35 research outputs found
Free Isotropic-Nematic Interfaces in Fluids of Charged Platelike Colloids
Bulk properties and free interfaces of mixtures of charged platelike colloids
and salt are studied within density-functional theory. The particles are
modeled by hard cuboids with their edges constrained to be parallel to the
artesian axes corresponding to the Zwanzig model. The charges of the particles
are concentrated in their center. The density functional is derived by
functional integration of an extension of the Debye-H\"uckel pair distribution
function with respect to the interaction potential. For sufficiently small
macroion charges, the bulk phase diagrams exhibit one isotropic and one nematic
phase separated by a first-order phase transition. With increasing platelet
charge, the isotropic and nematic binodals are shifted to higher densities. The
Donnan potential between the coexisting isotropic and nematic phases is
inferred from bulk structure calculations. Non-monotonic density and nematic
order parameter profiles are found at a free interface interpolating between
the coexisting isotropic and nematic bulk phases. Moreover, electrically
charged layers form at the free interface leading to monotonically varying
electrostatic potential profiles. Both the widths of the free interfaces and
the bulk correlation lengths are approximately given by the Debye length. For
fixed salt density, the interfacial tension decreases upon increasing the
macroion charge.Comment: 11 pages, submitted to J. Chem. Phy
Surface properties of fluids of charged platelike colloids
Surface properties of mixtures of charged platelike colloids and salt in
contact with a charged planar wall are studied within density functional
theory. The particles are modeled by hard cuboids with their edges constrained
to be parallel to the Cartesian axes corresponding to the Zwanzig model and the
charges of the particles are concentrated in their centers. The density
functional applied is an extension of a recently introduced functional for
charged platelike colloids. Analytically and numerically calculated bulk and
surface phase diagrams exhibit first-order wetting for sufficiently small
macroion charges and isotropic bulk order as well as first-order drying for
sufficiently large macroion charges and nematic bulk order. The asymptotic
wetting and drying behavior is investigated by means of effective interface
potentials which turn out to be asymptotically the same as for a suitable
neutral system governed by isotropic nonretarded dispersion forces. Wetting and
drying points as well as predrying lines and the corresponding critical points
have been located numerically. A crossover from monotonic to non-monotonic
electrostatic potential profiles upon varying the surface charge density has
been observed. Due to the presence of both the Coulomb interactions and the
hard-core repulsions, the surface potential and the surface charge do not
vanish simultaneously, i.e., the point of zero charge and the isoelectric point
of the surface do not coincide.Comment: 14 pages, submitte
Effective interaction between a colloid and a soft interface near criticality
Within mean-field theory we determine the universal scaling function for the
effective force acting on a single colloid located near the interface between
two coexisting liquid phases of a binary liquid mixture close to its critical
consolute point. This is the first study of critical Casimir forces emerging
from the confinement of a fluctuating medium by at least one soft interface,
instead by rigid walls only as studied previously. For this specific system,
our semi-analytical calculation illustrates that knowledge of the
colloid-induced, deformed shape of the interface allows one to accurately
describe the effective interaction potential between the colloid and the
interface. Moreover, our analysis demonstrates that the critical Casimir force
involving a deformable interface is accurately described by a universal scaling
function, the shape of which differs from that one for rigid walls.Comment: 19 pages, 11 figure
Self-diffusion and Cooperative Diffusion in Semidilute Polymer Solutions as measured by Fluorescence Correlation Spectroscopy
We present a comprehensive investigation of polymer diffusion in the
semidilute regime by fluorescence correlation spectroscopy (FCS) and dynamic
light scattering (DLS). Using single-labeled polystyrene chains, FCS leads to
the self-diffusion coefficient while DLS gives the cooperative diffusion
coefficient for exactly the same molecular weights and concentrations. Using
FCS we observe a new fast mode in the semidilute entangled concentration regime
beyond the slower mode which is due to self-diffusion. Comparison of FCS data
with data obtained by DLS on the same polymers shows that the second mode
observed in FCS is identical to the cooperative diffusion coefficient measured
with DLS. An in-depth analysis and a comparison with current theoretical models
demonstrates that the new cooperative mode observed in FCS is due to the
effective long-range interaction of the chains through the transient
entanglement network
Structure and equation of state of interaction site models for disc-shaped lamellar colloids
We apply RISM (Reference Interaction Site Model) and PRISM (polymer-RISM)
theories to calculate the site-site pair structure and the osmotic equation of
state of suspensions of circular or hexagonal platelets (lamellar colloids)
over a range of ratios of the particle diameter over thickness. Despite the
neglect of edge effects, the simpler PRISM theory yields results in good
agreement with the more elaborate RISM calculations, provided the correct form
factor, characterizing the intramolecular structure of the platelets, is used.
