonstrates that in
the case of polyquaternium- 28- treated hair there is no excess
of polymer that is rinsed from the surface. Nearly all of the
polymer that electrostatically interacts with the hair surface
during the treatment phase remains throughout the rinse cycle.
In the case of behentrimonium chloride, there is an excess at
the beginning of the rinse cycle that is progressively removed
during rinsing, although to a lesser extent than was the case
for the first treatment cycle rinsing (20- 35min). The rinse
cycle (80- 95 min) after the second SLES- CAPB treatment
shows results similar to that found after the first SLES- CAPB
treatment, although with slightly higher magnitude of positive
charge. Unlike behentrimonium chloride- treated hair, SLES
is more easily rinsed from the surface of polyquaternium- 28-
treated hair. A plateau is not reached during the SLES- CAPB
rinse cycle for behentrimonium chloride- treated hair as there
may be greater quantities of SLES counterions due to the
greater charge density on the surface of hair of behentrimo-
nium chloride relative to polyquaternium- 28.
The thickness of a deposited layer of polyquaternium- 28
was determined to be: 0.39 ± 0.01μm (first treatment); 0.13
± 0.14 μm (first SLES- CAPB treatment); 0.81 ± 0.11 μm
(second treatment); and 0.42 ± 0.01μm (second SLES- CAPB
treatment). After the first and second treatment cycle, the thick-
ness of the polymeric film is approximately in the range of the
cuticle step height (ca. 0.50μm). Interestingly, the thickness
decreases after shampooing, which suggests that initially after
treatment there are multiple layers of polymer on the hair—
polymer close to the surface adsorbed to the hair and polymer
above adsorbed to underlying polymer. After shampooing and
FIGURE 9 Zeta potential profile of
hair treated with high CD- high viscosity
and high CD- very low viscosity cationic
guar
TABLE 2 Calculated thickness of the adsorbed polymer layer according to Equation 2. Data are provided in μm
Cationic guar
Treatment 1
SLES- CAPB
Treatment 1
Cationic guar
Treatment 2
SLES- CAPB
Treatment 2
High CD- high viscosity 6.01 ± 1.30 2.97 ± 0.76 8.39 ± 2.17 5.35 ± 1.57
Medium CD- high viscosity 8.93 ± 0.89 4.50 ± 0.59 11.24 ± 1.00 7.75 ± 1.00
Low CD- high viscosity 7.00 ± 0.86 4.75 ± 0.79 9.24 ± 1.30 6.88 ± 0.89
Medium CD- low viscosity 1.02 ± 0.05 1.28 ± 0.53 1.88 ± 0.38 1.96 ± 0.78
High CD- very low viscosity 0.00 ± 0.00 0.18 ± 0.13 0.12 ± 0.10 0.21 ± 0.16
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11
MCMULLEN Et aL.
rinsing, much of the polymer adsorbed to polymer is removed
with only a thin layer remaining on the surface of hair.
Hair treated with cationic guar
The zeta potential curve for hair treated with a cationic guar
variant (low CD- high viscosity) is provided in Figure 7. For
comparison, the corresponding curve for polyquaternium- 28
is plotted alongside it. During the rinse cycle (20- 35min)
after the first treatment with the cationic polymers, it is ap-
parent that there are greater values of zeta potential and a
larger decreasing slope for polyquaternium- 28 as compared
to cationic guar. The difference in magnitude of zeta poten-
tial is more than likely due to the lower charge per unit area
on the surface of the hair for this particular cationic guar
variant, which is classified as low CD- high viscosity. The
values for zeta potential for cationic guar are nearly flat after
treatment and during rinsing. Such behaviour could sug-
gest that the colloidal particles of cationic guar are larger
than polyquaternium- 28. More than likely, cationic guar has
stronger intermolecular forces resulting in the deposition of a
larger colloid on the surface. Very little or no excess guar is
removed during rinsing. This phenomenon is similar to that
already touched upon in the previous section when compar-
ing behentrimonium chloride and polyquaternium- 28. In this
case, there may be some excess polyquaternium- 28, relative
to cationic guar, that is on or near the surface but is not elec-
trostatically bound and can be removed through the rinsing
step.
