Biomedical subjects

Terence Cosgrove

Publications and source records attributed to Terence Cosgrove.

At least 19 recordsLinked to original sources

PubMed2007

Rupturing polymeric micelles with cyclodextrins.

Small-angle neutron scattering has been used to investigate the associative structures formed by triblock copolymers of poly(ethylene oxide) (PEO)-polypropylene oxide (PPO)-poly(ethylene oxide) (PEO) (also known as Pluronics) and to monitor the structural changes occurring upon complexation with heptakis(2,6-di-O-methyl)-beta-cyclodextrin (hbeta-CD) over the temperature range from 5 to 70 degrees C. At low temperature, the Pluronics are dispersed as unimers. Close to ambient temperature, the hydrophobicity of PPO causes the aggregation of the polymers into spherical micelles with core sizes between 40 and 50 A and a high inclusion of solvent. The aggregation number increases with temperature as the hydrophobicity of the core is gradually enhanced. hbeta-CD spontaneously forms pseudopolyrotaxanes with the triblock copolymers either when in their unimer form or micellized. The complexation results in an increase in the effective critical micellar concentration. It is suggested that the cyclodextrins thread onto the polymer backbone to localize preferentially on the central PPO block, therefore improving its water solubility. At temperatures where the polymers exist in micellar form, complexation with hbeta-CD gives rise to a complete disruption of the aggregates. These processes are highly temperature-dependent. Above 50 degrees C, the break-up of the aggregates is inhibited, and large-scale aggregation is observed.

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PubMed2007

Steric interactions between physically adsorbed polymer-coated colloidal particles: soft or hard?

The steric interaction potential between colloidal particles imparted by adsorbed polymer layers is directly related to their structure. Due to the complexity of these interfacial structures, the steric potential may behave differently at different interparticle separations. In this study, we proposed a combined model of the equivalent hard-sphere model (EHS) and the Hayter-Penfold/Yukawa model (HPY) to describe the steric potential due to adsorbed homopolymers on colloidal particles. The EHS potential describes the dense train/small-loop region and the HPY potential the more diffuse tail/long-loop region. The steric potential was extracted from the structure factors measured by small-angle neutron scattering (SANS). It was found that this combined model gave better agreement with experimental data than either of its component models alone. This study also shows that the adsorbed polymer layer in a good solvent partially collapses when the layers approach one another, which is also supported by an NMR solvent relaxation study.

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PubMed2006

Effects of electrolytes on adsorbed polymer layers: poly(ethylene oxide)-silica system.

The effects of various electrolytes on the adsorption of poly(ethylene oxide) onto silica have been studied. The salts were the chlorides of Na+, Mg2+, Ca2+, and La3+. The methods used were adsorption isotherms, found using a depletion method with phosphomolibdic acid, photon correlation spectroscopy, and solvent relaxation NMR. All the salts increased the particle-polymer affinity and adsorbed amount according to the adsorption isotherms, and a linear relationship was found between the initial slope of the isotherms and the ionic strength of the solution. Final adsorbed amounts were approximately 0.4-0.5 mg m(-2). The polymer layer thicknesses as found by PCS were of the same order as the radius of gyration of the polymer and increased with both the concentration and the valency of the salt due to increased adsorption. Solvent relaxation NMR showed that NaCl is too weak to have a noticeable effect on the polymer train layer, but the divalent salts clearly did increase both the strength of solvent binding close to the silica surface and the amount of PEO required to reach the maximum train density.

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PubMed2006

Small-angle neutron scattering study of concentrated colloidal dispersions: the electrostatic/steric composite interactions between colloidal particles.

