Yasmine Adriaensen

Nanometer scale organization of mixed surfactin/phosphatidylcholine monolayers.

Mixed monolayers of the surface-active lipopeptide surfactin-C(15) and of dipalmitoyl phosphatidylcholine (DPPC) were deposited on mica and their nanometer scale organization was investigated using atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). AFM topographic images revealed phase separation for mixed monolayers prepared at 0.1, 0.25, and 0.5 surfactin molar ratios. This was in agreement with the monolayer properties at the air-water interface indicating a tendency of the two compounds to form bidimensional domains in the mixed systems. The step height measured between the surfactin and the DPPC domains was 1.2 +/- 0.1 nm, pointing to a difference in molecular orientation: while DPPC had a vertical orientation, the large peptide ring of surfactin was lying on the mica surface. The N/C atom concentration ratios obtained by XPS for pure monolayers were compatible with two distinct geometric models: a random layer for surfactin and for DPPC, a layer of vertically-oriented molecules in which the polar headgroups are in contact with mica. XPS data for mixed systems were accounted for by a combination of the two pure monolayers, considering respective surface coverages that were in excellent agreement with those measured by AFM. These results illustrate the complementarity of AFM and XPS to directly probe the molecular organization of multicomponent monolayers.

Physico-chemical mechanisms governing the adherence of starch granules on materials with different hydrophobicities.

The factors influencing the adherence of starch were examined to improve the understanding of the mechanisms affecting soiling and cleanability. Therefore an aqueous suspension of starch granules was sprayed on four model substrates (glass, stainless steel, polystyrene and PTFE) and dried, and the substrates were cleaned using a radial-flow cell. The morphology of the soiled surfaces and the substrate chemical composition were also characterized. By influencing droplet spreading and competition between granule-substrate and granule-granule interfaces regarding the action of capillary forces, substrate wettability affected the shape and compactness of the adhering aggregates, the efficiency of shear forces upon cleaning, and finally the adherence of soiling particles. The rate of drying had an influence explained by the duration left to capillary forces for acting. X-ray photoelectron spectroscopy demonstrated the presence of macromolecules, mainly polysaccharides, which were adsorbed from the liquid phase, or carried by the retracting water film and deposited at the granule-substrate interface. These macromolecules acted as an adhesive joint, the properties of which seemed to be influenced by the detailed history of drying and subsequent exposure to humidity. In summary, the substrate surface energy affects the adherence of starch aggregates by different mechanisms which are all linked together: suspension droplet spreading, action of capillary forces, direct interaction with starch particles and interfacial macromolecules.

Structure and Chemical Composition of Layers Adsorbed at Interfaces with Champagne

The structure and the chemical composition of the layer adsorbed at interfaces involving champagne have been investigated using native champagne, as well as ultrafiltrate (UFch) and ultraconcentrate (UCch) obtained by ultrafiltration with a 10(4) nominal molar mass cutoff. The layer adsorbed at the air/liquid interface was examined by surface tension and ellipsometry kinetic measurements. Brewster angle microscopy demonstrated that the layer formed on polystyrene by adsorption or drop evaporation was heterogeneous, with a domain structure presenting similarities with the layer adsorbed at the air/liquid interface. The surface chemical composition of polystyrene with the adlayer was determined by X-ray photoelectron spectroscopy (XPS). The contribution of champagne constituents varied according to the liquid (native, UFch, and UCch) and to the procedure of adlayer formation (evaporation, adsorption, and adsorption + rinsing). However, their chemical composition was not significantly influenced either by ultrafiltration or by the procedure of deposition on polystyrene. Modeling this composition in terms of classes of model compounds gave approximately 35% (w/w) of proteins and 65% (w/w) of polysaccharides. In the adlayer, the carboxyl groups or esters represent about 18% of carbon due to nonpolypeptidic compounds, indicating the presence of either uronic acids in the complex structure of pectic polysaccharides or of polyphenolic esters. This structural and chemical information and its relationship with the experimental procedures indicate that proteins alone cannot be used as a realistic model for the macromolecules forming the adsorption layer of champagne. Polysaccharides, the other major macromolecular components of champagne wine, are assembled with proteins at the interfaces, in agreement with the heterogeneous character of the adsorbed layer at interfaces.