Similar behaviour is observed for the two polymers
during the rinsing cycle (40- 55 min) after treatment with
SLES- CAPB. The zeta potential is lower (more negative)
for polyquaternium- 28, which could be due to the excess
presence of negatively charged SLES counterions. The sec-
ond treatment and subsequent rinse cycle (60- 75s) is very
similar to the first one for cationic guar. Curiously, after both
treatments there is a slight increase in zeta potential. This
behaviour is very common in some of the tested cationic
guar variants.
Influence of CD and MW on cationic
guar deposition
To better understand the deposition behaviour of cationic
guar on hair, we generated streaming potential data for sev-
eral variants with distinct CD and MW (characterized by
viscosity). Figure 8 contains a comparison of low and high
charge density variants with comparable molecular weight
characteristics. As anticipated, treatment with high CD- high
viscosity cationic guar left the hair surface more positively
charged than treatment with the low CD- high viscosity vari-
ant. This behaviour is observed after both rinsing cycles after
treatment with cationic guar. In the case of these two cationic
guar variants, we do not find a large difference between them
after SLES- CAPB treatment. This may suggest that SLES
forms a coacervate with cationic guar, which would mini-
mize the influence of anionic charge during its deposition on
the hair surface.
The effect of MW, or viscosity, on the deposition char-
acteristics of cationic guar are provided in Figure 9. High
CD- high viscosity cationic guar was compared to high CD-
very low viscosity cationic guar. There is a clear distinction
FIGURE 10 Combing work data
(integrated window regions) obtained from
wet combing curves for hair treated with
various cationic guar variants. The number
reported on each bar pair corresponds to the
difference between treated and untreated
hair for that particular treatment
12
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INTERACTIONS OF CATIONIC GUAR WITH HAIR
between the two variants. In addition to the difference in zeta
potential, the rinse cycle after treatment (16- 20min and 60-
75min) for the high CD- very low viscosity variant seems to
behave similar to a linear, flexible chain synthetic polymer.
The greatest difference between the two species when com-
paring the zeta potential plots after SLES- CAPB treatment
is the magnitude of charge on the surface. The zeta poten-
tial profile of the high CD- very low viscosity cationic guar
derivative is very similar to behentrimonium chloride after
SLES- CAPB treatment (Figure 6).
In general, we find that the zeta potential after treatment
with anionic surfactants (SLES- CAPB) of hair pre- treated
with a cationic polymer is greater than hair pre- treated with a
cationic surfactant. Cationic surfactants are removed from the
surface of hair after repeated shampooing. On the other hand,
once a cationic polymer (polysaccharide or flexible chain)
adsorbs to the surface of hair it is not easily removed by an-
ionic surfactant treatment. This phenomenon stems from two
principles regarding polymer adsorption. First, there is an
increase in entropy when a polyelectrolyte is adsorbed on a
surface from solution due to the release of counterions in the
solution from the polyelectrolyte and surface [25]. Therefore,
one would expect a greater entropic contribution with in-
creasing charge density of the polymer. Second, in order to
remove a cationic polymer from the surface, all of the elec-
trostatic bonds between the polymer and hair would have to
be broken simultaneously [11]. In addition, if a coacervate is
formed with cationic guar during the SLES- CAPB treatment
cycle, this would facilitate polymer deposition to an even
greater extent.
Examination of the thickness of the deposited layers on
the hair surface for cationic guar derivatives yielded inter-
esting results. Table 2 contains the deposited thin film thick-
ness data after the first and second treatments with cationic
guar followed by SLES- CAPB. Immediately apparent is the
dependence of the adsorbed polymer layer on the viscosity
(ie MW) of the cationic guar variant. The higher viscosity
samples leave considerably thicker films on the hair surface.
