Zoltán Hórvölgyi

Surface free energy of natural and surface-modified tropical and European wood species

To describe the wetting properties of various wood types from a practical point of view, the surface free energy of six tropical (guava, almond, teak, cloves, mango and neem) and six European wood species (English oak, Norway maple, hazel, ash, alder and Scots pine) were calculated using contact angles by the sessile-drop method. In order to provide water-repellent characteristics to the samples, they were silanized by a less used silylating reagent (using chloroform solution of trimethylsilyl N, N-dimethylcarbamate) and the results were compared with the effect of two commonly used reagents (chloroform solutions of chlorotrimethylsilane and octadecyltrichlorosilane). Since the Lifshitz–van der Waals/acid–base model is widely used in studies of biological surfaces, the energetics of the resultant wood surfaces were quantitatively described in terms of this model. For the mainly hydrophobic wood samples, anomalous surface behaviour (i.e. extremely high water contact angles (130–145°) and in certain cases unreasonably low surface free energy values) was found. Since the Lifshitz–van der Waals/acid–base model did not yield numerical results in some cases and the calculated surface free energies depended on the test liquid triplet used, the limitations in the applicability of this model are also discussed. For comparison, we analyzed our data also in terms of the Chang model.

Spreading of hydrophobic silica beads at water—air interfaces

Abstract Structured monoparticle layers, formed from 3 ± 1 μm diameter silica beads silylated to a greater (sample A) and a lesser (sample B) extent, have been investigated on water and on aqueous 1.0 M NaCl subphases in a Langmuir film balance. Hydrophobicities corresponding to the different extents of silylation were estimated by determining the wettabilities of identically treated 10 ± 1 μm silica beads. The advancing water contact angles of 98 ± 4° and 76 ± 4° have been calculated for samples A and B respectively. Visual observations of spreading and redispersity, and determinations of surface pressure—surface area isotherms, hysteresis and contact cross-sectional areas led to the assessment of the structural strengths of these two sets of silica beads. Appreciably stronger attractive interactions between neighboring silica beads have been found in structured monoparticle layers prepared from sample A than those formed from sample B. The less hydrophobic silica beads (sample B) were postulated to be separated from each other by a layer of water molecules in the structured monoparticle layer. This postulate has been supported by calculating the total interparticle interaction energies by applying the DLVO theory. Introduction of 1.0 M NaCl did not appreciably affect the behavior of structured monoparticle layers prepared from sample A. Significant creasing was observed, however, in the vicinity of the moving barrier upon the compression of structured monoparticle layers prepared from sample B.

Wetting behaviour of silanized glass microspheres at water-air interfaces: A Wilhelmy film balance study

Abstract The wetting behaviour of silanized glass microspheres (75±5 μm diameter) was studied at water-air interfaces in a broad hydrophobicity range (40–90°) using a Wilhelmy film balance. The wettability of surface modified particles was characterized by the determinations of static water contact angles using an optical microscopic method. In the course of the film balance investigations the direction of particles’ removal during the collapse was studied visually and surface pressure (Π) versus surface area (A) isotherms were obtained from which some important parameters, like collapse pressures, collapse areas and contact angles were calculated. Comparing the measured and calculated values of contact angles with each other and considering the unexpected direction of particles’ removal from the interface during the collapse of monoparticulate layer we came to the conclusion (concerning the medium hydrophobicity range) that there should had been an extra force which could hinder the immersion of particles into the water phase. The present conclusion partially supports our earlier investigations of wettability of similarly sized and hydrophobic glass spheres accomplished by a Langmuir film balance. However, the Wilhelmy film balance seems to be more sensitive for the study of wettability of the particles.

Large area self-assembled masking for photonic applications

Ordered porous structures for photonic application were fabricated on p- and n-type silicon by means of masking against ion implantation with Langmuir-Blodgett (LB) films. LB films from Stober silica spheres [J. Colloid Interface Sci. 26, 62 (1968)] of 350nm diameter were applied in the boron and phosphorus ion-implantation step, thereby offering a laterally periodic doping pattern. Ordered porous silicon structures were obtained after performing an anodic etch and were then removed by alkaline etching resulting in the required two-dimensional photonic arrangement. The LB silica masks and the resulting silicon structures were studied by field emission scanning electron microscope analysis.

On the universal growth of two-dimensional aggregates of hydrophobed glass beads formed at the (aqueous solution of electrolyte)-air interfaces

Abstract Two-dimensional aggregation of surface-modified glass beads was carried out at water (aqueous solution of electrolyte)—air interfaces. The effects of particle hydrophobicity and electrolyte concentration on the structure of the forming aggregates and on the growth process were investigated. The growth was characterized by density functions which demonstrate the change in mean particle number density as a function of aggregate size. The aggregation yielded fractal or non-fractal structures, depending on aggregate size. The fractal character for small aggregates was found to be dependent on particle hydrophobicity as well as on electrolyte concentration. However, above a certain electrolyte concentration, where restructuring of the growing aggregates was hindered slightly, practically the same values of fractal dimensions were obtained for investigated systems having different hydrophobicities. This result was attributed to the universal aspect of two-dimensional growth.

