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PubMed1997

The Effect of Neutral Polymer and Nonionic Surfactant Adsorption on the Electroacoustic Signals of Colloidal Silica

The effects of adsorbed neutral polymer and nonionic surfactant on the electroacoustic signals of silica particles have been determined. The dynamic mobility of the particles was measured using an AcoustoSizer (Matec Applied Sciences) during the addition of poly(vinyl alcohol) (PVA) and nonyl phenol ethoxylate (C9phiEN) to the system. Large changes in the dynamic mobility of the coated particles were observed. A theoretical treatment was developed to describe the electroacoustic behavior of an elastic gel layer of uncharged adsorbed polymer and provided a reasonable fit to the experimental dynamic mobility data. This analysis provided information about the thickness and elasticity of the adsorbed layers. The adsorbed layer thickness for PVA compared well with results of other techniques reported in the literature and appeared to be influenced by changes in pH. C9phiEN adsorbed in a relatively dense layer. For both PVA and C9phiEN, the thickness of the inner adsorbed layer remained approximately constant, while the outer layer became thicker with increasing concentration and molecular weight. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

Molecular Transport and SAPO-11 Crystal Growth in an i-Pr2NH-AlPO4-H3PO4-SiO2-H2O System

The crystal growth of SAPO-11 molecular sieve synthesized with i-Pr2NH as a novel template from mixtures of Si, Al, and P sources were studied by XRD, MAS-NMR, IR, and AAS techniques. It was found that the crystallization was improved partially by the desorption of i-Pr2NH from the crystallite products at high temperatures. The growth on (010) face was enhanced by stepwise adding external silica sol to the reaction mixtures during the synthesis process. The molecular transport and mechanism of the crystallization were discussed. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

Effect of Solvent Quality on Reverse Micelle Formation and Water Solubilization by Poly(ethylene oxide)/Poly(propylene oxide) and Poly(ethylene oxide)/Poly(butylene oxide) Block Copolymers in Xylene

In addition to associating into ("normal") micelles in aqueous solutions, amphiphilic polyoxyalkylene block copolymers can form "reverse" micelles in organic solvents at sufficiently high copolymer concentrations (above the critical micellization concentration (CMC)) and in the presence of some water. The effects of solvent quality on the copolymer micellization in an organic solvent and on the solubilization of water in such systems are examined here for representative poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) and poly(butylene oxide)-b-poly(ethylene oxide) (PBO-PEO) copolymers. The solvent quality is modulated by the addition of cosolutes and by a change in the temperature. A number of notable observations are reported: Worsening the aqueous solvent conditions by the addition of NaCl (10 wt% with respect to water) almost doubles the CMC and the water solubilization capacity (WS) of a PEO-PPO-PEO copolymer in p-xylene. An increase in temperature makes water a worse and xylene a better solvent for the copolymer. The combined result of heating is an increase of the CMC for all three copolymers studied. This indicates that the formation of reverse micelles is exothermic, which is opposite to what has been observed for normal micelles. The effects of temperature on the water uptake are nonmonotonic: WS increased with temperature for a PEO-PPO-PEO copolymer with 40% PEO but decreased for a copolymer with 20% PEO. An increase in WS with temperature, followed by a decrease, with the maximum WS efficiency occurring at the "effective" cloud point of the copolymer is proposed in order to explain this observation. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

Sorption of Metal Ions on Alumina

Precipitation and adsorption of metal ions (Co(II), Ni(II), Cu(II), and Cr(III)) on gamma-alumina were investigated experimentally. A surface chemical reaction model to calculate concentrations of aluminum ions, metal ions, and pH as variables depending on amount of alumina, volume of liquid and gas phase, initial metal concentration, and amount of acid or base added is presented. In the case of Co(II) the pH dependence of rest concentrations with and without alumina is equal; adsorption may be disregarded. For the other ions adsorption is important. Considering the charge of the surface does not improve the fit. In the pH region, where adsorption leads to lower rest concentrations than precipitation, adsorption may be described by a Henry isotherm. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

Surface Tension and Viscoelasticity of Alkane Solubilized Surfactant Monolayers

The behavior of solubilized alkanes at the surface of DoTAB solutions for various surfactant bulk concentrations below and above the cmc is described. Several techniques such as tensiometry, ellipsometry, and Brewster angle microscopy, as well as an excited capillary wave device are used for the study. Alkanes do not wet the surface completely and form a mixed monolayer with the surfactant. Rather unusual viscoelastic behavior is observed for the mixed monolayers. This suggests a new type of relaxation phenomenon, but its origin remains to be elucidated. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

