244 research outputs found
Recommended from our members
A Neutron Scattering Study of the Structure of Poly(dimethylsiloxane)-Stabilized Poly(methyl methacrylate) (PDMSâPMMA) Latexes in Dodecane
Hard-sphere particles in nonpolar solvents are an essential tool for colloid scientists. Sterically stabilized poly(methyl methacrylate) (PMMA) particles have long been used as the exemplary hard-sphere system. However, neither the particles themselves nor the poly(12-hydroxystearic acid) (PHSA) stabilizer necessary to prevent aggregation in nonpolar solvents are commercially available. To counter this, several alternatives have been proposed. In recent years, there has been an increased interest in poly(dimethylsiloxane) (PDMS) stabilizers as a commercially available alternative to PHSA, yet the structure of particles made in this way is not as well understood as those produced using PHSA. In this work, we employ small-angle neutron scattering to determine the internal structure of PDMS-stabilized PMMA particles, synthesized with and without an additional crosslinking agent. We report data consistent with a homogeneous PMMA core with a linearly decaying PDMS shell. The thickness of the shell was in excess of 50 nm, thicker than the PHSA layer typically used to stabilize PMMA but consistent with reports of the layer thickness for similar molecular weight PDMS at planar surfaces. We also show that the amount of the hydrogenous material in the particle core of the crosslinked particles notably exceeds the amount of added ethylene glycol dimethacrylate crosslinker, suggesting some entrapment of the PDMS stabilizer in the PMMA matrix
The Devil is in the details:Pentagonal bipyramids and dynamic arrest
Colloidal suspensions have long been studied as a model for atomic and
molecular systems, due to the ability to fluorescently label and individually
track each particle, yielding particle-resolved structural information. This
allows various local order parameters to be probed that are otherwise
inaccessible for a comparable molecular system. For phase transitions such as
crystallisation, appropriate order parameters which emphasise 6-fold symmetry
are a natural choice, but for vitrification the choice of order parameter is
less clear cut. Previous work has highlighted the importance of icosahedral
local structure as the glass transition is approached. However, counting
icosahedra or related motifs is not a continuous order parameter in the same
way as, for example, the bond-orientational order parameters and .
In this work we investigate the suitability of using pentagonal bipyramid
membership, a structure which can be assembled into larger, five-fold symmetric
structures, as a finer order parameter to investigate the glass transition. We
explore various structural and dynamic properties and show that this new
approach produces many of the same findings as simple icosahedral membership,
but we also find that large instantaneous displacements are often correlated
with significant changes in pentagonal bipyramid membership, and unlike the
population of defective icosahedra, the pentagonal bypyramid membership and
spindle number do not saturate for any measured volume fraction, but continues
to increase.Comment: accepted by JStat Mech: Theory and Experiment 201
Recommended from our members
Molecular exchange in spherical diblock copolymer colloids synthesised by polymerisation-induced self-assembly
Counterion Condensation on Spheres in the Salt-free Limit
A highly-charged spherical colloid in a salt-free environment exerts such a
powerful attraction on its counterions that a certain fraction condenses onto
the surface of a particle. The degree of condensation depends on the curvature
of the surface. So, for instance, condensation is triggered on a highly-charged
sphere only if the radius exceeds a certain critical radius \collrad^{*}.
