JournalTOCs

Colloid Journal

[3 followers] Follow
Full-text available via subscription

Subscription journal
ISSN (Print) 1608-3067 - ISSN (Online) 1061-933X
Published by Springer-Verlag

[2216 journals]

On the nonequilibrium thermodynamics of thermocrystallization motion of inclusions in solids
- Abstract: Abstract The methods of nonequilibrium thermodynamics are used to describe the temperature gradientinduced motion of inclusions and the cross effect in solids occurring in the state of premelting. Inclusion motion is caused by a nonuniform disjoining pressure in thin unfrozen interlayers that are formed at a nonuniformly heated inclusion. The cross effect is related to the generation of a local temperature field upon the inclusion motion under the action of an external force in a uniform solid. It is shown that the Onsager relations are valid for the nonequilibrium processes under consideration.
  PubDate: 2013-04-01
Kinetic study of the hetero Diels-Alder reaction in surfactant solutions
- Abstract: Abstract The kinetics of the cycloaddition of nitrosobenzene with 2,3-dimethyl-1,3-butadiene in solutions of anionic sodium di(2-ethyl-1-hexyl) sulphosuccinate (AOT) and cationic cetyltrimethylammonium bromide was investigated. The significant acceleration of the reaction rate by a factor of 5 compared to that in a pure ethylene glycol was observed. Isobaric activation parameters were determined, and the activation energy E a was shown to decrease upon increasing AOT concentration in solutions with low surfactant concentrations.
  PubDate: 2013-04-01
Effect of polyelectrolytes and polyelectrolyte mixtures on the electrokinetic potential of dispersed particles. 2. Electrokinetic potential of silica particles in electrolyte, polyelectrolyte and polyelectrolyte mixtures solutions
- Abstract: Abstract The effect pH, ionic strength (KCl concentration), weakly and medium charged anionic and cationic polyelectrolytes (PEs) as well as their binary mixtures on the electrokinetic potential of silica particles as a function of the polyelectrolyte/mixture dose, its composition, charge density (CD) of the PE, and way of adding the polymers to the suspension has been studied. It has been shown that addition of increasing amount of anionic PEs increases the absolute value of the negative zeta-potential of particles at pH > pH isoelectric point (IEP = 2.5); this increase is stronger the charge density of the polyelectrolyte is higher. Adsorption of cationic polyelectrolytes at these pH values gives a significant decrease in the negative ζ-potential and overcharging the particles; changes in the ζ-potential are more pronounced for PE samples with higher CD. In mixtures of cationic and anionic PE at pH > pHIEP, the ζ-potential of particles is determined by the adsorbed amount of the anionic polymer independently of the CD of PEs, the mixture composition and the sequence of addition of the mixture components. Unexpectedly, the ζ-potential of silica at pH = 2.1, i.e. < pHIEP, turned out to be positive in the presence of both anionic PE and cationic + anionic PE mixtures. This is explained by formation (and adsorption onto positively charged silica surface) of pseudo-cationic PEs from anionic ones due to transfer of protons from the solution to the amino-group of the anionic polymer. Considerations about the role of coulombic and non-coulombic forces in the mechanism of PE adsorption are presented.
  PubDate: 2013-04-01
Mechanical properties of one-phase microemulsion system
- Abstract: Abstract A theoretical approach for free Gibbs energy of microemulsion ternary systems water-oil-non-ionic surfactant of arbitrary size and shape is presented. We consider the Gibbs energy for three different geometries (flat, spherical and cylindrical) to estimate the free energy of surfactant-oil-water microemulsion. We also used a lattice-based self-consistent field (SCF) theory to calculate the free energy and the mechanical properties (the mean bending modulus k c , and the spontaneous curvature J 0, and the saddlesplay modulus $\bar k$ ) of such system. The values obtained by our theoretical approach were compared qualitatively with those predicted by SCF theory. The results of theoretical approach were in a good agreement with those of SCF in the region where the values of free energy approach zero for the flat geometry. We demonstrated that the parameters k c, J 0 and $\bar k$ strongly depend on the surfactant concentration.
