- [Show abstract] [Hide abstract] ABSTRACT: The history of colloid science, from its modern foundations in the mid-nineteenth century, has been strongly concerned with studies of the aggregation of colloidal particles. It was Thomas Graham (1861) who defined the word "colloid" (from the Greek word for glue) for those materials which could not pass through membranes, unlike smaller, truly-dissolved materials. Subsequently, Graham (1864), following earlier studies, principally by Selmi and Faraday, described "the power possessed by salts for destroying colloidal solutions". Although numerous, quantitative studies of particle aggregation were made in the years that followed, in particular, the determination of minimum electrolyte concentrations for the onset of particle aggregation and aggregation rates, no general theoretical framework emerged to account for these quantitative findings until the middle of the twentieth century. It was during and immediately following the Second World War that two sets of authors, Derjaguin and Landau, in Russia, and Verwey and Overbeek, in the Netherlands, independently came up with the theory that is now universally referred to as the DLVO theory of particle interactions and aggregation. All modern developments of the theory of particle aggregation use the DLVO theory as the keystone. However, the DLVO theory itself was based on a large body of experimental data in regard to particle aggregation obtained over the previous hundred years or so. This article is an attempt to review that body of experimental data and to show how this guided the DLVO authors in their thinking.
- [Show abstract] [Hide abstract] ABSTRACT: The absorption of two hydrophobically modified organic salts (HMOSs), containing azobenzene units, into poly(N-isopropylacrylamide-co-acrylic acid) microgel particles has been studied at pH 8 and 20 °C. These dispersions were then irradiated with UV light (wavelength 365 nm) for 10 min to observe the effect on the microgel particle properties, such as the adsorbed amount of the HMOS, the particle size, and the electrophoretic mobility. We show that irradiation of these dispersions with UV light can lead to induced, partial desorption of the HMOS molecules, with concomitant changes in the size and electrophoretic mobility of the microgel particles. This is due to a conformational switch (trans-form to cis-form) in the HMOS molecules, which reduces the strength of the hydrophobic interaction between the HMOS molecules and the isopropyl moieties within the microgel network. Moreover, the original absorbed amounts, size, and electrophoretic mobility values can be largely restored after storage in the dark for extended periods.
- [Show abstract] [Hide abstract] ABSTRACT: When aqueous suspensions of 1 μm, negatively charged polystyrene particles are subject to a 1 kHz alternating electric field of strength greater than 7 kV(rms) m(-1), dynamic elliptical clusters of particles spontaneously form. With potential applications in microchannel fluidics in mind, we characterize how cluster formation and particle circulation, driven by induced dipole-dipole interactions, is critically dependent on time, field strength, electrolyte concentration, and cell thickness. Logarithmic growth of cluster size is observed, and particle velocity within the clusters is found to be proportional to cluster length. Increasing cell thickness from 10 to 60 μm increases the projected cluster area but decreases cluster aspect ratio as the result of changing particle dispersal rates. Clusters are shown to generate significant fluid shear suitable for microchannel mixing applications. These clusters are observed to distort under transverse fluid flow and, above a critical flow rate, to undergo a transition to form regularly spaced particle streams, which may be suitable for two-dimensional visualization of fluid flow.
- [Show abstract] [Hide abstract] ABSTRACT: The uptake of gold [Au-MES] nanoparticles by pH-responsive poly(2-vinylpyridine) [P2VP] microgel particles, deposited on a surface, has been studied using a quartz crystal microbalance with dissipation monitoring [QCM-D]. QCM-D, which has not previously been applied to such systems, has been shown to be a promising technique for their analysis, especially when combined with a complementary technique such as atomic force microscopy [AFM]. The QCM-D technique, which provides information on mass and viscosity changes of material adsorbed to a surface, has been used to follow the adsorption of microgel particles to a surface, their subsequent swelling and collapse due to changing the system pH, and the uptake of Au-MES nanoparticles by the P2VP particles. Microgel particles with differing cross-linker content have been compared, following uptake by microgel particles in both their collapsed and swollen states. Qualitative differences in the QCM-D response were observed between microgels with differing cross-linker content (and hence different swelling properties). It was shown that uptake of Au-MES nanoparticles acted to fully and irreversibly collapse the swollen P2VP microgels, inhibiting their pH response. The Sauerbrey relationship was used to equate frequency changes measured by QCM-D to mass allowing sorption amounts for the uptake of Au-MES nanoparticles by P2VP particles to be obtained for these surface-constrained systems.
