Dilute Salt-free Solutions Research Articles

Conformational Transformation of pH-Responsive Hairy Cellulose NanoCrystalloids in Salt-Free Dilute Solutions.

Among biomaterials, pH-responsive nanoparticles have promising potential for overcoming nonspecific therapeutic delivery by taking advantage of the pH gradient between physiological and pathological states. This article discusses pH-dependent conformations of an organic nanoparticle that has a needle-shaped body from crystalline cellulose, sandwiched between two amorphous regions from chemically nanoengineered dicarboxylated cellulose (DCC). Computational study on a single free DCC chain elucidated that in a salt-free dilute solution, the chain undergoes a major transformation between pH ∼ 3 and ∼6.3. Through this transformation, the DCC chain nature varies from globular neutral polymer to coiled quasi-neutral polymer and finally to rodlike polyelectrolyte. Study on the particle nanostructure indicated that, at pH ∼ 3, the conformation of the amorphous regions is analogous to that of polymer brushes in poor solvents, whereas at pH ∼ 5, the conformation changes to that of quasi-neutral polymer brushes in good solvents. For pH > 6.3, the conformation transforms into that of star-like polyelectrolytes. The height of the amorphous region exhibits a regressive trend up to pH ∼ 6.3, followed by a progressive trend up to pH ∼ 10. Study on the hydrodynamic properties revealed a sharp decline in the diffusion rate as the pH varies from ∼3 to ∼5, followed by a plateau for higher pH. It was demonstrated that, at pH ∼ 3, the nanoparticle may form a coherent nanophase with a rodlike structure. These results may provide insight into designing pH-responsive nanocelluloses with a controlled expansion and diffusion coefficient.

Shape and Diffusion of Circular Polyelectrolytes in Salt-Free Dilute Solutions and Comparison with Linear Polyelectrolytes

The shape and diffusion of circular polyelectrolytes in salt-free dilute solutions are investigated over a large range of the Bjerrum length lB by mesoscale hydrodynamic simulations, where lB characterizes the strength of electrostatic interactions (EIs). A comprehensive comparison of linear and circular polyelectrolytes is also made to gain a deep understanding of the effects of topological constraints on the conformational and dynamical properties. As lB increases, counterions become increasingly important due to their condensations on the polyelectrolyte backbone. The shape of a circular polyelectrolyte changes from a prolate coil to an oblate ring at small lB, then to a prolate coil at intermediate lB, and finally to a dense coil at large lB; in contrast, the shape of a linear polyelectrolyte changes from a prolate coil to a rod, then to a prolate coil, and finally to a dense coil. By switching on/off hydrodynamic interactions (HIs), the simulations clarify the complex coupling effects of hydrodynamic...

Dynamics of single polyelectrolyte chains in salt-free dilute solutions investigated by analytical ultracentrifugation

The dynamics of polyelectrolytes in salt-free solution is an unsolved problem. We have investigated the sedimentation and diffusion of xanthan and poly(N-methyl 4-vinyl pyridine iodide) (P4VPI) in salt-free dilute solutions by analytical ultracentrifugation (AUC) using sedimentation velocity (SV) as a function of polyelectrolyte concentration (Cp). Our study reveals two concentration regimes distinguished in either polyanion (xanthan) or polycation (P4VPI) dilute aqueous solution. When Cp is below the Debye concentration (Cd) at which the chain separation (d) is close to the debye length (lD), the interchain electrostatic repulsion is negligible, and the reciprocal apparent sedimentation coefficient (1/s), apparent diffusion coefficient (D) or reciprocal apparent molecular weight (1/Mw) is linearly related to Cp. In the range Cp > Cd with d < lD, the interchain electrostatic repulsion is present, and the dynamics of polyelectrolytes becomes complex. The real sedimentation coefficient (s0), the diffusion coefficient (D0) and the molecular weight (Mw,0) of the single polyelectrolyte chain in salt-free dilute solution can be obtained by extrapolating the concentration to zero. The present study reveals that the complex dynamics of polyelectrolytes in salt-free dilute solutions arises due to the interchain electrostatic repulsion.

Diffusion behavior of polyelectrolytes in dilute solution: coupling effects of hydrodynamic and Coulomb interactions

The diffusion behavior of polyelectrolytes in dilute salt-free solution is studied through a hybrid mesoscale simulation technique that combines the molecular dynamics method and the multiparticle collision dynamics approach. To elucidate the effects of hydrodynamic interactions (HI), we compare results for hydrodynamic and random solvents. When HI are taken into account, we find that the chain diffusivity decreases initially and then increases gradually with the increasing strength of the Coulomb interaction. By contrast, when HI are switched off, the electrostatic-dependent diffusivity shows three distinct regions, and a plateau of approximately constant diffusivity manifests between two decreasing regions. The findings reveal that the dynamics of polyelectrolytes in dilute solution depend on the coupling effects of hydrodynamic and Coulomb interactions, and that these dynamics can be understood by considering the conformational changes of chains, the counterion condensation, and the dynamics of counterions.