Determination of the Surface Tension of Microporous Membranes Using Contact Angle Measurements

In this paper, a new method of determining the surface tension of the solid material that a microporous membrane is made from is introduced. The method is based on the well known determination of the so-called contact angle that is formed on the solid/liquid/gaseous three phase line. A nonideal state of the solid phase leads to a deviation of the contact angle that can be observed experimentally from the equilibrium angle that arises from the thermodynamically state of lowest energy, as it must be used to calculate the solid surface tension via the Young equation. The deviation caused from the porous structure of the solid material will be taken into account in this work. Doing so, we derived an equation that connects the surface porosity, the measured contact angle, and the equilibrium contact angle. Using this equation, the measured and therefore deviated contact angles can be corrected for the porosity of the solid material, yielding the contact angle observable on a surface made from the same but nonporous material. The equation derived was tested on different microporous membranes made from expanded poly(tetrafluoroethylene). The surface porosity needed was determined using scanning electron microscopy followed by computerized image analysis. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

Monte Carlo Study of Chemically Associating Polymerizing Two-Dimensional Fluids

NVT Monte Carlo simulations are reported for chemically associating two-dimensional fluids which can polymerize for certain interaction energies, due to the presence of two attractive sites per monomeric hard disc. The sites are fixed inside a hard core at a given valence angle and mutual penetration of discs is permitted. The type of products of polymerization depends on the parameters of the model; we observe the formation of small associates, as well as of extended chains, bent chains, and rings with different number of monomers. The values of valence angles and of association energy are of primary importance. The dependence of the structural properties of the model on fluid density and association energy is investigated. We performed detailed analysis of the clusters formed due to association in terms of fractions of singly and doubly bonded particles, of average numbers of chains and rings, and of their size. We also obtain the average end to end distance, the radius of gyration, and the persistent length of the products of polymerization. The pressure is calculated from the density profiles of particles of the polymerizing fluid near a hard "wall." The data can be used to develop the equation of state for chemically associating two-dimensional fluids. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

Assessing Extreme Models of the Stober Synthesis Using Transients under a Range of Initial Composition

29Si-NMR, conductimetry, and photon correlation spectroscopy are used to monitor the temporal profile of intermediate concentrations in Stober synthesis (i.e., ammonia-catalyzed hydrolysis of tetraethoxysilane in a batch reactor). Extreme models of the process are assessed by examining the effect of initial composition on these transients (over a wider range of composition than attempted previously). The trends with initial composition suggest that the nucleation is rate-limited by the hydrolysis of the singly hydrolyzed monomer, the product of which probably phase separates. Moreover, the trends are consistent with the aggregation model discussed by G. H. Bogush and C. F. Zukoski (J. Colloid Interface Sci. 142, 1, 19 (1991) and by M. T. Harris (Ph.D. dissertation, Univ. of Tennessee, 1992). The trends are not consistent with a growth model without aggregation. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

Spectroscopic Studies of Dextrin Adsorption onto Colloidal ZnS

The interaction of dextrin with colloidal ZnS has been investigated through adsorption studies and FT-IR spectroscopy in the 4000-400 cm-1 range. The adsorption capacity is estimated to be around 1 mg/m2. Maximum adsorption is found to be constant below pH approximately 7 and to increase with pH at least up to pH 11. Eighty percent of maximum adsorption is achieved within 3 min after addition of the dextrin. Based upon FT-IR studies and titration data, an adsorption mechanism is proposed. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

Electrokinetic Effects on Pressure-Driven Liquid Flows in Rectangular Microchannels

The effects of the electrical double layer near the solid-liquid interface and the induced electrokinetic field on the pressure-driven liquid flow through a rectangular microchannel are analyzed in this work. A nonlinear, two-dimensional Poisson-Boltzmann equation governing the electrical double layer field in the cross section of rectangular channels is numerically solved with the use of a finite-difference scheme. A body force caused by the electrical double field and the flow-induced electrokinetic field is considered in the equation of motion. An exact solution to this equation of motion in rectangular microchannels is obtained by employing the Green function formulation. The effects of the ionic concentration of the liquid, the zeta potential of the solid surface, and the size and the shape of microchannels on the fluid velocity distribution, streaming potential, volumetric flow rate, friction coefficient, and apparent viscosity are discussed. The results clearly show that for a liquid solution of low ionic concentration and a solid surface of high zeta potential the liquid flow in rectangular microchannels is significantly influenced by the presence of the electrical double layer field and hence deviates from the flow characteristics described by classical fluid mechanics. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

Preparation and Characterization of Surface-Covered Nanometer-Sized Catalyst by Carboxylate Phase Transfer

