Biomedical subjects

George V Franks

Publications and source records attributed to George V Franks.

8 recordsLinked to original sources

PubMed2006

Flocculation mechanism induced by cationic polymers investigated by light scattering.

Three cationic polymers with molecular weights and charge densities of 3.0 x 10(5) g/mol and 10%, 1.1 x 10(5) g/mol and 40%, and 1.2 x 10(5) g/mol and 100% were chosen as flocculants to aggregate silica particles (90 nm), under various conditions, including change in polymer dosage, particle concentration, background electrolyte concentration, and shear rate. The size and structure of flocs produced were determined using the static light scattering technique. On the basis of measurements of polymer adsorption and its effect on the zeta potential and floc properties, it has been found that the polymer charge density plays an important role in determining the flocculation mechanism. Polymers with a 10% charge density facilitate bridging, 40% charged polymers bring about either a combination of charge neutralization and bridging or bridging, depending on the polymer dosage, and polymers with the charge density of 100% induce electrostatic patch flocculation mechanism at the optimum polymer dosage and below but bring about bridging mechanism at the polymer dosage approaching the adsorption plateau value. Bridging aggregation can readily be affected by the particle concentration, and an increase in particle concentration results in the formation of larger but looser aggregates, whereas electrostatic patch aggregation is independent of particle concentration. The addition of a background electrolyte aids in bridging aggregation while it is detrimental to electrostatic patch aggregation. It has also been found that the effect of shear rate on the mass fractal dimension depends on polymer charge density.

Journal Article↗

PubMed2006

Charging behavior of the gibbsite basal (001) surface in NaCl solution investigated by AFM colloidal probe technique.

The charging behavior of the gibbsite gamma-Al(OH)3 basal (001) surface in aqueous solution is important for correctly modeling the overall charging properties of gibbsite particles which controls surface phenomena such as adsorption and crystal growth. However, the question of whether the hydroxyl groups on the basal plane are proton active has been raised recently both from experimental and theoretical points of view. Using gibbsite crystals prepared from industrial Bayer process, the surface potentials of cleaved (001) surfaces were calculated from forces measured by the colloidal probe technique in 1 mM NaCl solution with differing pH. It was surprisingly found that the basal plane is proton active in pH less than 7 and protonation seems to level off at about pH 5. The potential-pH data was accurately fitted with a single pKa surface protonation model with pK(a) = 5.9 +/- 0.2.

Journal Article↗

PubMed2005

Absence of specific cation or anion effects at low salt concentrations on the charge at the oil/water interface.

Surfactant-free 2 vol % hexadecane-in-water emulsions have been prepared at pH 9 in the presence of various alkali-metal salts. The surface charge and zeta potential of these emulsions are independent of the identities of the monovalent cations and anions up to 0.01 M electrolyte concentrations. The surface charge density of -5 microC cm(-2) is independent of the identity of the alkali-metal cation among Li, Na, and Cs. The zeta potentials decrease with the log of the salt concentration between 0.1 and 11 mM, independent of the identity of the anion of the sodium salts of iodide, bromide, chloride, fluoride, perchlorate, or iodate or of the identity of the cation of the chloride salts of Li, Na, or Cs. These results imply that neither hydration enthalpies nor ion dispersion potentials are significant in affecting the charge created by the hydroxide ion at the pristine oil/water interface at up to 0.01 M salt concentrations.

Alkanes↗

PubMed2005

Dipolar anions are not preferentially attracted to the oil/water interface.

Homogenization of hexadecane in water at pH 9 gives the same surface charge density in the presence of 0.2 mM thiocyanate or acetate anions as in the presence of chloride, indicating that these dipolar anions are not preferentially adsorbed at the oil/water interface. The decrease in the zeta potential of the emulsion droplets as the sodium salts of iodate, thiocyanate, or acetate are added from 0.1 to 10 mM is the same as that when sodium chloride is added, leading to the same conclusion. Increasing the sodium hydroxide concentration from pH 9 to 11.5 has a different effect on the zeta potential, consistent with the specific adsorption of hydroxide ion at the oil/water interface.