  PubDate: 2013-04-01
Formation of ring-shaped deposits as a result of evaporating droplets of dispersions of silver nanoparticles and study of their fine structure
- Abstract: Abstract Scanning electron microscopy is employed to study ring-shaped deposits formed at different time moments of evaporating droplets of silver colloidal solutions on a glass substrate. It is shown that, in addition to an external fringe, several rings are formed at intermediate stages of evaporation; these rings are, in the course of time, transformed into one dense fringe and a transition zone adjacent to its internal side. The time dependences of the height and width of the fringe are determined. It is established that an increase in the width and height of the fringe is due to the transfer of silver nanoparticles to this region, which play a key role in imparting specific structural organization to the fringe. It is noted that the height of the formed fringe may be qualitatively determined by energy dispersive X-ray microanalysis.
  PubDate: 2013-04-01
Synthesis of monolithic mesoporous silica with a regular structure (SBA-15) and macropores in neutral aqueous solution at room temperature
- Abstract: Abstract A monolithic biporous silica material of SBA-15 type has been synthesized by the sol-gel method in a neutral aqueous solution in the presence of glycerol without heating. Synthesis has been performed using P123 triblock copolymer and a novel silica precursor containing ethylene glycol residues.
  PubDate: 2013-04-01
Adsorption of ammonia by water clusters. Computer experiment
- Abstract: Abstract The method of molecular dynamics is used to study the adsorption of from one to six ammonia molecules by water clusters composed of 50 molecules. The adsorption of NH3 molecules markedly increases the IR absorption spectrum intensity, substantially decreases emission power in the frequency range of 0 ≤ ω ≤ 3500 cm−1, and transforms a continuous reflectance spectrum into a banded one. A rough surface formed by adsorbed ammonia molecules reduces the absorption coefficient and refractive index of the system of water-ammonia clusters in the entire frequency range. Adsorption of ammonia molecules by water clusters greatly diminishes the number of electrons that are active with respect to electromagnetic radiation.
  PubDate: 2013-04-01
Rheology of magnetite dispersions in oligo(propylene glycol) and the structure of the stabilizing layer on dispersed phase particles
- Abstract: Abstract Rheological measurements are employed to prove the Newtonian behavior of magnetite dispersions stabilized with oligomers and to determine the ratio between the volume of particles with stabilizing shells and the proper particle volume. The average diameter of magnetite particles is calculated by the magnetic granulometry method. The thickness of the stabilizing layer and the configuration of stabilizer molecules are estimated.
  PubDate: 2013-04-01
Molecular dynamics simulation of reverse micelles: A search for the most efficient strategy
- Abstract: Abstract A search has been performed for the efficient computer simulation of reverse micelles by the molecular dynamics method. The full-atomic and simplified approaches to the description of molecules and different kinds of initial geometry (preassembled micelles and randomly distributed molecules) have been compared. The structure of a simulated reverse micelle has been shown to be governed by its composition rather than an initial geometry. It has been noted that, when using preassembled micelles, the time scale must be at least 2 ns.
  PubDate: 2013-04-01
Liquid flow inside a cylindrical capillary with walls covered with a porous layer (Gel)
- Abstract: Abstract Viscous incompressible liquid flow in a long cylindrical capillary, the internal surface of which is covered with a permeable porous layer, is studied within the frameworks of three mathematical models. In the first model, the liquid flow in the porous layer is described by the Brinkman equation; according to the second one, the presence of the porous layer is taken into account using the Navier slip boundary conditions; and, in the third model, the Navier condition is imposed on the porous layer-liquid interface, with the flow inside the porous layer being excluded. The theoretical predictions are compared with the experimental data that one of us has obtained for liquid flow rates in porous capillaries. The validity and appropriateness of the application of the proposed models are discussed.
  PubDate: 2013-04-01
Tween 80-based mixed micelles as felodipine carriers in aqueous medium
- Abstract: Abstract UV spectroscopy, precise turbidimetry, and refractometry are employed to measure the solubilization of the lipophilic compounds felodipine (F) (drug) and nonionic surfactants glyceryl monooleate (GMO) and glyceryl monolaurate (GML) (efficient promoters of skin permeability (enhancers)) in aqueous solutions of Tween 80 (Tw). An increase in the concentration of Tw makes it possible to enhance the solubility of the drug by a factor of 102–103 and above as compared to F solubility in water. The effect becomes stronger in the presence of an enhancer. The following properties of Tw + F, Tw + GMO, Tw + GML, Tw + GMO + F, and Tw + GML + F micelles are studied: the aggregation numbers of the components, localization of solubilizates, diffusion coefficients, sizes, and degrees of hydration. Experimental data on the kinetics of drug mass transfer by micelles are in good agreement with the calculations performed within the framework of diffusion theory. Tw-based mixed micelles are found to be efficient carriers of F and enhancers in aqueous media, which is of importance for the creation of systems for delivery of lipophilic drugs in a bioavailable form.
