Hair surface interactions against different chemical functional groups as a function of environment and hair condition.

OBJECTIVE: The nature and magnitude of molecular interactions on hair surfaces underpin the design of formulated products, of which the application involves a competitive adsorption process between cationic surfactants, fatty alcohols and surface actives such as silicone. The knowledge of molecular interaction with hair surface will not only provide insight on the surface binding affinity but also offer an effective methodology in characterizing surface deposits. METHODS: Untreated and chemically treated hair samples were treated with either conditioner chassis alone (gel network) or conditioner chassis plus silicone (chassis/TAS). Hair surface interactions against four different chemical functional groups, namely methyl (-CH3 ), acid (-COOH), amine (-NH2 ) and hydroxyl (-OH), were quantified in both ambient and aqueous environment using Chemical Force Microscopy, a method based on atomic force microscopy (AFM). RESULTS: Surface adhesion on hair in ambient is dominated by capillary force that is determined by both the wettability of hair fibre (hydrophobic vs. hydrophilic), presence of any deposits and the chemical functionality of the AFM cantilever. Capillary force is diminished and replaced by electrostatic interaction when polar groups are present on both hair and AFM cantilever. A distinctively different force, hydrophobic interaction, plays a major role when virgin hair and hydrophobic functionalized AFM cantilever make contact in water. CONCLUSION: Results acquired by AFM cantilevers of different functional groups show that hydrophobic interaction is a key driver for deposition on virgin hair, whilst electrostatic interaction is the most important one for bleached hair. Interfacial conformation of chassis components upon deposition is determined by the hair surface properties. Our study highlights the possibility of a range of polar groups, not necessarily negatively charged, on the damaged hair. Unlike conventional surface chemical analysis method, it is possible to quantitatively evaluate the interfacial conformation of deposited surface actives on hair, which identifies the target moieties for conditioning products on different types of hair.

OBJECTIF: La nature et l'intensité des interactions moléculaires mesurables à la surface d'un cheveu, caractérisent l'effet de la formulation du produit initialement appliqué sur le cheveu. L'application du produit et son effet sur le cheveu repose sur un mécanisme d'adsorption complexe combinant l'effet de différents éléments tels que des surfactants cationiques, des alcools gras et des agents de surface tel que le silicone. L'étude et l'analyse des interactions moléculaires à la surface du cheveu permettent non seulement de déterminer l'affinité adhésive de surface, mais aussi d'offrir une méthode efficace pour caractériser les dépôts de surface. MÉTHODE: Des cheveux initialement traités et non-traités chimiquement, ont été analysés après l'application d'un après-shampoing chassis (structure en gel) ou d'un après-shampoing chassis avec du silicone (chassis/TAS). Les interactions entre la surface du cheveu et quatre groupes fonctionnels chimiques - méthyle (-CH3 ), acide (-COOH), amine (-NH2) et hydroxy (-OH) ont été quantifiées à l'air et en milieu aqueux par microscopie à force chimique, une méthode basée sur la microscopie à force atomique (AFM). RÉSULTATS: L'adhésion de surface sur cheveu à l'air est dominée par la force capillaire qui est déterminée par la mouillabilité de la fibre capillaire (hydrophobe vs. hydrophile), la présence de dépôts, et la fonction chimique du cantilever. La force capillaire diminue et est remplacée par des interactions électrostatiques quand des groupes polaires sont présents à la fois sur le cheveu et le cantilever. Une autre force - l'interaction hydrophobe, joue un rôle majeur quand un cheveu non-traité / vierge et un cantilever de fonction hydrophobe se rencontrent en milieu aqueux. CONCLUSIONS: Les résultats obtenus à partir de cantilevers de différentes fonctions chimiques, montrent que l'interaction hydrophobe joue un rôle clé dans l'application de produit capillaire sur cheveux non-traité tandis que les interactions électrostatiques sont prédominantes dans le cas de cheveux traités chimiquement. La conformation interfaciale des composés chassis avant déposition est déterminée par les propriétés de surface du cheveu. Notre étude souligne la présence potentielle de différents groupes polaires, pas nécessairement chargés négativement, sur la surface de cheveux endommagés. A l'inverse des méthodes conventionnelles d'analyse chimique de surface, il est possible d'évaluer quantitativement la conformation interfaciale de dépôts d'agents actifs présents à la surface du cheveu, ce qui permet d'identifier les zones cibles pour l'application d'après-shampoing sur différents types de cheveux.

