International Journal of Cosmetic Science, 2006, 28, 95–101
Charge density alterations in human hair fibers: an
investigation using electrostatic force microscopy
V. M. Longo*, V. F. Monteiro*, A. S. Pinheiro, D. Terci, J. S. Vasconcelos*, C. A. Paskocimas,
E. R. Leite*, E. Longo* and J. A Varela§
*CMDMC/LIEC/DQ/DEMA/UFSCar, Universidade Federal de São Carlos, São Carlos, SP 13565-905, Brazil,
Kosmoscience Ciência & Tecnologia Cosmética LTDA, Valinhos, SP 13273-125, Brazil, UFRN, Universidade Federal
do Rio Grande do Norte, Departamento de Engenharia Mecânica, Natal, RN 59 072-970, Brazil, and §Universidade
Estadual Paulista, Instituto de Quı́mica, Araraquara, SP 14801-907, Brazil
Received 1 March 2005, Accepted 15 June 2005
Keywords: atomic force microscopy, charge, cosmetic, hair, microscopy, polymer, surface
atomique (AFM) a eté utilisée simultanément pour
Synopsis
évaluer les changements de morphologie, de rugosité
A new method for high-resolution analyses of hair et d’épaisseur des cuticules. Les images EFM nous
surface charge density under ambient conditions is ont permis de visualiser la dègradation et la distribu-
presented in this paper. Electrostatic force microsco- tion du polymère sur la superficie des cheveux. Grâce
py (EFM) is used here to analyze changes in surface à la puissance analytique de l’image EFM, les diffé-
charge density in virgin hair, bleached hair, and rents degrés d’altération des fibres capilaires avec le
hair treated with a cationic polymer. The atomic polymère ont pu être observés. Les images de la den-
force microscopy technique is used concomitantly sité de charge superficielle et de la distribution du po-
to analyze morphological changes in hair roughness lymère sur la surface des cheveux ont été présentées
and thickness. The EFM images depict exactly how à la lumière de l’EFM.
the polymer is distributed on the surface of the hair
fiber. The EFM’s powerful analytical tools enabled
Introduction
us to evaluate the varying degrees of interaction
between the hair fiber surface charge density and Constant investigations for the technological devel-
the cationic polymer. The surface charge density opment of hair care products that surpass the con-
and the polymer’s distribution in the hair fibers are sumer’s expectations have long represented a
presented in the light of EFM measurements. challenge for researchers and scientists. Many
investigations have focused on developing effective
cosmetic products for human hair, and scientists
Résumé
have employed a variety of methods in their
Ce travail présente une nouvelle méthodologie pour attempts to understand the alterations and prove
analiser la densité de charge superficielle à tempéra- the cosmetic efficacy of these products [1].
ture ambiante avec une grande resolution. La micr- Human hair is structured in highly organized
oscopie à force électrostatique (EFM) a eté utilisée strata that are highly resistant to external stimuli.
pour mesurer la densité de charge supeficielle des However, morphological changes can occur
cheveux vierges, décolorés et traités avec un polymer through daily hair care routines [2]. Hair damage
cationique. La technique de microscopie à force leads to a change in the physical properties and
modifications in the surface charges of hair.
Correspondence: V. M. Longo, CMDMC/LIEC/DQ/DEMA/ Mechanical and electromechanical stresses
UFSCar, Universidade Federal de São Carlos, São Carlos,
induce the formation of submicrocavities within
SP 13565-905, Brazil. Tel./fax: +55 16 3361 5215;
e-mail: valerialongo@liec.ufscar.br polymer materials [3]. Electrons can then move
ª 2006 Society of Cosmetic Scientists and the Société Française de Cosmétologie 95
Charge density alterations in human hair fibers V. M. Longo et al.
without scattering within the submicrocavities, An up-to-date text on dielectrics emphasizes a
leading to further degradation. This raises the pos- fourth component of the electrical polarization of a
sibility of the formation of significant amounts of dielectric or insulator. This component is interfa-
carbon and oxygen anions and cations following cial polarization, the result of a local accumulation
polymer stressing, which would then be trapped in of charges resulting from the migration phenom-
the dielectric [4]. ena and concentrating around imperfections such
Cationic ingredients in general are highly sub- as impurities, defects, grain boundary, cuticle
stantive to hair because of its low isoelectric point, boundary, and others [13].
which is approximately pH ¼ 3.67 in cosmetically An analysis by EFM was performed to detect
unaltered hair, and even lower in bleached hair. electrostatic forces. Differences induced by electro-
Therefore, at any pH above the isoelectric, the sur- static forces on the surface of hair fibers during
face of hair bears a net negative charge and posi- scanning are detected and give a qualitative
tively charged (cationic) ingredients are attracted to measurement of the local charge density ‘J.S.