The RISM equation of state is sensitive to the number of sites used to model
the platelets, but saturates when the hard spheres, associated with the
interaction sites, nearly touch; the limiting equation of state agrees
reasonably well with available simulation data for all densities up to the
isotropic-nematic transition. When properly scaled with the second virial
coefficient, the equations of state of platelets with different aspect ratios
nearly collapse on a single master curve.Comment: 10 Pages, 11 Figures, Typesetted using RevTeX
Thermoresponsive Colloidal Molecules
We fabricated thermoresponsive colloidal molecules of ca. 250 nm size.
Electron- and scanning force microscopy reveal the dumbbell-shaped morphology.
The temperature dependence of the size and aspect ratio (ca. 1.4 to 1.6) is
analyzed by depolarized dynamic light scattering and found to be in good
agreement with microscopic evidence
Interaction strength between proteins and polyelectrolyte brushes : a small angle X-ray scattering study
We present an investigation of amp; 946; lactoglobulin adsorption onto spherical polyelectrolyte brushes SPBs by small angle X ray scattering SAXS . The SPB consists of a polystyrene core onto which long chains of poly styrene sulfonate are grafted. The amount and the distribution of proteins adsorbed in the brush layer at low ionic strength can be derived from SAXS. The analysis of the SAXS data reveals additionally that some of the protein molecules form aggregates of about six monomers in the adsorbed state. Furthermore, the position and the amount of slightly bound protein can be detected by the combination of the SAXS results and the SPB loading after extensive ultrafiltration. The total amount of adsorbed protein is compared to data derived from isothermal titration calorimetry. The comparison of both sets of data demonstrates that the protein molecules in the inner layers of the spherical polyelectrolyte brush are firmly bound. Proteins located in the outer layers are only weakly bound and can be washed out by prolonged ultrafiltratio
Trapping colloids near chemical stripes via critical Casimir forces
We study theoretically and experimentally the solvent-mediated critical
Casimir force acting on colloidal particles immersed in a binary liquid mixture
of water and 2,6-lutidine and close to substrates which are chemically
patterned with periodically alternating stripes of antagonistic adsorption
preferences. These patterns are experimentally realized via microcontact
printing. Upon approaching the critical demixing point of the solvent, normal
and lateral critical Casimir forces generate laterally confining effective
potentials for the colloids. We analyze in detail the rich behavior of the
spherical colloids close to such substrates. For all patterned substrates we
investigated, our measurements of these effective potentials agree with the
corresponding theoretical predictions. Since both the directions and the
strengths of the critical Casimir forces can be tuned by minute temperature
changes, this provides a new mechanism for controlling colloids as model
systems, opening encouraging perspectives for applications.Comment: Invited contribution to Molecular Physics Special Issue on Bob Evans'
65th birthda
Interaction of cylindrical polymer brushes in dilute and semi-dilute solution
We present a systematic study of flexible cylindrical brush-shaped macromolecules in a good solvent by small-angle neutron scattering (SANS), static light scattering (SLS), and by dynamic light scattering (DLS) in dilute and semi-dilute solution. The SLS and SANS data extrapolated to infinite dilution lead to the shape of the polymer that can be modeled in terms of a worm-like chain with a contour length of 380 nm and a persistence length of 17.5 nm. SANS data taken at higher polymer concentration were evaluated by using the polymer reference interaction site model (PRISM). We find that the persistence length reduce from 17.5 nm at infinite dilution to 5.3 nm at the highest concentration (volume fraction 0.038). This is comparable with the decrease of the persistence length in semi-dilute concentration predicted theoretically for polyelectrolytes. This finding reveals a softening of stiffness of the polymer brushes caused by their mutual interaction
Material and charge transport of large organic salt clusters and nanoparticles in electrospray ion beam deposition
Electrospray ion beam deposition (ES-IBD) or ion soft landing has been demonstrated as a technique suitable for processing nonvolatile molecules in vacuum under perfectly controlled conditions, an approach also desirable for the deposition of nanoparticles. Here, we present results from several approaches to generate, characterize, and deposit nanoparticle ion beams in vacuum for deposition. We focus on cluster ion beams generated by ESI of organic salt solutions. Small cluster ions of the salts appear in the mass spectra as defined peaks. In addition, we find nanoparticle-sized aggregates, appearing as a low intensity background at high m/z-ratio, and show by IBD experiments that these clusters carry the major amount of material in the ion beam. This transition from clusters to nanoparticles, and their successful deposition, shows that ES-IBD can in principle handle ion beams of very heavy and highly charged nanoparticles. In related experiments, however, we found the deposition of nanoparticles from dispersions to be of low reproducibility, due to the lack of control by mass spectrometry