Influence of pyrogenic silica hydration on water diffusion in surface layers

The surface hydration of pyrogenic silica (aerosil) has been studied by slow neutron scattering. It was shown that the rate of diffusion changes with the thickness of the layer monotonically. The mean square displacement of water molecules from the equilibrium position in aerosil hydration shell is smaller than in bulk water, but twice larger than in ice.     

Preparation of thin SnO2 layers by inorganic sol-gel process

SnO₂ sols were prepared in the following way: (1) precipitation of metastannic acid with aqueous ammonia from aqueous solutions of SnCl₄, (2) washing the precipitates with NH₄NO₃ solution and water, (3) peptization of precipitates in water, sometimes with an addition of HNO₃, at elevated temperature using mechanical stirring. In those sols, sometimes diluted with water or ethanol, substrates (glass or silica derived wafers) were dipped and withdrawn at various rates. Gel coatings were converted into crystalline SnO₂ by thermal treatment at 600°C. Coatings with thickness between 300–2000 Å were prepared.     

Ordered equilibrium structures of soft particles in thin layers

Considering a system of gaussian particles confined between two hard, parallel plates, we investigate at T = 0, ordered equilibrium configurations that the system forms as the distance D between the plates gradually increases. Using a very sensitive and reliable optimization technique that is based on ideas of genetic algorithms, we are able to identify the emerging sequences of the energetically most favorable structures. Although the resulting phase diagram is rather complex, its essential features can be reduced to the discussion of two archetypes of structural transitions: (i) a continuous transformation at a fixed number of layers, leading from a square to a centered rectangular and then to a hexagonal lattice; (ii) a discontinuous transition, transforming a hexagonal to a square lattice via complex intermediate structures, i.e., the so-called buckling transition, which is encountered as the system forms a new layer. Detailed Monte Carlo simulations are able to confirm the theoretical predictions on a semiquantitative level but are not able to grasp the tiny energetic differences between competing structures.     

Effect of reagents mixing on the thermal behavior of sol-gel precursors for silica-based nanocomposite thin films

The paper presents, based on TG-DTG-DSC data, some results of the thermal decomposition of some complex sol-gel precursors used for the deposition of mesoporous ZnO/SiO₂ nanocomposite thin films for gas sensing applications. The effect chemical composition of the sol and reagents mixing during the sol preparation is discussed. The chemical nature of ZnO source (zinc acetate solid salt, zinc acetate alcoholic solution or ZnO nanopowder) used for the sol preparation significantly affects the thermal decomposition of complex precursor and the microstructure and properties of the nanocomposite thin films.     

Deformation behavior of thin,compliant layers under tensile loading conditions

In this article, a connection is made between the behavior of thin layers of Newtonian liquids under tensile loading conditions and the behavior of highly deformable elastic or viscoelastic solids, which are more commonly used as adhesives. The behavior of Newtonian liquids is understood in the most quantitative detail and serves as a starting point for understanding the origins of fingering and cavitation instabilities that appear when the tensile deformation rates applied to these layers are sufficiently large. Similar instabilities appear in solid systems and can be attributed to common features of the stress distribution for incompressible liquids and solids. A unifying treatment is presented that can be used to understand the overall deformation behavior and adhesive performance of a wide variety of solid and liquid systems that are typically applied as thin layers. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4023–4043, 2004     

Synthesis and characterization of silica-silver core-shell composite particles with uniform thin silver layers

Silica-silver core-shell composite particles with uniform thin silver layers were successfully synthesized by a facile and one-step ultrasonic electrodeposition method. By electrolysis of the slurry consisting of preformed silica spheres and silver perchlorate without any additives, the homogenous composite particles can be prepared. The average size of single silver crystals in the composite is about 12 nm and the thickness of silver layer is 14±2 nm. Moreover, the continuity of Ag distribution, the surface roughness and the thickness of silver layer are controllable by adjusting the current density (I), the concentration of electrolyte (C) and the reaction time (t). Optical properties of the composite particles with different silver content were also investigated.     

Effect of hybrid phosphorus-doped silica thin films produced by sol-gel method on the thermal behavior of cotton fabrics

The aim of this work was to prepare hybrid organic-inorganic silica thin films to provide cotton fabrics with flame retardant properties and to investigate the films’ influence on the thermal and burning behavior of the treated samples. The fabrics were modified with three different sols in order to study the effect of pure silica sol-gel precursor, γ-aminopropyltriethoxysilane (APTES), and that of the hybrid sols consisting of the APTES and the phosphorus compound diethylphosphite. Furthermore, in order to improve the cross-linking degree and the phosphorus-nitrogen synergistic effect on flame retardancy of the P-doped silica thin film the melamine-based resin was added in the third sol. To evaluate the chemical structure of the coating material, pure xerogels of the treatment solutions were applied to glass slides and tested by ATR FT-IR spectroscopy. The cotton fabrics were impregnated with the sols by a padding-squeezing process and then dried. Thermal behavior of the treated cotton samples was investigated by thermogravimetric/differential scanning calorimetry analysis (TGA-DTG/DSC) and compared to the untreated one. The flame retardancy was tested according to the ASTM D 1230 standard method. The results showed a substantial enhancement of char-forming properties and flame retardancy for the fabrics modified with the thin films.     

