Dynamics simulation of disperse systems in the presence of a structure-related mechanical barrier resulting from surfactant adsorption

A computer model has been developed to refine the notions of the kinetics of structural transformations in surfactant-modified disperse systems, the mechanism of the influence of adsorption layers on viscosity, conditions of aggregation, and evolution of nonuniformities in microstructures. Classical concepts of the structure-related mechanical barrier providing disperse systems with stability to aggregation, as well as the criterion of adsorption-layer breakdown upon interparticle collisions under dynamic conditions, have been used. It has been shown that adsorption-layer breakdown under dynamic conditions may lead to the appearance of an extreme in the viscosity curve of a disperse system. The combined effect of additives of surfactants with different molecular masses successively added to the systems has been studied via taking into account the influence of surfactant adsorption layers on the dynamics of contact interactions. The simulation results make it possible to optimize the regulation of structure-related rheological properties of dispersions so as to decrease their apparent viscosity with a simultaneous increase in the uniformity of the structure.     

Fluidity and spreading of structured disperse systems

The regularities of flow and spreading of disperse systems as two interrelated processes are considered. Differences in the mechanisms of spreading of structured dispersions on solid surfaces are discussed in comparison with the processes of wetting and spreading of Newtonian and non-Newtonian unstructured liquids. It is shown that, for structured dispersions under consideration, their bulk structure-related rheological properties are of prime importance. Surface phenomena occurring at a boundary with a solid substrate affect the character of spreading of such dispersions to the same extent as the interaction of a structured disperse system with the surface affects variations in the properties of adjacent layers of the disperse system.     

Computer simulation of colloidal dispersions

This paper discusses recent applications of statistical mechanics to dispersions with particular emphasis on the computer simulation of the dynamic properties.Fundamental to any computation on a colloidal dispersion is the knowledge of the potential of mean force for at least a pair of suspended particles. At low-to-moderate particle concentrations for stable dispersions, statistical mechanical calculations based on the normal DLVO pair potential produce reasonable agreement with experiment for a number of equilibrium properties of simple latex dispersions. This phenomenon indicates that under these conditions the DLVO pair potential is a reasonable effective pair potential. However, recent Monte Carlo simulations and experimental measurements with liquids of spherical molecules suggest that the force between a pair of dispersed particles at very small separation may differ significantly from that predicted by DLVO theory.The computation of dynamic properties of dispersions involves problems not encountered in the above equilibrium calculations. In particular, one must include the effects of indirect hydrodynamic as well as direct interactions among the particles. This computation may be easily accomplished at moderately low particle concentrations and the results of such calculations are able to give a very detailed analysis of the results of Photon Correlation Spectroscopy measurements on ion exchanged polystyrene latex suspensions at low concentration. These computations also, once again, emphasize the usefulness of DLVO pair potentials as effective pair potentials for systems of strongly interacting particles.     

Effect of nanoparticles on the interfacial properties of liquid/liquid and liquid/air surface layers

An investigation is reported on the interfacial properties of nanometric colloidal silica dispersions in the presence of a cationic surfactant. These properties are the result of different phenomena such as the particle attachment at the interface and the surfactant adsorption at the liquid and at the particle interfaces. Since the latter strongly influences the hydrophobicity/lipophilicity of the particle, i.e., the particle affinity for the fluid interfacial environment, all those phenomena are closely correlated. The equilibrium and dynamic interfacial tensions of the liquid/air and liquid/oil interfaces have been measured as a function of the surfactant and particle concentration. The interfacial rheology of the same systems has been also investigated by measuring the dilational viscoelasticity as a function of the area perturbation frequency. These results are then crossed with the values of the surfactant adsorption on the silica particles, indirectly estimated through experiments based on the centrifugation of the dispersions. In this way it has been possible to point out the mechanisms determining the observed kinetic and equilibrium features. In particular, an important role in the mixed particle-surfactant layer reorganization is played by the Brownian transport of particles from the bulk to the interface and by the surfactant redistribution between the particle and fluid interface.     

