Colloids and Surfaces最新文献

The dissolution of solid particles in liquids involves the dissociation of solute molecules through a surface reaction and the subsequent diffusion of these molecules toward the bulk liquid phase. The conventional analysis is mainly based upon extreme cases, i.e. the rate of dissolution is either controlled by surface reaction or by molecular diffusion. Here, the analysis is generalized so that the diffusional resistance and the resistance for the surface reaction are considered simultaneously. Also, the effect of the variation of the solute concentration in the bulk liquid phase on dissolution is taken into account. The applicability of the present model is justified by analyzing the experimental results for the dissolution or human enamel powder in water. The evaporation of a water drop in air and the dissolution of 2-naphthol in water are also discussed. It is concluded that neglecting the effect of the surface reaction in the dissolution kinetics may lead to a significant deviation between experimental data and theoretical calculation.

Four different chitosans with different charge densities and different molecular weights were used for investigation of the mechanism involved in selective flocculation of cell debris particles in Escherichia coli disintegrates.

It was found that the main mechanism for flocculaticin is a “non-equilibrium” bridging process in which a very efficient removal of cell debris particles can be achieved with highly charged chitosans. The high molecular weight ((6.5–6.6)·10⁵) chitosans produce very large and shear-resistant floes suitable for filtration as well as settling and centrifugation. The low molecular weight (1·10⁵) product forms smaller and more shear-sensitive best suited for centrifugation. There are small differences in flocculation dosages depending on molecular weight, but large differences are found with significant changes in charge densities. A decrease in the deacetylation degree from 93 to 39; increased the flocculation dosages by 100–150;. A low molecular weight chitosan gave a much broader flocculation region than that of a high molecular weight. Flocculation by addition of urea revealed a hydrogen bonding capacity of chitosan toward cell debris particles which was not involved in chitosan's interaction with proteins or nucleic acids.

The purification of the enzyme β-galactosidase could be increased by a factor of 3.7 when using a two-step flocculation procedure. The enzyme yield was 82, and the enzyme solution was essentially free of both nucleic acids and cell debris particles.

A ternary water-in-oil microemulsion composed of xylene, water and a surface-active extractant, sodium bis-2-ethylhexyl phosphate, was investigated by small angle X-ray scattering (SAXS) and conductimetry.

In order to decide between two possible interpretations of the SAXS data, i.e. evidence for the occurrence of highly polydisperse, independent droplets or of slightly polydisperse and interacting ones, the effects of temperature, water content, dilution and type of solvent were investigated. The scattering profiles were simulated using a log—normal size distribution and the mean spherical approximation (MSA) for the interactions. This led us to proposes model of slightly polydisperse and aggregated particles with a mean radius of about 1.4 nm. The addition of water swells the micelles while concentration and aggregation number remain constant. This then generates an aggregation phenomenon which was confirmed by conductimetry measurements. This investigation makes it possible to optimize the conditions for producing a microemulsion such as is often used in liquid—liquid extraction of metals.

The adsorption of a non-ionic surfactant, alkylphenylpoly(oxyelhylene)alcohol from aqueous solutions onte industrial E-glass fibres has been followed by measurements of their surface properties (electron spectroscopy for chemical analysis (ESCA) technique), streaming/zeta potential and wetting characteristics. All three techniques indicate substantial changes in the interaction between glass fibres and water as a consequence of surfactant adsorption, although it seems that only a fraction of the total surface is covered by the surfactant. The streaming/zeta potential has been found to reflect the processability of the fibres in a realistic way, whereas the wettability of the fibres provides indirect information concerning the surface coverage and polarity of the fibres. Although the ESCA measurements are performed in ultra-high vacuum, the results are shown to give semi-quantitative information about the amount adsorbed on the surface, and correlate well with the wetting properties. All methods indicate, as expected, an enhanced absorption with increased surfactant concentration.

A mathematical model that accounts for the kinetics of flocculation and coalescence or emulsion drops was developed in order to quantify the kinetic stability of emulsions. The model is similar to that developed by van den Tempel with a correction to account for the coalescence in small flocs. The model identifies the conditions under which either the flocculation or coalescence rate controls the kinetics. The model also demonstrates that the rate-controlling mechanism could change from coalescence-rate controlling to flocculation-rate controlling during the course of the emulsion life. An extension to the model for concentrated emulsions was also developed. The model was used to fit two sets of experimental data to determine the kinetic constants that characterize the coalescence in the emulsions.

The nature of the surface charge and the effect of indifferent electrolytes (nitrates of alkali and alkaline earth metals) and non-indifferent electrolytes (HCl and KOH) on the surface charge density of SiC and TiC particles are established. It is shown that the adsorption of poly(ethylene oxide) (PEO) on these surfaces does not depend on pH over a wide range of pH values (4.0–11.6), has no effect on the surface charge density, and causes a considerable decrease in the electrokinetic potential of the particles owing to the shift of the shear plane towards the solution. The calculated “electrophoretic” thicknesses of PEO adsorbed layers are in good agreement with the dimensions of the end tails of adsorbed macromolecules estimated from the Scheutjens-Fleer theory of polymer adsorption.

