Aqueous Salt-free Research Articles

Chain flexibility and dynamics of alginate solutions in different solvents

Mechanical rheometry, specifically rotational rheometry, squeeze flow, and capillary rheometry, and two microrheology methods, namely multiple-particle tracking (MPT) and diffusing wave spectroscopy (DWS) have been used to get new insight into structural and dynamical properties of alginate dissolved in solvents widely used for bioprinting, namely deionized water, phosphate-buffered saline (PBS), and Dulbecco Modified Eagle Medium (DMEM) cell media. Results demonstrate that alginate rheological properties depend on the solvent quality at concentrations higher than 1 wt.%. In this high concentration regime, in aqueous salt-free and PBS solutions, experimental scaling exponents for the concentration dependence of the specific viscosity ηsp and the plateau modulus G0 agree well with theoretical predictions for neutral polymers in good solvent whereas for the terminal relaxation time TR, the exponent is slightly higher than theoretically predicted, presumably due to the formation of aggregates. For alginate dissolved in DMEM, all exponents for ηsp, G0, and TR agree with predictions for polymers in theta solvents, which might be related to the formation of polyelectrolyte complex as a result of interactions between alginate and amino acids. Chain persistence length lp values, as determined directly from high frequency rheometry for the first time, are independent of alginate concentration and temperature. Lower absolute lp values were found for DMEM solutions compared with the other solvents. Moreover, scaling exponents for ηsp, G0, and TR do not change with temperature, within 20 and 60 °C. These findings suggest no change in the conformation of alginate chains with temperature.Graphical abstractAlginate rheological properties depend on the solvent quality

Influence of solvation on the structure of highly charged nanoparticles in salt-free solutions

We investigate the influence of ion and nanoparticle solvation on the structure of an aqueous salt-free solution of highly charged nanoparticles. In particular, we perform molecular dynamics simulations of a minimal model of highly charged nanoparticles with an explicit solvent and counter-ions, where the relative affinity of the counter-ions and the nanoparticle for the solvent is tunable through the variation of the relative strength of the dispersion interactions of both the nanoparticle and counter-ions. We show that the competitive solvation of the counter-ions and nanoparticles leads to significant changes in the structure of the nanoparticle solution, which ranges from relatively uncorrelated conformations to self-assembled string- and sheet-like structures. Based on our coarse-grained model, we construct a morphology diagram identifying the different structures that emerge in our model with the variation of the solvent affinity for the charged species. The emergence of these different heterogeneous structures arising from the differential solvation of the charged species demonstrates the essential role of the solvent in the description of charged nanoparticle solutions, and provides guidance for the development of a more predictive theory of the thermodynamic and transport properties of these complex fluids.

Mathematical modeling of the temperature effect on the character of linking between monomeric proteins in aqueous solutions

The mathematical model was developed for taking into account the influence of the temperature of an aqueous salt-free solution on the character of dimer formation by different sections of small proteins: H2A–H2B and H3–H4 dimers, and the effect of temperature on various sections of the Bcl-xl protein were studied. The analysis of the numerical calculations obtained in the course of the developed mathematical model revealed a different behavior of the histone dimers H2B-H2A and H3–H4, as well as the Bcl-xl\(_{(1-212)}\)–Bcl-xl\(_{(1-212)}\), Bcl-xl–Bcl-xl, Bcl-xl\(_{(1-212)}\)–Bcl-xl\(_{(213-233)}\), Bcl-xl\(_{(213-233)}\)–Bcl-xl\(_{(213-233)}\) with an increase of temperature from 20 \(^\circ \)C to 40 \(^\circ \)C, as well as the contribution of different sections of the Bcl-xl protein to the formation of the biological complex.