\collrad^{*} is expected to be a simple function of the volume fraction of
particles. To test these predictions, we prepare spherical particles which
contain a covalently-bound ionic liquid, which is engineered to dissociate
efficiently in a low-dielectric medium. By varying the proportion of ionic
liquid to monomer we synthesise nonpolar dispersions of highly-charged spheres
which contain essentially no free co-ions. The only ions in the system are
counterions generated by the dissociation of surface-bound groups. We study the
electrophoretic mobility of this salt-free system as a function of the colloid
volume fraction, the particle radius, and the bare charge density and find
evidence for extensive counterion condensation. At low electric fields, we
observe excellent agreement with Poisson-Boltzmann predictions for counterion
condensation on spheres. At high electric fields however, where ion advection
is dominant, the electrophoretic mobility is enhanced significantly which we
attribute to hydrodynamic stripping of the condensed layer of counterions from
the surface of the particle.Comment: 13 pages, 9 figures and two table
Recommended from our members
Local structure in deeply supercooled liquids exhibits growing lengthscales and dynamical correlations
Glasses are among the most widely used of everyday materials, yet the process by which a liquidâs viscosity increases by 14 decades to become a glass remains unclear, as often contradictory theories provide equally good descriptions of the available data. Knowledge of emergent lengthscales and higher-order structure could help resolve this, but this requires time-resolved measurements of dense particle coordinatesâpreviously only obtained over a limited time interval. Here we present an experimental study of a model colloidal system over a dynamic window significantly larger than previous measurements, revealing structural ordering more strongly linked to dynamics than previously found. Furthermore we find that immobile regions and domains of local structure grow concurrently with density, and that these regions have low configurational entropy. We thus show that local structure plays an important role at deep supercooling, consistent with a thermodynamic interpretation of the glass transition rather than a principally dynamic description
Recommended from our members
Zwitterions fine-tune interactions in electrolyte solutions
Cellular organisms regulate electrolyte composition in the cytosol to optimize intracellular molecular interactions at the same time as balancing external osmotic pressure. While osmotic pressure can be tuned using multiple ionic, zwitterionic, and nonionic solutes, interactions between proteins and other macromolecules are sensitive to the precise composition of the medium. Nonetheless, the roles of individual ions and nonionic solutes in mediating cellular interactions remain relatively unexplored, and standard buffer solutions used in laboratory studies often contain only a few simple salts. Here, we report on model experiments investigating the combined effect of ionic and zwitterionic solutes on interaction forces across electrolytes, revealing a clear role for zwitterions in modifying interactions compared to simple salt solutions. First, we find that zwitterions act to disrupt water layering at interfaces, leading to smoothed interaction potentials. Second, we find that zwitterions strengthen electrostatic repulsions by enhancing effective surface charge. Third, zwitterions enhance the effective dielectric permittivity of the solution, and this âdielectricizerâ effect extends the range of electrostatic repulsions compared to solutions without zwitterion present. The latter two effects are likely important in stabilizing proteins and other macromolecules when external osmotic and mechanical pressure are very high and simple ionic solutes alone would lead to collapse
Recommended from our members
Hydrogen bond donors dictate the frictional response in deep eutectic solvents
Deep eutectic solvents (DESs) show promise as boundary lubricants between sliding surfaces, taking advantage of their physical stability, chemical stability, and tunability. Here, we study friction forces across nanofilms of two archetypal DES mixtures: choline chloride + ethylene glycol and choline chloride + glycerol. Using a surface force balance, we control the film thickness (to subnanometer precision) and determine the friction force simultaneously. Measurements are made at different mole fractions of the choline chloride salt and the molecular solvent, allowing us to determine the role of each species in the observed behavior. We find that the nature of the molecular solvent is dominant in determining the lubrication behavior, while the fraction of ChCl is relatively less important. By analyzing the steps in friction and the gradient of friction with load as the layers squeeze away from between the surfaces, we learn various mechanistic aspects of lubrication across the DES nanofilms of relevance to design and optimization of these promising fluids
Recommended from our members
Celebrating Soft Matter's 10th Anniversary: influencing the charge of poly(methyl methacrylate) latexes in nonpolar solvents
Sterically-stabilized poly(methyl methacrylate) (PMMA) latexes dispersed in nonpolar solvents are a classic, well-studied system in colloid science. This is because they can easily be synthesized with a narrow size distribution and because they interact essentially as hard spheres. These PMMA latexes can be charged using several methods (by adding surfactants, incorporating ionizable groups, or dispersing in autoionizable solvents), and due to the low relative permittivity of the solvents (Δr â 2 for alkanes to Δr â 8 for halogenated solvents), the charges have long-range interactions. The number of studies of these PMMA particles as charged species has increased over the past ten years, after few studies immediately following their discovery. A large number of variations in both the physical and chemical properties of the system (size, concentration, surfactant type, or solvent, as a few examples) have been studied by many groups. By considering the literature on these particles as a whole, it is possible to determine the variables that have an effect on the charge of particles. An understanding of the process of charge formation will add to understanding how to control charge in nonaqueous solvents as well as make it possible to develop improved technologically relevant applications for charged polymer nanoparticles
Experimental Determination of Configurational Entropy in a Two-Dimensional Liquid under Random Pinning
A quasi two-dimensional colloidal suspension is studied under the influence
of immobilisation (pinning) of a random fraction of its particles. We introduce
a novel experimental method to perform random pinning and, with the support of
numerical simulation, we find that increasing the pinning concentration
smoothly arrests the system, with a cross-over from a regime of high mobility
and high entropy to a regime of low mobility and low entropy. At the local
level, we study fluctuations in area fraction and concentration of pins and map
them to entropic structural signatures and local mobility, obtaining a measure
for the local entropic fluctuations of the experimental system
- âŠ