Segregation of Amine Oxide Surfactants in PVA Films.

The vertical depth distributions of amine oxide surfactants, N,N-dimethyldodecyl amine N-oxide (DDAO) and N,N-dimethyltetradecyl amine N-oxide (DTAO), in poly(vinyl alcohol) (PVA) films were explored using neutron reflectometry (NR). In both binary and plasticized films, the two deuterated surfactants formed a single monolayer on the film surface with the remaining surfactant homogeneously distributed throughout the bulk of the film. Small-angle neutron scattering and mechanical testing revealed that these surfactants acted like plasticizers in the bulk, occupying the amorphous regions of PVA and reducing its glass-transition temperature. NR revealed little impact of plasticizer (glycerol) incorporation on the behavior of these surfactants in PVA. The surfactant molecular area in the segregated monolayer was smaller for DTAO than for DDAO, indicating that the larger molecule was more densely packed at the surface. Surface tension was used to assess the solution behavior of these surfactants and the effect of glycerol incorporation. Determination of molecular area of each surfactant on the solution surface revealed that the structures of the surface monolayers are remarkably consistent when water is placed by the solid PVA. Incorporation of glycerol caused a decrease of molecular area for DDAO and increase in molecular area for DTAO both in solution and in PVA. This suggests that the head group interactions, which normally limit the minimum area per adsorbed molecule, are modified by the length of the alkyl tail.

Characteristics of respiratory microdroplet nuclei on common substrates.

To evaluate the role of common substrates in the transmission of respiratory viruses, in particular SARS-CoV-2, uniformly distributed microdroplets (approx. 10 µm diameter) of artificial saliva were generated using an advanced inkjet printing technology to replicate the aerosol droplets and subsequently deposited on five substrates, including glass, polytetrafluoroethylene, stainless steel, acrylonitrile butadiene styrene and melamine. The droplets were found to evaporate within a short timeframe (less than 3 s), which is consistent with previous reports concerning the drying kinetics of picolitre droplets. Using fluorescence microscopy and atomic force microscopy, we found that the surface deposited microdroplet nuclei present two distinctive morphological features as the result of their drying mode, which is controlled by both interfacial energy and surface roughness. Nanomechanical measurements confirm that the nuclei deposited on all substrates possess similar surface adhesion (approx. 20 nN) and Young's modulus (approx. 4 MPa), supporting the proposed core-shell structure of the nuclei. We suggest that appropriate antiviral surface strategies, e.g. functionalization, chemical deposition, could be developed to modulate the evaporation process of microdroplet nuclei and subsequently mitigate the possible surface viability and transmissibility of respiratory virus.

Design of tethered bilayer lipid membranes, using wet chemistry via aryldiazonium sulfonic acid spontaneous grafting on silicon and chrome.

We describe a bottom-up surface functionalization to design hybrid molecular coatings that tether biomembranes using wet chemistry. First, a monolayer was formed by immersion in a NH2-Ar-SO3H solution, allowing aryldiazonium salt radicals to spontaneously bind to it via strong C bonding. After formation of the air-stable and dense molecular monolayer (-Ar-SO3H), a subsequent activation was used to form highly reactive -Ar-SO2Cl groups nearly perpendicular to the monolayer. These can bind commercial surfactants, PEGylated oligomers and other inexpensive molecules via their -OH, -COOH, or -NH2 chain end-moieties, to build hybrid coatings. Metal and oxidized chromium, semi-conductor n-doped silicon (111), are the substrates tested for this protocol and the aromatic organic monolayers formed at their surface are characterized by X-ray photoelectron spectroscopy (XPS). XPS reveals unambiguously the presence of C-Cr and C-Si bonds, ensuring robustness of the coatings. Functional sulfur groups (-SO3H) cover up to 6.5×10-10 mol cm-2 of the silicon interface and 4.7×10-10 mol cm-2 of the oxidized chromium interface. These surface concentrations are comparable to the classic values obtained when the prefunctionalisation is driven by electrochemistry on conductors. Tethered lipid membranes formed on these coatings were analyzed by neutron reflectivity at the interface of functionalized n-doped silicon substrates after immersion in a solution of lipid vesicles and subsequent fusion. Results indicate a rather compact hybrid coating of Brij anchor-harpoon molecules that maintain a single lipid bilayer above the substrate, on top of a hydrated PEO cushion.

Influence of PVAc/PVA Hydrolysis on Additive Surface Activity.

