High-salt Limit Research Articles

Effect of shear rate, temperature, and salts on the viscosity and viscoelasticity of semi‐dilute and entangled xanthan gum

AbstractXanthan gum, a naturally‐occurring, high‐molecular‐weight, anionic polyelectrolyte, exhibits significant drag‐reducing properties at low concentrations in water, suggesting promising applications in geothermal networks. The flow behavior of xanthan solutions are explored, specifically at concentrations in the semi‐dilute (1000 ppm by mass) and entangled (4000 ppm) regimes as well as in both water and various salt solutions across a temperature range of 5–85°C. The viscosity and viscoelastic properties of xanthan solutions examine relationships between polyelectrolyte concentration, temperature, and salt concentration. Viscosity decreases by two orders of magnitude in 4000 ppm xanthan solutions in deionized water and with 50 mM NaCl (in the high salt limit) as the temperature increases from 5 to 85°C. Next, the addition of salts affects viscosity differently in the two concentration regimes. Specifically, at 25°C, the zero‐shear rate viscosity upon salt addition decreases by 63% for 1000 ppm solutions (from 0.54 to 0.2 Pa s) but increases by 280% for 4000 ppm solutions (from 28 to 107 Pa s). At 1000 ppm, salt ions cause chain collapse, reducing viscosity; At 4000 ppm, entanglements led to structural changes, resulting in higher viscosity. Furthermore, three salts commonly found in geothermal waters, namely NaCl, Na2CO3, and NaHCO3, exhibited similar changes in viscosity and shear thinning behavior in the entangled regime across temperatures from 5 to 85°C.

Chain Flexibility and Dynamics of Polysaccharide Hyaluronan in Entangled Solutions: A High Frequency Rheology and Diffusing Wave Spectroscopy Study

We have investigated the linear viscoelastic properties of high molecular weight hyaluronan in aqueous solution using an experimental approach combining mechanical rheometry and optical microrheology. The complex shear modulus has been measured over a broad frequency range from 10(-1) to 10(7) rad/s. Chain flexibility is characterized by the persistence length lpand this parameter has been determined for the first time in the entangled regime here from high frequency modulus data. At NaHA concentrations above the entanglement concentration ce, lp is essentially independent of polymer concentration, temperature, and ionic strength. The latter is consistent with the Odijk-Skolnick-Fixman theory. The scaling exponent describing the concentration dependence of the plateau modulus G0 agrees well with predictions for polymers in good solvents. The scaling exponents for the specific viscosity ηsp and relaxation time TR are slightly higher than theoretically predicted for polyelectrolytes in the high salt limit, indicating, that molecular aggregation occurs at higher polymer concentrations.

Increasing viscosity in entangled polyelectrolyte solutions by the addition of salt

The viscosity of several polyelectrolytes is measured in both salt free solutions and solutions in the high salt limit. At low polymer concentrations, the zero shear rate viscosity decreases as much as 100-fold upon addition of a monovalent salt, namely NaCl. However, as polymer concentration increases, the viscosity difference between polymer in salt free and in monovalent salt solution diminishes. Further, the zero shear rate viscosity becomes independent of added monovalent salt at the critical polyelectrolyte concentration c D . Above c D , the addition of monovalent salt increases the zero shear rate viscosity of the entangled polyelectrolyte solutions. The viscosity increase agrees with viscosity scaling theory for polyelectrolytes in the entangled regime. Polyelectrolytes exhibiting an increase in viscosity above c D in the presence of monovalent salt include three natural anionic polyelectrolytes (xanthan, carrageenan, welan), one synthetic anionic polyelectrolyte (hydrolyzed polyacrylamide), and one natural cationic polyelectrolyte (chitosan). Generally, these polyelectrolytes are relatively high molecular weight (>1 M Dalton), which makes c D experimentally accessible (e.g., c D = 0.2 wt% for xanthan). The magnitude of the viscosity increase is as high as 300% for xanthan and nearly independent of monovalent salt concentration in the high salt limit. The increase in viscosity in monovalent salt solution and magnitude of c D appear to be heavily influenced by the molecular characteristics of the polymers such as monomer weight, molecular structure, and chain conformation.

