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1.
Monte Carlo simulations are used to study the non-uniform equilibrium charge distribution along a single annealed polyelectrolyte
chain under θ solvent conditions and with added salt. Within a range of the order of the Debye length charge accumulates at
chain ends while a slight charge depletion appears in the central part of the chain. The simulation results are compared with
theoretical predictions recently given by Castelnovo et al. In the parameter range where the theory can be applied we find almost perfect quantitative agreement.
Received 7 March 2002 and Received in final form 28 May 2002 相似文献
2.
Single- and double-stranded DNA and many biological and synthetic polyelectrolytes undergo two structural transitions upon
increasing the concentration of multivalent salt or molecules. First, the expanded-stretched chains in low monovalent salt
solutions collapse into nearly neutral compact structures when the density of multivalent salt approaches that of the monomers.
With further addition of multivalent salt the chains redissolve acquiring expanded-coiled conformations. We study the redissolution
transition using a two-state model (F.J. Solis, M. Olvera de la Cruz, J. Chem. Phys. 112, 2030 (2000)). The redissolution occurs when there is a high degree of screening of the electrostatic interactions between
monomers, thus reducing the energy of the expanded state. The transition is determined by the chemical potential of the multivalent
ions in the solution, μ and the inverse screening length, κ. The transition point also depends on the charge distribution
along the chain but is nearly independent of the molecular weight and degree of flexibility of the polyelectrolytes. We generate
a diagram of μversusκ2 where we find two regions of expanded conformations, one with charged chains and the other with overcharged (inverted charge)
chains, separated by a collapsed nearly neutral conformation region. The collapse and redissolution transitions occur when
the trajectory of the properties of the salt crosses the boundaries between these regions. We find that in most cases the
redissolution occurs within the same expanded branch from which the chain precipitates.
Received 15 May 2000 and Received in final form 28 June 2000 相似文献
3.
A novel polymer system containing charged diblock polymers grafted onto a planar surface, simple salt and solvent has been
considered in the framework of a mean-field lattice theory. On the basis of predicted volume fraction profiles of polymer
segments, free ends, block junctions, and small ions, a detailed picture of the system has emerged. It is found that the structure
of the polymer brush is decisively dependent of the relative charge of the blocks. For certain conditions, bimodal profiles
appear which demonstrate the simultaneous presence of two types of chain conformations, one coiled and one stretched.
Received 30 August 2000 相似文献
4.
J.J. Arenzon J.F. Stilck Y. Levin 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(1):79-82
We present a simple model for the possible mechanism of appearance of attraction between like charged polyions inside a polyelectrolyte
solution. The attraction is found to be short ranged, and exists only in the presence of multivalent counterions. It is produced
by the correlations in layers of condensed counterions surrounding each polyion and is only weakly temperature dependent.
We find the attraction to be maximum at zero temperature and dimish as the temperature is raised. The attraction is only possible
if the number of condensed counterions exceeds the threshold, , where is the valence of counterions and Z is the polyion charge.
Received 10 March 1999 and Received in final form 20 April 1999 相似文献
5.
6.
We formulate the exact non-linear field theory for a fluctuating counter-ion distribution in the presence of a fixed, arbitrary
charge distribution. The Poisson-Boltzmann equation is obtained as the saddle-point of the field-theoretic action, and the
effects of counter-ion fluctuations are included by a loop-wise expansion around this saddle point. The Poisson equation is
obeyed at each order in this loop expansion. We explicitly give the expansion of the Gibbs potential up to two loops. We then
apply our field-theoretic formalism to the case of a single impenetrable wall with counter ions only (in the absence of salt
ions). We obtain the fluctuation corrections to the electrostatic potential and the counter-ion density to one-loop order
without further approximations. The relative importance of fluctuation corrections is controlled by a single parameter, which
is proportional to the cube of the counter-ion valency and to the surface charge density. The effective interactions and correlation
functions between charged particles close to the charged wall are obtained on the one-loop level.
Received 8 February 1999 and Received in final form 15 May 1999 相似文献
7.
