Biomedical subjects

Zhenghe Xu

Publications and source records attributed to Zhenghe Xu.

12 recordsLinked to original sources

PubMed2006

Colloidal interactions between Langmuir-Blodgett bitumen films and fine solid particles.

In oil sand processing, accumulation of surface-active compounds at various interfaces imposes a significant impact on bitumen recovery and bitumen froth cleaning (i.e., froth treatment) by altering the interfacial properties and colloidal interactions among various oil sand components. In the present study, bitumen films were prepared at toluene/water interfaces using a Langmuir-Blodgett (LB) upstroke deposition technique. The surface of the prepared LB bitumen films was found to be hydrophobic, comprised of wormlike aggregates containing a relatively high content of oxygen, sulfur, and nitrogen, indicating an accumulation of surface-active compounds in the films. Using an atomic force microscope, colloidal interactions between the LB bitumen films and fine solids (model silica particles and clay particles chosen directly from an oil sand tailing stream) were measured in industrial plant process water and compared with those measured in simple electrolyte solutions of controlled pH and divalent cation concentrations. The results show a stronger long-range repulsive force and weaker adhesion force in solutions of higher pH and lower divalent cation concentration. In plant process water, a moderate long-range repulsive force and weak adhesion were measured despite its high electrolyte content. These findings provide more insight into the mechanisms of bitumen extraction and froth treatment.

Journal Article↗

PubMed2006

Structural effects recorded for AFM tips interacting with individual nanoparticles and their clusters deposited on substrates.

Mica and alumina were coated with nanoparticles using aqueous suspensions while managing attractive substrate-particle electrostatic forces. Using nanoparticle-coated substrates, structural forces were measured for 10 nm silica particles deposited on the alumina substrate and 5-80 nm alumina particles on mica using an atomic force microscopy technique. For nanoparticles forming clusters, oscillation of structural forces was recorded with a periodicity that is close to the size of nanoparticles used. Positioning the AFM tip over the single particles allowed, on the other hand, the study of probe-nanoparticle colloidal forces.

Journal Article↗

PubMed2006

Colloidal interactions between asphaltene surfaces in aqueous solutions.

Asphaltene at oil/water interfaces plays a dominant role in the recovery of crude oil. In this study, asphaltene monolayer films were deposited on hydrophobic silicon wafers and silica spheres from oil-water interfaces using a Langmuir interfacial trough. The morphology of the deposited asphaltene films was characterized with an atomic force microscope (AFM). The colloidal forces between the prepared asphaltene films in aqueous solutions were measured with AFM to shed light on the stabilization of water or oil droplets coated with asphaltene films. Factors such as solution pH, KCl concentration, calcium addition, and temperature all showed a strong impact on colloidal forces between the prepared asphaltene films. The findings provided a better understanding of asphaltene interfacial films at an oil/water interface in stabilizing bitumen-in-water and water-in-bitumen emulsions.

Journal Article↗

PubMed2006

Adhesion of single polyelectrolyte molecules on silica, mica, and bitumen surfaces.

In a recent study (Energy Fuels 2005, 19, 936), a partially hydrolyzed polyacrylamide (HPAM) was used as a process aid to recover bitumen from oil sand ores. It was found that HPAM addition at the bitumen extraction step not only improved bitumen recovery but also enhanced fine solids settling in the tailings stream. To understand the role of HPAM, single-molecule force spectroscopy was employed for the first time to measure the desorption/adhesion forces of single HPAM molecules on silica, mica, and bitumen surfaces using an atomic force microscope (AFM). Silicon wafers with an oxidized surface layer and newly cleaved mica were used, respectively, to represent sand grains and clays in oil sands. The force measurements were carried out in deionized water and in commercial plant process water under equilibrium conditions. The desorption/adhesion forces of HPAM obtained on mica, silica, and bitumen surfaces were approximately 200, 40, and 80 pN in deionized water and approximately 100, 50, and 40 pN in the plant process water, respectively. The measured adhesion forces together with the zeta potential values of these surfaces indicate that the polymer would preferentially adsorb onto clay surfaces rather than onto bitumen surfaces. It is the selective adsorption of HPAM that benefits both bitumen recovery and tailings settling when the polymer was added directly to the bitumen extraction process at an appropriate dosage.