Low-frequency collective exchange mode in the dielectric spectrum of salt-free dilute polyelectrolyte solutions

We study the frequency-dependent dielectric response of flexible polyelectrolyte chains in aqueous salt-free dilute solution as a function of polymer concentration and length by means of Brownian dynamics simulations including hydrodynamic interactions. We show that condensed and uncondensed counter ions are characterized by different dielectric response spectra and thus confirm long-held scaling assumptions used for the interpretation of experimental data. In addition to the well-known relaxation modes of condensed and uncondensed counter ions we observe for a single polyelectrolyte chain, we find in many-chain simulations a novel spectral feature at low frequencies that essentially corresponds to an exchange of counter ions between neighboring polyelectrolytes. Our results suggest that the experimental low-frequency dielectric mode might not be due to condensed counter ions, as is commonly assumed, but rather due to a collective many-chain process. This could resolve a long-standing puzzle in the comparison of experimental results and scaling predictions.

Synthesis by self-condensing AGET ATRP and solution properties of arborescent poly(sodium 2-acrylamido-2-methyl-N-propane sulfonate)

Randomly branched (arborescent) poly(sodium 2-acrylamido-2-methyl-N-propanesulfonates) (NaPAMPS) were synthesized via self-condensing vinyl polymerization using activators generated by electron transfer atom transfer radical polymerization (AGET ATRP). The controlled self-condensing AGET ATRP of NaAMPS was realized in the presence of 2-(2-bromopropionyloxy)ethyl acrylate (BPEA) as a branching monomer (inimer) in water/pyridine (35–50% of Py) mixed solvents. The content of BPEA in the reaction feed was varied from 10 to 30 wt% allowing the synthesis of NaPAMPS with different degree of branching. SEC determined molecular weight of the prepared NaPAMPS was Mw = 94 000–120 000 g/mol, and the accompanying polydispersity index PDI ranged from 1.84 to 2.47. The definite evidence of highly branched structure of NaPAMPS was provided by the dependence of radius of gyration Rg on weight-average molecular weight Mw with characteristic slope a = 0.38–0.42, and by small-angle X-ray scattering (SAXS) analysis. Molecular parameters, conformation and dynamics of the branched NaPAMPS in dilute salt-free solutions and in the presence of a salt were elucidated by static and dynamic light scattering and SAXS.

Re-examination of Dynamics of Polyeletrolytes in Salt-Free Dilute Solutions by Designing and Using a Novel Neutral−Charged−Neutral Reversible Polymer

By using the Staudinger ligation to attach 4-{5′-[1′′-(dimethylamino)-ethylideneamino]pentyl}-1-methyl-2-(diphenylphosphino)terephthalate to poly(p-azidomethylstyrene)-co-polystyrene (PAMS-co-PS), we successfully prepared a novel polymer that can undergo a neutral−charged−neutral transition in DMF with 0.5% H2O when the solution is alternatively bubbled with CO2 and N2. Such a reversible change is confirmed by its sharp conductivity variation. Armed with this polymer, we re-examined dynamics of salt-free polyelectrolyte dilute solutions by using laser light scattering (LLS). As expected, there exists only one diffusive relaxation mode in the neutral state. The bubbling of CO2 decreases the scattering intensity and splits this initial diffusive relaxation mode into a fast and a slow diffusive mode, and their scattering intensity contributions are independent of the scattering angle, indicating that the slow mode is not related to some scattering objects larger than the LLS observation length (∼35 nm < 1/q < ∼190 nm). The bubbling of N2 gradually diminishes the slow mode and returns both the scattering intensity and solution dynamics back to their initial neutral state. The change from dilute to semidilute solution slows the fast relaxation down and suppresses the slow mode. On the other hand, the addition of LiBr (50 mM) can completely suppress electrostatic interaction.

STUDIES ON SELF-ASSOCIATIVE BEHAVIOR OF A NOVEL CATION AMPHIPHILIC POLYMER

A novel associating polymer P(AEBA) was synthesized by radical polymerization of the cationic amphiphilic monomer, 4-(2-(acryloyloxy)ethoxy)benzyl tri-ethyl ammonium bromide (AEBA), in aqueous solutions. P(AEBA) displays a strong tendency for self-association in aqueous solutions and is sensitive to the external stimulation such as added salt. The associative properties and morphologies of P(AEBA) were studied by fluorescnece probe technique, viscometry and TEM. In dilute salt-free solutions P(AEBA) behaves as polyelectrolyte, while its behavior is similar to that of the polysoap as salt added. Accompanying increasing polymer concentration, polymer aggregation conformation changes from an extended necklace-like structure to a compact globular aggregate corresponding to the viscosity reduction.