Surface-covered nanometer-sized CuO/Al2O3, CuO-ZnO/Al2O3, and CuO/ZnO catalysts were prepared by phase transfer with carboxylate. The surface-covered structure was studied using XPS, XRD, TEM, and high-resolution electron microscope. The results indicated that amorphous CuO and ZnO were dispersed on the surface of nanometer-sized Al2O3 particles in catalysts CuO-ZnO/Al2O3 and CuO/Al2O3, respectively. The thickness of its surface layer is about several angstroms. No spinel structure was found in the catalyst. The particle size of the surface-covered CuO/Al2O3 catalyst was about 2-3 nm. Al2O3 in the catalyst was amorphous. Surface-covered material CuO in the catalyst with low CuO content was also armophous. A large amount of copper carboxylate resulted in crystalline CuO and Cu2O. The protective ability of carboxylate to sol particles differs from the metal element of carboxylate. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

Additive Effects on the Microstructure Evolution in Hexadecyltrimethylammonium Bromide Solution and Its Rheological Properties

The effects of various additives on the microstructure evolution in hexadecyltrimethylammonium bromide micellar solution and its rheological properties are investigated. The additives considered in the present study are primary, secondary, and tertiary heptanols and sodium salicylate (NaSal). The microstructure is developed via the two-step shape transitions in the micellar solution; first, the initial spherical micelles undergo shape transition to rod-like or disc-like micelles as the micelles tend to be packed compactly with increase in the surfactant concentration; then further increment in the surfactant concentration makes the anisotropic rod-like micelles overlap each other. Solutions in these states exhibit typical non-Newtonian behaviors such as shear thinning at high shear rates. In the present study, the additive effects on the microstructure evolution are investigated by employing various techniques including a phase modulated flow birefringence, a dynamic light scattering and a dynamic oscillatory rheometry. The results show that addition of the solubilized additives enhances the microstructure transitions, which are affected by the additive concentration and its chemical structure. Presence of the cosurfactants such as heptanols with hydrophobic alkyl chains reduces the repulsion by forming surfactant-alcohol mixed micelles. The primary heptanol can penetrate easily into the micelles and aligned parallel to the surfactant molecules compared with the secondary and tertiary heptanols. Thus, the primary heptanol enhances the two-step shape evolutions more effectively than the other types. In the presence of NaSal, on the other hand, the micellar solution exhibits the viscoelastic properties and the yield stress owing to the formation of networked or worm-like micelles. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

Percolating Behavior of Solid Bimodal (Polymethylmethacrylate + Inert) Particle Packings during Polymerization

A percolation-based thermodynamical model was able to predict the break-down of bimodal PMMA + I solid particle packings (P-MMA, polymethylmethacrylate; I, solid phase inert to MMA polymerization) due to the polymerization process of the MMA monomers. By slip casting of biphasic PMMA + I aqueous suspensions, homogeneous samples of polymethylmethacrylate and various inert solid aggregates (I = Ca3(PO4)2, ZrO2, Si3N4, and Al2O3) were prepared and embedded in liquid methylmethacrylate to start the kinetics of the network formation. For any I component, breakdown of samples did not occur in well defined PMMA mass concentration ranges, which were strongly related to the signs of PMMA and I surface charges (i.e., isoelectric points) of the aqueous solid monodispersed systems. As application of percolation theory in composite materials suggest, when the samples resisted the polymerization kinetics the occurrence of a percolative behavior was expected. Accordingly, the model was performed by dealing thermodynamically with the physicochemical features of the starting aqueous suspensions (e.g., adsorption from solution at the solid/liquid interface and solid agglomeration) at the percolation threshold in bicontinuous PMMA + I systems during the progress in the polymerization reaction. Thresholds have been regarded as percolation of cubic units and related to the sign of the solid PMMA and I surface charge in aqueous monodispersed systems, namely, to the attractive, neutral, repulsive character of the electrostatic interparticle forces. Once site percolation thresholds were correctly estimated it was shown that the assumption of percolation is compatible with the theoretical model when the solid PMMA mass concentration values are ranging just in the critical ranges experimentally obtained. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

