Condensation or Desublimation: Nanolevel Structural Look on Two Frost Formation Pathways on Surfaces with Different Wettabilities.

Processes of water condensation and desublimation on solid surfaces are ubiquitous in nature and essential for various industrial applications, which are crucial for their performance. Despite their significance, these processes are not well understood due to the lack of methods that can provide insight at the nanolevel into the very first stages of phase transitions. Taking advantage of synchrotron grazing-incidence wide-angle X-ray scattering (GIWAXS) and environmental scanning electron microscopy (ESEM), two pathways of the frosting process from supersaturated vapors were studied in real time for substrates with different wettabilities ranging from highly hydrophilic to superhydrophobic. Within GIWAXS, a fully quantitative structural and orientational characterization of the undergoing phase transition reveals the information on degree of crystallinity of the new phase and determines the ordering at the surfaces and inside the films at the initial stages of water/ice nucleation from vapor onto the substrates. The diversity of frosting scenarios, including direct desublimation from the vapor and two-stage condensation-freezing processes, was observed by both GIWAXS and ESEM for different combinations of substrate wettability and vapor supersaturations. The classical nucleation theory straightforwardly predicts the pathway of the phase transition for hydrophobic and superhydrophobic substrates. The case of hydrophilic substrates is more intricate because the barriers in Gibbs free energy for nucleating both liquid and solid embryos are close to each other and comparable to thermal energy kBT. At that end, classical nucleation theory allows concluding a relation between contact angles for ice and water embryos on the basis of the observed frosting pathway.

Opportunities and new developments for the study of surfaces and interfaces in soft condensed matter at the SIRIUS beamline of Synchrotron SOLEIL.

The SIRIUS beamline of Synchrotron SOLEIL is dedicated to X-ray scattering and spectroscopy of surfaces and interfaces, covering the tender to mid-hard X-ray range (1.1-13 keV). The beamline has hosted a wide range of experiments in the field of soft interfaces and beyond, providing various grazing-incidence techniques such as diffraction and wide-angle scattering (GIXD/GIWAXS), small-angle scattering (GISAXS) and X-ray fluorescence in total reflection (TXRF). SIRIUS also offers specific sample environments tailored for in situ complementary experiments on solid and liquid surfaces. Recently, the beamline has added compound refractive lenses associated with a transfocator, allowing for the X-ray beam to be focused down to 10 µm × 10 µm while maintaining a reasonable flux on the sample. This new feature opens up new possibilities for faster GIXD measurements at the liquid-air interface and for measurements on samples with narrow geometries.

Structural investigation of titanium oxide nanowires with unconventional optoelectronic behaviour.

Grazing incidence wide angle X-ray scattering measurements on aligned titanium oxide nanowires displaying anisotropic optical-electronic properties are carried out. Elemental and thermal analyses provide a chemical composition corresponding to H2Ti3O7·nH2O with n ≈ 1 while the crystallographic data indicate a monoclinic cell with a lamellar substructure. Cell parameters are close to those of H2Ti3O7 notwithstanding a doubling of the lattice in the layer plane. A comparison of the band gap energy values and the electronic transition modes between the two polymorphs displays differences that could be ascribed to the structural variation.

Gel-to-gel non-variant transition of an organogel caused by polymorphism from nanotubes to crystallites.

An amide based gelator forms gels in trans-decalin. Below concentrations of 1 wt% the gels melt at temperatures varying with concentration. Above a concentration of 1 wt%, upon heating, the gel transforms into an opaque gel at an invariant temperature, and melts at higher temperature. The gel-to-gel transition is evidenced by several techniques: DSC, rheology, NMR, OM and turbidimetry. The phase diagram with the domain of the existence of both morphs was mapped by these techniques. Optical and electronic microscopy studies show that the first gel corresponds to the self-assembled nanotubes while the second gel is formed by crystalline fibers. The fibers are crystalline, as shown by the presence of Bragg peaks in the scattering curves. Both morphs correspond to a different H-bonding pattern as shown by FTIR. The first gel forms at a higher cooling rate, is metastable and transforms slowly into the second one. The second gel is stable. It forms at a low cooling rate, or by thermal annealing or aging of the first gel.

Bending elastic modulus of a polymer-doped lyotropic lamellar phase.