it [5]. Most often, the active component of the con- Vasconcelos et al., unpublished data’.
ditioning formulation is a polymeric (such as poly- When a voltage is applied between the tip and
quaternium 6) or monomeric (such as trimethyl the sample, an interaction occurs between the
ammonium bromide, CETAB) cationic quaternary charges and energy variations in the sample’s
compound, which has a great affinity for the negat- stored capacitances. The force the hair fiber is sub-
ively charged hair fiber surface. These conditioners jected to under an applied dc bias is due to the
adsorb strongly by electrovalent interaction with charge–charge interaction, and changes in capa-
sulfonic acid groups on the hair surface. Interaction city energy, which is given by:
between the conditioner and the keratin fiber can
q q 1 dC
lead to different degrees of interaction on the surface s tForce ¼ þ ðVapplied  V 2contactÞ
4" z2 2 dz
of the hair fiber [6]. Jachowicz et al. [7] studied the 0
generation of static charge on intact and modified were qs is the surface charge, qt is the charge induced
human hair fibers in the rubbing mode, using a on the tip, z is the tip–sample separation, C is the
variety of metal and polymer contact probes. capacitance between the tip and the sample, Vapplied
Andre et al. [8] studied the adsorption of condi- is the applied voltage, and Vcontact is the contact
tioning polymers employing atomic force microsco- voltage ‘J.S. Vasconcelos et al., unpublished data’.
py (AFM) to characterize the polymer/sufactant In the present work, we evaluate the surface
complex. Pfau et al. [9] used the AFM to charac- charge density of virgin hair and bleached hair
terize the adsorption and desorption behavior and measured by EFM. AFM is used simultaneously to
morphology of a set of polyquaternium polymers study the topography of the fiber’s surface. In the
on human hair. Results were in line with a simple same way, we evaluate the different degrees of
model of coulombic interaction between hair and charge density interaction of the cationic polymer
polymer and were interpreted on this basis. (PQ-6) on the hair fiber surface.
Electrostatic phenomena in insulators have been
known for the past four centuries, but many rela-
Experimental
ted questions are still unanswered, for instance:
what is the distribution of electric potentials across
Materials
an organic polymer or ionic non-conducting
material, and how does it contribute to the Untreated black Caucasian hair was obtained from
mechanical, optical, adhesion, and electrically De Meo Brothers, New York, USA. Hair tresses
insulating properties of the solid? [10]. weighing approximately 1.0 g and a length of
A new possibility to address these questions was 25.0 cm each were prepared for this work. The hair
created recently, thanks to advances in analytical samples were pretreated with 3% lauryl ammonium
electron microscopy [11]. The electrostatic force sulfate (LAS) at 25C and then air-dried.
microscope (EFM), for instance, maps the variation
and potential energy difference between a tip and
Bleaching
a sample arising from non-uniform charge distri-
butions and local variations in surface work func- The bleach solution was prepared by mixing a
tion [12]. 6% hydrogen peroxide (H2O2) solution with a
96 ª 2006 International Journal of Cosmetic Science, 28, 95–101
Charge density alterations in human hair fibers V. M. Longo et al.
concentrated ammonium hydroxide solution and voltage was applied to the tip and the hair fibers
ammonium persulfate powder in a 2:1:1 weight were grounded. The tip–sample separation, z, was
proportion, respectively. The tresses were left in set at 25 nm. The EFM and AFM measurements
the bleach solution for 30 min at 25C. After were taken simultaneously.
bleaching, the tresses were rinsed in abundant
distilled water, washed out with a solution of 3%
Statistical treatment
LAS, and air-dried. This treatment turned the
tresses light brown with a reddish tone. The roughness and thickness values of the cuticle
layers were treated statistically by means of the
Prism 2.01 software (GraphPad Prism version
Treatment
4.00 for Windows, GraphPad Software, San Diego,
The virgin and bleached hair tresses were washed CA, USA), using Tukey’s multiple comparison test.
five times with a solution containing 3% of PQ-6. The symbol P indicates the probability that one
Figure 1 shows the structure of the monomer of hair is statistically equal to or different from
the PQ-6 polymer. The PQ-6 polymer is a homo- another hair. One minus P represents the level of
polymer with an average molecular weight of confidence that the sets of hair were really different.
approximately 100 000 and a charge density of These data confirmed the varying level of confi-
approximately 126. dence that the sets of hair were really different [1].
Two hundred microliters of the solution were
left for 30 s in contact with the hair fibers, which
Results and discussion
were then rinsed in distilled water for 1 min and
air-dried at 25C. This procedure corresponded to Figure 2 shows the AFM and EFM images of virgin
one washing. hair, and virgin hair treated with PQ-6. Note that
the virgin hair (Fig. 2A) has uniform, regular-
shaped cuticle layers oriented longitudinally and
Atomic force microscopy
aligned along the length of the fiber.