Molecular Dynamics Simulations of Dendritic Polyelectrolytes with Flexible Spacers in Salt Free Solution

We present the results of molecular dynamics simulations of dendritic polyelectrolytes in dilute salt-free solutions. The dendritic polyelectrolytes are modeled as an ensemble of regular-branched bead-spring chains of neutral and charged Lennard-Jones particles with explicit counterions. A wide range of molecular variables of the dendritic polyelectrolytes such as generation number, spacer length, and charge density were considered in the simulations. The effect of dendrimer size on relaxation time, the conformation of spacers, and the size dependence of the dendrimer on molecular variables are discussed and compared with a Flory type theory. The osmotic coefficients of the dilute dendritic polyelectrolyte solutions, as well as the profiles of monomers and counterions, are calculated directly from the simulations. Our simulation results show that the inner spacers of the dendrimers are extensively stretched, and the size dependence on the molecular weight deviates from the scaling prediction that assumes a Gaussian elasticity of the spacer.

Mean field theory for the intermolecular and intramolecular conformational transitions of a single flexible polyelectrolyte chain

The diMarzio theory has been extended to elucidate the intermolecular and intramolecular phase segregations of a single flexible chain polyelectrolyte in dilute salt-free solutions. At the long chain limit, this theory yields the formalism obtained from the more sophisticated Edward Hamiltonian for polyelectrolyte problems. The calculated phase diagram exhibits the features of a first-order phase transition, with continuous and discontinuous transitions separated by a critical point. Under the discontinuous transition, the polyelectrolyte chain exhibits coexistent expanded and collapsed conformational states, same as intermolecular phase segregation. For a limiting long chain, the mean chain size at critical point is roughly 90% of the size of an ideal chain. Such a result implies that partial contraction within a chain molecule is required to collapse a flexible polyelectrolyte chain. Moreover, the theory predicts that for a longer chain, intramolecular segregated conformations differ significantly from intermolecular segregated conformations, but the difference becomes small for shorter chains. Besides, the charge needed to induce intramolecular segregation is smaller than that of intermolecular segregation for a given chain length. These findings are consistent with previous literature results.

Osmotic Coefficient Calculations for Dilute Solutions of Short Stiff-Chain Polyelectrolytes

Osmotic properties of dilute salt-free solutions of stiff-chain polyelectrolytes are studied using computer simulations. A number of factors affecting the charge distribution around the macroions are analyzed quantitatively for parameters of an aqueous solution of a typical synthetic strongly charged rodlike polyelectrolyte. Departing from a mean-field treatment of an infinitely long linear charged rod, we significantly refine the model description. We found that incorporation of effects such as electrostatic correlations between counterions, chain flexibility, and addition of salt decreases the osmotic coefficient slightly. On the other hand, a model of a finite macroion with a noncentral charge distribution predicted significantly higher pressure. We quantify all these contributions separately and show what deviation one should expect from the prediction of the Poisson−Boltzmann solution of the infinite rod cell model. We further comment on the discrepancy between our simulation results and experimentally measured values.

The formation of planar ribbonlike aggregates from stiff polyanions in the presence of anisotropic cations

A dilute salt-free solution of rodlike polyanions in the presence of anisotropic (chain) cations consisting of neutral tails and charged heads is studied. Using Monte Carlo simulation within the framework of the primitive model, different Coulomb coupling regimes were considered. While aggregation in the strong coupling limit is expected, we report new morphology, namely, the formation of ribbonlike nanostructures. At strong electrostatic interaction, the system is found to undergo the self-organization resulting in the formation of planar aggregates that look like a "ladder" of polyanions sandwiched between cationic chains. We investigate the stability of different morphologies and find that these aggregates are thermodynamically stable. Focus has been made on how the chemical structure of anisotropic cations affects the morphology of the aggregates.

Computer simulations of diffusion and dynamics of short-chain polyelectrolytes

Brownian dynamics simulations are conducted to investigate the diffusional and dynamic properties of polyelectrolytes in dilute salt-free solutions. The polyelectrolyte molecule is represented by a bead-spring chain in a primitive model. The long-range hydrodynamic and Coulomb interactions are both taken into consideration through the Ewald summations for the first time. The major finding of our simulations is that the dependence of the long-time chain diffusivity on the Coulomb interaction strength is very different from that of the Kirkwood short-time diffusivity, which simply shows a trend nearly opposite to the chain size. When ignoring the hydrodynamic interaction (HI), the coupling effect between the chain and its counterions gives rise to a noticeable increase in the long-time diffusivity at intermediate electrostatic interaction strengths. However, the incorporation of HI suppresses this effect to a degree that one can no longer discern it. Moreover, the rotational relaxation is found to show a dependence opposite to that of the gyration radius relaxation.

