Rheological behavior of moderately concentrated silica suspensions in the presence of adsorbed poly(ethylene oxide)

Adsorption and flocculation properties have been investigated for a well-characterized aqueous system of precipitated nanometric silicabeads in the presence of a high molecular-weight poly(ethylene oxide). Particular attention was given to the evolution of these properties with increasing concentration. In addition, the corresponding consequences on the rheological behavior in the homogeneous domains of the phase diagram of the systems were studied. Important rheological effects can actually be obtained with such systems, even at moderate values of the volume fraction of the dispersed phase (5·10^–35·10^–2).According to the adsorption level, the number of particles per macromolecular chain, and the free polymer equilibrium concentration, quite different phenomena were observed such as rheopectic shear-induced gelation, Newtonian flow or thixotropic shear-thinning.     

The DLVO theory in microbial adhesion

Adhesion of microorganisms to various interfaces has been explained by the classical Derjaguin–Landau–Verwey–Overbeek (DLVO) theory of colloid stability. The theory has been used as a qualitative model, but also in a quantitative way to calculate adhesion free energy changes involved in microbial adhesion. In this paper some important investigations will be review that show how the DLVO theory is used in microbiology, mainly for bacteria. Other models have also been developed in order to predict adhesion, such as the thermodynamic approach and later the extended DLVO theory. These will be discussed in relation to the ‘classical’ DLVO theory. The theories assume that microbial cells behave as inert particles. The implications that follow from the fact that they are biologically active and have heterogeneous cell surfaces will also be exemplified and discussed.     

镁铝类水滑石/阳离子淀粉分散体系流变学振荡行为研究

采用稳态剪切和小振幅振荡剪切实验研究了镁铝类水滑石化合物(Mg-Al HTlc)/阳离子淀粉(CS)分散体系(HTlc与CS质量之比R分别为0, 0.02, 0.05和0.08)的流变学性质．实验结果表明: 上述体系在实验pH值(5～11)范围内均为剪切稀释非牛顿流体, 其中, Mg-Al HTlc/CS (R＝0, 0.02和0.05)分散体系的流变曲线符合幂律模型, 而Mg-Al HTlc/CS (R＝0.08)体系符合Herschel-Bulkley模型. 上述体系先于1000 s^－1剪切180 s, 再于1 s^－1(或者3 s^－1)低速剪切时均表观出粘度随时间周期变化的振荡现象. 振荡现象缘于CS分子结构的周期变化. pH值、剪切速率和HTlc含量的变化可以改变振荡周期(频率)和振幅. 低pH值条件下, Mg-Al HTlc/CS (R＝0.08)体系先于1000 s^－1剪切180 s, 再于小振幅振荡剪切(剪切应力0.03 Pa, 频率0.5 Hz)时储能模量G?和损耗模量G?也表现出随时间周期变化的振荡现象.     

Palladium nanoparticles from solvated atoms—stability and HRTEM characterization

Palladium particles of nanometric dimensions were synthesized by the chemical liquid deposition method in which the Pd atoms were co-deposited at 77 K with 2-propanol, acetone, and tetrahydrofurane vapor to obtain colloidal dispersions. The colloidal dispersions were characterized by UV–visible spectrophotometry, transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). The palladium colloids synthesized by use of these solvents are very stable. A strong absorption band in the UV region suggests that quantum confinement occurs in the nanoparticles obtained by this procedure. Studies of TEM micrographs reveal average size distributions between 1 and 5 nm for all Pd colloids. Whereas particle sizes in Pd–2-propanol colloids are not very sensitive to concentration change, the particle-size average in Pd–acetone and Pd–THF increases by 0.5 nm when the concentration increases from 10^–3 to 10^–2 mol l^–1. The HRTEM results show the high crystallinity of Pd nanoparticles and three low-energy structure shapes were found: cuboctahedron, tetrakaidecahedron, and icosahedron.     

Estimation of kinetic rate constants by turbidity and nephelometry techniques in a homocoagulation process with different model colloids

 In this work turbidimetric and nephelometric techniques have been used to study the homocoagulation of aqueous dispersions of uniform spherical particles of surfactant-free latexes. Cationic and anionic latexes of similar particle sizes (361 and 370 nm) and different surface charge densities (+16.4 and −3.6 μC/cm²) were used throughout. The kinetic constants which control the aggregation processes when the electrical repulsion disappears were estimated by both techniques at different particle concentration and wavelength in order to establish the experimental conditions which provided reliable and similar values for the coagulation rate constant. Both experimental techniques (turbidity and nephelometry) and two ways of fitting the data have been used with both latexes. For the first method, the initial slope of turbidity or total scattered intensity versus time curves were used to calculate the kinetic constants. In the second method, the whole turbidity or total scattered intensity versus time curves were fitted and the kinetic constants calculated. An unambiguous experimental value for the doublet rate constant in diffusion conditions is obtained by turbidity and nephelometry techniques. By nephelometry both data treatments have permitted a distinction between the doublet rate constant and the global rate constant in diffusion conditions. Received: 2 June 1997 Accepted: 14 August 1997     