Small-angle neutron scattering was used to investigate the interactions in concentrated colloidal dispersions containing silica or polystyrene latex with adsorbed polyethyleneoxide (PEO). In these dispersions of charged particles, both electrostatic and steric repulsions are present. The PEO layer was made invisible to neutrons through contrast matching. The effect of the interparticle repulsion was clearly shown in the scattering spectra by the appearance of a peak at low Q. The effective potentials can be well described by the Hayter-Penfold/Yukawa (HPY) potential. In the silica dispersions studied, the layer thickness is small, hence the electrostatic potential dominates and the potential has a lower concentration dependence. In the dispersions of polystyrene latex, the adsorbed layer is thicker; consequently, the electrostatic potential dominates at low volume fraction (the potential has a lower concentration dependence), and the steric potential dominates at higher volume fraction (the potential has a higher concentration dependence). This study also suggests that when more than one potential is present the stronger one has a dominant influence in determining the structure factor. This finding makes it possible to describe the multicomponential interactions by a single function.

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PubMed2006

A small-angle X-ray scattering study of the interactions in concentrated silica colloidal dispersions.

The structure factors of colloidal silica dispersions at rather high volume fractions (from 0.055 to 0.22) were measured by small-angle X-ray scattering and fitted with both the equivalent hard-sphere potential model (EHS) and the Hayter-Penfold/Yukawa potential model (HPY). Both of these models described the interactions in these dispersions successfully, and the results were in reasonable agreement. The strength and range of the interaction potentials decreased with increasing particle volume fractions, which suggests shrinkage of the electrical double layer arising from an increase in the counterion concentration in the bulk solution. However, the interactions at the average interparticle separation increased as the volume fraction increased. The interaction ranges (delta) determined by the two models were very similar. Structure factors were also used to determine the size and volume fraction of the particles. The values of the size obtained from the structure factors were slightly larger than those obtained from the form factors; this difference is ascribed to the nonspherical shape and polydispersity of the colloidal particles. The volume fractions measured by these two methods were very similar and are both in good agreement with the independently measured results.

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PubMed2005

Small-angle neutron scattering studies of an adsorbed non-ionic surfactant (C12E24) on hydrophobised silica particles in water.

Small-angle neutron scattering (SANS) studies of aqueous dispersions of Stöber silica particles (which have been hydrophobised by having 1-octadecanol grafted to their surface), carrying an adsorbed layer of the nonionic surfactant C12E24, in water, have been performed as a function of temperature. Using mixtures of D2O and H2O, the composition of the continuous phase was adjusted to have the same scattering length density as the silica particles. Hence, only the scattering from the 1-octadecanol and C12E24 layers was detected. The data have been analyzed using both a surface Guinier analysis and a two-layer structure model. It has been found that a step profile best describes the inner combined adsorbed layer (1-octadecanol grafted chains, plus the penetrating alkyl chains from the surfactant) and a semi-Gaussian profile the extended poly(ethylene oxide) outer layer. Both analyses demonstrated that the combined surface layer contracted with increasing temperature.

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PubMed2005

Solubility of cyclomaltooligosaccharides (cyclodextrins) in H2O and D2O: a comparative study.

Cyclomaltooligosaccharides (cyclodextrins, CDs) are cyclic oligomers having six, seven, or eight units of alpha-D-glucose, named as cyclomaltohexaose (alpha-CD), cyclomaltoheptaose (beta-CD) and cyclomaltooctaose (gamma-CD), respectively. The molecule of CD has a cavity in which the interior is hydrophobic relative to its outer surface. The solubility of cyclodextrins in water is unusual, as an irregular trend is observed in the series of the cyclic oligomers of glucose. beta-CD is at least nine times less soluble than the others CDs. This intriguing behavior has been investigated, and some interesting explanations in terms of the effect caused by CD on the water lattice structure have been proposed. In this work a comparative study on the solubility of alpha, beta, and gamma-cyclodextrins was carried out in H2O and D2O and reveals a much lower solubility of the three CDs in D2O. The solid-phase structure of the CDs in equilibrium with the solution is quite similar with both solvents. The results are discussed in terms of the CD molecular structure and the differences in the hydrogen bonds formed between H2O and D2O.

