1,136 research outputs found
Coarse-graining polymers as soft colloids
We show how to coarse grain polymers in a good solvent as single particles,
interacting with density-independent or density-dependent interactions. These
interactions can be between the centres of mass, the mid-points or end-points
of the polymers. We also show how to extend these methods to polymers in poor
solvents and mixtures of polymers. Treating polymers as soft colloids can
greatly speed up the simulation of complex many-polymer systems, including
polymer-colloid mixtures.Comment: to appear in Physica A, special STATPHYS 2001 edition. Content of
invited talk by AA
Density profiles and surface tensions of polymers near colloidal surfaces
The surface tension of interacting polymers in a good solvent is calculated
theoretically and by computer simulations for a planar wall geometry and for
the insertion of a single colloidal hard-sphere. This is achieved for the
planar wall and for the larger spheres by an adsorption method, and for smaller
spheres by a direct insertion technique. Results for the dilute and semi-dilute
regimes are compared to results for ideal polymers, the Asakura-Oosawa
penetrable-sphere model, and to integral equations, scaling and renormalization
group theories. The largest relative changes with density are found in the
dilute regime, so that theories based on non-interacting polymers rapidly break
down. A recently developed ``soft colloid'' approach to polymer-colloid
mixtures is shown to correctly describe the one-body insertion free-energy and
the related surface tension
Accurate effective pair potentials for polymer solutions
Dilute or semi-dilute solutions of non-intersecting self-avoiding walk (SAW)
polymer chains are mapped onto a fluid of ``soft'' particles interacting via an
effective pair potential between their centers of mass. This mapping is
achieved by inverting the pair distribution function of the centers of mass of
the original polymer chains, using integral equation techniques from the theory
of simple fluids. The resulting effective pair potential is finite at all
distances, has a range of the order of the radius of gyration, and turns out to
be only moderately concentration-dependent. The dependence of the effective
potential on polymer length is analyzed in an effort to extract the scaling
limit. The effective potential is used to derive the osmotic equation of state,
which is compared to simulation data for the full SAW segment model, and to the
predictions of renormalization group calculations. A similar inversion
procedure is used to derive an effective wall-polymer potential from the center
of mass density profiles near the wall, obtained from simulations of the full
polymer segment model. The resulting wall-polymer potential turns out to depend
strongly on bulk polymer concentration when polymer-polymer correlations are
taken into account, leading to a considerable enhancement of the effective
repulsion with increasing concentration. The effective polymer-polymer and
wall-polymer potentials are combined to calculate the depletion interaction
induced by SAW polymers between two walls. The calculated depletion interaction
agrees well with the ``exact'' results from much more computer-intensive direct
simulation of the full polymer-segment model, and clearly illustrates the
inadequacy -- in the semi-dilute regime -- of the standard Asakura-Oosawa
approximation based on the assumption of non-interacting polymer coils.Comment: 18 pages, 24 figures, ReVTeX, submitted to J. Chem. Phy
Topological methods for searching barriers and reaction paths
We present a family of algorithms for the fast determination of reaction
paths and barriers in phase space and the computation of the corresponding
rates. The method requires the reaction times be large compared to the
microscopic time, irrespective of the origin - energetic, entropic, cooperative
- of the timescale separation. It lends itself to temperature cycling as in
simulated annealing and to activation-relaxation routines. The dynamics is
ultimately based on supersymmetry methods used years ago to derive Morse
theory. Thus, the formalism automatically incorporates all relevant topological
information.Comment: 4 pages, 4 figures, RevTex
The Asakura-Oosawa model in the protein limit: the role of many-body interactions
We study the Asakura-Oosawa model in the "protein limit", where the
penetrable sphere radius is much greater than the hard sphere radius
. The phase behaviour and structure calculated with a full many-body
treatment show important qualitative differences when compared to a description
based on pair potentials alone. The overall effect of the many-body
interactions is repulsive.Comment: 9 pages and 11 figures, submitted to J. Phys.: Condensed Matter,
special issue "Effective many-body interactions and correlations in soft
matter
Prospects of Transition Interface Sampling simulations for the theoretical study of zeolite synthesis
The transition interface sampling (TIS) technique allows to overcome large
free energy barriers within reasonable simulation time, which is impossible for
straightforward molecular dynamics. Still, the method does not impose an
artificial driving force, but it surmounts the timescale problem by an
importance sampling of true dynamical pathways. Recently, it was shown that the
efficiency of TIS to calculate reaction rates is less sensitive to the choice
of reaction coordinate than those of the standard free energy based techniques.
This could be an important advantage in complex systems for which a good
reaction coordinate is usually very difficult to find. We explain the
principles of this method and discuss some of the promising applications
related to zeolite formation.Comment: 9 pages, accepted for publication in Phys. Chem. Chem. Phys. for the
special issue of the CECAM workshop: Computational aspects of building
blocks, nucleation, and synthesis of porous materials Aug. 29 2006 to Aug. 31
200
Phase behavior of a system of particles with core collapse
The pressure-temperature phase diagram of a one-component system, with
particles interacting through a spherically symmetric pair potential in two
dimensions is studied. The interaction consists of a hard core plus an
additional repulsion at low energies. It is shown that at zero temperature,
instead of the expected isostructural transition due to core collapse occurring
when increasing pressure, the system passes through a series of ground states
that are not triangular lattices. In particular, and depending on parameters,
structures with squares, chains, hexagons and even quasicrystalline ground
states are found. At finite temperatures the solid-fluid coexistence line
presents a zone with negative slope (which implies melting with decreasing in
volume) and the fluid phase has a temperature of maximum density, similar to
that in water.Comment: 11 pages, 15 figures included. To appear in PRE. Some figures in low
quality format. Better ones available upon request from [email protected]
- …