Biomedical subjects

John Ralston

Publications and source records attributed to John Ralston.

14 recordsLinked to original sources

PubMed2006

High-resolution in situ x-ray study of the hydrophobic gap at the water-octadecyl-trichlorosilane interface.

The knowledge of the microscopic structure of water at interfaces is essential for the understanding of interfacial phenomena in numerous natural and technological environments. To study deeply buried liquid water-solid interfaces, high-energy x-ray reflectivity measurements have been performed. Silicon wafers, functionalized by a self-assembled monolayer of octadecyl-trichlorosilane, provide strongly hydrophobic substrates. We show interfacial density profiles with angstrom resolution near the solid-liquid interface of water in contact with an octadecyl-trichlorosilane layer. The experimental data provide clear evidence for the existence of a hydrophobic gap on the molecular scale with an integrated density deficit rhod = 1.1 A g cm(-3) at the solid-water interface. In addition, measurements on the influence of gases (Ar, Xe, Kr, N(2), O(2), CO, and CO(2)) and HCl, dissolved in the water, have been performed. No effect on the hydrophobic water gap was found.

Journal Article↗

PubMed2006

Directed crystallisation of zinc oxide on patterned surfaces.

Patterned self-assembled monolayers of functionalised alkane thiols were prepared on gold substrates, using UV-photolithography. Two alkane thiols, 11-mercaptoundecanoic acid (MUA) and a fluorinated decane thiol (FDT, CF3(CF2)7CH2CH2SH) were used to fabricate chemically structured surfaces which served as templates for zinc oxide (ZnO) crystallisation. When these patterns, containing high (MUA) and low (FDT) surface energy regions were exposed to a 10 mM zinc nitrate crystallising solution, nucleation occurred selectively on the low energy regions. After 90 min, hexagonal prisms had grown upright on these areas. The crystal growth is uniform with a crystal length of about 1 mum and a diameter between 50 and 100 nm. We attribute the selective growth to a combination of crystallographic frustration of the zinc ions on the high energy regions and an accumulation of hydroxide ions on the low energy regions.

Journal Article↗

PubMed2006

The influence of polymer structure and morphology on talc wettability.

Advancing water contact angles were measured on freshly cleaved talc faces as well as on talc particles. The intrinsic hydrophobicity of talc was shown to be due to the dominance of the apolar components of the work of adhesion. Polyacrylamides and polysaccharides adsorb onto the surface of talc, displaying strikingly different morphologies. Adsorbed amount, apparent layer thickness, and polymer structure control talc wettability.

Journal Article↗

PubMed2006

Electrowetting of ionic liquids.

We have successfully demonstrated that imidazolium- and pyrrolidinium-based commercial room-temperature ionic liquids can electrowet (with a dc voltage) a smooth fluoropolymer (Teflon AF1600) surface. Qualitatively, the process is analogous to the electrowetting of aqueous electrolyte solutions: the contact angle versus voltage curve has a parabolic shape which saturates at larger voltages (positive or negative). On the other hand we observed several peculiarities: (i) the efficiency is significantly lower (by about an order of magnitude); (ii) the influence of the bulky cation is larger and the importance of the smaller anion is lesser, especially with respect to electrowetting saturation; (iii) there is an asymmetry in the saturation contact angles found for positive and negative voltages. The asymmetry may be correlated with the cation-anion asymmetry of the ionic liquids. The low efficiency may be caused by the presence of water and other impurities in these commercial materials.

Electrochemistry↗

PubMed2005

Light-induced surface wettability of a tethered DNA base.

We show that the DNA base thymine and other uracil derivatives, when alkylated with a hydrocarbon chain and assembled at a gold interface, dimerize when subjected to UV irradiation. The process is robust and reversible and is accompanied by a substantial decrease in wettability as well as a marked decrease in acidity constant when the dimer is formed. There is a concerted molecular reorientation that accompanies photodimerization, with the dimer displaying a marked affinity for the gold surface. The spatial requirements for this reorganization are satisfied during dimerization by the reduction in intermolecular distance that occurs when the cyclobutane ring is formed between adjacent bases. The structural changes observed here for a tethered DNA base provide a direct route for exploring reactions, in two dimensions, that are of central interest in biology.

DNA↗

PubMed2005

An in situ ATR-FTIR study of polyacrylamide adsorption at the talc surface.