- [Show abstract] [Hide abstract] ABSTRACT: Phase separation, induced by attractive depletion interactions, in mixtures of two sizes of charged silica particles (30 and 200 nm diameter), dispersed in dimethylformamide (DMF), upon the addition of a non-adsorbing, monodispersed polymer (polystyrene) has been studied, for two polymer molecular weights (number-average 115,000 and 543,000). The repulsive electrostatic interaction between the silica particles was reduced through addition of 40 mM lithium chloride. This paper is a follow-up to a recent short communication by the same authors (J. Zhou, J. S. van Duijneveldt and B. Vincent, Phys. Chem. Chem. Phys. 13, 110–113 (2010)), for the same silica particles, plus the lower molecular weight polymer, in DMF. In that paper it was reported that, for certain concentration ranges of the ternary system, three co-existing phases are formed at equilibrium, and in others two. In this work it is shown that, despite the well-known importance of the polymer/colloid size ratio on the phase behaviour of their mixtures, using the higher molecular weight polymer leads to broadly similar behaviour, although the phase boundaries are shifted slightly. The main new finding is that of distinct regions in the phase separation process for systems resulting in two co-existing equilibrium phases (fluid + solid). On increase of polymer concentration, initially only the large particles aggregate. At higher polymer concentration, mixed aggregates are formed consisting of both large and small particles. While this finding can be rationalized in terms of relative strengths of depletion interactions, such scenarios have not been predicted theoretically to the authors’ knowledge.
- [Show abstract] [Hide abstract] ABSTRACT: A series of four hydrophobically modified, diphenylazo-based organic salts have been prepared and characterized. To achieve this a C(x) (x = 4, 6, 8, or 10) hydrocarbon chain was inserted between the diphenylazo moiety and the quaternary ammonium headgroup of the salt. The absorption of each of the four modified organic salts into anionic microgel particles of poly(N-isopropylacrylamide-co-acrylic acid) has been studied at pH 8. In addition, the hydrodynamic diameters and electrophoretic mobilities of the microgel particles have been studied as a function of the organic salt concentration, also at pH 8. In addition to the electrostatic attraction between the quaternary ammonium head groups of the organic salts and the anionic groups within the microgel particles, hydrophobic association between the chains of the organic salts within the microgel particles plays a role, with this effect increasing strongly from x=4 to 10. Desorption of the x=4 and 6 organic salts occurs readily on changing, in situ, the pH from 8 to 2.5 (and thereby eliminating the electrostatic interaction) but is only partially achieved for the x=8 and 10 organic salts. Indeed, for the x=10 organic salt, only about 80% of the organic salt is desorbed upon dilution of the microgel particles into a large excess of water.
- [Show abstract] [Hide abstract] ABSTRACT: Intensified efforts to decipher the origin of disease at the molecular level stimulate the emergence of more efficient proteomic technologies. To complement this, attempts are being made to identify new predictive biomarkers for building more reliable biomarker patterns. As biomarker research gathers pace an immediate interest becomes focused on platforms, which although based on mainstream approaches, are more amenable to specialist tasks. Particularly relevant this is for disease-specific biomarkers, which are present at very low concentrations in multicomponent biological fluids and require depletion protocols enabling their separation from high-abundance components. In this report, we describe a new strategy allowing the rapid detection of target protein biomarkers by MALDI-ToF mass spectrometry. The approach relies on selective sequestering of target proteins from complex media by engineered microgels, which select proteins by their size (<30 kDa) and isoelectric points (protein pI <6.5). Subsequently, biomarker-loaded microgels are subjected to direct mass-spectrometric analysis without the need for preceding protein extraction. Exemplified by a natural protein-folding motif, coiled-coil, the monitoring of hierarchical folding-dependent macromolecular systems by the approach is also shown. The described strategy offers a general rationale for versatile platforms for high throughput proteomics and holds promise for proteome fingerprinting of biomolecular interactions.
