Computer simulation of configurational properties of partially ionized polyelectrolytes (original) (raw)
The effect of added salt on polyelectrolyte structure
We present results of molecular dynamics simulations of strong, flexible polyelectrolyte chains in solution with added salt. The effect of added salt on the polyelectrolyte chain structure is fully treated for the first time as a function of polymer density. Systems above and below the Manning condensation limit are studied. The chain structure is intimately tied to the ion density near the chain even for chains in the counterion condensation (CC) regime. The end-to-end distance is demonstrated to be a function of the inverse Debye screening length and the Bjerrum length. The ion density near the polymer chain depends on the amount of added salt, and above the condensation limit the chains significantly contract due to added salt.
Polyelectrolytes in Solution-Recent Computer Simulations
Arxiv preprint cond-mat/9812152, 1998
We present a short overview over recent MD simulations of systems of fully flexible polyelectrolyte chains with explicitly treated counter ions using the full Coulomb potential. The main emphasis is given on the conformational properties of the polymers, with a short discussion on counter ion condensation.
Polyelectrolytes in Salt Solutions: Molecular Dynamics Simulations
We present results of the molecular dynamics simulations of salt solutions of polyelectrolyte chains with number of monomers N = 300. Polyelectrolyte solutions are modeled as an ensemble of beadÀspring chains of charged Lennard-Jones particles with explicit counterions and salt ions. Our simulations show that in dilute and semidilute polyelectrolyte solutions the electrostatic induced chain persistence length scales with the solution ionic strength as I À1/2 . This dependence of the chain persistence length is due to counterion condensation on the polymer backbone. In dilute polyelectrolyte solutions the chain size decreases with increasing the salt concentration as R µ I À1/5 . This is in agreement with the scaling of the chain persistence length on the solution ionic strength, l p µ I À1/2 . In semidilute solution regime at low salt concentrations the chain size decreases with increasing polymer concentration, R µ c p À1/4 , while at high salt concentrations we observed a weaker dependence of the chain size on the solution ionic strength, R µ I À1/8 . Our simulations also confirmed that the peak position in the polymer scattering function scales with the polymer concentration in dilute polyelectrolyte solutions as c p 1/3 . In semidilute polyelectrolyte solutions at low salt concentrations the location of the peak in the scattering function shifts toward the large values of q* µ c p 1/2 while at high salt concentrations the peak location depends on the solution ionic strength as I À1/4 . Analysis of the simulation data throughout the studied salt and polymer concentration ranges shows that there exist general scaling relations between multiple quantities X(I) in salt solutions and corresponding quantities X(I 0 ) in salt-free solutions, X(I) = X(I 0 )(I/I 0 ) β . The exponent β = À1/2 for chain persistence length l p , β = 1/4 for solution correlation length ξ, and β = À1/5 and β = À1/8 for chain size R in dilute and semidilute solution regimes, respectively.
Ionization Equilibria and Conformational Transitions in Polyprotic Molecules and Polyelectrolytes
The Journal of Physical Chemistry B, 2006
The coupling between proton binding and conformational degrees of freedom in polyprotic molecules and polyelectrolytes is studied theoretically. Our approach combines the classical rotational isomeric state (RIS) model developed by Flory and the site binding (SB) model used to treat proton binding equilibria. The properties of the resulting SBRIS model, which treats conformational degrees of freedom and proton binding on equal footing, are studied with statistical mechanical techniques. Quantities of interest, such as titration curves, conformational probabilities, or macroscopic binding constants, are expressed as thermal averages and are evaluated by direct enumeration of states or by transfer matrix techniques. We further demonstrate that in the SBRIS model conformational degrees of freedom can be averaged out, leading to the contracted description within the SB model. In most cases, this contraction leads to higher order interactions, which may not be present at the SBRIS level (e.g., triplet interactions). Several examples are discussed to illustrate the concepts developed. The case of succinic acid exemplifies the situation in its simplest form. The model can further rationalize the very different titration behavior of poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA). In particular, the characteristic "jump" in the titration curve of PMAA is described quantitatively and is interpreted in terms of a conformational transition.
Chain Structure in Polyelectrolyte Solutions at Nonzero Concentrations
Macromolecules, 1997
We present an analytic theory for the structure of intrinsically flexible polyelectrolyte chains in solution at arbitrary concentrations, with or without added salt. We use a self-consistent approach based on an idea suggested by generalized Debye-Hü ckel theory, namely, that the screening length from polyions is wavevector-dependent, due to chain connectivity. At high wavevectors (short length scales), only counterions contribute to screening, but at lower wavevectors, both polyions and counterions contribute. The results of the theory are in quantitative agreement with molecular dynamics simulations, with no adjustable parameters.
Monte Carlo Simulation to Investigate the Cascade Transition of a Permuted Polyelectrolyte Chain
2012
Using Monte Carlo simulations, the cascade transition of a single polyelectrolyte chain having permuted charge distribution is investigated. The polyelectrolyte chain is simulated at various solvent regimes. The effect of the electrostatic interaction strength is studied. The structures are analyzed with respect to the scaling laws. The effect of permuted charge distribution is investigated in comparison to the other charge distribution in the literature.
MRS Advances, 2016
ABSTRACTWe explore the impact of monovalent counter-ions on the molecular conformation of highly charged flexible polyelectrolytes for a range of molecular topologies (linear chains, stars, and unknotted and trefoil rings) by molecular dynamics simulations that include an explicit solvent having short range interaction with the polyelectrolyte. In particular, we investigate how the counter-ions near the polyelectrolytes with variable mass influence the average molecular shape. We also characterize the interfacially “bound” counter-ions by calculating the time-averaged number of interfacial counter-ions, as well as the degree to which the polyelectrolytes wrap around the counter-ions by calculating the number of contacts between the counter-ions and the polyelectrolyte.
Gels, 2018
The study of the coupling between the conformational properties of a polyelectrolyte chain and the distribution of counter-ions surrounding the chain is important in developing predictive theories for more complex polymer materials, such as polyelectrolyte gels. We investigated the influence of solvent affinity to counter-ions and the polyelectrolyte backbone on the conformational properties of highly charged flexible polymer chains using molecular dynamics simulations that include both ions and an explicit solvent. We find that the solvation of the polyelectrolyte backbone can be achieved by either increasing the solvent affinity for the polyelectrolyte segments or by increasing the solvent affinity for the counter-ions. However, these two mechanisms influence the conformational properties of the polyelectrolyte chain in rather different ways, suggesting the inadequacy of polyelectrolyte solution models that treat the solvent as a continuum medium.
Molecular Dynamics simulations of rod-like polyelectrolytes
We present theoretical and numerical results on sti , linear polyelectrolytes within the framework of the cell model. With the help of an in ection point criterium we discuss counterion condensation for systems with added salt. Molecular dynamics simulations of the cell model are used to show, under which circumstances the mean-eld Poisson-Boltzmann theory fails qualitatively. We present measurements of the osmotic coe cient, which can become negative when multivalent counterions are present, leading to e ective attractions. To facilitate the comparison to experiments, we compute the osmotic coe cient for poly(para-phenylenes). We also present rst investigations of ion-ion correlations and compare them to predictions of the recently advocated Wigner crystal theories.