Nawel Baghdadli

Adsorption of polyelectrolytes and polyelectrolytes-surfactant mixtures at surfaces: a physico-chemical approach to a cosmetic challenge.

The use of polymer and polymer - surfactant mixtures for designing and developing textile and personal care cosmetic formulations is associated with various physico-chemical aspects, e.g. detergency and conditioning in the case of hair or wool, that determine their correct performances in preserving and improving the appearance and properties of the surface where they are applied. In this work, special attention is paid to the systems combining polycations and negatively charged surfactants. The paper introduces the hair surface and presents a comprehensive review of the adsorption properties of these systems at solid-water interfaces mimicking the negative charge and surface energy of hair. These model surfaces include mixtures of thiols that confer various charge densities to the surface. The kinetics and factors that govern the adsorption are discussed from the angle of those used in shampoos and conditioners developed by the cosmetic industry. Finally, systems able to adsorb onto negatively charged surfaces regardless of the anionic character are presented, opening new ways of depositing conditioning polymers onto keratin substrates such as hair.

Adsorption of conditioning polymers on solid substrates with different charge density.

The adsorption processes of polymers that belong to two different families (neutral hydrophilic polymers and cationic polysaccharide polymers) onto solid surfaces with different charge density have been studied using dissipative quartz crystal microbalance (D-QCM) and ellipsometry. The polymers studied are very frequently used in the cosmetic industry as conditioning agents. The adsorption kinetics of the polymers involves at least two steps. The total adsorbed amount depends on the charge density of the surface for both types of polymers. The comparison of the adsorbed mass on each layer obtained from D-QCM and from ellipsometry has allowed calculating the water content of the layers that reaches high values for the polymers studied. The analysis of D-QCM results also provided information about the shear modulus of the layers, whose values have been found to be typical of a rubber-like polymer system. The main driving force of the adsorption was found to be the energy of the interactions between chains and surface.

Localization of Human Hair Structural Lipids Using Nanoscale Infrared Spectroscopy and Imaging

Atomic force microscopy (AFM) and infrared (IR) spectroscopy have been combined in a single instrument (AFM-IR) capable of producing IR spectra and absorption images at a sub-micrometer spatial resolution. This new device enables human hair to be spectroscopically characterized at levels not previously possible. In particular, it was possible to determine the location of structural lipids in the cuticle and cortex of hair. Samples of human hair were embedded, cross-sectioned, and mounted on ZnSe prisms. A tunable IR laser generating pulses of the order of 10 ns was used to excite sample films. Short duration thermomechanical waves, due to infrared absorption and resulting thermal expansion, were studied by monitoring the resulting excitation of the contact resonance modes of the AFM cantilever. Differences are observed in the IR absorbance intensity of long-chain methylene-containing functional groups between the outer cuticle, middle cortex, and inner medulla of the hair. An accumulation of structural lipids is clearly observed at the individual cuticle layer boundaries. This method should prove useful in the future for understanding the penetration mechanism of substances into hair as well as elucidating the chemical nature of alteration or possible damage according to depth and hair morphology.

Keratin network modifications lead to the mechanical stiffening of the hair follicle fiber

Significance Mechanical properties of tissues often emerge from fibrous protein networks spanning multiple cell lengths. For the first time, to our knowledge, atomic force microscopy was used to measure the mechanical properties of the human hair follicle. We find a considerable stiffening along the first millimeter that we link to changes in the keratin network architecture and composition. In early keratinization stages, the thickening, densification, and increasing orientation of fibers are responsible for the mechanical stiffening, whereas in later stages, intermolecular cross-linking becomes predominant. Our results corroborate the known biological and structural events during hair keratinization and underline the link between the mechanical properties of the hair follicle and its multiscale tridimensional organization. The complex mechanical properties of biomaterials such as hair, horn, skin, or bone are determined by the architecture of the underlying fibrous bionetworks. Although much is known about the influence of the cytoskeleton on the mechanics of isolated cells, this has been less studied in tridimensional tissues. We used the hair follicle as a model to link changes in the keratin network composition and architecture to the mechanical properties of the nascent hair. We show using atomic force microscopy that the soft keratinocyte matrix at the base of the follicle stiffens by a factor of ∼360, from 30 kPa to 11 MPa along the first millimeter of the follicle. The early mechanical stiffening is concomitant to an increase in diameter of the keratin macrofibrils, their continuous compaction, and increasingly parallel orientation. The related stiffening of the material follows a power law, typical of the mechanics of nonthermal bending-dominated fiber networks. In addition, we used X-ray diffraction to monitor changes in the (supra)molecular organization within the keratin fibers. At later keratinization stages, the inner mechanical properties of the macrofibrils dominate the stiffening due to the progressive setting up of the cystine network. Our findings corroborate existing models on the sequence of biological and structural events during hair keratinization.

Detection of corneodesmosin on the surface of stratum corneum using atomic force microscopy

Abstract:  Corneodesmosin, a protein known to be present in the stratum corneum (SC), plays an important role in its physical integrity. Here, a specific antibody to corneodesmosin was tethered via a flexible linker to an atomic force microscopy tip, and the interaction forces between this tip and the surface of the SC were successfully measured. Using the recently developed technique of simultaneous topography and recognition imaging, we were able to map the distribution of corneodesmosin on the surface of the SC at the nanoscale.

Modeling of Polyelectrolyte Adsorption from Micellar Solutions onto Biomimetic Substrates

Depositing cationic polyelectrolytes (PEs) from micellar solutions that include surfactants (SU) onto surfaces is a rich, complex, highly relevant, and challenging topic that covers a broad field of practical applications (e.g., from industrial to personal care). The role of the molecular architecture of the constituents of the PEs are often overruled, or at least and either, underestimated in regard to the surface properties. In this work, we aim to evaluate the effect of a model biomimetic surface that shares the key characteristics of the extreme surface of hair and its concomitant chemo- and physisorbed properties onto the deposition of a complex PEs:SU system. To tackle out the effect of the molecular architecture of the PEs, we consider (i) a purely linear and hydrophilic PE (P100) and (ii) a PE with lateral amphiphilic chains (PegPE). Using numerical self-consistent field calculations, we show that the architecture of the constituents interfere with the surface properties in a nonintuitive way such that, depending on the amphiphilicity and hydrophilicity of the PEs and the hydrophobicity of the surface, a re-entrant adsorbing transition can be observed, the lipid coverage of the model hair surface being the unique control parameter. Such a behavior is rationalized by the anticooperative associative properties of the coacervate micelles in solution, which is also controlled by the architecture of the PEs and SU. We now expect that PEs adsorption, as a rule, is governed by the molecular details of the species in solution as well as the surface specificities. We emphasize that molecular realistic modeling is essential to rationalize and optimize the adsorption process of, for example, polymer conditioning agents in water-rinsed cosmetic or textile applications.