Nawel BAGHDADLI, Françoise FIAT, Gustavo S

NEW ADVANCES IN THE CHEMICAL STRUCTURE
OF THE HAIR SURFACE
Nawel BAGHDADLI, Françoise FIAT, Gustavo S. LUENGO
L’Oréal Research & Innovation, Aulnay-sous-bois, France
INTRODUCTION
Human Hair is a complex and unique physico-chemical composite, with well characterized microstructure. The cuticle is formed with cuticle cells, in from of flat square
sheets 0.5 µm thick and ~ 50 µm in length. It is formed from keratin proteins, characterized by a high content of cystine, and amino acid rich on disulfide bonds, also
in peptide bonds and the abundant COOH and NH2 groups 1. The cuticle of native hair contain an important lipid called 18-methyleicosanoic acid (18-MEA), which is
covalently attached to the protein via thioester bonds to the cysteine residues 2,3. This surface can be affected by chemical treatments and daily-life routines with a direct
impact on cuticle quality, by oxidizing the proteins and removing the 18-MEA layer 4.
In this study we investigated hair surface modifications, before and after a damaging process, with High resolution microscopy techniques such as : scanning (SEM) and
transmission (TEM) electron microcopies to describe the morphology and structure and changes. And we used Atomic Force Microscopy (AFM) and a novel technique,
dynamic Chemical Force Microscopy (dCFM), with hydrophobic and ionic sensitivity was applied to obtain chemical information in correlation with topography mapping
on the hair surface with a high lateral resolution. Chemically modified AFM-tips, CH3- and NH2- terminated, were applied to achieve a defined chemical contrast
(hydrophobic and ionic) in aqueous medium. The hydrophobic/hydrophilic distribution on hair before and after treatments was investigated at exactly the same position.
This enabled us to get exact information about the homogeneity of the chemical and physic-chemical changes induced by a bleaching treatment at a local scale. A
comparative FTIR-study identified the active groups on the hair surface was done on the same hair samples.
MATERIALS AND METHODS
Fig. 1
Morphological
degradation on hair
surface, images
obtained from the
analysis of the same
fiber before virgin hair.
From top to down :
SEM, AFM
and TEM images.
HAIR SAMPLES PREPARATION AND LIPID EXTRACTION
Virgin Caucasian hair collected from healthy volunteers and cleaned with a regular shampoo.A
simplex bleaching treatment of hydrogen peroxide solution (H2O2, 30%) was used.
SCANNING (SEM) and TRANSMITION (TEM) ELCTERON MICROSCOPIES OBSERVATIONS
For SEM fibers were coated with platinium and examined on a SEM JEOL 6300 F.
For TEM, samples were fixed and cross ultrathin sections were cuted and contrasted with special buffer.
The grids were observed under a ZEISS CEM 902 at 80kv microscope.
FOURIER TRANSFORMED INFRARED SPECTROSCOPY (FTIR)
Hair bundles were investigated with a FTIR Bruker Vektor 22, investigated area ca. 3x3 mm
and spectra range from 600 to 4500 cm-1.
AFM and dCFM
For standard AFM measurments CPII Research (Veeco) (contact mode -buffer pH 7) was used.
For dCFM, Scanning Force Microscope (SFM) (Veeco, Multimode) was used. Digital pulsed
force controller (Witec GmbH)9,10 was used to control sinusoidal tip modulation and data
acquisition. Tips were chemically modified with gold plated (≈5nm Cr + ≈35nm Au) silicon
tips: to hydrophobic, using self assembly techniques with thiolated molecules SH-(CH2)11-CH3
(Dodecanethiol) and to ionic using SH-(CH2)2-NH2 (Aminoethanethiol) .
RESULTS AND DISCUSSION
HAIR SURFACE MORPHOLOGY MODIFICATION (Fig.1 )
Obvious morphological degradation of the hair surface, was observed at different scales, after the
bleaching process.
