PubMed Health⌕ Search

Biomedical subjects

S Biggs

Publications and source records attributed to S Biggs.

At least 19 recordsLinked to original sources
PubMed2003

The influence of chain length and electrolyte on the adsorption kinetics of cationic surfactants at the silica-aqueous solution interface.

The equilibrium and kinetic aspects of the adsorption of alkyltrimethylammonium surfactants at the silica-aqueous solution interface have been investigated using optical reflectometry. The effect of added electrolyte, the length of the hydrocarbon chain, and of the counter- and co-ions has been elucidated. Increasing the length of the surfactant hydrocarbon chain results in the adsorption isotherm being displaced to lower concentrations. The adsorption kinetics indicate that above the cmc micelles are adsorbing directly to the surface and that as the chain length increases the hydrophobicity of the surfactant has a greater influence on the adsoption kinetics. While the addition of 10 mM KBr increases the CTAB maximal surface excess, there is no corresponding increase for the addition of 10 mM KCl to the CTAC system. This is attributed to the decreased binding efficiency of the chloride ion relative to the bromide ion. Variations in the co-ion species (Li, Na, K) have little effect on the adsorption rate and surface excess of CTAC up to a bulk electrolyte concentration of 10 mM. However, the rate of adsorption is increased in the presence of electrolyte. Slow secondary adsorption is seen over a range of concentrations for CTAC in the absence of electrolyte and importantly in the presence of LiCl; the origin of this slow adsorption is attributed to a structural barrier to adsorption.

Journal Article↗
PubMed2003

Mechanism of cationic surfactant adsorption at the solid-aqueous interface.

Until recently, the rapid time scales associated with the formation of an adsorbed surfactant layer at the solid-aqueous interface has prevented accurate investigation of adsorption kinetics. This has led to the mechanism of surfactant adsorption being inferred from thermodynamic data. These explanations have been further hampered by a poor knowledge of the equilibrium adsorbed surfactant morphology, with the structure often misinterpreted as simple monolayers or bilayers, rather than the discrete surface aggregates that are present in many surfactant-substrate systems. This review aims to link accepted equilibrium data with more recent kinetic and structural information in order to describe the adsorption process for ionic surfactants. Traditional equilibrium data, such as adsorption isotherms obtained from depletion approaches, and the most popular methods by which these data are interpreted are examined. This is followed by a description of the evidence for discrete aggregation on the substrate, and the morphology of these aggregates. Information gained using techniques such as atomic force microscopy, fluorescence quenching and neutron reflectivity is then reviewed. With this knowledge, the kinetic data obtained from relatively new techniques with high temporal resolution, such as ellipsometry and optical reflectometry, are examined. On this basis the likely mechanisms of adsorption are proposed.

Adsorption↗
PubMed2002

Viscosity effect on the structural compactness of latex flocs formed under weak depletion attractions.

Dilute aqueous dispersions of colloidal polystyrene latex spheres were flocculated by adding a nonadsorbing polymer sample, poly(acrylic acid). The structural compactness of the flocs thus formed was characterized in terms of their mass fractal dimension using the small-angle static light scattering technique. It was found that with low poly(acrylic acid) concentrations and thus weak depletion attraction forces, the dispersion medium viscosity had a marked effect on the floc structure. An increase in the viscosity led to formation of denser flocs. This was revealed in three sets of depletion flocculation experiments: (a) adjusting the background electrolyte concentration at a fixed level of poly(acrylic acid), (b) using water and 30% (w/w) glycerol as the respective solvents, and (c) inducing latex flocculation with two poly(acrylic acids) of different molecular weights at the respective critical polyacid concentrations. Direct force measurements were made with atomic force microscopy to isolate the influence of viscosity on floc structure from that of interparticle interaction energies. We conclude that the formation of denser flocs with increasing medium viscosity can be attributed to the reduced diffusivity of particles in the solution. The latter resulted in an enhanced rate of floc restructuring (through relaxation of attached particles) relative to floc growth.

Colloids↗
PubMed2001

Microscopic and macroscopic aspects of stick-slip motion in granular shear.

An annular shear cell has been used to investigate a number of factors known to influence stick-slip motion in an assembly of near monosized, spherical glass beads. In this paper, both the sample shear stress and volumetric strain were recorded, allowing new insights into the possible mechanics of stick-slip motion in a granular body. Rather than the commonly presented mechanism of sample dilation and fluidization accompanying the slip events, in the material studied here, sample dilation occurred during the preslip deformations of the granular body, while the slip event was accompanied by assembly contraction. Drive velocity and applied normal pressure were both found to influence the magnitude of the stick-slip spikes in a manner analogous to previous studies of stick-slip in assemblies of confined, near-spherical lubricant systems. Finally, atmospheric relative humidity was found to have a marked effect on the magnitude of the stick-slip motion. To investigate this mechanism more fully, the atomic force microscopy was employed to measure the particle-particle interaction forces as a function of atmospheric relative humidity. A water meniscus was found to form under all humidities, from less than 5% to greater than 95%. However, its influence on the adhesive forces varied by an order of magnitude. While most previous studies of stick-slip phenomena have attempted to remove atmospheric humidity as a variable, here we present a useful link between the role of relative humidity on particle-particle interaction forces and the macroscopic response of the granular assembly.

