Showing papers in "Langmuir in 1992"

Mechanism of formation of two-dimensional crystals from latex particles on substrates

Abstract: The dynamics of two-dimensional ordering of micrometer-size polystyrene latex spheres on a horizontal glass substrate has been directly observed by means of optical microscopy. It turns out that the ordering starts when the thickness of the water layer containing particles becomes approximately equal to the particle diameter. By variation of the electrolyte concentration, the charge of the particles, and their volume fraction, it is proven that neither the electrostatic repulsion nor the van der Waals attraction between the particles is responsible for the formation of two-dimensional crystals. The direct observations revealed the main factors governing the ordering-the attractive capillary forces (due to the menisci formed around the particles) and the convective transport of particles toward the ordered region. The control of the water evaporation rate turns out to allow obtaining either well-ordered monolayers or well-ordered domains consisting of multilayers (bilayers, trilayers, etc.).

Synthesis and characterization of colloidal dispersions of fluorescent, monodisperse silica spheres

Abstract: Stable dispersions of monodisperse colloidal silica spheres containing a dye or fluorophore have been synthesized according to a general procedure and dispersed in polar and apolar liquids. The procedure consists of the coupling of the dye to a silane coupling agent, (3++minopropyl)triethoxysilane, and the controllable incorporation of the reaction product into the silica sphere. The silica spheres are prepared from tetraethoxysilane in mixtures of ammonia, water, and ethanol. The composition of the silica spheres can be controlled in such a way that the organic groups can be placed on the surface, in a thin shell inside the particle or distributed through the volume of an inner core. Fluorescein isothiocyanate was used to make easily bleachable, fluorescent silica spheres. Hydrophilic charge stabilized and organophilic sterically stabilized 1-octadecanol-coated dyed silica systems were synthesized and dispersed in several solvents. All the particles were characterized after the several reaction steps by static and dynamic light scattering and transmission electron microscopy. The fluorescent spheres were further characterized by fluorescence spectroscopy and confocal scanning laser fluorescence microscopy. Great effort was taken to prepare monodisperse dispersions free of clusters of particles. Such model dispersions are required for (scattering) studies of interparticle interactions in (concentrated) systems. Therefore, the several steps of the synthesis and optical characterization are described in detail.

Shear-dependent slippage at a polymer/solid interface

Abstract: We discuss shear flows of a polymer melt near a solid surface onto which a few chains (chemically identical to the melt) have been grafted. At low shear rates a a*. This transition may be important in the processing of polymers, where a few chains from the melt can be bound on an extruder wall and play the role of the grafted chains.

Mechanism of adsorption and desorption of water vapor by homoionic montmorillonite. 1. The sodium-exchanged form

Abstract: The structural changes which occur during the intracrystalline swelling of a well-characterized Wyoming sodium montmorillonite have been investigated using controlled-rate thermal analysis, nitrogen adsorption volumetry, water adsorption and desorption gravimetry, immersion microcalorimetry in water, and X-ray diffraction under controlled humidity conditions. The experimental X-ray powder patterns of the 001 reflections have been compared with the theoretical simulations to determine the structural change of the montmorillonite during hydration and dehydration. 85 refs., 13 figs., 4 tabs.

Equilibrium measurements of oscillatory disjoining pressures in aqueous foam films

Abstract: Disjoining pressure isotherms are measured for single, isolated foam films stabilized with sodium dodecyl sulfate above the critical micelle concentration (cmc). The measured isotherms exhibit an equilibrium oscillatory component that extends out to film thicknesses as large as 50 nm. The oscillations originate from surfactant structuring within the film and are responsible for the multiple black films extensively studied by Perrin in 1918. We find that as film thickness decreases, the slopes and peak heights of the oscillations increase for each subsequent black film encountered. The discrete changes in film thickness observed at each black film transition range from 6 to 10 nm and are a function of the ionic strength and surfactant concentration. Pressures required to rupture the multiple black films are several orders of magnitude lower than the final film-rupture pressure. Finally, the oscillatory form of the disjoining pressure permits quantitative interpretation of stepwise thinning behavior observed in films containing surfactant above the cmc. 50 refs., 13 figs.

Electrokinetics of the silica-solution interface: a flat plate streaming potential study

Abstract: Details for the construction of a flat plate streaming potential apparatus are presented. Measurements using this apparatus on fused silica slides are reported and analyzed on the basis of a Gouy-ChapmanStern-Grahame (GCSG) model for the electrical double layer at the silica-aqueous solution interface. It is found that the GCSG model can only partly account for the experimental results. Possible reasons for the discrepancy between experiment and theory are discussed.

pH-dependent interactions between adsorbed chitosan layers

Abstract: Chitosan is a cationic polyelectrolyte obtained after N-deacetylation of chitin by alkaline treatment. The adsorption of chitosan and the interaction between chitosan layers have been investigated ...