When comparing the three high viscosity samples, there is
no apparent dependence on CD. For comparison, the low and
very low viscosity cationic guar derivatives result in much
thinner deposition layers, approaching levels anticipated for
a flexible polymer. In the case of the high viscosity samples,
subsequent treatment with SLES- CAPB appears to reduce
the thickness of adsorbed polymer layer. However, in the case
of the low and very low viscosity samples the thickness stays
the same after SLES- CAPB treatment. In the case of most
polymers we have investigated by the streaming potential
technique, there is normally a slight increase in the thickness
of the deposited layer after the second treatment, but this nor-
mally reaches a plateau after several treatments.
Mechanical wet combing analysis of hair
treated with cationic guar derivatives
One of the most common techniques to measure the sur-
face properties of hair is wet combing analysis. The forces
encountered during wet combing— as opposed to dry
combing— are thought to arise from swelling of the fibres
and increased adhesion between the fibres, which both
would impede the motion of a comb through a hair fibre
assembly [28]. Most conditioning agents reduce wet comb-
ing forces. In part, the surface tension properties of a treat-
ment are believed to contribute to the level of wet combing
forces [29].
To better understand the effects of cationic guar
MW and CD on the conditioning properties of hair, we
FIGURE 11 Zeta potential profile
for European dark brown and Mulatto hair
treated with low CD- high viscosity cationic
guar
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13
MCMULLEN Et aL.
conducted wet combing analysis on fine hair bleached in
two regions of the tress. The use of fine hair allowed us
to minimize the amount of error usually associated with
wet combing measurements. Measurements of untreated,
bleached hair were conducted first, followed by treatment
of the entire tress with cationic guar, and then additional
combing measurements. In the analysis, the two regions
of the combing curves corresponding to the window re-
gions (where bleaching was administered) were integrated
before and after treatment. Average values were obtained
by measuring three tresses twice. Again, this special tech-
nique (fine hair bleached in the windows region) was cho-
sen due to its ability to discern small differences between
treatments. Therefore, it should be noted that the stream-
ing potential data presented throughout this report were
collected using virgin dark brown hair. We investigated
the effects of bleaching and subsequent treatment with
cationic guar on the streaming potential profiles (data not
shown) and only found differences in the magnitude of the
various parameters as compared to virgin hair.
Figure 10 contains a graph of the combing work values
reported in gf⋅cm for a number of cationic guar variants.
The order of sequence in the bar chart corresponds to the
greatest (left side) to least (right side) difference between
untreated bleached hair and bleached hair treated with the
respective cationic guar variant. The actual difference in
combing work is provided on the chart and ranges from
0.0138 to 0.0695 gf⋅cm. Overall, it appears that there is a
trend demonstrating that CD plays a greater role than MW
at determining the combing reduction properties of cationic
guar.
Behaviour of cationic guar on different
types of hair
The interaction of cationic guar with hair was investigated
for various hair types including African, Chinese, European
dark brown, European 65% grey, Indian and Mulatto hair.
In previous work, it was reported that the amino acid
composition is the same for all hair types [30]. Further,
a recent study carried out with an ethnically diverse pool
of individuals from South Africa detailed the proteomic
evaluation of hair and did not find racial differences in
the quantitative evaluation of keratins, keratin- associated
proteins, histone proteins and desmosomes [31]. Despite
these findings, data by several groups, including our own,
show that the lipid content of African hair is higher than
for other hair types [32]. Asian hair has the most circu-
lar cross section while African hair is the most elliptical.
In addition, African hair has a lower radial swelling rate
than Asian and Caucasian hair, which could be attributed
to its higher lipid content. In terms of mechanical (tensile)
properties, it is generally found that the break stress and
elongation at break is lower in African hair than Asian and
Caucasian hair. Most of the investigated differences in hair
types from various racial origin focus on the bulk proper-
ties of the fibre imparted by the cortex. With the exception
of lipid analysis, differences in the outermost layers of the
cuticle have yet to be reported. Nevertheless, we investi-
gated the deposition properties of cationic guar on various
types of hair.