Viscosity and Scaling of Semiflexible Polyelectrolyte NaCMC in Aqueous Salt Solutions

We investigate the viscosity dependence on concentration and molecular weight of semiflexible polyelectrolyte sodium carboxymethylcellulose (NaCMC) in aqueous salt-free and NaCl solutions. Combining new measurements and extensive literature data, we establish relevant power laws and crossovers over a wide range of degree of polymerization (N) as well as polymer (c) and salt (cs) concentrations. In salt-free solution, the overlap concentration shows the expected c* ∝ N–2 dependence, and the entanglement crossover scales as ce ∝ N–0.6±0.3, in strong disagreement with scaling theory for which ce ∝ c* is expected, but matching the behavior found for flexible polyelectrolytes. A second crossover, to a steep concentration dependence for specific viscosity (ηsp ∝ c3.5±0.2), commonly assigned to the concentrated regime, is shown to follow c** ∝ N–0.6±0.2 (with c**/ce ≃ 6) which thus suggests instead a dynamic crossover, possibly related to entanglement. The scaling of c* and ce in 0.01 and 0.1 M NaCl shows neutra...

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.

Conductometric and Light Scattering Studies on the Complexation between Cationic Polyelectrolyte Nanogel and Anionic Polyion

This work aims to provide a basic understanding of the water dispersibility of a 1:1 stoichiometric polyelectrolyte complex (SPEC) in water in the absence of low-molecular-weight salts. We studied the complexation of a linear polyanion, potassium poly(vinyl alcohol sulfate) (KPVS), with a cationic polyelectrolyte nanogel (CPENG) composed of a lightly cross-linked copolymer of N-isopropylacrylamide and 1-vinylimidazole, in an aqueous salt-free solution (pH 3 and 25 °C), as a function of the molar mixing ratio (Mmr) of anionic to cationic groups. Also studied for comparison was the complexation of KPVS with poly(diallyldimethylammonium chloride) (PDDA), which is a standard reaction in colloid titration. Turbidimetric and conductometric measurements were used in combination of dynamic light scattering (DLS). An abrupt increase of turbidity curve and a break of conductivity curve were observed at Mmr =1 when KPVS was added to the CPENG or PDDA solution, indicating the formation of SPEC. All the complexes formed at Mmr ≤ 1 were water-dispersible and hence characterized by DLS. The CONTIN analysis of DLS data showed that (i) an increase of Mmr causes a decrease of the hydrodynamic radius (R(h)) of the nanogel complex particle but (ii) the R(h) of the PDDA complex remains unchanged at Mmr < 0.8. Taking these into account, we discussed the conductometric results in terms of the random model (RM) and all-or-none model (AONM) in polyelectrolyte complex formations. It was found that KPVS and PDDA yield a water-dispersible SPEC particle at each Mmr, accompanying the uptake of counterions (K(+) and Cl(-)) by the complex. This uptake amount was about 7% of the stoichiometric release of the counterions. In the nanogel system, a complete release of the counterions was observed at Mmr < 0.2 at which one or two KPVS chains were bound to a CPENG particle, but further KPVS binding led to about 20% of the counterion uptake to maintain electroneutrality. Thus, we suggest that the counterion uptake becomes a key factor to understand the water dispersibility of SPEC particles.

Light-Scattering Study of Polyelectrolyte Complex Formation between Anionic and Cationic Nanogels in an Aqueous Salt-Free System