Persistency of single-stranded DNA: The interplay between base sequences and base stacking

The chain persistency of single-stranded (ss) DNA at a high-salt limit mainly arises from the so-called base-stacking interaction between consecutive bases along the strand. Stacking is appreciable for purine–purine (e.g., A– A) and purine–pyrimidine stacks (e.g., A– T), but it is weak for pyrimidine stacks (i.e., T– T, T– C, and C– C). We study how base stacking can stiffen the strand by classifying bases into two subclasses: stacking pairs (i.e., purine–purine and purine–pyrimidine) and non-stacking (i.e., pyrimidine–pyrimidine) pairs. With this simplification, we develop an exactly solvable model for calculating the stacking-induced persistence length ℓ stack of heterogeneous ssDNA. It is shown that ℓ stack is mainly determined by the occurrence rate of purines; intrinsic correlations in real DNA sequences barely influence ℓ stack . Our approach leads to a reasonable estimate of ℓ stack ≈ 2 b – 3 b (under typical conditions), where b is the inter-base distance.

Solution Rheological Behavior and Electrospinning of Cationic Polyelectrolytes

The influence of polyelectrolyte rheological behavior on the electrospinning process was determined for a series of poly(2-(dimethylamino)ethyl methacrylate hydrochloride) (PDMAEMA·HCl) aqueous solutions in the presence of added NaCl. Solution rheological studies revealed that PDMAEMA·HCl in an 80/20 w/w water/methanol cosolvent displayed polyelectrolyte behavior based on the scaling relationship between specific viscosity (ηsp) and concentration in the semidilute unentangled and semidilute entangled regimes. The entanglement concentration (Ce) increased with NaCl concentration due to screening of the electrostatic repulsive forces along the PDMAEMA·HCl backbone, which enabled the PDMAEMA·HCl chains to adopt a flexible, coillike conformation. Moreover, the scaling behavior in the semidilute entangled regime shifted from polyelectrolyte (ηsp ∼ C1.5) to neutral polymer behavior (ηsp ∼ C3.75) in the high salt limit. The electrospinning performance of PDMAEMA·HCl solutions was also dependent on NaCl concentration, and NaCl-free PDMAEMA·HCl solutions did not form fibers at concentrations less than 8Ce. The minimum concentration for fiber formation decreased as the level of NaCl was increased due to screening of the repulsive, electrostatic interactions between charged repeating units that served to stabilize the electrospinning jet. Moreover, because of the high electrical conductivity of the polyelectrolyte solutions, the electrospun polyelectrolyte fibers were 2−3 orders of magnitude smaller in diameter compared to fibers that were electrospun from solutions of neutral polymers of equal zero shear viscosity (η0) and normalized concentration (C/Ce).

Translocation of a confined polymer through a hole.

Based on an analogy between polymer translocation across a free energy barrier associated with polymer worming through a hole and classical nucleation and growth process, the escape time tau is predicted asymptotically to be N(N/rho)(1/3nu). N is the polymer length, rho is the monomer density prior to escape, and nu is the radius of gyration exponent. Monte Carlo simulation data collected in the high salt limit (nu approximately 3/5) are in agreement with the asymptotic law and provide vivid details of the escape.

Rheology of sodium hyaluronate under physiological conditions.

Sodium hyaluronate NaHA in phosphate-buffered saline behaves as a typical polyelectrolyte in the high-salt limit, as Newtonian viscosities are observed over a wide range of shear rates. There is no evidence of intermolecule hydrogen bonding causing gel formation in NaHA solutions without protein present. The concentration dependences of viscosity, relaxation time, and terminal modulus are consistent with observations on flexible, neutral polymers in good solvents, which are known to be in the same universality class as flexible polyelectrolytes in the presence of excess salt.