Acrylic Resins↗

PubMed2005

Adsorption of bituminous components at oil/water interfaces investigated by quartz crystal microbalance: implications to the stability of water-in-oil emulsions.

Silica-gel-coated QCM crystals oscillating in a thickness shear mode are used to measure adsorption of bituminous components in water-saturated heptol (1/1 vol ratio of a heptane/toluene mixture) at the oil/water interface. In addition to the viscoelasticity of the adsorbed film, the effects of the bulk liquid density and viscosity as well as the liquid trapped in interfacial cavities are taken into account for the calculation of adsorbed mass. Asphaltenes in heptol adsorb continuously at the oil/water interface, while resins (the surface-active species in maltenes) show adsorption saturation in the same solvent. For Athabasca bitumen in heptol, two adsorption regimes are observed depending on concentration. At low concentrations, a slow, non-steady-state, and irreversible adsorption takes place. At high concentrations, a steady-state adsorption with limited reversibility results in a quick adsorption saturation. The threshold concentration between these adsorption regimes is 1.5 wt % and 8 wt % for oil/water and oil/gold interfaces, respectively. The threshold concentration, the total adsorbed amount, and the flux of non-steady-state adsorption depend on the resin-to-asphaltene ratio. The threshold concentration is related to the earlier reported critical bitumen concentration characterizing the rigid-to-flexible transition of the interfacial film. We propose a new mechanism based on the change of the effective resin-to-asphaltene ratio with dilution to explain both the adsorption behavior and emulsion stability.

Journal Article↗

PubMed2005

Interaction forces in bitumen extraction from oil sands.

Water-based extraction process (WBEP) has been successfully applied to bitumen recovery from Athabasca oil sand ore deposits in Alberta. In this process, two essential steps are involved. The bitumen first needs to be "liberated" from sand grains, followed by "aeration" with air bubbles. Bitumen "liberation" from the sand grains is controlled by the interaction between the bitumen and sand grains. Bitumen "aeration" is dependent, among other mechanical and hydrodynamic variables, on the hydrophobicity of the bitumen surface, which is controlled by water chemistry and interactions between bitumen and fine solids. In this paper, the interaction force measured with an atomic force microscope (AFM) between bitumen-bitumen, bitumen-silica, bitumen-clays and bitumen-fines is summarized. The measured interaction force barrier coupled with the contacted adhesion force allows us to predict the coagulative state of colloidal systems. Zeta potential distribution measurements, in terms of heterocoagulation, confirmed the prediction of the measured force profiles using AFM. The results show that solution pH and calcium addition can significantly affect the colloidal interactions of various components in oil sand extraction systems. The strong attachment of fines from a poor processing ore on bitumen is responsible for the corresponding low bitumen flotation recovery. The identification of the dominant non-contact forces by fitting with the classical DLVO or extended DLVO theory provides guidance for controlling the interaction behavior of the oil sand components through monitoring the factors that could affect the non-contact forces. The findings provide insights into megascale industrial operations of oil sand extraction.

Journal Article↗

PubMed2005

Surface charge controlled magnetic nanoparticles with grafting of poly(4-vinylpyridine).

Poly(4-vinylpyridine)-grafted magnetic nanoparticles were prepared by the "graft to" method. Surface-modified magnetic particles are covered with a hydrophobic layer because it can be dispersed in organic solvents such as methanol, toluene, benzene, and chloroform, but not in water. Surface charge control with magnetic nanoparticles of various isoelectric point was achieved by quaternization of pyridyl groups, and the positively charged particles can be dispersed in water within a wide range of pH.

Crystallization↗

PubMed2004

Effect of charged colloidal particles on adsorption of surfactants at oil-water interface.

A change of oil/water interfacial tension in the presence of cationic or anionic surfactants in an organic phase was observed due to the addition of charged fine solids in the aqueous phase. The charged fine solids in the aqueous phase adsorb surfactants diffused from the oil phase, thereby causing an increase in the bulk equilibrium surfactant concentration in the aqueous phase, governed by the Stern-Grahame equation. Consequently, surfactant adsorption at the oil-water interface increases, which was demonstrated from the measured reduction of the oil-water interfacial tension. The increased surfactant partition in the aqueous phase in the presence of the charged particles was confirmed by the measured decrease in the surface tension for the collected aqueous solution after solids removal, as compared with the cases without solids addition.