Characterisation of silica nanoparticulate layers with scanning-angle reflectometry

Scanning-angle reflectometry using p-polarized laser light is a sensitive in situ method to characterize the structure of very thin surface layers when the reflectance is scanned around the Brewster angle of the substrate. Monolayers of silica particles (of size 40 nm and of different hydrophobicity) on the water surface were investigated with this method. We used both a uniform-layer model to evaluate the measured reflectance curves and a “gradient refractive index layer” approximation. The uniform-layer approximation yields both an effective refractive index and a layer thickness, but it breaks down when the layer is strongly inhomogeneous, i.e. the size of particles is comparable with the wavelength and/or the refractive index of the particles differs much from that of the host material. At the air–water interface, the particles are partially submerged in the liquid phase and partially surrounded by air, so there is a steep refractive index change inside the layer. We elaborated a model which takes in depth inhomogeneity into account, and the structure parameters, such as the immersion depth, the particle diameter, the nearest-neighbour distance and the surface coverage, were determined by fitting the theoretical reflectivity to the measured reflectance curves. The effective refractive index of the hydrophilic layers was considerably higher than that of the hydrophobic ones, and that proved to correlate with the immersion depth of the particles. The parameters also changed with the applied surface pressure.

Langmuir and Langmuir−Blodgett Films of Bidisperse Silica Nanoparticles

We present the studies on the structure and optical properties of bidisperse Stöber silica nanoparticulate Langmuir films prepared at the air/water interface in a Wilhelmy film balance and transferred onto glass slides using the Langmuir-Blodgett technique. Three different compositions (covered area ratios: 4:1; 1:1, and 1:4) of two bidisperse systems were used in the experiments. Bidisperse samples (B1 and B2) were prepared by mixing the appropriate amount of monodisperse sols of particles with 61 and 100 nm diameters (B1) and those with 37 and 100 nm diameters (B2). By surface pressure-area isotherms and (transmission and scanning) electron microscopy images we provide information about the structure of the films. Optical properties of the supported films were measured with UV-vis spectroscopy and the transmittance spectra were evaluated in terms of an optical model which allows monotonous in-depth variation of the refractive index across the film. (1) We have found that the refractive index decreased from the substrate-layer interface toward the air-layer interface when the smaller particles were in majority, and increased otherwise. That would suggest that the smaller particles of each bidisperse system can be positioned at the air side of the film if they are in minority in the sample and they can be situated on the substrate if they are in majority. The scanning electron microscope images of bidisperse films supported the in-depth film structure suggested by optical studies.

Use of the optical admittance function and its WKB approximation to simulate and evaluate transmittance spectra of graded-index colloidal films

A method is presented for the simulation of transmittance spectra of graded-index dielectric layers deposited on a transparent substrate of finite thickness, and the simulation process is applied to films of silica and zinc oxide nanoparticles. The method is based on the optical admittance by solving its differential equation for an appropriate refractive index profile. When the refractive index changes with depth z according to the function N(z) = N(0)/(1+z/ζ)2, an exact solution exists which is equivalent to the WKB (geometric optical) approximate solution derived for the electromagnetic field in weakly inhomogeneous media. Using this solution allows us to characterize transparent graded-index films by two optical parameters—the mean refractive index and the grade of inhomogeneity, in addition to the film thickness through a fast curve fitting process.

Experimental study of the aggregate structures formed in the boundary layer of water-air phases

Abstract Two-dimensional aggregation of surface-modified glass beads was carried out in the boundary layer of water—air phases. The effect of the hydrophobicity of the beads on the structure of forming aggregates and on the growth process was investigated. The structure of the aggregates and their growth were characterized by a density function which expresses the change of mean density of the aggregates as a function of size. The growth yielded fractal or non-fractal structures in the systems studied. The fractal structure of the aggregates was dependent on restructuring processes controlled by the surface properties of the beads.

Complex Langmuir−Blodgett Films of SiO2 and ZnO Nanoparticles with Advantageous Optical and Photocatalytical Properties

Multifunctional Langmuir-Blodgett (LB) films were fabricated on the surface of glass substrates using sol-gel derived ZnO and SiO2 particles. ZnO particles of 6 and 110 nm diameter were synthesized according to the methods of Meulenkamp and Seelig et al. (Meulenkamp, E. A. J. Phys. Chem. B 1998, 102, 5566; Seelig, E. W.; Tang, B.; Yamilov, A.; Cao, H.; Chang, R. P. H. Mater. Chem. Phys. 2003, 80, 257). Silica particles of 37 and 96 nm were prepared by the Stober method (Stober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62). Alternate deposition of monoparticulate Langmuir films of SiO2 and ZnO nanoparticles provided complex (six- and nine-layered) LB films with both antireflective and photocatalytic properties. The LB films were investigated with scanning electron microscopy (morphology and structure) and UV-vis spectroscopy (optical properties and stability). The photocatalytic activity was measured by immersing the UV-irradiated films into an aqueous solution of Methyl Orange and following the photodegradation of the dye by optical spectroscopy. Adding ZnO particles to the silica films slightly lowered the antireflection property but ensured strong photocatalytic activity. Both the photocatalytic activity and antireflection properties were proved to be sensitive to the sequence of the silica and ZnO layers, with optimum properties in the case of nine-layered films with a repeated (SiO2-ZnO-ZnO) structure.