Titanium coated with high-performance nanocrystalline ruthenium oxide synthesized by the microwave-assisted sol–gel procedure

Ruthenium oxide coating on titanium was prepared by the sol–gel procedure from well-defined colloidal oxide dispersions synthesized by the microwave (MW)-assisted hydrothermal route under defined temperature and pressure heating conditions. The dispersions were characterized by dynamic light scattering (DLS) measurements and scanning electron microscopy (SEM). The electrochemical properties were analyzed as capacitive performances gained by cyclic voltammetry and electrochemical impedance spectroscopy and as the electrocatalytic activity for oxygen evolution from acid solution. The obtained dispersions were polydisperse and contained regular particles and agglomerates of increasing surface energy and decreasing particle size as the MW-assisted heating conditions were intensified. Owing to these features of the precursor dispersions, the obtained coatings had considerably improved capacitive performances and good electrocatalytic activity for oxygen evolution at high overpotentials.     

Preparation, characterization, surface modification and redox reactions of silver nanoparticles in the presence of tryptophan

The synthesis and characterization of water-soluble dispersions of Ag nanoparticles by the reduction of AgNO(3) using tryptophan under alkaline synthesis conditions are reported. The Ag nanoparticle formation was very slow at low concentration and rapid at extremes. For surface modification and redox reactions, manipulating the interparticles interaction controlled the size of Ag nanoparticles aggregates. Our results suggest that the replacement of the BH(4)(-) ions adsorbed on the nanoparticle surface by tryptophan destabilizes the particles and further caused aggregation. A mechanism is proposed for the formation of silver nanoparticles by tryptophan. The experimental results are supported by theoretical calculations. The Ag nanoparticles were characterized by UV-vis absorption, dynamic light scattering and transmission electron microscopy techniques.     

Redox equilibria of iron oxides in aqueous-based magnetite dispersions: effect of pH and redox potential

The effect of pH and redox potential on the redox equilibria of iron oxides in aqueous-based magnetite dispersions was investigated. The ionic activities of each dissolved iron species in equilibrium with magnetite nanoparticles were determined and contoured within the Eh-pH framework of a composite stability diagram. Both standard redox potentials and equilibrium constants for all major iron oxide redox equilibria in magnetite dispersions were found to differ from values reported for noncolloidal systems. The "triple point" position of redox equilibrium among Fe(II) ions, magnetite, and hematite shifted to a higher standard redox potential and an equilibrium constant which was several orders of magnitude higher. The predominant area of magnetite stability was enlarged to cover a wider range of both pH and redox potentials as compared to that of a noncolloidal magnetite system.     

Serum-resistant Gene Transfer Activity of Mannosylated Dendrimer/α-Cyclodextrin Conjugate (G3)

Interactions in aqueous dispersion between dipalmitoyl phosphatidyl choline (DPPC) liposomes and dissolved cyclodextrins (CD) of different chemical compositions, respectively, were studied. Liposomal dispersions with small unilamellar vesicles (SUV) of monomodal size distribution were prepared and the physical stability of the vesicles both in the presence and absence of cyclodextrin was investigated. For the characterization of the kinetic stability of the dispersions under various conditions, size-distribution functions determined by photon-correlation spectroscopy (PCS) were used. The affinity of cyclodextrins to the liposomes was characterized by ‚binding isothermsȁ9 determined under equilibrium conditions at 25 °C. Based on the quantity of the cyclodextrins bound to the DPPC bilayers, stability constants for the associates were estimated. The physical stability of the liposomes and the possible control of stability were also investigated.     