Figure 11 contains a zeta potential plot comparing
European dark brown and Mulatto hair treated with low
FIGURE 12 Conductivity profiles
of hair treated with various cationic guar
derivatives
14
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INTERACTIONS OF CATIONIC GUAR WITH HAIR
CD- high viscosity cationic guar. At first glance, both hair
types appear to behave similarly when treated with the cat-
ionic guar variant. However, there are subtle differences in
the zeta potential profiles that are very reproducible from
one experiment to another. For example, there is flatter pla-
teau at the beginning of both rinse steps (20- 35min and 60-
75min) right after treatment with cationic guar for Mulatto
as compared to European dark brown hair. After making
this observation on numerous occasions, we came to the
conclusion that Mulatto hair has a more open structure al-
lowing cationic guar to more quickly bind to the surface.
Likewise, the zeta potential values at the beginning of the
rinsing cycle after treatment with SLES- CAPB demonstrate
that there is a greater negative charge present in the case of
European dark brown hair. In this case, it probably indicates
that SLES- CAPB rinses from Mulatto with greater ease than
from European dark brown hair due to its more open struc-
ture. In fact, we made the same observation for African and
European 65% grey hair. Overall, our findings suggest that
African, European 65% grey, and Mulatto hair might more
quickly desorb SLES- CAPB than Chinese, European dark
brown and Indian hair. This might be a structural attribute
of the different hair types. For example, African, European
65% grey and Mulatto hair may have a more open structure
that allows for quicker sorption and desorption (or diffusion
into or out of the fibre) of ingredients.
Conductivity and pH of the streaming
potential solution
Thus far, we have shown data of the zeta potential of hair
treated with anionic surfactants, cationic surfactants and
cationic polymers with various degrees of MW and CD. The
determination of zeta potential of hair is dependent on the
streaming potential solution conductivity. The pH also af-
fects zeta potential since changes in pH can alter the surface
chemistry of hair— protonating or deprotonating pendant and
terminal acid and amide groups of the protein amino acids—
thereby influencing the streaming potential values. As noted
in the experimental section of this report, we monitor pH,
conductivity, streaming potential, temperature and flow rate
during each experiment. At very low concentrations, one
might expect the KCl electrolyte solution to be very sensi-
tive to small changes in pH or conductivity caused by adding
surfactants or polymers to the streaming potential cell.
Figure 12 contains a plot of streaming potential solution
conductivity as a function of the duration of the experiment.
At the far left of the plot, there is a section labelled as un-
treated. This is the rinse stage of pre- washed untreated hair.
The conductivity of the streaming potential solution after
passing through the various hair plugs of untreated hair (1-
15 min) is reproducible from experiment to experiment.
Treatment with cationic guar is administered between 16 and
20min. After treatment (16- 20min), the conductivity begins
to decrease until it plateaus between 2.7 and 5.5 µS/cm for
the different cationic guar variants. When the surfactant solu-
tion of SLES- CAPB is introduced to the system (36- 40min),
there is a spike in the conductivity. However, there is a pla-
teau of the conductivity after thorough rinsing (41- 55min).
As a result of the second cycle of cationic guar treatment
(56- 60min), there is another small peak in the conductivity
curves, which again seems to level off during the rinse cycle
(61- 75min). Finally, a second SLES- CAPB treatment is ad-
ministered (76- 80min), which causes an increase in the con-
ductivity for all cationic guar variants, which is slightly higher
FIGURE 13 pH profile of hair treated
with high CD- very low viscosity cationic
guar
|
15
MCMULLEN Et aL.
than that observed in the first cycle. During the final rinse
stage (81- 95 min), the conductivity appears to plateau for
nearly all of the samples, except one of the high CD variants.
Our results suggest that the choice of cationic guar species
used to treat the hair influence the measured conductivity.
The conductivity of an electrolyte solution is a measure of
its ability to conduct electricity due to the ion concentration.