Atomic force microscopy studies were performed The EFM images show negative charges varying
using a Digital III-Instrument under atmospheric from magenta (more negative) to red (less negat-
conditions at room temperature. Imaging was ive), and positive charges varying from green
obtained in the contact mode, using silicon nitride (more positive) to yellow (less positive). In this
profile tips mounted in a cantilever at a 3.6 nN instance, the bias was switched to 0.0 V. Fig-
force constant. The AFM images were processed ure 2(B) presents the EFM image of a virgin hair
with Nanoscope software (Digital Instruments, with an overall homogeneous positive and negat-
Inc., Santa Barbara, CA, USA), which was also ive charge distribution on the surface and a slight
used to measure the roughness and thickness of accumulation of negative charges at cuticle edges.
the cuticle layers. These charges were attributed to the structures of
the hair’s amino acids. Cuticle edges are more sus-
ceptible to the accumulation of charges because
Electrostatic force microscopy
they are interfacial regions and therefore prone to
The measurements by electrostatic force micro- concentrate defects.
scopy were taken as function of tip voltage and Figure 2(C) shows the AFM images of virgin hair
characterized by the contrast of the EFM signal, treated with PQ-6, revealing a morphology similar
which is altered with variations in the tip bias. to that of Fig. 2(A), although the roughness of the
The EFM signal is obtained by changes in the fiber is reduced, as shown in Fig. 3. This reduction
resonant frequency in the oscillating tip. Bias may be attributed to the deposition of polymer on
the surface of the hair fibers. The mean roughness,
Rm, is defined as the arithmetical average of the
* *
– absolute values of the surface deviations, measured
N+ Cl from the mean plane within the box cursor [1]. The
H3C CH3 hair’s surface roughness revealed in the variousn
AFM images was calculated for three regions, using
Figure 1 Polymer PQ-6. AFM’s Nanoscope software, which was also used
ª 2006 International Journal of Cosmetic Science, 28, 95–101 97
Charge density alterations in human hair fibers V. M. Longo et al.
A B
15.0 15.0
10.0 10.0
5.0 5.0
0 0
0 5.0 10.0 15.0 0 5.0 10.0 15.0
µm µm
C D
15.0 15.0
10.0 10.0
5.0 5.0
Figure 2 AFM images of virgin
0 0 hair (A, B) and EFM images of vir-
0 5.0 10.0 15.0 0 5.0 10.0 15.0
µm µm gin hair treated with PQ-6 (C, D).
Roughness  (nm) required to define how the deposition of the polymer
70
53
60 on the hair’s surface occurs, i.e. does the polymer
38
50 become homogeneously distributed, forming a poly-
32
40
23 meric film on the hair’s surface, or is the deposition
30
20 site-specific?
10 The EFM image in Fig. 2(D) depicts the hair
0 fiber treated with polymer and may provide an
Virgin Virgin + polymer Bleached Bleached +
polymer answer to this question. As can be seen, the image
differs significantly from Fig. 2(B). The deposition
Figure 3 EFM images of cuticle surface roughness of
of the polymer on the hair’s surface completely
hair subjected to different treatments. Tukey’s multiple
altered the charge distribution in the fiber. This
comparison test: virgin vs. virgin + polymer (P < 0.001);
bleached vs. bleached + polymer (P < 0.001). image reveals two very distinct regions: at the
cuticle edges there is an accumulation of negative
charges (magenta), whereas the cuticle surface
to evaluate cuticle thinning based on cross-sectional shows an accumulation of positives charges
analysis. Three cross-sections of each region were (green). The accumulation of negative and positive
prepared for analysis, and revealed in different pro- charges indicates the presence of effective barriers
files according to the various treatments. The cuticle in the material ‘J.S. Vasconcelos et al., unpub-
thickness values were calculated by measuring the lished data’. The commercial polymer PQ-6 is cati-
maximum peak distance in each cross-section of the onic and interacts with the anionic regions of the
different profiles (Fig. 4). The procedure of cross-sec- hair fibers, appearing green in the EFM image. The
tioning is given in Fig. 5. The lesser thickness of vir- regions where cationic polymer was not deposited
gin hair treated with polymer was likely evidence of appear to be negatively charged. This finding leads
the deposition of a polymeric film on the surface of to the assumption that, in the regions where poly-
the treated hair. However, further information is mer was not deposited, a migration of charges
98 ª 2006 International Journal of Cosmetic Science, 28, 95–101
Charge density alterations in human hair fibers V. M. Longo et al.
Thickness cuticle (nm)
400 topography and thickness of virgin hair (close to
328 330 nm, Fig. 4).