The effect of inserting a neutral water-soluble adsorbing polymer on the flexibility of amphiphilic bilayers in a lamellar phase is investigated. The Lα system is a stack of charged undulating bilayers composed of sodium dodecyl sulfate (SDS) and octanol separated by aqueous solutions of polyethylene glycol (PEG). The mean bending elastic modulus κ is determined from the quadrupole splittings in the solid state NMR spectra of the perdeuterated octanol chains embedded in the membranes that undergo collective fluctuations. Parameters for describing the membrane behavior (bilayer thickness, elastic compressibility modulus, order parameter) are obtained by supplementing the NMR data with complementary experiments (x-ray scattering), NMR spectral simulations, and theoretical considerations. A fairly complete picture of the membrane rigidity emerges for any location in the lamellar phase thanks to a broad sweep of the lamellar domain by systematically varying the membrane fraction along dilution lines as well as the polymer composition. The most remarkable result is the difference between dilute and semi-dilute regimes. In the dilute PEG solution, no (or slight positive shift) polymer contribution to the rigidity curvature of the layered system is noted within the experimental resolution (≤0.3 kBT) and κ remains around 2.7 kBT. In contrast, the membrane rigidity increases steadily upon polymer addition once the crossover concentration cp* is exceeded, amounting to a 60% increase in κ at polymer concentration 2.5 cp* in the aqueous interlayers. These results are discussed with regard to the theoretical expectation of membrane rigidification upon irreversible polymer adsorption.

The long-range attraction between hydrophobic macroscopic surfaces.

Direct measurements of the long-range strongly attractive force observed between macroscopic hydrophobic surfaces across aqueous solutions are reexamined in light of recent experiments and theoretical developments. The focus is on systems in the absence of submicroscopic bubbles (preexistent or induced) to avoid capillary bridging forces. Other possible interferences to the measurements are also eliminated. The force-distance profiles are obtained directly (no contributions from electrical double layer or hydrodynamics) between symmetric identical hydrophobic surfaces, overall charge-neutral, at the thermodynamic equilibrium and in a quenched state. Therefore in the well-defined geometry of crossed-cylinders, sphere-flat, or sphere-sphere, there is no additional interaction to be considered except the ever-present dispersion forces, negligible at large separations. For the three main categories of substrates rendered hydrophobic, namely surfaces obtained with surfactant monolayers physically adsorbed from solution to deposited ones, and substrates coated with a hydrophobizing agent bonded chemically onto the surface, the interaction energy scales as A exp (-2κD)/2κD at large separations, with measured decay lengths in accord with theoretical predictions, simply being half the Debye screening length, κ-1/2, at least in non vanishing electrolyte. Taken together with the prefactor A scaling as the ionic strength, the interaction energy is demonstrated to have an electrostatic origin in all the systems. Thanks to our recent SFAX coupling force measurements with x-ray solution scattering under controlled nano-confinement, the microstructuration of the adsorbed film emerges as an essential feature in the molecular mechanism for explaining the observed attraction of larger magnitude than dispersion forces. The adsorption of pairs of positive and negative ions on small islands along the interface, the fluctuation of the surface charge density around a zero mean-value with desorption into or adsorption from the electrolyte solution, the correlations in the local surface ion concentrations along the surfaces, the redistribution of counterions upon intersurface variation, all contribute and are tuned finely by the inhomogeneities and defects present in the hydrophobic layers. It appears that the magnitude of the interacting energy can be described by a single master curve encompassing all the systems.

Cationic-Surfactant-Coated Mica Surfaces below the Critical Micellar Concentration: 1. Patchy Structures As Revealed by Peak Force Tapping AFM Mode.

The morphology and structure of the self-assembled surfactant aggregates at the solid-liquid interface remain controversial. For the well-studied system of cationic cetyltrimethylammonium bromide (C16TAB) adsorbed onto the opposite negatively charged, atomically smooth mica surface, a variety of surface aggregates have been previously reported: AFM imaging pointing to cylinders and surface micelles as opposed to mono/bilayer-like structures revealed by neutron and X-ray reflectometry, NMR, spectroscopic techniques, and numerical simulations. To reconcile with the latter results, we revisit the morphometry of the C16TAB-coated mica surfaces using the recent peak force tapping (PFT-AFM) mode that allows fragile structures to be imaged with the lowest possible applied force. The evolution of the structural organization at the mica-water interface is investigated above the Krafft boundary over a wide concentration range (from 1/1000 to 2 cmc) after long equilibration times to ensure thermodynamic equilibrium. A complex but fairly complete picture has emerged: At very low concentrations, the C16TAB surfactants adsorb as isolated molecules before forming small clusters. Above 1/140 cmc, monolayer-like stripes are formed. As the concentration is increased, a connected network of these patches progressively covers the mica substrate. Above 1/80 cmc, bilayer-like patches build on top of the underlying monolayer, and ultimately a complete bilayer (at about half the cmc) covers the entire mica substrate. Thanks to the less invasive PFT-AFM imaging mode, our observations not only agree with the theoretical predictions and numerical simulations but also reconcile, at last, the direct observations by means of the AFM imaging technique with the results obtained with other techniques.