Journal Article↗

PubMed2003

Effect of chemical composition on electrokinetics of diaspore.

The effect of chemical composition of diaspores on their electrokinetics was studied. Increasing SiO(2) content in a diaspore sample was found to decrease its isoelectric point. The X-ray diffraction and SEM microanalysis showed the absence of distinct SiO(2) phases. A linear correlation was found to exist between the measured isoelectric point and alumina to silica mass ratio in diaspore samples. The linear regression analysis of the experimental data suggests a more significant impact of silicon content than aluminum content, indicating a preferential adsorption of dissolved silicon on diaspore samples and/or preferential dissolution of aluminum from diaspore samples.

Journal Article↗

PubMed2003

Studies of Athabasca asphaltene Langmuir films at air-water interface.

Asphaltenes are present in heavy oils and bitumen. They are a mixture of hydrocarbons having complex structures of polyaromatic rings and short side chains. In general, the high-molecular-weight asphaltene is the most aromatic fraction with the highest number of side chains and the low-molecular-weight asphaltene contains the lowest number of side chains, while the number of side chains of the whole asphaltene fraction lies in between. In this study, asphaltenes were extracted and/or fractionated from Athabasca oil sand bitumen. Subfractions of high and low molecular weight and the whole asphaltenes were characterized using a Langmuir trough and complementary techniques such as VPO, FTIR, AFM, and contact angle measurements. At an air-water interface, amphiphilic asphaltene molecules can form a monolayer. Various fractions (high, low, and whole) of the asphaltene molecules behave similarly at the air-water interface, characterized by close resemblance of their surface pressure-area, hysteresis, and relaxation isotherms. The high-molecular-weight asphaltene is the most expanded fraction, while the low-molecular-weight asphaltene fraction is the most condensed, with the whole asphaltene lying in between. At the air-water interface a monolayer of the low-molecular-weight asphaltene relaxes at a faster rate than one of the high-molecular-weight asphaltene.

Air↗

PubMed2003

Effect of surface mobility on the particle sliding along a bubble or a solid sphere.

The sliding velocity of glass beads on a spherical surface, made either of an air bubble or of a glass sphere held stationary, is measured to investigate the effect of surface mobility on the particle sliding velocity. The sliding process is recorded with a digital camera and analyzed frame by frame. The sliding glass bead was found to accelerate with increasing angular position on the collector's surface. It reaches a maximum velocity at an angular position of about 100 degrees and then, under certain conditions, the glass bead leaves the surface of the collector. The sliding velocity of the glass bead depends strongly on the surface mobility of a bubble, decreasing with decreasing surface mobility. By a mobile surface we mean one which cannot set up resistive forces to an applied stress on the surface. The sliding velocity on a rigid surface, such as a glass sphere, is much lower than that on a mobile bubble surface. The sliding velocity can be described through a modified Stokes equation. A numerical factor in the modified Stokes equation is determined by fitting the experimental data and is found to increase with decreasing surface mobility. Hydrophobic glass beads sliding on a hydrophobic glass sphere were found to stick at the point of impact without sliding if the initial angular position of the impact is less than some specific angle, which is defined as the critical sticking angle. The sticking of the glass beads can be attributed to the capillary contracting force created by the formation of a cavity due to spontaneous receding of the nonwetting liquid from the contact zone. The relationship between the critical sticking angle and the particle size is established based on the Yushchenko [J. Colloid Interface Sci. 96 (1983) 307] analysis.

Journal Article↗

PubMed2002

Physical characterization of a chitosan-based hydrogel delivery system.

Hydrogel formation from a mixture of biocompatible chitosan, beta-glycerol phosphate, and hydroxyethyl cellulose was studied. The rheological properties of the formed hydrogels were examined using a Bohlin VOR rheometer. The effect of hydrogel composition and temperature on both the gelation rate and the elastic strength of hydrogels was investigated, from which possible hydrogel formation mechanisms were inferred. The formed hydrogels as potential vehicles for delivering pilocarpine were examined. The advantages and disadvantages of chitosan/hydroxyethyl cellulose hydrogel as a delivery system are discussed.

Chitin↗