300
214 Figure 2(D) depicts exactly how the polymer is
176 distributed on the surface of the hair fiber, clearly
200
137 indicating that, on the surface of the cuticles, the
100 polymer forms a discontinuous and positively
charged film.
0 Figure 6 shows AFM and EFM images of
Virgin Virgin + polymer Bleached Bleached +
polymer bleached hair, and bleached hair treated with PQ-
6. The morphology revealed in the AFM images
Figure 4 AFM images of cuticle thickness of hair fibers
indicates that the cuticle layer in the bleached
subjected to different treatments. Tukey’s multiple com-
hair (Fig. 6A) underwent a process of chipping,
parison test: virgin vs. virgin + polymer (P < 0.001);
extraction and erosion, resulting in dryness and
bleached vs. bleached + polymer (P < 0.001).
greater susceptibility to further damaging action,
which may involve large segments or sections of
occurred due to the intensification of negative scales being ripped from the hair. The surface
charges in those regions. Those regions appear to roughness (Rm), which was calculated as des-
be the cuticle edges. The explanation for this cribed earlier, increased with the bleaching pro-
apparent discrepancy, as the cuticle edges have a cess, as demonstrated in Fig. 3. The cuticle
tendency to concentrate negatively charged defects thickness evaluation results showed lower values
and should therefore interact preferentially with for bleached than for virgin hair. This observation
the cationic polymer, is that there may be a phys- is consistent with the destruction of the cuticle
ical obstacle to the polymer’s adherence. This structure and layers due to the deterioration of
would make the polymer’s adsorption difficult in protein, which causes reductions in cuticle
the sheath of cuticles edges due to the cuticle’s thickness.
Section analysis
0 10.0 20.0
µM
Spectrum
Figure 5 Proceeding of cross-sec-
tion analysis of AFM images. DC Min
ª 2006 International Journal of Cosmetic Science, 28, 95–101 99
–2500 0 2500
Charge density alterations in human hair fibers V. M. Longo et al.
A B
15.0 15.0
10.0 10.0
5.0 5.0
0 0
0 5.0 10.0 15.0 0 5.0 10.0 15.0
µm µm
C D
15.0 15.0
10.0 10.0
5.0 5.0
Figure 6 AFM images of bleached
hair (A, B) and EFM images of
0 0
0 5.0 10.0 15.0 bleached hair treated with PQ-6 (C,0 5.0 10.0 15.0
µm µm D).
The whole-fiber amino acid composition of lower thickness values (Fig. 4) of treated hair
human hair when bleached is described in the result from this deposition. Again, the AFM images
literature. These data suggest that the primary and cuticle thickness and roughness values do not
chemical differences between extensively bleached provide sufficient information about the distribu-
hair and unaltered hair are lower cystine content, tion of polymer on the fiber.
higher cysteic acid content, and lower amounts of The EFM image (Fig. 6D) indicates a consider-
tyrosine and methionine in bleached hair. These able change in charge distribution, with an almost
findings suggest that the reaction of bleaching total absence of negative charges, due to the inter-
agents with human hair protein occurs primarily action of the cationic polymer with the anionic site
in the disulfide bonds [5]. created in the bleaching process. The recovery of
Figure 6(B) depicts an EFM image of bleached polymer in the fiber is practically complete. The
hair, revealing that the bleached fiber contains a deposition is homogeneous but not continuous,
higher concentration of negative charges at the with a few negative charges distributed punctually
cuticle edges. This indicates that cystine degrada- along the film. Unlike the virgin hair, the bleached
tion occurs preferentially at the cuticle edge, hair cuticle is less thick (close to 175 nm, Fig. 4)
which is an interfacial region. The concentration and shows a considerable accumulation of negat-
of charges in cuticle edges is visual confirmation ive charges at the edges, physically and chemically
of this degradation. The negative charges are ani- facilitating the adhesion of polymer in the cuticle
onic sites on the fiber’s surface and evidence of layers. The penetration of low-molecular-weight
cysteic acid and other products of degradation. components of the cationic conditioning com-
When the bleached hair fibers were treated with pounds into the intercuticular regions leads to
polymer, the general appearance of the cuticle’s plasticization of the cuticular sheath [6]. There-
integrity seemed improved (Fig. 6C) as a result of fore, polymer treatments are more effective in
the deposition of polymer on their surfaces. The degraded hair fibers, which have a greater number
adsorption of polymer in hair fibers reduces their of anionic active sites for interaction, lower cuticle
roughness (Fig. 3) and hence, their porosity. The thickness, and greater roughness.
100 ª 2006 International Journal of Cosmetic Science, 28, 95–101
Charge density alterations in human hair fibers V. M. Longo et al.
Conclusions appearance. III. Generation of light-scattering factors
in hair cuticles and the influence on hair shine.
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