Cyclodextrins↗

PubMed2005

Small-angle neutron scattering study of concentrated colloidal dispersions: the interparticle interactions between sterically stabilized particles.

Small-angle neutron scattering (SANS) was used to investigate the interparticle interactions in concentrated dispersions of colloidal silica stabilized either by steric or by electrostatic repulsive interactions. In 10 mM NaCl, an adsorbed PEO layer is required to prevent flocculation, and particles are stabilized by steric repulsions. The adsorbed layer was made invisible to neutrons by contrast matching with the aqueous continuous phase. Dispersions of the same particles at the same concentrations but in the absence of added salt and adsorbed PEO were also studied. In both cases, the SANS spectra of concentrated dispersions show a peak at low Q, which is due to interparticle interactions: a structure factor. The SANS data can be described rather well by a homogeneous spherical form factor and a structure factor based on the Hayter-Penfold/Yukawa potential model. The steric potential was compared to the electrostatic potential obtained by fitting the SANS data of the bare silica dispersions. The steric potential shows a greater dependence on the particle volume fraction, which we ascribe to the penetration and compression of the adsorbed PEO layer as the particles approach.

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PubMed2005

Small-angle neutron scattering study of adsorbed pluronic tri-block copolymers on laponite.

The adsorption of selected poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) tri-block copolymers on synthetic clay particles (laponite) has been investigated. The adsorbed amount and distribution of polymer was determined as a function of relative block composition and size, using the technique of contrast variation small-angle neutron scattering. The pluronic molecules appear to adsorb via a preferential segregation of hydrophobic PPO segments at the surface, with hydrophilic PEO segments dangling into solution. The effect of the PPO segments is substantial with large increases in adsorbed amount and layer thickness as the anchor fraction decreases/PEO chain length increases. This is in direct contrast to the behavior observed for PEO homopolymer adsorption (of much higher molecular weights) where the adsorbed amount and layer thickness are smaller and change little with molecular weight.

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PubMed2005

Wormlike micelles mediated by polyelectrolyte.

Aqueous solutions of the anionic surfactant potassium oleate (K-oleate) were studied using small-angle neutron scattering (SANS), steady-state rheology, and cryogenic transmission electron microscopy (cryo-TEM). The micellar structural changes induced by the addition of potassium chloride (KCl) and sodium polystyrenesulfonate (PSS) of different molecular weights were investigated. Upon addition of KCl, a transition from spherical to wormlike micelles was detected from the SANS data and confirmed by the cryo-TEM pictures. The rheological measurements revealed a strong dependence of the low-shear viscosity on the concentration of salt: a broad maximum in the viscosity curve was observed upon addition of KCl, characteristic of the growth of micelles into long worms, followed by branching. The addition of PSS to salt-free solutions of K-oleate had a significant effect on the scattering patterns, revealing partial growth of the spherical micelles into rodlike micelles. In contrast, in the presence of high salt concentrations, addition of PSS to solutions of wormlike micelles did not bring any noticeable modifications in the scattering. However, in the same salt conditions, a clear effect was observed on the low shear viscosity upon addition of PSS, which was found to depend significantly on molecular weight. This suggests a novel way of impacting the viscosity of solutions of wormlike micelles.

Micelles↗

PubMed2005

Giant micellar worms under shear: a rheological study using SANS.

Flow-SANS experiments were performed on viscoelastic aqueous solutions of erucyl bis(hydroxyethyl) methylammonium chloride in the presence of potassium chloride. This cationic surfactant has the ability to form very long and flexible wormlike micelles upon addition of salt. The effects of the key-parameters-shear rate, temperature, surfactant and salt concentration-on the ability of the micelles to align in the flow-field were investigated. The scattering data were analyzed in terms of an anisotropy factor (Af). It was found that the wormlike micelles aligned in the direction of the applied shear rate and that the anisotropy factor increased with shear rate. In addition, an increase in temperature caused a decrease of the anisotropy factor (Af) due to the formation of shorter worms. Furthermore, the branching of the micelles at high ionic strength caused the anisotropy factor to decrease in comparison with the values obtained from linear wormlike micelles, hence revealing that the formation of 3-way junctions restricts the alignment of the micelles in the shear-flow. Furthermore, the total surfactant concentration was found to affect the shear-induced patterns significantly, and different behaviors were observed depending on the ionic strength.