To some extent, the number of ions present in solution is
proportional to the amount of charge transferred between the
electrodes of a conductivity meter. It should be noted that not
all ions conduct electricity to the same degree. For example,
ions that move slower through solution do not conduct elec-
tricity as well as ions that have greater speed and mobility.
Comparing the different cationic guar variants in Figure
12, the MW (viscosity) and CD appear to affect the conductiv-
ity. Treatment of hair with the higher molecular weight species
results in conductivity values greater than lower molecular
variants. Within the higher MW samples (low CD- high vis-
cosity, medium CD- high viscosity and high CD- high viscos-
ity), treatment of hair with lower CD cationic guar results in
higher conductivity readings. It could be that the lower CD of
the macroion results in less binding of the ions from the elec-
trolyte solution with the polymer- coated hair fibre. Therefore,
more of the ions from the streaming potential solution would
be free and detectable during the conductivity measurements.
The same trend is observed when comparing the lower MW
species (medium CD- low viscosity and medium CD- very
low viscosity). As for the differences between high and low
MW cationic guar treatment, ions from the streaming poten-
tial solution may be able to move more freely and access the
charge sites of the lower molecular weight analogs. Again,
this might leave less free ions in the streaming potential
solution, and therefore, decrease the conductivity. Likewise,
conductivity measurements of behentrimonium chloride and
polyquaternium- 28- treated hair resulted in similar fluctua-
tions, although the recovery time is slightly greater probably
due to their higher CDs than the cationic guar variants.
The pH profile for the streaming potential solution after
treatment of hair with high CD- very low viscosity cationic
guar is provided in Figure 13. Essentially, treatment with all
of the cationic guar variants resulted in very similar pH pro-
files. The pH of the streaming potential solution after pass-
ing through the streaming potential cell for untreated hair is
generally pH 5.5. There is a small change in pH after treat-
ment with guar. In some cases, there is a slight increase while
in others a small decrease is observed. The pH change due
to cationic guar treatment only fluctuates during the online
treatment cycle (ie at 21- 35min and 56- 60min) and returns
to normal during the rinsing stage. Similarly, treatment with
SLES- CAPB (ie 36- 40 and 76- 80min) causes the pH to rise
to just below pH 6.5, but only during the treatment stage. As
in the case with the cationic guar treatment, the pH returns to
a normal steady value during the rinsing step. Similar trends
in pH were observed for treatments with behentrimonium
chloride and polyquaternium- 28.
CONCLUSIONS
In this study, we present data on the electrokinetics of the
deposition of cationic guar variants on human hair. The
deposition behaviour is controlled by CD and MW of the
sample. CD has a direct effect on the measured zeta po-
tential of the treated fibres. On the other hand, MW has
greater influence on the thickness of the deposited thin
film. Mechanical wet combing experiments of hair tresses
demonstrated that CD actually plays the dominant role in
determining the amount of combing force reduction of hair
fibre assemblies in the form of a tress. We also examined
the behaviour of cationic guar and SLES- CAPB on various
types of hair. We found that curly (African and mulatto)
and European 65% grey hair behaved differently as com-
pared to Chinese, European dark brown and Indian hair.
For example, SLES- CAPB desorbs more quickly from
curly and grey hair than the other types we tested, which
may suggest they have a more open structure and morphol-
ogy, at least at the cuticle level.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the technical contribu-
tions of Drs. Janusz Jachowicz and Marek Zielinski in the
design and construction of the streaming potential instru-
ment. Also, much gratitude goes to Dr. Michael Franzke
and Ms. Lidia Kulcsar from Ashland for critically reading
the text and offering their insight. The authors would like
to acknowledge the financial support provided by Ashland
Specialty Ingredients, G.P. This article is dedicated to our
coauthor Donna Laura, one of the most meticulous scientists
with whom we have had the pleasure to collaborate.
ORCID
Roger L. McMullen https://orcid.
org/0000-0002-0084-1753
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How to cite this article: McMullen RL, Laura D,
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