Ribbonlike nanostructures from stiff polyanions and short cationic chains

A Monte Carlo simulation is used to study the formation of finite-size nanostructures in a dilute salt-free solution of stiff polyanions in the presence of short cationic diblock chains consisting of neutral tails and charged heads. At strong electrostatic interaction, the system is found to undergo the self-organization resulting in the formation of planar aggregates that look like a ‘ladder’ of polyanions sandwiched between cationic chains. We investigate the stability of different morphologies and find that these aggregates are thermodynamically stable.

Dilute Solutions of Strongly Charged Flexible Polyelectrolytes in Poor Solvents: Molecular Dynamics Simulations with Explicit Solvent

The behavior of dilute salt-free solutions of charged flexible polymer molecules in poor solvents is studied using molecular dynamics simulations. The polymer molecule is modeled as a chain of char...

Hierarchical Self-Organization of ABC Terpolymer Constituted of a Long Polyelectrolyte End-Capped by Different Hydrophobic Blocks

The self-organization of a mono hydrophilic ABC terpolymer, which is constituted of a central long polyelectrolyte end-capped by two different hydrophobic short blocks (i.e., polystyrene-b-poly(acrylic acid)-b-poly(n-butyl methacrylate), PS−PAA−PnBMA), was examined in aqueous salt-free dilute solutions by static and dynamic light scattering, scanning electron microscopy, and rheology. The neutralized PS−PAA−PnBMA terpolymer is self-organized hierarchically from flowerlike micelles to dendronlike micellar aggregates and eventually to a three-dimensional physical network. The rheological properties of the resulting physical gel were also investigated and discussed.

Charged designed copolymers in the presence of multivalent counterions: a molecular dynamics study

We present the results of molecular dynamics simulations of charged proteinlike hydrophobic–hydrophilic (HP) copolymers in a dilute salt-free solution with multivalent counterions under poor solvent conditions. The primary sequence of these copolymers is constructed such that they can self-assemble into a segregated core–shell microstructure thus resembling some of the basic properties of globular proteins. The results are compared with those obtained earlier for the system containing monovalent counterions. The processes of coil-to-globule transition, aggregation of polyions, and counterion condensation are studied in detail as a function of temperature. The main attention is paid to the influence of the counterions of different charge on the aggregation of the copolymers. It is found that multivalent counterions considerably increase the aggregation of the chains, which form in their presence the finite-size aggregates built up from several polyions. However, the striking feature of the aggregation is that this process does not appear to lead to macroscopic phase separation. The mechanism that prevents the phase separation is discussed.

Hydrophobically Modified Polyelectrolytes in Dilute Salt-Free Solutions. Volume 33, Number 21, October 17, 2000, pp 8097−8105.

Hydrophobically Modified Polyelectrolytes in Dilute Salt-Free Solutions. Volume 33, Number 21, October 17, 2000, pp 8097−8105.

Hydrophobically Modified Polyelectrolytes in Dilute Salt-Free Solutions

The universal diagram of states of hydrophobically modified polyelectrolytes in dilute salt-free solutions is calculated in the framework of the scaling approach. The equilibrium conformations of these polymers were classified using two parametersgeometric and energetic ratios of hydrophobic and polyelectrolyte blocks in unassociated states (the ratio of block sizes and the ratio of block energies). The hydrophobically modified polyelectrolytes self-organize either into necklaces of hairy, crew-cut, or braided micelles or into wormlike cylindrical micelles depending on the values of these two parameters. The universal diagram of states is modified by the counterion condensation.

Screening in Solutions of Star-Branched Polyelectrolytes

Equilibrium conformations of star-branched polyelectrolytes in dilute solutions are studied on the basis of a numerical self-consistent-field (SCF) approach and analytical theories. It is shown that, even in a dilute salt-free solution, the intramolecular Coulombic repulsion in many-armed stars is strongly screened by counterions which are localized preferentially in the intrastar space. As a result, the dependence of the star size on the number of branches levels off for many-armed stars. Addition of salt results in additional screening and in contraction of the stars. The scaling prediction R ∼ cs-1/5 for the star size as a function of the salt concentration cs is well confirmed by SCF calculations. A decrease in the star size can also be induced by an increase in the concentration of the polyelectrolyte in the solution. We have observed significant contraction of the stars with increasing concentration below the overlap threshold, i.e. in dilute solutions. The latter effect is more pronounced for stars with a small number of branches. The screening of the intramolecular Coulombic repulsion due to added salt is compared with that occurring upon increasing the concentration of the polyelectrolyte.