  PubDate: 2013-04-01
Effect of the adsorption component of the disjoining pressure on foam film drainage
- Abstract: Abstract The present work is trying to explain a discrepancy between experimental observations of the drainage of foam films from aqueous solutions of sodium dodecylsulfate and the theoretical DLVO-accomplished Reynolds model. It is shown that, due to overlap of the film adsorption layers, an adsorption component of the disjoining pressure is important for the present system. The pre-exponential factor of this adsorption component was obtained by fitting to the experimental drainage curves. It corresponds to a slight repulsion, which reduces not only the thinning velocity as observed experimentally but corrects also the film equilibrium thickness.
  PubDate: 2013-04-01
An NMR-relaxation study of the effect of albumin on aggregation of magnetic iron oxide nanoparticles
- Abstract: Abstract The dependence of the aggregation of magnetic iron oxide nanoparticles in aqueous suspensions under the action of human serum albumin is analyzed based on the data of proton magnetic relaxation. It is shown that albumin adsorption on magnetic nanoparticles gives rise to the formation of a protein corona and clusters of magnetic nanoparticles, decreasing the aggregation stability of the suspension in a 7.1-T magnetic field. Clustering of magnetic iron oxide nanoparticles enhances the relaxation efficiency of magnetic suspensions during NMR measurements.
  PubDate: 2013-04-01
Association and colloid-chemical properties of humic acids in aqueous solutions
- Abstract: Abstract Colloid-chemical properties of humic acids in aqueous solutions have been studied as functions of their concentration, medium pH, and counterion nature. It has been established that, at pH < 4, Na-form humic acids exist in aqueous solutions as 30–120-nm aggregates. At pH > 4, the state of humic acids depends on the solution concentration. At low concentrations, they exist as individual molecules; as the concentration is increased, they associate to form supramolecular structures at 5 mg/dm3 and micelles at 8 g/dm3. An increase in the counterion charge has been shown to decrease the critical concentration of supramolecular structuring. Supramolecular structures have increased adsorbability on clay minerals and promote the manifestation of solubilizing properties of humic acids in solutions even at a concentration of 5 mg/dm3, which is three orders of magnitude lower than the critical micelle concentration (8 g/dm3).
  PubDate: 2013-04-01
Effect of polyelectrolytes and polyelectrolyte mixtures on the electrokinetic potential of dispersed particles. 1. Electrokinetic potential of polystyrene particles in solutions of surfactants, polyelectrolytes and their mixtures
- Abstract: Abstract The effect of cationic and anionic surfactants, as well as cationic and anionic polyelectrolytes (PE), their binary mixtures on the electrokinetic potential of monodisperse carboxylated polystyrene (PS) particles as a function of the reagents dose, pH, the charge density (CD) of polymers, the surfactant/PE and binary PE mixture composition, and sequence of components addition to the suspension has been studied. It has been shown that addition of increasing amount of anionic surfactant/polyelectrolytes increases the absolute value of the negative zeta-potential of PS particles; this increase is stronger the CD of the PE and pH of the system are higher. Adsorption of cationic surfactant/polyelectrolytes leads to a significant decrease in the negative ζ-potential and to overcharging the particles; changes in the ζ-potential are more pronounced for PE samples with higher CD and for suspensions with lower pH values. In mixtures of cationic and anionic PE, in a wide range of mixture composition, the ζ-potential of particles is determined by the adsorbed amount of the anionic polymer independently of the CD of PEs and the sequence of addition of the mixture components. The isoelectric point of the surface is reached at the adsorbed amount of positive charges of PE that is approximately equal to the surface CD of particles. The laws observed were explained by features of macromolecules conformation in adsorbed mixed PE layers. Considerations about the role of coulombic and non-coulombic forces in the mechanism of anionic/cationic PE adsorption are presented.