- [Show abstract] [Hide abstract] ABSTRACT: Whilst binary colloid-polymer mixtures have been studied in detail over the past few decades, here the first results are presented on a ternary mixture involving two particle sizes. Novel and unusual phase separation kinetics are found, with a liquid phase separating from an aggregate phase.
- [Show abstract] [Hide abstract] ABSTRACT: The interaction between lysozyme and colloidal poly(NIPAM-co-acrylic acid) microgels is investigated in aqueous solutions at neutral pH. Lysozyme binding isotherms, obtained within the ionic strength range 10-220 mM, indicate that the maximum uptake at 10 mM is 2.4 g lysozyme per gram dry gel, and that the uptake capacity decreases with increasing ionic strength to approximately 0 at 220 mM. Swelling isotherms, obtained from photon correlation spectroscopy measurements, show that the binding is accompanied by a substantial deswelling of the microgels. The microgel suspension is stable up to a protein-to-polymer charge ratio in the microgels of about 0.6, largely independent of ionic strength, whereas flocculation/sedimentation occurs at higher charge ratios. The charge ratio 0.6 corresponds to a zeta-potential of about -6 mV, as obtained from measurements of electrophoretic mobility. Binding and swelling isotherms are analyzed in detail and compared with predictions of theoretical model calculations. The influence of protein-protein attraction is highlighted, as well as the interplay between electrostatic interactions and network elasticity.
- [Show abstract] [Hide abstract] ABSTRACT: The interaction between negatively charged organic salts, with one two and three sulfonate groups respectively, and positively charged poly(2-vinylpyridine) microgel particles has been investigated. Absorption isotherms are used to describe the uptake of organic salt into the microgel network and the particle size and electrophoretic mobility of the dispersions have been discussed in terms of the electrostatic attraction between the organic salts and microgel network. The results suggest that all organic salts interact weakly with the microgel particles, and there is a tendency towards more noticeable changes in the microgel dispersion properties at lower concentration as the number of sulfonate groups within the organic salts increases.
- [Show abstract] [Hide abstract] ABSTRACT: Depletion-induced phase separation in mixtures of charged silica particles and nonadsorbing polymer near theta conditions (polystyrene in dimethylformamide) was studied. The colloid-polymer size ratio q was varied through the particle size and the electrical double layer thickness (kappa(-1)) through addition of lithium chloride (LiCl). The dependence of the phase boundaries, and the nature of the separated phases, on q and kappa is reported and is found to be in good agreement with recent theoretical predictions (Gogelein, C.; Tuinier, R. Eur. Phys. J., E 2008, 27, 171-184).
- [Show abstract] [Hide abstract] ABSTRACT: Here we present novel double shell composite microcapsules (melamine formaldehyde (MF) polymer inner shell and ripened CaCO(3) nanoparticle outer shell) prepared using a method based on in situ polymerisation to form a MF polymer shell inside the ripened CaCO(3) nanoparticulate microcapsules wall.
- [Show abstract] [Hide abstract] ABSTRACT: The interactions of negatively charged gold nanoparticles with positively charged poly(2-vinylpyridine) microgel particles, initially in either their collapsed (pH 6) or swollen (pH 3) states, have been studied in aqueous dispersion. When the gold nanoparticles are mixed with the collapsed microgel particles, at pH 6, they adsorb around the surface of the microgel particles, having no appreciable affect on the size or structure of the microgel particles. However, this uptake of the anionic nanoparticles onto the surface of the microgel particles does lead to a reversal of the surface charge of the latter. When the gold nanoparticles are mixed with swollen cationic microgel particles at pH 3, the anionic gold nanoparticles are initially taken up into the interior of the cationic polymer network, due to electrostatic attraction. Addition of further gold particles leads to the (irreversible) collapse and reversal of sign of the microgel particles. The amount of gold particles absorbed into the interior of the microgel particles is found to decrease with increasing cross-linker content of the P2VP particles.