2-D and 3-D Interactions in Random Sequential Adsorption of Charged Particles

We explore the influence of electrolyte concentration on the adsorption of charged spheres using modeling techniques based on random sequential adsorption (RSA). We present a parametric study of the effects of double layer interactions between the charged particles and between the particle and the substrate on the jamming limit using a two-dimensional RSA simulation similar to that of Z. Adamczyk et al. (1990, J. Colloid Interface Sci. 140, 123) along with a simple method of estimating jamming limit coverages. In addition, we present a more realistic RSA algorithm that includes explicit energetic interaction in three dimensions, that is, particle-particle and particle-surface interactions during the approach of a particle to the substrate. The calculation of interaction energies in the 3-D RSA model is achieved with the aid of a three-body superposition approximation. The 3-D RSA model differs from the 2-D model in that the extent of coverage is controlled by kinetic rather than energetic considerations. Results of both models capture the experimentally observed trend of increased surface coverage with increased electrolyte concentration, and both models require the value of a key model parameter to be specified for a quantitative match to experimental data. However, the 3-D model more effectively captures the governing physics, and the parameter in this case takes on more meaningful values than for the 2-D model. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

Properties of a Mixed Micellar System of Sodium Dodecyl Sulfate and Octylglucoside

Properties of a binary mixed surfactant system of a nonionic surfactant, octylglucoside, and an anionic one, sodium dodecyl sulfate, were investigated by measurement of the surface tension of their aqueous solutions containing 20, 75, and 150 mM sodium chloride. Changes in the critical micelle concentration with the composition of the two surfactants thus determined were analyzed according to the conventional regular solution theory for mixed micelles as well as with reference to Maeda's formulation for ionic/nonionic mixed micelles (H. Maeda, J. Colloid Interface Sci. 172, 98 (1995)). For the three salt concentrations, the properties of the mixed micellar system were found to be well described by the above theories, and the parameters in Maeda's formulation, B1 and B2, which are equivalent to the interaction parameter in the conventional theory of regular solution approach were determined. The effect of salt concentration on the properties of the mixed micellar system is discussed. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

Evaluation of Surface Plasmon Resonance (SPR) for Heparin Assay

The concentration of heparin, an anticoagulant in blood, is usually inferred from clotting type assay, which determines a parameter related to the heparin activity. Because of the heterogeneity of heparin, however, it is desirable to monitor the absolute concentration of heparin directly in the clinical range of 0-2 U/ml. Surface plasmon resonance (SPR) provides a optical direct method of monitoring binding events. Gold films, as required for SPR, were modified with protamine; the immoblized protamine interacts electrostatically with heparin so that the heparin adsorption is dependent on the absolute concentration. The "thickness" of the immobilized protamine layer determined the linear range of the sensor's response and the sensitivity. Less densely packed layers of protamine showed a lower detection limit for heparin, suggesting a mixing of the heparin into the incomplete protamine layer. On the other hand, thicker, denser protamine layers did not show a low concentration sensitivity to heparin although their maximum heparin binding capacity was increased. It was shown that the linear response range of the protamine modified SPR device to heparin could be modulated by altering both the protamine loading and its method of immobilization. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

Evaluation of Surface Plasmon Resonance (SPR) for Heparin Assay

Heparin adsorption onto polyethylene imine (PEI) films was investigated. The adsorption was followed with time-dependent surface plasmon resonance (SPR) for PEI cast on gold films, and the thickness of the adsorbed heparin was determined by fitting the angular reflection curves to Fresnel's equations. The average thickness of the PEI determined the heparin adsorption range and it was found that both heparin binding capacity and sensitivity increased with PEI thickness. At an appropriate thickness of 10 ± 2 nm, adsorption of heparin in the clinically applicable range of 0-2 U/ml caused a linear concentration dependent change in the SPR reflectivity at a fixed angle, with excellent sensitivity (0.05 U/ml). The results are compared to heparin adsorption on immobilized protamine and the surface coverage and probable heparin distribution discussed. Copyright 1997 Academic Press. Copyright 1997Academic Press

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PubMed1997

Determining Surface Areas from Linear Adsorption Isotherms at Supercritical Conditions

Here we demonstrate that there is a wide range of pressures at supercritical temperatures where there is essentially monolayer coverage (i.e., there is nearly a complete layer of molecules adsorbed adjacent to the surface, but very little excess density in the second and subsequent layers). In this regime, the dependence of the Gibbs adsorption on the density is linear, and coefficients of this linear function depend on the monolayer capacity and surface area. It is shown that measurements of the Gibbs adsorption in this regime can be used to determine surface area without knowledge of the area per molecule or a model for the adsorption isotherm. This new method has the potential to provide reliable values of specific surface area for macroporous, mesoporous, and microporous systems without the limitations of previous methods. Lattice-theory calculations suggest that the best conditions for surface area measurements are in the supercritical region with temperatures about twice the critical temperature and pressures well above the critical pressure. These conditions allow nearly complete monolayer coverage but there is not multilayer adsorption because the temperature is sufficiently above the critical temperature of the adsorbate. Copyright 1997 Academic Press. Copyright 1997Academic Press

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