Journal Article↗
PubMed2001

Assessing the health impact of age-specific housing.

It has been claimed that 'retirement communities', defined in this instance as voluntary communities of older people living in shared, purpose-built housing, combine the best attributes of residential and community living. Subjective health status may thereby be improved through a culture in which independence and autonomy are actively promoted. Concern has also been raised that age-segregated communities of this sort might produce 'ghettos' of increasing dependency and service demand. This study, conducted over a 12-month time period, found that when compared to older people living in the local neighbourhood, the retirement community population maintained their physical and mental health (utilising measures including the SF36, Life Satisfaction Index, and 18 semantic differentials). Investigation of these findings indicated that peer support and safety/security, and 'autonomy with inclusion' were key factors in maintaining health status.

Aged↗
PubMed2001

A Foucauldian analysis of old age and the power of social welfare.

It is argued that the question of social welfare is a key, if often overlooked, component in the construction of power relations and identities in later life that can take its place next to debates on bioethics and consumer lifestyle. Foucault's (1977) claim, that identities are kept in place through the deployment of integrated systems of power and knowledge and a routine operation of surveillance and assessment, is critically examined in this context. Trends in social welfare in the United Kingdom are used as a case example that sheds light on wider contemporary issues associated with old age. Finally, implications for the creation of particular narratives about later life are discussed and grounded through Foucault's (1988) notion of "technologies of self."

Aged↗
PubMed2000

Lateral Force Microscopy Study of the Friction between Silica Surfaces.

The friction between a single silica glass sphere and a flat silica glass surface has been investigated using lateral force microscopy. Two types of silica surface were investigated, one fully hydroxylated and the other partially dehydroxlated. These samples have markedly different wetting properties and are known as hydrophilic and hydrophobic silica, respectively. The lateral friction force as a function of applied normal load has been collected for five different-sized spheres for both surface types. Analysis of the friction versus load data indicates that the hydrophilic surfaces have a molecularly smooth contact. For the hydrophobic surfaces, it is not clear whether the friction-load response is caused by a smooth or an asperity-dominated contact zone. Copyright 2000 Academic Press.

Journal Article↗
PubMed2000

Force Calibration in Lateral Force Microscopy.

An analysis of the contact mechanics and the forces of interaction in lateral force microscopy measurements is presented. This analysis allows for a new method of interpretation of the frictional forces, the lateral contact stiffness, and the contact shear strength. The technique was developed for the interpretation of frictional data obtained with colloidal probes, although results are presented which illustrate its ability to interpret measurements recorded with both colloidal probes and standard atomic force microscopy tips. The technique is found to compensate for the variations in the contact geometry, giving repeatable results for probes of different sizes. A critical review of other techniques which have been employed to interpret the frictional force in lateral force microscopy is also presented. Copyright 2000 Academic Press.

Journal Article↗
PubMed2000

Modification of a Commercial Atomic Force Microscope for Nanorheological Experiments: Adsorbed Polymer Layers.

The atomic force microscope (AFM) has previously been applied to the measurement of surface forces (including adhesion and friction) and to the investigation of material properties, such as hardness. Here we describe the modification of a commercial AFM that enables the "stiffness" of interaction between surfaces to be measured concurrently with the surface forces. The stiffness is described by the rheological phase difference between the response of the AFM tip to a driving oscillation of the substrate. We present the interaction between silica surfaces bearing adsorbed polymer, however, the principles could be applied to a wide variety of materials including biological samples.

Journal Article↗
PubMed2000

Ultrasonic initiation of polystyrene latex synthesis

Polystyrene latex samples have been synthesised from an oil-in-water (o/w) emulsion of the monomer using ultrasonic irradiation as the initiation source. No added chemical initiators were used in any of the reactions. The free radicals formed from the cavitation process under the action of the ultrasound are sufficient to initiate the polymerisation. In the current study, the effects of varying the input intensity of the ultrasound were investigated. A marked increase in the monomer conversion rate as a function of time was seen as the power was increased. Despite this increase in conversion rate, the increasing intensity did not affect the resultant latex particle sizes. In all cases, the particle diameters at the conclusion of a polymerisation were seen to be in the range of 40 to 50 nm. The effects of changing the concentration of surfactant in the polymerisation medium and of changing the temperature were also probed. Increases in both were seen to lead to an increased rate of conversion at a fixed input intensity. Again, the particle diameters were invariant as a function of changing conditions. The small particle sizes and high conversion rates are thought to be due to a continuous nucleation process that is caused by the scavenging of free radicals throughout the polymerisation by freshly formed small oil droplets.