Aggregation of Alkyltrimethylammonium Surfactants in Aqueous Poly(styrenesulfonate) Solutions

Abstract: The aggregation of alkyltrimethylammonium surfactants &TA+ and ClsTA+ in dilute water solutions of sodium poly(styrenesu1fonate) has been investigated. Aggregation numbers were estimated with the time-resolved fluorescence quenching technique. In the calculations, results from binding isotherms and solubility measurements were used. Binding isotherms for dodecyltrimethylammonium bromide to the polyelectrolyte were determined using a surfactant-selective electrode. The aggregation numbers were found to be independent of the concentration of surfactant and type of counterion, but to increase with increasing surfactant tail length. From the kinetics of the quenching of pyrene fluorescence with hydrophobic and hydrophilic quenchers, it was concluded that compact aggregates with net negative charge were formed, in which the polyelectrolyte is intimately associated with the surfactant. The aggregates are joined by surfactant-free parta of the polyelectrolyte chain, the lengths of which depend on the amount of bound surfactant. The quencher dimethylbenzophenone was found to migrate between the aggregates at the highest concentration of the long-tailed surfactant.

Effect of surfactant on the solution properties of hydrophobically modified polyacrylamide

Abstract: The interactions, in aqueous solution, of sodium dodecyl sulfate (SDS) with copolymers of acrylamide (AM) and N-(4-ethylphenyl)acrylamide have been examined in the semidilute concentration range by rheology, fluorescence, and conductometry. Data are discussed with regard to changes in the hydrophobic content, the molecular weight, and the microstructure of the copolymers

Borohydride reductions of metal ions. A new understanding of the chemistry leading to nanoscale particles of metals, borides, and metal borates

Abstract: The reduction of CoClz(aq) with NaBH4 yields ultrafine particles of CozB as the primary product if the two reagents are rapidly mixed and the product is handled under argon. However, the CozB is converted to Co(s) particles plus boron oxides if exposed to oxygen (sacrificial reaction where boron is oxidized while cobalt is reduced). Still a third product, Co(BOz)z, can be formed if time is allowed for the Co2+/CozB catalysis of the aqueous BH4BOz-(aq) oxidation to occur before the major part of the CoClz(aq) is added. The Co(B02)~ is converted to Co3(B0& and small amounts of Co on heat processing at 500 "C.

Use of the Washburn equation for surface free energy determination

Abstract: The thin-layer wicking technique was used to determine the solid surface free energy components for thin-layer chromatography silica (Merck) by the Washburn equation. It has been found that wick-wetting is a reproducible process and the results of the wetting rates expressed as the relationship of the penetrated distances squared, x2 , are a linear function of time, t. However, in some systems an inflection appeared at a distance 3 5 x 5 5 cm, giving thus two straight-line sections with somewhat different slopes. nAlkanes, diiodomethane, water, and formamide were used as the penetrating liquids. Van Oss et al.'s approach to the surface free energy was applied. Thus, apolar, ysLw, and polar electron donor, ys, and electron acceptor, ys+, components were determined for both the laboratory-made glass plates with a deposited Si02 thin layer as well as commercial ones. Practically the same value of ysLw was obtained for both kinds of plates. However, the commercial plates showed higher values of the yscomponent. It is probably due to the presence of some adhesive additives. The polar ys+ component is small for both plates. It is believed that this technique and the approach to the surface free energy components are very useful for testing the wetting properties of solids by liquids.

Imaging and Etching of Self-Assembled n-Octadecanethiol Layers on Gold with the Scanning Tunneling Microscope

Abstract: Self-assembled monolayers of n-octadecanethiol on Au( 11 1) were studied by scanning tunneling microscopy (STM). The STM images, taken with a high bias (- 1 V) and small (- 1 nA) tunneling current, show self-assembled monolayers with defect sites (pits). Continued scanning in this mode caused small lateral expansion of the pits and eventually a complete morphological change of the n-octadecanethiolcovered surface. Purposeful etching of the self-assembled monolayers could be achieved by bringing the STM probe tip closer to the substrate using a low bias potential (10 mV) and a high tunneling current (10 nA) or by the application of a high bias potential (3 V) for a few seconds. A positive substrate bias (+3 V) pulse always led to pit formation, while a negative (-3 V) substrate bias pulse produced mounds. These mounds could be removed with a positive bias pulse. The modifications induced by potential pulses were attributed to field evaporation of material from substrate or tip material. The atomic structures of etched Au surfaces using an STM tip or strong oxidizing agents, such as chromic acid and peroxide (H2SO4:HzO2 = 7:3) solutions, were remarkably different. The surface structure of the etched area using an STM tip showed complete distortion from the Au(ll1) surface, whereas peroxide solution etching preserved the original Au(ll1) structure.