  PubDate: 2013-04-01
Cationic electrolyte adsorption layers on hydrophilic and hydrophobic surfaces
- Abstract: Abstract The capillary electrokinetics method (measurements of streaming potential and current in original and hydrophobized fused quartz capillaries with radii of 5–7 μm) is employed to study the formation of adsorption layers upon contact with solutions containing a cationic polyelectrolyte, poly(diallyldimethylammonium chloride). It is shown that polyelectrolyte adsorption causes the charge reversal of both hydrophilic and hydrophobic surfaces, with a smaller amount of the substance being adsorbed on the hydrophobic than on the hydrophilic surface. The adsorption on both surfaces increases with the polymer solution concentration. The cationic polyelectrolyte adsorption on the pure quartz surface occurs mainly due to the electrostatic attraction, while, in the case of the hydrophobic surface, the contribution of hydrophobic interactions increases. The study of the layer deformability shows that, on the hydrophilic surfaces, the layer ages and its structure depends on the polymer solution concentration. On the modified surface, the deformation of even freshly formed layers is slight, which suggests that a denser layer is formed on the hydrophobic surface. In contrast to the hydrophilic surface, the polyelectrolyte is partly desorbed from the hydrophobic surface.
  PubDate: 2013-04-01
Surface and micellar properties of some amphiphilic drugs in various salt solutions
- Abstract: Abstract In this work the effect adding of various electrolytes on the solution properties of three amphiphilic drugs was investigated. The critical micelle concentration (cmc) values, determined on the basis of surface tension measurements, were found to decrease with the increasing salt concentration; this decrease was dependent upon the nature of added ion. Coions (K+ and Na+) remained in the aqueous phase and were almost equally efficient in decreasing the cmc. Counterions (Cl− and Br−) concentrated on the micellar surface and reduced electrostatic repulsion. The smaller the hydrated counterions, the stronger they interacted with the micellar surface. The decrease of cmc was therefore more pronounced for Br− ions than that for Cl− ones. The observed changes in A min and Γmax were explained in terms of the cmc behavior. The values of both ΔG m pO and ΔG ads pO suggest that the drugs tend to form micelles more readily in the presence of salts.
  PubDate: 2013-04-01
Diffusion and magnetotransport in ferrofluids containing chain-shaped aggregates
- Abstract: Abstract The effect of chain structures on particle diffusion in nanodisperse ferrofluids subjected to external magnetic fields is studied. It is shown that the chain formation imparts very strong anisotropy to the diffusion properties of ferrofluids: the diffusion coefficient along a field appears to be approximately two orders of magnitude higher than that in the transverse direction. The presence of the chains changes the particle magnetophoresis coefficient significantly.
  PubDate: 2013-01-01
Assembly and characterization of Ag nanoparticles in PAM-g-PVA/PVP semi-interpenetrating network hydrogels
- Abstract: Abstract Polyacrylamide grafted poly(vinyl alcohol)/polyvinylpyrrolidone (PAM-g-PVA/PVP) semi-interpenetrating network (semi-IPN) hydrogels were designed and prepared via a simple free radical polymerization reaction process initiated by a PVA-(NH4)2Ce(NO3)6 redox system. The structure of the PAM-g-PVA/PVP semi-IPNs was characterized by a Fourier transform infrared spectroscopy. The morphologies of PAM-g-PVA/PVP hydrogels and PAM-g-PVA/PVP/Ag nanocomposite hydrogels were examined by scanning electron microscopy and transmission electron microscopy (TEM). The experimental results indicated that the PAM, PVA or PVP chains can efficiently act as stabilizing agents for Ag nanoparticles. TEM investigation of sample morphology showed the presence of nearly spherical-, square- or rectangular-shaped Ag nanoparticles with diameters ranging from 10 to 60 nm. The characteristic surface plasmon resonance band appeared at 390–400 run as a result of the immobilization of Ag nanoparticles within the hydrogel matrices. The self-assembly of Ag nanoparticles and the swelling behavior of the resulting nanocomposites can be controlled and modulated by altering the mole fraction of PVP in the PAM-g-PVA/PVP semi-IPNs.
  PubDate: 2013-01-01
On the theory of structuring magnetic suspensions
- Abstract: Abstract The results of the three-dimensional computer and analytical simulation are presented for the kinetics of chain-shaped aggregate growth in suspensions of magnetizable non-Brownian particles. The results of the computer experiment show that, when the volume fraction of particles is no larger than 2–3%, chain-shaped aggregates are formed in the suspensions under the action of a field. The dependence of average number <n> of particles in a chain on time t is adequately described by the power law <n> = Ct k . The experiment indicates that, in contract to the common power approximations, in which exponent k is considered to be a universal constant parameter, it depends on the concentration of particles and their interactions with walls bounding a suspension. At concentrations noticeably exceeding 2–3%, dense bulk aggregates are formed in suspensions. The kinetics of their growth depends on the sizes of a suspension-containing vessel.
  PubDate: 2013-01-01