Properties of TiO2-polyacrylic acid dispersions with potential for molecular recognition

Titanium dioxide (TiO₂)/polyacrylic acid (PAA) (TiO₂/PAA) particles were formed by mixing PAA and an acidic solution of TiO₂ nanoparticles in dimethylformamide (DMF) followed by heat treatment. TEM and particle analysis showed that the resulting particles had a narrow size distribution. The colloid was very stable and aggregation was not observed over a wide pH range (3–9) or at high salt concentration. The residual carboxylic acid of PAA could be modified via EDC/NHS activation to form an amide bond with a protein. An antibody was attached to the hybrid nanoparticle and specific binding to antigen was monitored by surface plasmon resonance. The results suggest that TiO₂/PAA nanoparticles are candidates as the base component of a photocatalytic system with potential for substrate selectivity.     

Comparison of the Rheological Behavior of Solutions and Formulated Oil-in-Water Emulsions Containing Carboxymethylcellulose (CMC)

In this study, it was aimed to compare the rheological properties of carboxymethylcellulose (CMC) in aqueous solutions and their corresponding emulsions containing 0.05, 0.1, 0.25, and 0.5% CMC in the aqueous phase. Samples with 0.05 and 0.1% CMC showed Newtonian behavior, but shear-thinning behavior was observed in CMC solutions and emulsions with increasing CMC concentrations to 0.25% and 0.5%. Rheological behavior of all samples were modeled by Power law (R ² = 0.986–197) and Casson models (R ² = 0.968–1). According to the Ostwald–de Waele model, the consistency index of all samples was increased and the flow behavior index decreased with increasing CMC concentration. Comparison of our data with four predicting models (Einstein, Larson, Pal, and Dougherty-Krieger equations) showed that the viscosity of continuous phase controls the viscosity of emulsions with high CMC concentrations and these models are not applicable for such situations. Addition of CMC increased the emulsion stability of O/W emulsions. This stability was increased with increasing CMC concentrations.     

Colloid stability of electrostatically stabilized sols

 The dependence of the stability of SiO₂ and Al₂O₃ sols on the pH of their medium has been studied. Vapor adsorption isotherms on powders were prepared from acidic and basic sols, the immersion enthalpy (heat) of samples containing preadsorbed water have been determined and, also, the reversibility of the sol ⇌ xerogel transformation, i.e. the peptizability of the powders, has been investigated. Based on the flocculation values determined with KCl, the sols have been classified into three groups. The stability of highly hydrophilic sols (acidic SiO₂- and Al₂O₃-sols) is ensured by a thick continuous diffuse lyosphere formed around the particles, as the continuity principle by Ostwald–Buzágh suggests. In this case, no electric charge is needed for ensuring stability. These sols are thermodynamically stable (group 1). Sols with medium stability are stabilized by the electrical double layer around the particles and by 1–2 layers of adsorbed water. The flocculation value of these sols is determined by the electrostatic interaction, whereas the peptizability of the flocs is related to formation of water layers. Such sols are the basic SiO₂- and Al₂O₃-sols (group 2). Sols of low stability are of hydrophobic nature. Their flocculation value with 1:1 electrolytes is smaller than 0.1 molkg^-1. The transformation process sol ⇒ floccule ⇒ xerogel is mostly irreversible. There are a lot of such sols (group 3). The existence of a hydrosphere is proved by the almost identical value of the hydration energy for both the acidic and the basic SiO₂ sols, in spite of an order of magnitude difference in the flocculation value. The remnants of adsorbed water after drying hinders sintering of the particles and ensures the peptizability of powders. The highly hydrophilic sols (sequence of hydrophilicity: SiO₂> Al₂O₃>FeO OH ⋅ 0.5 H₂O) are all oxides which are formed in acidic media. Received: 25 May 1997 Accepted: 13 October 1997     

DLVO and steric contributions to bacterial deposition in media of different ionic strengths