N. Lee S. Obukhov 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):371-376
We study the configurational statistics of a ring polyampholyte chain made of N randomly charged monomers with elementary charge .To a large extent, the overall structure of a polyampholyte is controlled by a total sum Q of all charges. When the total charge is smaller than , the polyampholyte has a compact globular structure. At charges larger than , the configuration has the form of a ring of small globules (beads) connected by strings. Between Q1 and Q2 we find a remarkable diversity of meta-stable configurations having the shapes of irregular clusters of small globules connected
by the strings. We estimate the number of these configurations and the energy barriers between them. Between Q1 and Q2, the minimum energy configurations are completely controlled by randomness in the charge distribution along the chain. There
are hysteresis effects in the shapes of the clusters. As the total charge increases, the linearly extended configurations
become dynamically more preferable. When the charge decreases, the circular shapes are preferred. We remark on the probable
connection with the multiple phase transitions observed in polyampholyte gels.
Received: 10 July 1997 / Accepted: 13 October 1997 相似文献
8.
O.V. Borisov F. Hakem T.A. Vilgis J.-F. Joanny A. Johner 《The European physical journal. E, Soft matter》2001,6(1):37-47
We present a scaling theory for the adsorption of a weakly charged polyelectrolyte chain in a poor solvent onto an oppositely
charged surface. Depending on the fraction of charged monomers and on the solvent quality for uncharged monomers, the globule
in the bulk of the solution has either a spherical conformation or a necklace structure. At sufficiently high surface charge
density, a chain in the globular conformation adsorbs in a flat pancake conformation due to the Coulombic attraction to the
oppositely charged surface. Different adsorption regimes are predicted depending on two screening lengths (the Debye screening
length monitored by the salt concentration and the Gouy-Chapman length monitored by the surface charge density), on the degree
of ionization of the polymer and on the solvent strength. At low bulk ionic strength, an increase in the surface charge density
may induce a transition from an adsorbed necklace structure to a uniform pancake due to the enhanced screening of the intra-chain
Coulombic repulsion by the counterions localized near the surface.
Received 12 April 2001 相似文献
9.
We study the cylinder to sphere morphological transition of diblock copolymers in aqueous solution with a hydrophobic block
and a charged block. We find a metastable undulated cylinder configuration for a range of charge and salt concentrations which,
nevertheless, occurs above the threshold where spheres are thermodynamically favorable. By modeling the shape of the cylinder
ends, we find that the free-energy barrier for the transition from cylinders to spheres is quite large and that this barrier
falls significantly in the limit of high polymer charge and low solution salinity. This suggests that observed undulated cylinder
phases are kinetically trapped structures. 相似文献
10.
We have performed molecular-dynamics simulations to study the effect of an external electric field on a macroion in the solution
of multivalent Z : 1 salt. To obtain plausible hydrodynamics of the medium, we explicitly make the simulation of many neutral particles along
with ions. In a weak electric field, the macroion drifts together with the strongly adsorbed multivalent counterions along
the electric field, in the direction proving inversion of the charge sign. The reversed mobility of the macroion is insensitive
to the external field, and increases with salt ionic strength. The reversed mobility takes a maximal value at intermediate
counterion valence. The motion of the macroion complex does not induce any flow of the neutral solvent away from the macroion,
which reveals screening of hydrodynamic interactions at short distances in electrolyte solutions. A very large electric field,
comparable to the macroion unscreened field, disrupts charge inversion by stripping the adsorbed counterions off the macroion.
Received 5 December 2001 and Received in final form 10 April 2002 相似文献
11.
P.G. Khalatur A.R. Khokhlov S.A. Prokhorova S.S. Sheiko M. Möller P. Reineker D.G. Shirvanyanz N. Starovoitova 《The European physical journal. E, Soft matter》2000,1(1):99-103
Conformational properties of comb-like polymers strongly adsorbed on a flat solid surface were investigated using computer
simulation and scanning force microscopy. The computer simulation showed that the macromolecules with asymmetric distribution
of the side chains relatively to the backbone are effectively in a collapsed state even under conditions of a good solvent.
They formed peculiar helical superstructures which could be observed by scanning force microscopy of cylindrical brushes of
polymethylmethacrylate on mica.
Received 13 September 1999 相似文献
12.