- [Show abstract] [Hide abstract] ABSTRACT: This paper uses the plane-wave expansion and finite-difference time-domain methods to study tunable 3D photonic crystals for use in display applications. The paper calculates particle diameter and refractive index for operation across the visible spectrum and estimates the reflectivity achievable for one to five layers of particles. The effects of disorder in particle position and diameter are then studied and the results show the system to be robust against such effects. Finally, the paper discusses the potential performance of this display technology in terms of reflectivity, color gamut, and contrast ratio.
- [Show abstract] [Hide abstract] ABSTRACT: The preparation is described of water-core/silica-based shell particles, from W/O emulsion droplets, by adding alkoxysilanes to the oil-continuous phase, to form the shell by an interfacial condensation reaction at the W/O interface. In order to form relatively thick (and hence stronger) shells, it is found necessary to use a mixture of tetraethoxysilane (TEOS) and diethoxydimethylsilane (DEODMS), rather than TEOS alone. It is suggested that, in the former case, trans-shell diffusion of the alkoxysilane monomers (from the oil side) and water molecules (from the aqueous side) can continue, as a result of the higher permeability of the shells to these small molecules, thus allowing the interfacial condensation reaction to continue, even when the reaction would have ceased for a harder shell, having a much lower permeability, as occurs when TEOS alone is used. A successful method of transferring the water-core/silica-based shell particles from oil into water is described, based on the direct centrifugation of the particles from an upper oil phase (containing the dispersed particles initially) into a lower aqueous phase placed beneath, which contains a surfactant capable of adsorbing onto the particles and making them water-wetted.
- [Show abstract] [Hide abstract] ABSTRACT: The use of novel polyampholyte microgel particles for the controlled absorption and release of a cationic surfactant has been investigated. The addition of cetylpyridinium chloride (CPC) to aqueous dispersions of poly(2-diethylamino)ethyl methacrylate-co-methacrylic acid (DEAEM-co-MAAc) microgel particles has been studied with respect to CPC concentration and solution pH. CPC was found to absorb into the polyampholyte microgel particles, resulting in reduced hydrodynamic diameter and electrophoretic mobility, when added to microgel dispersion at pH 11. Strong desorption could be induced by switching the pH from 11 to 3, with most of the desorption occurring in the region of the isoelectric pH of the particles. The properties of surface-adsorbed monolayers of polyampholyte microgel particles were also investigated, both in the presence and absence of CPC. The substrate surface charge was found to influence the swelling profile of the adsorbed microgel monolayers. The interaction of CPC surfactant with monolayers of adsorbed microgel particles shows strong correlations with the interaction of CPC surfactant with microgel particles in dispersion.
- [Show abstract] [Hide abstract] ABSTRACT: Demand for improved fuel efficiency, decreased pollution emissions, and prolonged engine lifespan have given rise to the need for improved fuel and engine oil properties. In particular, the dispersion of carbon deposits is an important consideration. To this end, the adsorption of eight non-ionic, potential fuel-additive surfactants on carbon black particles from iso-octane solutions has been examined and compared. The surfactants all contain poly(iso-butylene) (PIB) tails of varying lengths and head groups based on phenol or amine-substituted phenols of various kinds. For the amine-substituted phenol surfactants, changes in the surfactant structure (number and type of amine groups in the head group and the PIB tail length) were observed to have no significant effect on the heat of adsorption, determined calorimetrically. However, changes were observed in the maximum adsorbed amount for the surfactants with different head group structures and PIB tail lengths. These are explained in terms of changes in the varying extents of solvent rejection of the different surfactants. In hydrocarbon media containing these surfactants, the only surfactants that showed anything other than marginal improvement in the stability to aggregation of dispersions of carbon black were those containing primary or secondary amine groups. Those surfactants with only tertiary amine groups showed no significant improvement in dispersion stability compared to dispersions of the bare particles. In the case of the primary and secondary amine groups, this is explained in terms of proton exchange between the head groups of the surfactant molecules and acid groups on the surface.