Journal Article↗
PubMed2000

Microscopic and macroscopic effects of surface lubricant films in granular shear

Experiments were conducted to investigate the link between particle-particle interaction forces and the bulk properties of granular shear using an idealized system of near-spherical, monosized glass beads. The atomic force microscopy colloidal probe technique was employed to investigate the adhesion and friction between a single bead and a second glass surface, while the annular shear cell was used to measure the shear properties of the bulk granular material. A covalently bound monomolecular film of aliphatic chains was introduced to alter the tribological interactions between the particles. The atomic force microscope was used to measure the reduction in the particle-particle surface forces resulting from the addition of the boundary lubricant, while the shear cell showed that the effect of the lubricant film was to reduce the coefficient of internal friction and the dilation during shear. This is an experimental study to provide quantitative data linking particle-particle interaction forces and the shear properties of a granular body.

Journal Article↗
PubMed1996

Butyl Acrylate/Vinyl Acetate Copolymer Latex Synthesis Using Ultrasound As an Initiator

The polymerization of oil-in-water (o/w) emulsions of monomeric species has been performed at 30°C (±5°C) under ultrasonic irradiation, in the absence of any added chemical initiators. Emulsions of butyl acrylate and vinyl acetate as well as emulsified mixtures of the two monomers have been reacted in this way. In all cases, the radicals formed as a result of the ultrasonic cavitation were sufficient to cause polymerization. Stable blue-white or white dispersions of polymer latex particles were obtained. The kinetics of the copolymerization process were monitored. The data obtained here show that the polymerization rate depends strongly upon the monomer concentration dissolved in the aqueous phase and on its vapor pressure. The more volatile of the monomers examined, vinyl acetate, was seen to have a markedly lower polymerization rate at equivalent monomer concentrations when compared to the butyl acrylate. This was attributed to monomer evaporation into the cavities formed by the ultrasound, causing a dampening of the cavitation process and hence a lower radical density. Data of particle sizes and polymer molecular weights for the latex samples support this hypothesis.

Journal Article↗
PubMed1996

The Direct Measurement of the Forces of Interaction between a Colloid Particle and an Oil Droplet

An atomic force microscope was used to measure the forces between a silica sphere (diameter 10 μm) and a n-decane droplet (diameter 0.3 mm) in water. Force-distance profiles showed a weak attraction and adhesion due to van der Waals forces. When the anionic surfactant sodium dodecyl sulfate was added, there was electrostatic repulsion at all separations due to the adsorption of the anionic surfactant at the oil-water interface, and no adhesion of the sphere to the oil droplet was observed upon retraction. Fitting the repulsive curves to the nonlinear Poisson-Boltzmann equation for heterogeneous surfaces yielded surface potentials on the oil surface which were consistently lower than those values found from electrokinetic experiments on oil drops under similar conditions. The measured Debye lengths were also found to be significantly different from those calculated from the surfactant and electrolyte conditions employed. Possible reasons for the discrepancies are outlined.

Journal Article↗
PubMed1996

ZG-1494 alpha, a novel platelet-activating factor acetyltransferase inhibitor from Penicillium rubrum, isolation, structure elucidation andbiological activity.

A novel inhibitor of platelet-activating factor (PAF) acetyltransferase, an essential enzyme in the remodeling pathway of platelet-activating factor synthesis, was identified by a high throughout screen of natural product extracts of microbial origin. The compound, ZG-1494 alpha, was isolated from an ethyl acetate extract of a culture broth of Penicillium rubrum through bioassay guided fractionation. The structure of ZG-1494 alpha was determined by spectroscopic methods. A key feature of the structure, which is relatively rare among natural products, is the 5-hydroxy-3-pyrrolin-2-one moiety. A 13C-13C INADEQUATE was utilized to unambiguously determine the regiochemistry of this molecule.

Acetyltransferases↗
PubMed1993

Expression cloning and signaling properties of the rat glucagon receptor.

Glucagon and the glucagon receptor are a primary source of control over blood glucose concentrations and are especially important to studies of diabetes in which the loss of control over blood glucose concentrations clinically defines the disease. A complementary DNA clone for the glucagon receptor was isolated by an expression cloning strategy, and the receptor protein was expressed in several kidney cell lines. The cloned receptor bound glucagon and caused an increase in the intracellular concentration of adenosine 3', 5'-monophosphate (cAMP). The cloned glucagon receptor also transduced a signal that led to an increased concentration of intracellular calcium. The glucagon receptor is similar to the calcitonin and parathyroid hormone receptors. It can transduce signals leading to the accumulation of two different second messengers, cAMP and calcium.

Amino Acid Sequence↗