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PubMed2005

Structure of a hydrophilic-hydrophobic block copolymer and its interactions with salt and an anionic surfactant.

The aggregation of a hydrophilic-hydrophobic diblock copolymer consisting of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(methyl methacrylate) (PMMA) in aqueous solution has been investigated by small-angle neutron scattering. This polybase is extensively protonated at low pH and forms micelles with a dense core of PMMA and a diffuse coronal layer of cationic PDMAEMA. Addition of salt induced micellar growth, brought about by charge screening and more efficient packing of the chains. As a result, the aggregation number increased from 8 up to 31. A similar effect was observed at low concentrations of an anionic surfactant, sodium dodecyl sulfate (SDS) since the net cationic charge in the hydrophilic coronal layer was reduced due to surfactant binding. However, at higher surfactant concentrations, a drastic structural reorganization occurred, as the PMMA became solubilized into the SDS micellar cores and the PDMAEMA chains interacted with the surfactant micelles, resulting in a "pearl-necklace" structure. The presence of the cationic polyelectrolyte significantly increased the population of SDS micelles by effectively lowering the critical micelle concentration of this anionic surfactant.

Journal Article↗

PubMed2005

SANS study of the interactions among DNA, a cationic surfactant, and polystyrene latex particles.

The compaction of DNA by a cationic surfactant both in the bulk and adsorbed on the surface of latex particles was followed for the first time by SANS. In the bulk, a decrease in the overall size of the DNA coil in the presence of the cationic surfactant was observed at a negative-to-positive charge ratio far below the phase separation region, at a negative-to-positive charge ratio of 18. Additionally, large surfactant aggregates seem to form within the DNA-surfactant complex. On the other hand, DNA adsorbs onto the surface of latex particles, forming a thick layer, as evidenced by the fitting of the SANS data to a core-shell form factor. Addition of a cationic surfactant to the DNA-coated latex particles at a negative-to-positive charge ratio of 38 induces a slight decrease in the size of the particle layer, where the cationic surfactant is evenly distributed within the adsorbed layer. A further decrease of the negative-to-positive charge ratio to 18 induces a dramatic change in the SANS data that suggests significant compaction of the adsorbed layer and the formation of large surfactant aggregates, similar to those detected in the bulk.

Adsorption↗

PubMed2004

Dynamic light scattering studies of poly(ethylene oxide) adsorbed on Laponite: layer conformation and its effect on particle stability.

Dynamic light scattering has been used to determine the hydrodynamic thickness of poly(ethylene oxide) (PEO) adsorbed on synthetic anisotropic clay particles (Laponite) as a function of molecular weight. The layer thicknesses, and their increase with molecular weight, indicate that the conformation of the adsorbed layer is very compact and is much smaller than those normally observed for polymer adsorption on flat interfaces. The aggregation kinetics of the polymer coated particles in 5 mM NaCl was analyzed in a quantitative manner, revealing that the potential barrier to aggregation is strongly enhanced when polymer is present.

Adsorption↗

PubMed2004

Mixed spherical and wormlike micelles: a contrast-matching study by small-angle neutron scattering.