We studied complex formation in an aqueous salt-free system (pH approximately 3 and at 25 degrees C) between nanogel particles having opposite charges. Anionic gel (AG) and cationic gel (CG) particles consist of lightly cross-linked N-isopropylacrylamide (NIPA) copolymers with 2-acrylamido-2-methylpropane sulfonic acid and with 1-vinylimidazole, respectively. The number of charges per particle was -4490 for AG and +20 300 for CG, as estimated from their molar masses (3.33 MD for AG and 11.7 MD for CG) by static light scattering (SLS) and their charge densities (1.35 mmol/g for AG and 1.74 mmol/g for CG) by potentiometric titration. The complexes were formed through the addition of AG to CG and vice versa using a turbidimetric titration technique. At the endpoint of the titration, the aggregate formed was a complex based upon stoichiometric charge neutralization: CG(n)()(+) + xAG(m)()(-) --> CG(n)()(+) (AG(m)()(-))(x)() where x = (n)()/(m)(). At different stages of the titration before the endpoint, the resulting complexes were examined in detail using dynamic light scattering, SLS, and electrophoretic light scattering (ELS). The main results are summarized as follows: (i) When AG with a hydrodynamic radius (R(h)) of 119 nm is added to CG (R(h) approximately 156 nm), the (R(h)) of the complex size decreases from 156 to 80 nm. (ii) In contrast to this (R(h)) change, the molar mass increases from 11.7 MD to 24 MD with increasing amounts of added AG. (iii) Upon addition of CG to AG, the complex formed has the same size ((R(h)) approximately 80 nm) and the same molar mass (55 +/- 2.5 MD) until 55 +/- 5% of AG has been consumed in the complexation. To understand these results, we used the following two models: the random model (RM), in which the added AG particles uniformly bind to all of the CG particles in the system via a strong electrostatic attraction, and the all-or-none model (AONM), in which part of the AG particles in the system preferably bind to the added CG particles to neutralize their electric charges but the other AG particles are uncomplexed and remain in the system. The complex formations upon addition of AG to CG and CG to AG were elucidated in terms of RM and AONM, respectively.

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.

Effects of Surface Charge Distribution of Proteins in Their Complexation with Polyelectrolytes in an Aqueous Salt-Free System

Complexation between protein and polyelectrolyte was studied by a combination of three main experimental techniques: turbidimetric titration, quasi-elastic light scattering (QELS), and static light scattering (SLS). Proteins of two sorts, lysozyme (Lyz) and ribonuclease (RNase), were chosen by considering the following characteristics: (i) Both proteins have the same number (19) of basic groups; (ii) their distribution is almost homogeneous on Lyz but not on RNase; (iii) there is little difference in the molar mass between both proteins. Potassium poly(vinyl alcohol) sulfate (KPVS) with different molecular weights (MPE) and various degrees of esterification (De) was used as the polyelectrolyte. We employed a salt-free aqueous medium and adjusted it to pH 2, the level of which forces to completion the protonation of all of the basic groups. As the titration of proteins with KPVS proceeded, the absorbance (A) as an indication of turbidity increased linearly and then rapidly at a certain titrant volume, r...

Monte Carlo simulations on potentiometric titration of cylindrical polyelectrolytes: Introduction of a method and its application to model systems without added salt

A novel method of the Monte Carlo simulation on the potentiometric titration behavior is developed for an aqueous salt-free solution of rod-like polyelectrolytes in a cylindrical cell system. The trials are attempted in an iterative loop for the ionization and deionization of each ionizable group on the polymer rod as well as for the random motion of small ions within the cell free volume by using the Monte Carlo criterion. In the simulation, pH value of the aqueous solution is assumed as a fixed parameter that determines the excess energy with ionization process. The interaction between charged species is modeled as a combination of a point charge on an impenetrable rod surface and a charged hard sphere with several sizes. The averaged degree of ionization, the ion distribution around the polyion and other thermodynamic quantities are finally obtained as those of the equilibrium state by repeating iteration after the convergence is fulfilled. The results are compared with the analytical solutions of the Poisson-Boltzmann (PB) equation for the uniformly charged cylindrical polyelectrolytes. The counterion distribution agrees with each other, whereas the titration curve is deviated from the PB results. The deviation is interpreted as the difference in the electrostatic free energies between both models. The characteristics as well as the limitations of the present method are also discussed.

Sodium ion activity and electrical conductivity of poly(maleic acid) and poly(isobutylene-alt-maleic acid) in aqueous salt-free solution

The effect of charge density and distribution of charged groups in the polyelectrolyte chain on the counterion binding was studied. The values of the activity coefficient of Na + in poly(maleic acid) (PMA) under salt-free conditions deviated from those predicted by the theories of Manning and Katchalsky-Lifson. The same tendency was observed in the copolymer (PIM) for higher dissociation degrees regions. The results suggest that the bound sodium ions are fairly labile for dissociation degrees higher than 0.5 in dilute, salt-free solutions of PMa and PIM.