Investigation of the doping efficiency of poly(styrene sulfonic acid) in poly(3,4‐ethylenedioxythiophene)/poly(styrene sulfonic acid) dispersions by capillary electrophoresis

CE can efficiently separate poly(3,4‐ethylenedioxythiophene)/poly(styrene sulfonic acid) (PEDOT/PSS) complexes and free PSS in dispersions and can be used to estimate the degree of PSS doping. We investigated the doping efficiency of PSS on PEDOT in dispersions using CE and its effect on the conductivity of the resulting PEDOT/PSS films. Results of this study indicate that dispersions containing 1:2.5–3 EDOT:PSS feed ratio (by weight) exhibiting 72–73% PSS doping generate highly processable and highly conductive films. Conductivity can be optimized by limiting the time of reaction to 12 h. At this point of the reaction, the PEDOT/PSS segments, appearing as broad band in the electropherogram, could still exist in an extended coil conformation favoring charge transport resulting in high conductivity. Above a threshold PEDOT length formed at reaction times longer than 12 h, the PEDOT/PSS complex, appearing as spikes in the electropherogram, most likely have undergone a conformational change to coiled core‐shell structure restricting charge transport resulting in low conductivity. The optimal conductivity (5.2 S/cm) of films from dispersions synthesized for 12 h is significantly higher than those from its commercial equivalent Clevios P and other reported values obtained under similar conditions without the addition of codopants.     

Properties of Aqueous Suspensions of Highly Dispersed Silica in the Presence of Polyvinylpyrrolidone

The effect of polyvinylpyrrolidone (PVP) on stability, rheological, electrokinetic and electrophysical properties of aqueous dispersions of A-300 pyrogenic silica was studied. The suspensions were shown to be most stable at PVP concentrations in the range from 0.12 to 0.24 wt % that was provided by structure-related mechanical factors. In the presence of PVP, the value of -potential decreases, the ac conductivity increases, and rheological properties vary similarly to variations of oxide dispersity.     

The retention of some azoles in reversed-phase liquid chromatography

The influence of the structure-related physicochemical properties of derivatives of nitrogen-containing heterocyclic compounds on their retention under the conditions of reversed-phase liquid chromatography was studied. The dependence of the chromatographic retention of azole derivatives on the composition of the mobile phase was determined. The presence of azole protonated forms was substantiated by conductometric data.     

Stability of mixtures of charged silica, silica-alumina, and magnetite colloids

We report experiments on the stability of aqueous mixtures of charged colloidal magnetite and charged silica and silica covered with alumina particles of similar size. First, positively charged magnetite dispersions were mixed with negatively charged silica dispersions at pH 4, at different volume ratios and low colloid volume fractions, producing mixtures which were stable over a period of weeks despite the expected electrostatic attraction between the oppositely charged particles. When magnetite particles were mixed with positively charged silica covered with alumina at pH 4 under exactly the same conditions, some of the systems separated to form a magnetite sediment. When the volume fraction of the initial dispersions was increased, the behavior of the mixtures was the opposite: positive magnetite/negative silica mixtures were unstable at intermediate volume ratios. The unexpected behavior of the mixtures was investigated by means of electrophoretic mobility, initial susceptibility, and dynamic light scattering measurements as well as sedimentation experiments.     

The Syntheses of Triazinone and Pyrimidinone Biphenyltetrazoles as Angiotensin II Receptor Antagonists

A series of biphenylyltetrazole substituted triazinones and structure-related pyrimidinones are systhesized, and their binding affinities for angiotensin II receptor are reported.     

Physicochemical characterization of natural-like branched-chain glycosides toward formation of hexosomes and vesicles