Silica nanoparticles synthesized by the controlled, base-catalyzed hydrolysis of tetraethoxysilane (TEOS) in cyclohexane-polyoxyethylene(5) nonylphenyl ether—water reverse microemulsions exhibit a bimodal size distribution when a relatively high water/surfactant molar ratio (R) is utilized. During the particle formation process there is a partial expulsion of water from the microemulsion phase and this creates a second phase of bulk water which apparently induces the formation of new silica nuclei. This reaction-promoted phase instability is attributable to the ability of ethanol, a product of TEOS hydrolysis, to shift the water solubility limit of the phase diagram to higher temperatures. At relatively low water/TEOS ratios, the resulting silica dispersions become unstable with the passage of time and eventually large flocs form. This behavior is rationalized in terms of a water-shell model according to which the presence of a water film around the silica particles facilitates the ionization of the surface silanol groups, which in turn permits the development of the necessary electrostatic stabilization. Investigation of the solubilization locale of ethanol with fluorescence techniques indicates that at low R there is a preferential partitioning of the alcohol to the continuous cyclohexane phase. Thus, under these conditions, ethanol molecules (which can also support silica ionization) are not available to substitute for water in the water shell.

The Langmuir—Hinselwood (L-H) equation is the simplest kinetic equation which is consistent with Langmuir's equilibrium isotherm. This kinetic equation cannot describe well the dynamic surface tension data for octanol, sodium dodecyl sulfate (SDS), and other surfactants. A new kinetic equation for the rate of adsorption from the subsurface (dl/dt = k^a_Lc(θ,t)(1−θ) exp(−Bθ)−k^d_L Γ exp(−Bθ). where θ is the fractional surface coverage Γ/Γ_m, c(θ,t) is the subsurface concentration, and k^a_L, k^d_L, and B are constants) includes modification of the kinetics but not of the equilibrium isotherm. The new equation describes better the capture efficiency of the interfacial monolayer for additional surfactant, and can describe activation barriers for adsorption and desorption, or cooperative adsorption caused by primarily attractive interactions between the monolayer and the dissolved surfactant. This equation was used in a new model of mixed kinetics for one-dimensional diffusion/adsorption/desorption. For octanol and heptanol, the initial adsorption rate is controlled by intrinsic adsorption/desorption kinetics (slow adsorption/desorption). With increasing surface coverage, dynamic adsorption gets closer to the diffusion-controlled limit (fast adsorption/desorption relative to diffusion). This indicates attractive and cooperative interactions of alcohol molecules in the monolayer. For sodium di-2-ethylhexylsulfosuccinate (DESS or AOT) and SDS, adsorption is much slower than predicted by diffusion-controlled models. The modified L-H equation in a mixed-kinetics model can fit the data well. The capture efficiency factor, k^a_L exp(−bθ), increases with increasing SDS concentration c_SDS or NaCl concentration c_s, indicating that adsorption is strongly affected by electrostatic barriers. For c_s = 0 and c_SDS = 1.7 to 5.9 mM (for θ_c<0.4), the estimated surface electrical potential is in the range 150–230 mV, and is consistent with classical double-layer theory. For θ_c > 0.4 and a high salt concentration, the parameter B may involve substantial steric or other interactions.

The effect of a hydrophobically modified anionic polymer (a maleic anhydride α-olefin copolymer. DAPRAL), which has a comb-like structure, on the stability of alumina suspensions was studied. In the absence of the polymer the suspension stability is controlled by the electrostatic repulsion between particles. Addition of DAPRAL was found to have a significant effect only when the electrostatic repulsion was not sufficient to maintain a stable suspension. The zeta potential of alumina particles and the adsorption density of DAPRAL on alumina were also measured in order to develop the underlying mechanisms for the stability changes. The results suggest that interactions between hydrocarbon chains on the dangling loops and tails do reduce the effect of electrostatic repulsion between alumina particles at low polymer concentrations. At higher polymer concentrations, the hydrocarbon chains on such loops and tails form micellar aggregates; the stability of the suspension is drastically enhanced under such conditions by both steric and electrostatic repulsion.

A new physicochemical method for preparing homogeneous liposomes with high trapping efficiency has been developed. A lamellar liquid crystalline (LLC) phase formed in the egg yolk lecithin (EPC)/propylene glycol/glycerol/water four-component system is utilized in this method. The ratio of propylene glycol to glycerol is important for the preparation of fine liposomes. The roles of propylene glycol and glycerol in the preparation process were studied using phase diagrams, small angle X-ray diffraction and video-enhanced microscopy. It has been shown that propylene glycol and glycerol control the hydrophile-lipophile balance (HLB) of lecithin and that the liposome size changes according to the HLB. The lamellar liquid crystalline phase with a 1:1 propylene glycol:glycerol weight ratio expands for a low concentration of lecithin, and liposomes with minimum size (116 nm) are prepared by adding water to the LLC phase. It was found that the trapping efficiency of the liposomes prepared by this new method was 19.5%, while the trapping efficiency was 0.2% for the sonication method and 2.1% for the extrusion method.