Surface functionalization of PEEK films studied by time-of-flight secondary ion mass spectrometry and x-ray photoelectron spectroscopy

Using the wet chemistry method, the surface of poly(aryl ether ether ketone) (PEEK) him aas selectively modified to produce PEEE-OH, PEEE-COOH, PEEK-glutamine, PEEK-NH2 and PEEK-SO3H samples displaying, respectively hydroxyl, carboxyl, amino acid, amine and sulphonyl functions. All the samples were analysed by XPS and time-of-flight (ToF) SIMS; the experimental data provided by both techniques were in good agreement, and allowed the chemical nature and the yield of the functional groups introduced by the different surface derivatizations to be determined. Copyright (C) 1999 John Wiley & Sons, Ltd.

A photocleavable stabilizer for the preparation of PHEMA nanogels by dispersion polymerization in supercritical carbon dioxide

A new photo-sensitive diblock copolymer composed of a hydrophilic sequence of poly(ethylene oxide) (PEO) linked to a CO2-philic sequence of poly(1H,1H,2H,2H-heptadecafluorodecyl acrylate) (PFDA) by a light sensitive o-nitrobenzyl group was successfully synthesized by RAFT polymerization and used as a stabilizer for the free radical polymerization of 2-hydroxyethyl methacrylate (HEMA) dispersion in α,α,α-trifluorotoluene and supercritical carbon dioxide (scCO2). Thanks to this fluorinated stabilizer, well-defined particles of PHEMA down to 350 nm diameter were produced in scCO2. Advantageously, the photocleavable group at the block junction of the stabilizer could be cleaved by exposing the particles to UV light so that the fluorinated block could be extracted in TFT or scCO2. As supported by X-ray photoelectron spectroscopy (XPS) analysis, up to 80% of the fluorinated block of the stabilizer can be removed, leading to efficient swelling and dispersion of the resulting PHEMA nanogels in water.

Fabrication of surfaces with bimodal roughness through polyelectrolyte/colloid assembly

From bioengineering to optics and electronics, a great deal of work has been conducted on the development of new materials with structured surfaces. A large range of methods has been used, such as plasma etching, electron beam and colloidal lithography (Denis et al., 2002), electrical deposition (Yang et al., 2009), phase separation (Dekeyser et al., 2004) and polyelectrolyte assembly in order to produce structured surfaces (Agheli et al., 2006). The combination of different methods is also more and more explored. Schaak et al. (2004) described a simple approach to achieve colloidal assembly on a patterned template obtained by lithography. Densely packed layers of colloidal particles can be produced by lifting a substrate vertically from a suspension (Fustin et al., 2004; Li J. et al., 2007) or by spin coating (Yang et al., 2009). Colloidal lithography utilizes the ability of particles to adhere on oppositely charged surfaces (Johnson & Lenhoff, 1996; Hanarp et al., 2001, 2003)), possibly using surface modification by inorganic or organic polyelectrolytes. The surface coverage is influenced by several factors: ionic strength, particle size and time. The adhesion of microbial cells on various substrates was also achieved by surface treatments with inorganic or organic polycations (Changui et al., 1987; Van haecht et al., 1985) or with positively charged colloidal particles (Boonaert et al., 2002). A review on colloidal lithography and biological applications was published recently (Wood, 2007). Adsorption of polyelectrolytes is influenced by ionic strength, pH and the polyelectrolyte characteristics (molecular mass, charge density) (Lindquist & Stratton, 1976; Davies et al., 1989; Choi & Rubner, 2005). At low ionic strength, highly charged polyelectrolytes adopt extended conformations and are fairly rigid due to the strong repulsion between charged units. The maximum adsorbed amount and the adsorbed layer thickness do not vary markedly according to molecular weight. As the salt concentration is increased and the electrostatic intrachain repulsion is decreased, the polyelectrolyte becomes more coiled. In this case, the maximum amount adsorbed (expressed in mass) increases as a function of molecular weight (Roberts, 1996; Lafuma, 1996; Claesson et al., 2005; Boonaert et al., 1999). Build up of polyelectrolyte films may be achieved using layer-by-layer assembly through alternating adsorption of oppositely charged polyelectrolytes (Decher & Hong, 1991).