Nonlinear behavior during semi-quantitative analysis of thin organic layers by laser desorption mass spectrometry

Characterization of the surface coverage and thickness of an organic thin film is particularly important in organic electronics and optoelectronics. For surface coverage down to the submonolayer level there is still a need for characterization methods which are easily applicable. In the present work we report on the evaluation of laser desorption mass spectrometry (LD-MS) for its use in thickness determination of organic thin films. Whereas LD-MS is well established as a soft ionization method for small molecules, its capability for use in quantitative analysis is nearly unexplored. We carried out experiments with two different molecules, 7,7,8,8-tetracyanoquinodimethane and hexabenzocoronene, in a series of experiments with increasing surface coverage. The obtained data were analyzed by plotting the LD signal intensities versus the relative layer thickness and they reveal a nonlinear behavior, which can be classified into regions of different desorption/ionization efficiencies. Visualization by atomic force microscopy reveals that the first efficiency change corresponds to the transition between incomplete and complete coverage of the metal surface by analyte molecules. A second transition is observed at high layer thickness where the signal intensity stays constant, independent of further thickness increments, and this is attributed to the limited penetration depth of the laser beam. The intermediate region between the two transitions shows a linear behavior and can thus be used for semi-quantitative thickness measurements. The efficiency change observed at the point of complete surface coverage is particularly useful for thin layer preparation of organic field effect transistors, where complete surface coverage is a minimum requirement.     

Impact of adsorption layers on thin liquid films

The present review is focused on most recent studies, in which the interconnections of interfacial layer and foam film properties are innate for the performance of the respective systems and have been clarified to highest possible degree. These investigations cover fundamental issues as the initial onset of premicelles in single-surfactant solutions, explore intricate topics as the interplay of competitive and cooperative adsorption in surfactant and protein–surfactant mixtures and give hints for fine-tuning the design of various formulations to be applied in industrial applications.     

Efficient synthesis of asymmetric particles by sol-gel process

This paper presents a novel and facile method of synthesizing polymer/silica particles with controlled asymmetric morphologies. Our approach is based on the sol–gel process in which cross-linked polystyrene particles (CPS) are adopted as templates and 3-mercaptopropyltriethoxysilane is used as a single silica source. The reaction process causes silane oligomer to preferentially grow on the local surface of CPS, giving rise to polystyrene/thiol-functionalized silica composite particles with a tunable shape. It is found that the morphologies of particles can be easily tailored by changing the ratio of ethanol/water in the reaction medium. In addition, the amount of cross-linker used during the polymerization also plays a key role in the formation of various complex-shaped particles. Controlled geometries of these organic/inorganic composite particles will allow a broad range of potential applications, such as photonic crystals, Pickering emulsifier, sensors, and so on.     

Electrochemically induced sol-gel deposition of zirconia thin films

A novel electrochemical method for deposition of ZrO(2) thin films is described. The films, 50-600 nm thick, were obtained by applying moderate positive or negative potentials (+2.5 V to -1.5 V versus SHE) on conducting surfaces immersed in a 2-propanol solution of zirconium tetra-n-propoxide [Zr(OPr)(4)] in the presence of minute quantities of water (water/monomer molar ratios in the range of 10(-5) to 10(-1)), which was the limiting reagent. Oxidative electrochemical formation of solvated H(+) and reductive formation of OH(-) catalyze the hydrolysis and condensation of the metal alkoxide precursor. The magnitude of the applied potential and its duration provide a convenient way of controlling the film thickness. The films consist of an amorphous phase, as revealed by XRD measurements. The effects of different parameters, such as the applied potential and its duration, the amount of added water and the current-time characteristics, were studied. A mechanism for the electrodeposition of the zirconia films which is in accordance with our findings is proposed.     

In-situ fluorescence imaging of sol-gel thin film deposition

Pyranine was used as a fluorescence probe to monitor the chemical evolution in-situ during thin film deposition by the dip coating process. The sensitivity of the pyranine luminescence to protonation/deprotonation effects was used to quantify changes in the water/alcohol ratio in real time within the depositing film as the substrate was withdrawn from the coating reservoir. The spatially resolved spectral results clearly showed that preferential evaporation of alcohol occurred with increasing distance from the reservoir and that the maximum water content reached rather high values near the drying line. Correlation of the luminescence results with the interference pattern of the drawn films allows the solvent composition in the film to be mapped as a function of film thickness. These experiments demonstrate for the first time that luminescent organic molecules may be applied to the processing science of sol-gel thin film deposition.     

Structure of water in thin layers

This paper considers the physical behavior of the boundary layers of water near the hydrophilic and hydrophobic surfaces. The overlapping of these layers leads to the generation of structural forces of repulsion, in the first case, and those of attraction, in the second. Inclusion of structural forces in the DLVO theory and the theory of heterocoagulation allows one to explain the results of measurements of contact angles for water solutions at different concentrations, pH values, and temperatures. The maximum influence of structural effects on colloid stability, wetting phenomena, and liquid flow in thin pores is to be expected in two extreme cases—that of a highly lyophobic system and that of a highly lyophilic one.     

Computer simulations of thin polymer layers

Molecular dynamics calculations are used to explore the structure of dense monolayers of long-chain molecules supported on a planar surface. As a model we consider ensembles of flexible chains consisting of N segments (N=32, 64 and 128) in a box with lateral (x, y) periodicity conditions. The effect of surface coverage on the conformational properties of chains is studied. At high coverages, the results of the simulations show that each chain is strongly stretched along the normal to the surface and the mean layer thickness is linear in N. The segment density distribution along the normal is found to be an universal function A^2/3 f (zA^1/3 N), where A is the surface area per chain. The high-coverage distribution has a well defined broad plateau, in agreement with the so-called blob model. In contrast to the predictions of this model, however, we observe that the chains are strongly stretched at all space scales. Differences between the results of simulations and those predicted by the mean-field theory are also discussed.