- [Show abstract] [Hide abstract] ABSTRACT: The synthesis of nine, nonionic, fuel-soluble poly(isobutylene)-based surfactants, and an assessment of their ability to remove and prevent carbonaceous deposit build up on light-duty vehicle engine valves, is described. Surfactants with varying surfactant head group polarity and structure, and tail length, were assessed for their potential fuel surfactant efficacy by establishing their adsorption kinetics and adsorption isotherms, from iso-octane, onto stainless steel surfaces, using both solution depletion and ellipsometry measurements. Increasing head group polarity from a simple phenol group, by the inclusion of an amine group, increased the adsorption affinity and the maximum adsorbed amount. Inclusion of additional amine groups also increased them maximum adsorbed amount, albeit only marginally. Replacing N-Me groups by N-H groups also increased the adsorbed amounts. Surfactant adsorption was found to be independent of tail length and the maximum adsorbed amount corresponded more-or-less to the surface concentration of strong acid groups on the steel surface, as determined by a Boehm titration. Additionally, all the surfactants studied appeared to adsorb onto the steel surface “irreversibly.”
- [Show abstract] [Hide abstract] ABSTRACT: We compare the behavior of a new two-dimensional aqueous colloidal model system with a simple numerical treatment. To the first order the attractive interaction between the colloids induced by an in-plane rotating ac electric field is dipolar, while the charge stabilization leads to a shorter ranged, Yukawa-like repulsion. In the crystal-like "rafts" formed at sufficient field strengths, we find quantitative agreement between experiment and Monte Carlo simulation, except in the case of strongly interacting systems, where the well depth of the effective potential exceeds 250 times the thermal energy. The "lattice constant" of the crystal-like raft is located approximately at the minimum of the effective potential, resulting from the sum of the Yukawa and dipolar interactions. The experimental system has display applications, owing to the possibility of tuning the lattice spacing with the external electric field. Limitations in the applied field strength and relative range of the electrostatic interactions of the particles result in a reduction in tunable lattice spacing for small and large particles, respectively. The optimal particle size for maximizing the lattice spacing tunability was found to be around 1000 nm.
- [Show abstract] [Hide abstract] ABSTRACT: The encapsulation of one material by another, to form core-shell particles (microcapsules), has many applications, principally the containment, protection, and distribution of an active material. This work describes the development of core-shell particles with silicone oil cores and solid silica-like shells of controlled thickness. Oligomeric polydimethylsiloxane (PDMS) emulsions are employed as the core templates for the formation of the solid shells. The core templates are prepared by the surfactant-free, condensation polymerization of diethoxydimethylsilane (DEODMS) that leads to the formation of monodisperse silicone oil/water emulsions. Solid silica-like, composite shells were formed through condensation of tetraethoxysilane (TEOS) and DEODMS onto the core templates. The shell thickness may be controlled by manipulation of relative TEOS and DEODMS concentrations or by quenching the shell development step. It is possible to incorporate a dye into the core prior to shell formation, which does not seem to permeate the shell. The coated PDMS particles were subjected to a controlled compression stress using a micromanipulation technique. The capsule breaking force was found to be proportional to the shell thickness, as quantified using scanning electron microscopy (SEM) ultramicrotomy.
University of Bristol
Bristol, England, United Kingdom
- • School of Chemistry
- • Department of Electrical and Electronic Engineering
Wageningen, Gelderland, Netherlands
- Laboratory of Physical Chemistry and Colloid Science