The adsorption of a low molecular weight unmodified polyacrylamide (Polymer-N) and a hydroxyl-substituted polyacrylamide (Polymer-H) onto talc was studied using in situ particle film ATR-FTIR spectroscopy in the multiple internal reflection mode. Spectra of the adsorbed polymer were collected as a function of increasing concentration and as a function of time. Measurement of the peak intensities of the adsorbed polymer allowed adsorption isotherms and adsorption kinetics to be determined for both polymers. Langmuir adsorption isotherm analysis of in situ data yielded Gibbs free energies of adsorption (deltaG0(ads)) for Polymer-N and Polymer-H of -44.5 and -45.7 kJ/mol, respectively, which correlate well with similar values determined from ex situ adsorption isotherms. Kinetic analysis indicated that the adsorption of both polymers was a pseudo-first-order process. The apparent rate constants for Polymer-N and Polymer-H were 0.10 and 0.15 min(-1), respectively. Absence of spectral shifts in the spectra of adsorbed polymer is indicative of a hydrophobic interaction between the polyacrylamides and the talc surface.

Journal Article↗

PubMed2005

Marangoni effects in aqueous polypropylene glycol foams.

The foam behavior of three polypropylene glycols covering the molecular weight range between 192 and 725 g/mol has been examined. Static and dynamic surface tension data, as well as bubble size distribution and retention time in the foam, were incorporated into a simple model of foam stability. The latter clearly indicates that surface tension differences between the plateau border and lamellar region adjacent to the bubble surface are the dominant factor in controlling foamability, causing liquid flow in the direction opposite to liquid drainage, a process termed the Marangoni effect.

Journal Article↗

PubMed2005

Colloid stability of synthetic titania and the influence of surface roughness.

The colloid stability of synthetic titania particles was studied as a function of KCl concentration at pH values of 6.3, 6.7, and 8.4, using static light scattering to obtain stability ratios. Standard DLVO theory was then used to calculate the stability ratios as a function of salt concentration. Reasonable agreement between theory and experiment could only be obtained if an effective interaction radius, corresponding to surface asperities on the titania particles, was used in the calculation. High-resolution TEM images suggest that the effective interaction radius corresponds to the size of surface crystallites formed during synthesis.

Journal Article↗

PubMed2005

Morphology of adsorbed polymers and solid surface wettability.

The adsorption of a polyacrylamide (MW 14600) and two polysaccharides (MW 9260 and 706 x 10(3)) onto model silica surfaces of different hydrophobicities was investigated. In all cases, adsorption adhered to the Freundlich isotherm, reflecting the heterogeneous character of the solid substrates. The latter strongly influenced the character of the adsorbed polymer, with morphologies from chainlike structures to thin films and patches being observed. Surface roughness, polymer type, and molecular weight also play roles in controlling adsorbed polymer morphology. Surface wettability is strongly influenced by the thickness of the adsorbed layer.

Journal Article↗

PubMed2005

Contact angle saturation in electrowetting.

Electrowetting is the phenomenon of contact angle decrease under the influence of an external voltage applied across the solid/liquid interface. Electrowetting offers an interesting possibility to enhance the wettability of hydrophobic materials without altering the chemical composition of the system and thus could be incorporated in various microfluidic devices. Electrowetting is fundamentally an electrocapillary effect occurring on an insulated solid electrode (hence the change of the solid/liquid interfacial tension with voltage follows Lippmann's equation). A limiting contact angle value larger than zero is achieved even at very large external voltages. Saturation precludes full wetting of the substrate and restricts the magnitude of the capillary force variation. Contact angle saturation has been given various interpretations (e.g., charge trapping, air ionization) but appears to reflect a natural thermodynamic limit rather than being simply a defective property. The limiting value of the contact angle is given by the Young equation when the value of the solid/liquid interfacial tension reaches zero. The model is in excellent agreement with our own results and often gives an adequate description of published data. It also suggests that the saturation limit is determined by the material properties of the system and electrowetting at voltages exceeding this threshold is essentially a nonequilibrium process.

Electrochemistry↗

PubMed2005

Solution behavior of IFN-beta-1a: an empirical phase diagram based approach.

An empirical phase diagram approach has been developed as a practical tool to aid macromolecular preformulation/formulation studies. This method employs an eigenvector based procedure to visualize and interpret complex data sets. Human Inteferon-beta-1a, an important therapeutic protein, was used to further develop the method and test its utility. The protein was characterized in solution as a function of pH (2-8), temperature (10 degrees C-85 degrees C) and ionic strength (I = 0.1 and 1.0) using intrinsic and ANS fluorescence, Far-UV circular dichroism (Far-UV CD), Fourier Transform Infrared spectroscopy (FTIR) and derivative UV absorbance spectroscopies, as well as differential scanning calorimetry (DSC) to supplement spectroscopic thermal stability studies. Derivative UV absorbance data were initially used to construct a pH-temperature phase diagram at each ionic strength. Three distinctive phases at I = 0.1 and two major phases at I = 1.0 were identified corresponding to different conformation/aggregation states of the protein. For the first time, heterogeneous data sets (i.e., data from different techniques) including Far-UV CD, fluorescence and UV absorbance results were used to generate empirical phase diagrams. Results from different data sets are compared; precautions in applying the method and its overall utility are discussed.