N. Lee D. Thirumalai 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(4):599-605
The effect of electrostatic interactions on the stretching of DNA is investigated using a simple worm like chain model. In
the limit of small force there are large conformational fluctuations which are treated using a self-consistent variational
approach. For small values of the external force f, we find the extension scales as where is the Debye screening length. In the limit of large force the electrostatic effects can be accounted for within the semiflexible
chain model of DNA by assuming that only small excursions from rod-like conformations are possible. In this regime the extension
approaches the contour length as where f is the magnitude of the external force. The theory is used to analyze experiments that have measured the extension of double-stranded
DNA subject to tension at various salt concentrations. The theory reproduces nearly quantitatively the elastic response of
DNA at small and large values of f and for all concentration of the monovalent counterions. The limitations of the theory are also pointed out.
Received 13 October 1998 and Received in final form 9 June 1999 相似文献
13.
G. Fischer E. Dormann 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,15(1):21-27
High-resolution 13C nuclear magnetic resonance with 1H cross polarization and 1H decoupling under magic angle spinning is measured for the quasi-one dimensional organic conductor diperylene hexafluorophosphate
(including tetrahydrofurane solvent molecules) at temperatures between 160 K and 270 K. Ab initio molecular orbital calculations are used for chemical shift analysis and for assignment of Knight shifted lines and individual
carbon positions. The coexistence of neutral perylene molecules and perylene radicals in the same radical cation salt is revealed.
From Knight and chemical shifts we were able to distinguish two inequivalent perylene radicals within the conducting stack.
The spin density distribution of the molecular electronic wave function is determined quantitatively for these radicals.
Received 29 June 1999 and Received in final form 4 November 1999 相似文献
14.
G. Migliorini N. Lee V. Rostiashvili T.A. Vilgis 《The European physical journal. E, Soft matter》2001,6(3):259-270
We study the properties of polyelectrolyte chains under different solvent conditions, using a variational technique. The free
energy and the conformational properties of a polyelectrolyte chain are studied by minimizing the free energy FN, depending on N(N - 1)/2 trial probabilities that characterize the conformation of the chain. The Gaussian approximation is considered for
a ring of length 24 < N < 28 and for an open chain of length 50 < N < 200 in poor- and theta-solvent conditions, including a Coulomb repulsion between the monomers. In theta-solvent conditions
the blob size is measured and found in agreement with scaling theory, including charge depletion effects, expected for the
case of an open chain. In poor-solvent conditions, a globule instability, driven by electrostatic repulsion, is observed.
We notice also inhomogeneous behavior of the monomer-monomer correlation function, reminiscence of necklace formation in poor-solvent
polyelectrolyte solutions. A global phase diagram in terms of solvent quality and inverse Bjerrum length is presented.
Received 7 June 2001 and Received in final form 17 October 2001 相似文献
15.
A. Milchev J.P. Wittmer D.P. Landau 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(2):241-251
We use an off-lattice microscopic model for solutions of equilibrium polymers (EP) in a lamellar shear flow generated by means
of a self-consistent external field between parallel hard walls. The individual conformations of the chains are found to elongate
in flow direction and shrink perpendicular to it while the average polymer length decreases with increasing shear rate. The
Molecular Weight Distribution of the chain lengths retains largely its exponential form in dense solutions whereas in dilute
solutions it changes from a power-exponential Schwartz distribution to a purely exponential one upon an increase of the shear
rate. With growing shear rate the system becomes increasingly inhomogeneous so that a characteristic variation of the total
monomer density, the diffusion coefficient, and the center-of-mass distribution of polymer chains of different contour length
with the velocity of flow is observed. At higher temperature, as the average chain length decreases significantly, the system is shown to undergo an order-disorder transition
into a state of nematic liquid crystalline order with an easy direction parallel to the hard walls. The influence of shear
flow on this state is briefly examined.
Received 22 October 1998 and Received in final form 12 April 1999 相似文献
16.
T. Lachhab C. Weill 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(1):59-69
Mechanical properties of packings of deformable spheres of polyelectrolyte gel are studied experimentally. These particles
are plunged into a brine. They have the property to swell and shrink when the concentration of salt of the solution is varied.