Synthetic branched-chain glycolipids have become of great interest in biomimicking research, since they provide a suitable alternative for natural glycolipids, which are difficult to extract from natural resources. Therefore, branched-chain glycolipids obtained by direct syntheses are of utmost interest. In this work, two new branched-chain glycolipids are presented, namely, 2-hexyldecyl β(α)-D-glucoside (2-HDG) and 2-hexyldecyl β(α)-D-maltoside (2-HDM) based on glucose and maltose, respectively. The self-assembly properties of these glycolipids have been studied, observing the phase behavior under thermotropic and lyotropic conditions. Due to their amphiphilic characteristics, 2-HDG and 2-HDM possess rich phase behavior in dry form and in aqueous dispersions. In the thermotropic study, 2-HDG formed a columnar hexagonal liquid crystalline phase, whereas in a binary aqueous system, 2-HDG formed an inverted hexagonal liquid crystalline phase in equilibrium with excess aqueous solution. Furthermore, aqueous dispersions of the hexagonal liquid crystal could be obtained, dispersions known as hexosomes. On the other hand, 2-HDM formed a lamellar liquid crystalline phase (smectic A) in thermotropic conditions, whereas multilamellar vesicles have been observed in equilibrium with aqueous media. Surprisingly, 2-HDM mixed with sodium dodecyl sulfate or aerosol OT induced the formation of more stable unilamellar vesicles. Thus, the branched-chain glycolipids 2-HDG and 2-HDM not only provided alternative nonionic surfactants with rich phase behavior and versatile nanostructures, but also could be used as new drug carrier systems in the future.     

On the Possibility of Flow Curve Analysis of Structured Disperse Systems Using the Catastrophe Theory

The possibility of analyzing the processes of the formation and destruction of disperse structures under dynamic conditions was considered using concepts of synergetics and the catastrophe theory. Such an approach is demonstrated by the results of studying phenomena typical of the shear flow of structured dispersions.     

DISPERSION OF CHOLESTEROL IN AQUEOUS SURFACTANT SOLUTIONS: INTERPRETATION OF VISCOSITY DATA

The viscosity of cholesterol dispersions in aqueous buffered surfactant solutions has been reported under progel conditions. The viscosity versus concentration curves pass through maximum which corresponds to the beginning of solubilization of cholesterol into the surfactant solutions. The stability of the dispersions has been explained by a mechanism involving formation of association complex between cholesterol and the surfactants through hydrogen bonding.     

Contact angles of protein black foam films under dynamic and equilibrium conditions

The contact angles of protein Newton black foam films from ALG (alpha-lactalbumin), BLG (beta-lactoglobulin) and BSA (bovine serum albumin) are measured here within. The measurements are carried out under dynamic and equilibrium conditions. For all proteins, a strong hystheresis effect of the contact angle is observed under dynamic conditions. An attempt is made to explain these results by the slow adsorption and desorption kinetics of the protein bilayers and by the dynamic structure and the rheology of the protein network forming the bubble walls. In addition, we propose a modification of the experimental device reported previously for contact angle measurements of large flat films in equilibrium. The advantages of this method are discussed in detail. Some shortcomings (precision, reproducibility) of this preliminary variant of the device in this initial stage of its application, do not allow one to draw reliable conclusions about the interactions of these films. Some improvements of the measurement quality are proposed.      

Coherent Disperse Structures in Magnetic Suspensions

Results of computer and physical simulation of cooperative behavior of a system of interacting ferroparticles in a liquid matrix under the action of rotating magnetic field are reported. It is shown that, under far from equilibrium (with respect to orientational variables) conditions, the phenomenon of dissipative self-organization arises resulting in the formation of coherent disperse structures.     

Electrophoretic mobility of poly(acrylic acid)-coated alumina particles

The effect of poly (acrylic acid) (PAA) adsorption on the electrokinetic behavior of alumina dispersions under high pH conditions was investigated as a function of polymer concentration and molecular weight as well as the presence, concentration and ion type of background electrolyte. Systems of this type are relevant to nuclear waste treatment, in which PAA is known to be an effective rheology modifier. The presence of all but the lowest molecular weight PAA studied (1800) led to decreases in dynamic electrophoretic mobility at low polymer concentrations, attributable to bridging flocculation, as verified by measurements of particle size distribution. Bridging effects increased with polymer molecular weight, and decreased with polymer concentration. Increases in background electrolyte concentration enhanced dynamic electrophoretic mobility as the polymer layers were compressed and bridging was reduced. Such enhancements were reduced as the cation was changed from K(+) to Na(+) to Cs(+).