This aims to establish design rules for the influence of complex polymer matrices on the surface properties of small molecules. Here, we consider the dependence of the surface behaviour of some model additives on polymer matrix hydrophobicity. With stoichiometric control over hydrolysis, we generate systematic changes in matrix chemistry from non-polar, hydrophobic PVAc to its hydrolysed and hydrophilic analogue, PVA. With the changing degree of hydrolysis (DH), the behaviour of additives can be switched in terms of compatibility and surface activity. Sorbitol, a polar sugar-alcohol of inherently high surface energy, blooms to the surface of PVAc, forming patchy domains on surfaces. With the increasing DH of the polymer matrix, its surface segregation decreases to the point where sorbitol acts as a homogeneously distributed plasticiser in PVA. Conversely, and despite its low surface energy, octanoic acid (OA) surprisingly causes the increased wettability of PVAc. We attribute these observations to the high compatibility of OA with PVAc and its ability to reorient upon exposure to water, presenting a hydrophilic COOH-rich surface. The surfactant sodium dodecyl sulfate (SDS) does not show such a clear dependence on the matrix and formed wetting layers over a wide range of DH. Interestingly, SDS appears to be most compatible with PVAc at intermediate DH, which is consistent with the amphiphilic nature of both species under these conditions. Thus, we show that the prediction of the segregation is not simple and depends on multiple factors including hydrophobicity, compatibility, blockiness, surface energy, and the mobility of the components.

Electrodes for Membrane Surface Science. Bilayer Lipid Membranes Tethered by Commercial Surfactants on Electrochemical Sensors.

Commercial surfactants, which are inexpensive and abundant, were covalently grafted to flat and transparent electrodes, and it appears to be a simple functionalization route to design biomembrane sensors at large-scale production. Sparsely tethered bilayer lipid membranes (stBLM) were stabilized using such molecular coatings composed of diluted anchor-harpoon surfactants that grab the membrane with an alkyl chain out of a PEGylated-hydrogel layer, which acts as a soft hydration cushion. The goal of avoiding the synthesis of complex organic molecules to scale up sensors was achieved here by grafting nonionic diblock oligomers (Brij58 = C xH2 x+1(OCH2CH2) nOH with x = 16 and n = 23) and PEO short chains ((OCH2CH2) nOH with n = 9 and n = 23) from their hydroxyl (-OH) end-moiety to a monolayer of -Ar-SO2Cl groups, which are easy to form on electrodes (metals, semiconducting materials, ...) from aryl-diazonium salt reduction. A hybrid molecular coating on gold, with scarce Ar-SO2-Brij58 and PEO oligomers, was used to monitor immobilization and fusion kinetics of DOPC small unilamellar vesicles (SUV) by both quartz crystal microbalance (QCM) and surface plasmon resonance (SPR) techniques. Using flat and transparent thin chromium film electrodes, we designed biosensors to couple surface sensitive techniques for membranes, including X-ray reflectivity (XRR), atomic force microscopy (AFM) with subnanometer resolution, and optical microscopy, such as fluorescence recovery after photobleaching measurements (FRAP), in addition to electrochemistry techniques (cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)). The advantages of this biomembrane-sensing platform are discussed for research and applications.

Grafting Commercial Surfactants (Brij, CiEj) and PEG to Electrodes via Aryldiazonium Salts.

Grafting commercial surfactants appears to be a simple way to modify electrodes and conducting interfaces, avoiding the synthesis of complex organic molecules. A new surface functionalization route is presented to build surfactant coatings with monolayer thickness grafting molecules considered as nonreactive. A monolayer of -SO2Cl functions (from a p-benzenesulfonyl chloride) was first electrografted. It showed a high reactivity toward weak nucleophiles commonly found on surfactant end-moieties such as hydroxyl groups (-OH), and it was used to covalently graft the following: (1) nonionic diblock oligomers (Brij or CiEj, CxH2x + (OCH2CH2)nOH with x = 16 and n = 23 for Brij58, x = 16 and n = 10 for Brij C10, and x = 16 and n = 2 for Brij52); (2) poly(ethylene glycol) (PEG) short chains (PEO9 for (OCH2CH2)nOH with n = 9) and mixed formula. The surface modification due to these molecular coatings was investigated in terms of wetting properties and interfacial electrochemistry characteristics (charge transfer resistivity, capacity, and ions dynamics). Built on flat and transparent thin chromium films, Brij and PEO mixed coatings have been proven to be promising coatings for electrochemical biosensor application such as for stabilizing a partially tethered supported biomimetic membrane.