An oedometric compression is performed imposing cycles of deformation at constant speed and constant salinity Cs. Under many different conditions, we study the laws of deformation relating the macroscopic compression force F, to the macroscopic strain . We find empirical non linear relations of the type . The values of this exponent m are discussed and compared to the results of measurements on a single sphere compressed on
a plane as well as to the results of experiments and simulations on dry model granular assemblies. The swelling and deswelling
properties of the spheres are used to perform isotropic compression tests. In this situation we determine the relation between
the force at equilibrium and the macroscopic strain . The results are compared with those obtained in the oedometric compression tests.
Received 27 January 1998 相似文献
17.
S.A. Prosandeev V.S. Vikhnin S. Kapphan 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,15(3):469-474
Experimental data on the temperature and electric field dependence of the Second Harmonic Generation Intensity in the solid
solution KTaO3:Li is analyzed in the framework of a new percolation-type approach which considers the history of cluster formation averaged
over a large number of samples. In order to take into account that two cluster dipole moments turn when joining clusters together,
a constraint is imposed on the cluster growth. This constraint leads to cluster-size saturation just after the percolation
threshold. Owing to this circumstance, the connected cluster coexists with free clusters in some range of the impurity concentration
and temperature. There is qualitative agreement between the computational results and experiments on Second Harmonic Generation
intensities. The algorithm was written in C++ which allowed realistic computations to be performed on a standard PC.
Received 26 August 1999 and Received in final form 27 December 1999 相似文献
18.
S. Residori N. Olivi-Tran E. Pampaloni 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2000,12(1):15-20
In the case of 2D optical patterns, frustration comes from the interplay between the physical constraints (light-matter interaction)
and the geometrical constraints (cavity length and structure). Depending on the dynamical parameters, we are able to single
out two distinct behaviors. For small diffusion and close to threshold, the system is forced to fulfill the geometrical constraints
giving rise to a phase dynamics of quasicrystals. For larger diffusion, the system fragmentates into spatial domains giving
rise to a competition between different patterns. By means of a geometrical argument, we show that the spatial distribution
of domains is related to the symmetry imposed by the geometrical constraint and that the domain borders are disinclination
defects. These defects being the nucleation centers of spatial domains, they trigger the onset of pattern competition.
Received 27 December 1999 and Received in final form 29 March 2000 相似文献
19.
A.N. Semenov 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(3):497-507
Secondary domain superstructures in correlated random block copolymers are considered theoretically using the concept of the
second order parameter related to fluctuations of the local mean block length. It is shown that the size of secondary domains,
, is much larger than the primary domain size, L: , while , where is a small parameter defining the composition asymmetry. Different secondary morphologies are characterized. It is also shown
that separation of the system in two macroscopic phases with different primary morphologies predicted earlier using the free
energy expansion up to ( is the usual order parameter related to local composition) is an artifact of this widely accepted theoretical model.
Received 15 July 1998 and Received in final form 18
January 1999 相似文献
20.
A. Albersdörfer E. Sackmann 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(4):663-672
In this paper we study the effect of monovalent and divalent ions on the swelling behavior and viscoelastic parameters of ultrathin layers of the natural polyelectrolyte hyaluronic acid
covalently coupled to glass substrates. A colloidal probe technique is applied for this purpose based on latex beads, hovering over the polymer cushion. By analyzing the vertical Brownian motion of these beads with reflection
interference contrast microscopy (RICM) we determined the equilibrium layer thickness (with 3 nm vertical resolution), the
interfacial interaction potential, and the characteristic mesh size limiting the hydrodynamic flow within the polyelectrolyte
film as a function of the ionic strength. The experimental results are interpreted in terms of three different theoretical
models: the polyelectrolyte brush approximation of Pincus [#!ref1!#], a modified polyelectrolyte brush approximation in the
high salt concentration limit of Ross and Pincus [#!ref2!#] and the simple scaling approximation for neutral adsorbed polymers
of de Gennes [#!ref3!#]. Within experimental error all of these different models fit our experimental data and yield comparable
results for the equilibrium layer thickness. Moreover we determine a thickness dependent, effective surface coverage from
both brush models. The hydrodynamic properties of the films are interpreted in terms of the Brinkmann model of elastic porous
media by assuming an effective mesh size, which depends linearly on the Debye screening length. The salt induced condensation
of the polyelectrolyte films can be described microscopically in terms of a progressive contraction of the mesh size with
increasing salt concentration.
Received 10 September 1998 and Received in final form 30 November 1998 相似文献