SEM images (A/D) shows failure of scales, local lift and loss of cuticle edge. AFM images (B/E)
(10µm x 10µm), shows damages for the same scale after treatment (“puffiness, bubbles”) and edge
(broken). Where, TEM observations (C/F) of hair cuts shows degradations in the cortex area and
important defects on the cuticle layers.
CHEMICAL SURFACE ANALYSIS : HYDROPHOBIC MAPPING WITH dCFM (Fig.2)
Topography (A/B) and the corresponding adhesion (C/D) images measured by dCFMa with
CH3-modified tip shows significant decreasing of surface hydrophobicity after treatment
(loose of 18-MEA). Hair surface becomes hydrophilic, except for some spots in the range of
approximately 200 nm. Analysis of different areas shows that an average adhesion ratio of 1/1,8
between bleached and natural hair. According to this ratio and with assumption that a model
of lipid modified gold surfaces has a similar surface energy to the natural lipid on hair, i.e.≈24
mN/m, the surface energy of bleached hair can be estimated to be approx. 43,6 mN/m.
FOURIER TRANSFORMED INFRARED SPECTROSCOPY (FTIR) (Fig.3).
Relative comparison of oxidation and lipid levels of hair surface investigated at 1040 cm-1 for
SO3 [ν(S=O)sym] and at 2918 cm-1 for CH2 [ν(CH2)asym] shows : increase of SO3 adsorption
after bleaching. This suggests a high level of cysteine acid formation on the bleached hair
surface with an oxidation (SH→SO3) process. The hydrophobic protection layer seems to be
removed as shown with the dCFM hydrophobic mapping and the surface shows SO3 moieties.
Fig. 2 - Virgin (A, C) and Bleached (B,D hair surface measured with a CH3- modified tip with dynamic CFM, (A, B) 2d topography images and (C,D)
2d adhesion images.
Fig. 3 - Relative comparison of the determined
oxidation and lipid levels.
IONIC MAPPINGWITH dCFM (Fig.4)
(NH3+) positively charged tips was used to detect negative (SO3-) groups and their distribution
on the bleached hair. High and local scale adhesive images with important adhesion forces
(green) highly negatively charged density and low interactions areas (red) without charges were
obtained. Chemical mapping of the surface can be done, since molecule requires an area of
~ 0,25 nm². Mean density of cysteine acid on the bleached hair surface can be calculated :
δmean ≈ 2,5 molecules/nm² or if the SO3- group is part of cysteine acid δmean ≈ 707,6 µg/m²
(cysteine acid). AFM high resolution shows two relative homogenous phases. Homogeneity
of the negatively charged regions and the sharp chemical borders suggest that the cysteine
acid molecules (SO3-) in these clustered regions are densely packed similar to a “Self Assembly
Monolayer” (almost 16 molecules/nm2). This leads to the conclusion that the structure of the
hydrophobic lipid layer on the virgin hair surface is densely packed forming a monolayer.
CONCLUSION
We investigated the way how the hydrophobic
protection layer of the natural human hair is
affected by chemical treatment (bleaching)
with high lateral resolution using dynamic
Chemical Force Microscopy (hydrophobic and
ionic dCFM). We conclude that the bleaching
process is removing the hydrophobic top layer,
resulting in a mainly hydrophilic, SO3-group
(cysteine acid) terminating top surface. This indicates that thioester bonds of fatty acids
were removed due to the oxidation. On molecular level our results suggest a clustered
“Self Assembled Monolayer” like distribution of the cysteine acid. The surface energy of the
bleached hair could be estimated to be approximately 44 mN/m. The main information and
results of our study can be represented in a schematic way in figure below.
Fig. 4 - Adhesive image of the ionic
interaction on bleached hair with
the corresponding topography.
Aknowledgements and References
Dr. Michael KORTE, Dr. Sabri AKARI (NANOCRAFT Gmb, Germany) for their experimental support, discussions and advice. Marcelle
HUART, Sophie VICIC and Isabelle PASINI are acknowledged for her valuable technical assistance.
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surface of bleached human hair fibers. Submitted 2014.