Calorimetry, Differential Scanning↗

PubMed2004

Temperature influence of nonionic polyethylene oxide and anionic polyacrylamide on flocculation and dewatering behavior of kaolinite dispersions.

Nonionic polyethylene oxide (PEO) and anionic polyacrylamide (PAM) flocculation of kaolinite dispersions has been investigated at pH 7.5 in the temperature range 20-60 degrees C. The surface chemistry (zeta potential), particle interactions (shear yield stress), and dewatering behavior were also examined. An increase in the magnitude of zeta potential of kaolinite particles, in the absence of flocculant and at a fixed PEO and PAM concentration, with increasing temperature was observed. The zeta potential behavior of the flocculated particles indicated a decrease in the adsorbed polymer layer thickness, while at the same time, however, the adsorbed polymer density showed a significant increase with increasing temperature. These results suggest that polymer adsorption was accompanied by temperature-influenced conformation changes. The hydrodynamic diameter and supernatant solution viscosity of both polymers decreased with increasing temperature, consistent with a change in polymer-solvent interactions and conformation, prior to adsorption. The analysis of the free energy (DeltaG(ads)) of adsorption showed a strong temperature dependence and the adsorption process to be more entropically than enthalpically driven. The polymer conformation change and increased negative charge at the kaolinite particle surface with increasing temperature resulted in decreased polymer bridging and flocculation performance. Consequently, the shear yield stress and the rate and the extent of dewatering (consolidation) of the pulp decreased significantly at higher temperatures (>40 degrees C). The temperature effect was more pronounced in the presence of PEO than PAM, with 40 and 20 degrees C indicated as the optima for enhanced performance of the latter and former flocculants, respectively. The results demonstrate that a temperature-induced conformation change, together with polymer structure type, plays an important role in flocculation and dewatering behavior of kaolinite dispersions.

Journal Article↗

PubMed2003

Bubble particle heterocoagulation under turbulent conditions.

An analytical model that enables the calculation of the flotation rate constant of particles as a function of particle size with, as input parameters, measurable particle, bubble, and hydrodynamic quantities has been derived. This model includes the frequency of collisions between particles and bubbles as well as their efficiencies of collision, attachment, and stability. The generalized Sutherland equation collision model and the modified Dobby-Finch attachment model developed previously for potential flow conditions were used to calculate the efficiencies of particle-bubble collision and attachment, respectively. The bubble-particle stability efficiency model includes the various forces acting between the bubble and the attached particle, and we demonstrate that it depends mainly on the relative magnitude of particle contact angle and turbulent dissipation energy. The flotation rate constants calculated with these models produced the characteristic shape of the flotation rate constant versus particle size curve, with a maximum appearing at intermediate particle size. The low flotation rate constants of fine and coarse particles result from their low efficiency of collision and low efficiencies of attachment and stability with gas bubbles, respectively. The flotation rate constants calculated with these models were compared with the experimental flotation rate constants of methylated quartz particles with diameters between 8 and 80 micro m interacting with gas bubbles under turbulent conditions in a Rushton flotation cell. Agreement between theory and experiment is satisfactory.

Journal Article↗

PubMed2003

Influence of hydrolyzable metal ions on the interfacial chemistry, particle interactions, and dewatering behavior of kaolinite dispersions.

The influence of hydrolyzable metal ions (Mn(II) and Ca(II)) adsorption on the surface chemistry, particle interactions, flocculation, and dewatering behavior of kaolinite dispersions has been investigated at pH 7.5 and 10.5. Metal ion adsorption was strongly cation type- and pH-dependent and significantly influenced the zeta potential, anionic polyacrylamide-acrylate flocculant (PAM) adsorption, shear yield stress, settling rate, and consolidation of kaolinite slurries. The presence of Mn(II) and Ca(II) ions alone led to a systematic reduction in zeta potential due to specific adsorption of positively charged metal ion-based hydrolysis products at the kaolinite-water interface. Metal ion-mediated zeta potential changes were reflected by lower dispersion shear yield stresses and improved clarification (higher settling rates) but had no detectable effect on dispersion consolidation. The adsorption of PAM was significantly improved by prior addition of the metal ions. In the presence of Mn(II) or Ca(II) ions, the flocculant adsorption density was enhanced at pH 7.5 for Mn(II) and pH 10.5 for Ca(II). Optimum flocculation conditions, involving partial rather than complete particle surface coverage by both metal ions and flocculant, were identified. As a consequence, the metal ions and flocculant acted synergistically to enhance dewatering, producing particle interactions that were more conducive to high settling rates and greater consolidation of kaolinite dispersions at pH 7.5 than 10.5.

Journal Article↗