Sliding tethered ligands add topological interactions to the toolbox of ligand-receptor design.

Adhesion in the biological realm is mediated by specific lock-and-key interactions between ligand-receptor pairs. These complementary moieties are ubiquitously anchored to substrates by tethers that control the interaction range and the mobility of the ligands and receptors, thus tuning the kinetics and strength of the binding events. Here we add sliding anchoring to the toolbox of ligand-receptor design by developing a family of tethered ligands for which the spacer can slide at the anchoring point. Our results show that this additional sliding degree of freedom changes the nature of the adhesive contact by extending the spatial range over which binding may sustain a significant force. By introducing sliding tethered ligands with self-regulating length, this work paves the way for the development of versatile and reusable bio-adhesive substrates with potential applications for drug delivery and tissue engineering.

Interactions between charged lamellae in aqueous solution.

Interactions between charged surfaces in aqueous solutions, widespread in soft matter and biology, are very complex and, despite many efforts, their full explanation remains challenging. We support the idea that, in contrast to extremely small separations (d≤2 nm), where many effects, prominently those linked to the structure of liquid water, interfere, electrostatics alone rules over larger distances (d≥5 nm) at low ionic strength. We set up specially designed surface force apparatus (SFA) experiments to measure the elastic compressibility modulus of a stack of charged membranes with monovalent counterions, directly and with high precision. We demonstrate that electrostatics alone, if implemented beyond Poisson-Boltzmann theory, fully accounts for the data, nonelectrostatic contributions playing at best a minor role.

Size-controlled polyelectrolyte complexes: direct measurement of the balance of forces involved in the triggered collapse of layer-by-layer assembled nanocapsules.

Polyelectrolyte multilayers composed of poly(allylamine hydrochloride) and poly(styrene sulfonate) were assembled on 13 nm gold nanoparticles and characterized by Transmission Electron Microscopy and Atomic Force Microscopy. The direct measurement of the interactions at the molecular level using a Surface Force Apparatus revealed that the colloidal stability of such coated particles in aqueous media is brought about concomitantly by electrostatic and steric repulsive interactions. The cyanide induced dissolution of the gold cores yields either hollow nanocapsules or collapsed nanospheres, two species which are very difficult to distinguish. In contrast to the established micron sized hollow capsules, the dissolution of the nanosized gold cores may induce a substantial swelling of the polyelectrolyte complex into the central void as induced by the temporary local increase of the ionic strength. At least three layer pairs are required to maintain the structural integrity of the polyelectrolyte shells to yield hollow nanospheres. At about three layer pairs, thin nanocapsules are mechanically compressible and may collapse on themselves following mechanical stimulation to form even smaller spherical polyelectrolyte complex particles that retain the small polydispersity of the gold cores. Thus, the templating of polyelectrolyte shells around, e.g., gold nanoparticles followed by the dissolution of the respective cores constitutes a new method for the synthesis of extremely small polyelectrolyte complex particles with very low polydispersity.

Glassy state of polystyrene sulfonate/polyallylamine polyelectrolyte multilayers revealed by the surface force apparatus.

The interactions between two poly(allylamine)/poly(styrene sulfonate) multilayers made of 4.5 and 5 bilayers are investigated by the surface force apparatus (SFA). As the two surfaces approach, one reaches a threshold point where a repulsion sets in, until they become barely compressible. Repetitive load/unload cycles show that, once compressed, the films remain almost in their compressed state. This indicates that the poly(allylamine)/poly(styrene sulfonate) films are in a glassy state, in marked difference with the SFA findings on poly-(L-lysine)/poly-L-glutamic acid) multilayers. These results are discussed in the light of linearly and exponentially growing films.