Letter↗

PubMed2005

Stimulant sensitive flocculation and consolidation for improved solid/liquid separation.

A novel method of flocculation resulting in both rapid sedimentation and low sediment moisture is described. It relies on changing the inter-particle forces from repulsive to attractive (aggregation and fast settling results) and then back to repulsive (densification of sediment then occurs). The change in inter-particle force is controlled by a stimulus such as pH or temperature. The technique is demonstrated without polymer using the isoelectric point and pH as the stimulus. The polyelectrolyte chitosan produces faster sedimentation and clearer supernatants as a pH sensitive flocculant. Methylcellulose is an effective temperature sensitive flocculant. The sediment bed volumes can be reduced by between 10 and 45% depending upon the conditions which, as yet, have not been fully optimized.

Journal Article↗

PubMed2005

High resolution AFM images of the single-crystal alpha-Al2O3(0001) surface in water.

The (0001) surface of alpha-Al(2)O(3) single crystals has been imaged by atomic force microscopy in water. The observed hexagonal lattice arrangement has a period of 4.7 A, in good agreement with the known bulk unit cell. The sample cleaning procedure was found to be crucial in obtaining clean terraces and achieving lattice resolution.

Journal Article↗

PubMed2004

Dispersion stability of a ceramic glaze achieved through ionic surfactant adsorption.

The adsorption of cetylpyridinium chloride (CPC) and sodium dodecylbenzenesulfonate (SDBS) onto a ceramic glaze mixture composed of limestone, feldspar, quartz, and kaolin has been investigated. Both adsorption isotherms and the average particle zeta potential have been studied in order to understand the suspension stability as a function of pH, ionic strength, and surfactant concentration. The adsorption of small amounts of cationic CPC onto the primarily negatively charged surfaces of the particles at pH 7 and 9 results in strong attraction and flocculation due to hydrophobic interactions. At higher surfactant concentrations a zeta potential of more than +60 mV results from the bilayered adsorbed surfactant, providing stability at salt concentrations < or = 0.01 M. At 0.1 M salt poor stability results despite substantial zeta potential values. Three mechanisms for SDBS adsorption have been identified. When anionic SDBS monomers either adsorb by electrostatic interactions with the few positive surface sites at high pH or adsorb onto like charged negative surface sites due to dispersion or hydrophobic interactions, the magnitude of the negative zeta potential increases slightly. At pH 9 this increase is enough to promote stability with an average zeta potential of more than -55 mV, whereas at pH 7 the zeta potential is lower at about -45 mV. The stability of suspensions at pH 7 is additionally due to steric repulsion caused by the adsorption of thick layers of neutrally charged Ca(DBS)2 complexes created when the surfactant interacts with dissolved calcium ions from the calcium carbonate component.

Adsorption↗

PubMed2002

Zeta potentials and yield stresses of silica suspensions in concentrated monovalent electrolytes: isoelectric point shift and additional attraction.

The zeta potentials and yield stresses of silica suspensions were measured over a wide range of monovalent electrolyte concentrations. The counterions investigated were Li(+), Na(+), K(+), and Cs(+), while the co-ion was always Cl(-). The poorly hydrated ions (Cs(+) and K(+)) adsorb in greater quantity to the silica surface than the well-hydrated ions (Li(+) and Na(+)) and produce lower magnitude negative zeta potentials at high pH. At high electrolyte concentrations and at low pH the poorly hydrated counterions adsorb in great enough quantity to reverse the sign of the zeta potential from negative to positive. This specific adsorption of counterions shifts the iep to higher values. The shift in the iep is directly related to the hydration of the counterion with the least hydrated ions creating the greatest iep shift. The yield stresses of silica suspensions at high pH were found to increase in the order Li(+)<Na(+)<K(+)<Cs(+). The magnitude of the yield stress correlates with the amount of adsorbed ions; i.e., the greatest yield stresses are observed with the least hydrated ions. The yield stresses measured at high pH with high salt concentrations are greater than can be accounted for with just van der Waals attraction. An attractive ion-ion correlation force is presumed responsible for the additional attraction necessary to explain the observed results.

Journal Article↗