The deposition of eight bacterial strains on Teflon and glass in aqueous media with ionic strengths varying between 0.0001 and 1 M was measured and interpreted. Two types of interactions were considered: (1) those described by the DLVO theory, which comprise van der Waals attraction and electrostatic repulsion (bacteria and surfaces are both negatively charged); and (2) steric interactions between the outer cell surface macromolecules and the substrata. As a trend, at low ionic strength (<0.001 M), deposition is inhibited by DLVO-type electrostatic repulsion, but at high ionic strength (≥0.1 M) it is dominated by steric interactions. The ionic strength at which the transition from the DLVO-controlled to the sterically controlled deposition occurs, is determined by the extension of the macromolecules into the surrounding medium, which varied between 5 and 100 nm among the bacterial strains studied. The steric interactions either promote deposition by bridging or inhibit it by steric repulsion. Between Teflon and hydrophobic bacteria, bridging is generally observed. The surface of one bacterial strain contains amphiphilic macromolecules that form bridges with Teflon but induce steric repulsion on glass. The presence of highly polar anionic polysaccharide coatings on the cell impedes attachment on both glass and Teflon. For practice, the general conclusion is that the deposition of most bacteria is: (1) strongly inhibited by DLVO-type electrostatic repulsion in aqueous environments of low ionic strength such as rain water, streams and lakes; (2) controlled by DLVO and/or steric interactions in systems as domestic waste waters and saliva; and (3) determined by steric interactions only in more saline environments as milk, urine, blood and sea water.     

Effect of diutan microbial polysaccharide on the stability and rheological properties of O/W nanoemulsions formed with a blend of Span20-Tween20

The stability and rheological behavior of oil-in-water (O/W) nano-emulsions formed with a blend of Span20-Tween20 have been studied with and without diutan microbial polysaccharide. It is found that there exist thresholds for the water content and emulsifier to obtain stable nano-emulsions using the emulsion inversion phase (EIP) method. The viscosity of the nano-emulsion is proportional to the emulsifier content and inversely proportional to the water content. High emulsifier content is not conducive to the thermal stability of the nano-emulsion. The addition of diutan gum with negative charge into the nano-emulsions increases the electrostatic repulsion between droplets and makes the droplet size smaller and more unifom, slowing down the coalescence and Ostwald ripening of the nano-emulsions. Due to the association of the diutan gum double helix, a three-dimensional network structure is formed in the continuous phase of nano-emulsions, which improves the stability of nano-emulsions and is also the main factor giving the nano-emulsion high viscoelasticity at high temperature. This study offers new insight into the nano-emulsion containing microbial polysaccharide and may serve as a guideline for practical applications of new nano-emulsion systems.     

Effect of liquid oils on the properties of multiple emulsions containing liquid crystals

Recently, many cosmetic researchers have been focused on multiple emulsions due to better performance. Limited application of multiple emulsions has been attributed to their instability, which can be resolved by the presence of liquid crystals. Multiple emulsions containing liquid crystals are affected by various formulation parameters, such as liquid oils. In this paper, the influence of liquid oils on the formation mechanism was studied. Besides, stability, small-angle x-ray scattering (SAXS) spectra analysis, and rheological analysis of the emulsions were investigated as well. The results showed that when the gap of the polarity between inner oils and external liquid oils is greater, the multiple structures were more easily formed. Multiple emulsions containing liquid crystals were superior in stability to multiple emulsions prepared in the same way with liquid oils that did not form liquid crystals. SAXS indicated that the liquid crystal orientation was lamellar. Rheological analysis indicated that the different structure emulsions showed shear-thinning behavior. The presence of liquid crystal decreased the viscosity and resulted in pseudoplastic enhancement. Both the storage modulus (G′) and the loss modulus (G″) of multiple emulsions were slightly higher than those of O/W-type emulsions, implying the existence of multiple structures.     

Stability and Rheological Properties of Model Low-Fat Salad Dressing Stabilized by Salep

A model low-fat salad dressing including salep as thickening agent was prepared. In order to obtain an stable salad dressing, different variables each one at three levels like salep content (0.5, 2.0, and 3.5% w/w), oil volume fraction (7.50, 16.25, and 25.00% w/w), pH (3, 5, and 7), salt concentration (0.3, 0.9, and 1.5% w/w), and egg yolk content (2, 4, and 6% w/w) were chosen and their effect on the creaming index of salad dressing was studied. It was observed that samples with highest salep content at pH 3 were the most stable during storage time (15 days). The microstructure of some samples was considered. Rheological measurements were performed for stable samples. Oil fraction and salt content increased zero shear viscosity and G′ modulus of samples.      

Protein adsorption at liquid/liquid interface with low interfacial tension

Adsorption of proteins at the interface of two-liquid systems composed of aqueous ammonium sulfate solution and tert-butanol by phase separation was investigated by drop shape tensiometry. The change of interfacial tension with time and protein concentration as well as upon compression of the adsorbed layer were compared for bovine serum albumin, ovalbumin, β-lactoglobulin, lysozyme, trypsin and horse radish peroxidase. A correlation between the dilatational moduli of various protein films and the partitioning of proteins in the two-liquid system was found, which provides evidence to the role of emulsion stability in protein separation by three-phase partitioning.