Small-angle neutron scattering studies were used to investigate the effect of adding an alcohol ethoxylate nonionic surfactant (d-C12E20) to aqueous solutions of a cationic surfactant, erucyl bis(hydroxyethyl) methylammonium chloride (EHAC), with and without salt (KCl). The systematic use of contrast-matching, by alternately highlighting or hiding one of the surfactants, confirms that mixed micelles are formed. In salt-free solutions, mixed spherical micelles are formed and a core-shell model combined with a Hayter-Penfold potential was used to describe the data. The core radius is dominated by the EHAC tails and the outer radius determined by the ethoxylate headgroups of the nonionic surfactant. Addition of KCl promotes micellar growth; however, results of varying the solvent contrast revealed that when the nonionic surfactant is incorporated into the wormlike structure micellar breaking is promoted. Thus, mixed wormlike micelles with shorter contour lengths compared to the pure EHAC worms are formed.

Journal Article↗

PubMed2004

Formation of a supramolecular gel between alpha-cyclodextrin and free and adsorbed PEO on the surface of colloidal silica: effect of temperature, solvent, and particle size.

Aqueous solutions of alpha-cyclodextrin (alpha-CD) complex spontaneously with poly(ethylene oxide) (PEO), forming a supramolecular structure known as pseudopolyrotaxane. We have studied the formation of the complex obtained from the threading of alpha-CD onto PEO, both free in solution and adsorbed on colloidal silica. The kinetics of the reaction were studied by gravimetric methods and determined as a function of temperature and solvent composition for the PEO free in solution. PEO was then adsorbed on the surface of colloidal silica particles, and the monomers were displaced by systematically varying the degree of complexation, the concentration of particles, and the molecular weight of the polymer. The effect of the size of the silica particles on the yield of the reaction was also studied. With the adsorbed PEO, the complexation was found to be partial and to take place from the tails of the polymer. The formation of a gel network containing silica at high degrees of complexation was observed. Small-angle X-ray and neutron scattering experiments were performed to study the configuration of the polymeric chains and confirmed the partial desorption of the polymer from the surface of the silica upon complexation.

Adsorption↗

PubMed2004

Impacting the length of wormlike micelles using mixed surfactant systems.

The effect of adding an alcohol ethoxylate nonionic surfactant (C(18)E(18)) to aqueous solutions of a cationic surfactant, erucyl bis(hydroxyethyl) methylammonium chloride (EHAC,CH(3)(CH(2))(7)(CH)(2)(CH(2))(12)N(+)-(CH(2)CH(2)OH)(2)CH(3)Cl(-)), was studied using small-angle neutron scattering (SANS), steady-state rheology, and cryo-transmission electron microscopy (Cryo-TEM). This cationic surfactant has the ability to self-assemble into giant wormlike micelles in the presence of an electrolyte, such as KCl. In salt-free solutions, the mixture of the two surfactants gave rise to spherical micelles. The scattering curves obtained were fitted with a polydisperse core-shell model combined with a Hayter Penfold potential. The inner and outer radii were found to be dependent on the surfactant ratio. In the presence of KCl, mixed wormlike micelles were formed. However, further addition of C(18)E(18) promoted the breaking of the micellar worms with the appearance of a structure peak in the scattering curves. In addition, it was found that the low shear viscosity is decreased upon addition of the alcohol ethoxylate nonionic surfactant. These findings are in good qualitative agreement with the Cryo-TEM images. The results show that the addition of the nonionic surfactant to the system is a method of controlling the worm length.

Alcohols↗

PubMed2004

Detailed modeling of the volume fraction profile of adsorbed polymer layers using small-angle neutron scattering.

A detailed analysis of on and off-contrast small-angle neutron scattering from poly(ethylene oxide)s adsorbed on polystyrene latex is presented. The results have been fitted to an exponential decay, self-consistent mean-field (SCF) and scaling models of the volume fraction profile. As the chain length increases a clear self-similar layer emerges in both the SCF and scaling profiles. The RMS thickness of the adsorbed layer in the plateau of the adsorption isotherm varies as M0.4 for both the SCF and scaling profiles but the exponential one however gives a much lower exponent consistent with the neglect of the tail or distal region. The new constrained version of the Scheutjens-Fleer model was able to predict the structure of the layer and hence the scattering with great accuracy.

Journal Article↗