Fraction of Condensed Counterions around a Charged Rod: Comparison of Poisson−Boltzmann Theory and Computer Simulations

We investigate the phenomenon of counterion condensation in a solution of highly charged rigid polyelectrolytes within the cell model. A method is proposed whichbased on the charge distribution functionidentifies both the fraction of condensed ions and the radial extension of the condensed layer. Within salt-free Poisson−Boltzmann (PB) theory it reproduces the well-known fraction 1−1/ξ of condensed ions for a Manning parameter ξ > 1. Furthermore, it predicts a weak salt dependence of this fraction and a breakdown of the concept of counterion condensation in the high-salt limit. We complement our theoretical investigations with molecular dynamics simulations of a cell-like model, which constantly yield a stronger condensation than predicted by PB theory. While the agreement between theory and simulation is excellent in the monovalent, weakly charged case, it deteriorates with increasing electrostatic interaction strength and, in particular, increasing valence. For instance, at a high concentration of divalent salt and large ξ our computer simulations predict charge oscillations, which mean-field theory is unable to reproduce.

Light scattering and fluorescence photobleaching recovery study of poly(amidoamine) cascade polymers in aqueous solution

Diffusion coefficients of polyamidoamine cascade polymers (PAMAMs) were measured in aqueous solutions by dynamic light scattering and, after labeling with fluorescein isothiocyanate, by fluorescence photobleaching recovery. The dynamic light scattering results depended weakly on pH at a high salt concentration, but varied strongly with the concentration of added salt in the low-salt limit. The fluorescence photobleaching recovery values were almost independent of salt concentration. The difference between the two techniques is that thermodynamic nonideality strongly affects light scattering at the concentrations that are experimentally accessible. The hydrodynamic sizes from fluorescence photobleaching recovery were somewhat smaller than those from dynamic light scattering in the high-salt limit, despite attachment of the dye. Nevertheless, fluorescently tagged PAMAMs should make suitable markers and diffusion probes. © 1996 John Wiley & Sons, Inc.

Elastic Modulus and Equilibrium Swelling of Polyelectrolyte Gels

We present a scaling theory for the modulus G of polyelectrolyte gels as a function of strand length between cross-links, monomer concentration c, salt concentration cs, and preparation conditions (monomer concentration c0 and salt concentration cs°). The theory assumes affine displacement of the junction points when the concentration is changed. With no added salt (cs = cs° = 0), we predict a new concentration dependence of the modulus G ∼ c5/6. In the high-salt limit, we predict the usual concentration dependence for uncharged polymers but a novel dependence on salt concentration, G ∼ c7/12cs1/4. We also predict the modulus to decrease as charge is added to the gel strands. The predicted effects of added salt and charge on modulus have recently been observed by Candau and co-workers. At low concentrations, we discuss the strong stretching of network strands and its effect on modulus and equilibrium swelling.

Conformation of LiDNA in solutions of LiCl.

AbstractWe have derived radii of gyratin, Rg, from the absolute intensity of the scattered light of mondisperse linear Col E1 LiDNA in solution at various LiCl concentrations up to 5M. The second virial coefficients, A2, decrease strongly with increasing LiCl concentration, and vanish between 3 and 5M LiCl. It was thus possible to calculate a limiting value at a high salt concentration of 28.5 nm for the persistence length, a0, of LiDNA, without the necessity of applying excluded‐volume corrections. The value obtained is in good agreement with the value previously obtained for NaDNA at high NaCl concentrations, and can be identified with the high salt limit of DNA flexibility, with long‐range electrostatic interactions effectively screened. Sedimentation coefficients in the ultracentrifuge and apparent and translational diffusion coefficients (at finite and vanishing scattering vectors, respectively) from dynamic laser‐light scattering have also been obtained up to 5M LiCl. From the sedimentation and apparent diffusion, D(90), (at 90° scattering angle only) above 5M, and up to 9M LiCl, it could be shown that the solutions are stable for reasonable periods of time, and the molecular parameters vary smoothly and moderately at high salt. Conformational transitions were not observed and precipitation occurs between 9 and 10M LiCl.