Diffusion and Viscosity of Unentangled Polyelectrolytes

Quantification of Nanoplastics and Inorganic Nanoparticles via Laser-Induced Breakdown Detection (LIBD)

Nanoparticles with diverse characteristics are difficult to quantify at low concentrations in the water environment (106-109 particles mL-1 for nanoplastics originating from the breakdown of plastic debris) for the evaluation of effective treatment methods. This study examines the sensitivity, or limit of detection (LOD), of laser-induced breakdown detection (LIBD) for the counting of nanoparticles, including nanoplastics. For polystyrene (PS) standards with sizes of 20-400 nm, LIBD shows relatively low LODs (for example, 2 × 106 particles mL-1 for 100 nm particles) compared with turbidity monitoring, UV-vis spectroscopy (both 6 × 108 particles mL-1), and nanoparticle tracking analysis (2 × 107 particles mL-1). For nanoplastics (PS, polypropylene, and polyethylene terephthalate), the detection limits are 104 - 105 particles mL-1, one to two orders of magnitude lower than the PS standards. LIBD can quantify inorganic nanoparticles, such as zeolite, titania, and hematite. The sensitivity increases (i.e., LOD reduces) with increasing particle density, while some particles are prone to artifacts. The low LODs make LIBD a robust technique for counting nanoparticles of various types and sizes, even at the concentrations found in the permeate of membrane-based water treatment systems. Given the high sensitivity, LIBD has the potential to be applied in membrane integrity monitoring and fundamental studies on membrane mechanisms.

Viscosity of Polyelectrolytes: Influence of Counterion and Solvent Type

We study the viscosity of polystyrenesulfonate with sodium and tetrabutylammonium counterions in aqueous and organic solvent media. We find that at low concentrations the Fuoss law (ηsp ∼ c1/2) is approximately obeyed, but at higher concentrations, an exponential dependence on the polymer volume fraction sets in. These findings are discussed in terms of Fujita's free volume theory.

Extreme dependence of dynamics on concentration in highly crowded polyelectrolyte solutions

Charge-carrying species, such as polyelectrolytes, are vital to natural and synthetic processes that rely on their dynamic behavior. Through single-particle tracking techniques, the diffusivity of individual polyelectrolyte chains and overall system viscosity are determined for concentrated polylysine solutions. These studies show scaling dependences of D ~ c-6.1 and η ~ c7.2, much stronger than theoretical predictions, drawing the applicability of power law fits into question. Similar trends are observed in concentrated solutions prepared at various pH and counterion conditions. These hindered system dynamics appear universal to polyelectrolyte systems and are attributed to the large effective excluded volumes of polyelectrolyte chains inducing glassy dynamics. The framework of the Vrentas-Duda free-volume theory is used to compare polyelectrolyte and neutral systems. Supported by this theory, excluding counterion mass from total polymer mass results in all environmental conditions collapsing onto a common trendline. These results are applicable to crowded biological systems, such as intracellular environments where protein mobility is strongly inhibited.

Dilute polyelectrolyte solutions: recent progress and open questions

Polyelectrolytes are a class of polymers possessing ionic groups on their repeating units. Since counterions can dissociate from the polymer backbone, polyelectrolyte chains are strongly influenced by electrostatic interactions. As a result, the physical properties of polyelectrolyte solutions are significantly different from those of electrically neutral polymers. The aim of this article is to highlight key results and some outstanding questions in the polyelectrolyte research from recent literature. We focus on the influence of electrostatics on conformational and hydrodynamic properties of polyelectrolyte chains. A compilation of experimental results from the literature reveals significant disparities with theoretical predictions. We also discuss a new class of polyelectrolytes called poly(ionic liquid)s that exhibit unique physical properties in comparison to ordinary polyelectrolytes. We conclude this review by listing some key research challenges in order to fully understand the conformation and dynamics of polyelectrolytes in solutions.

Coarse-grained explicit-solvent molecular dynamics simulations of semidilute unentangled polyelectrolyte solutions

We present results from explicit-solvent coarse-grained molecular dynamics (MD) simulations of fully charged, salt-free, and unentangled polyelectrolytes in semidilute solutions. The inclusion of a polar solvent in the model allows for a more physical representation of these solutions at concentrations, where the assumptions of a continuum dielectric medium and screened hydrodynamics break down. The collective dynamic structure factor of polyelectrolytes, S(q, t), showed that at [Formula: see text], where [Formula: see text] is the polyelectrolyte peak in the structure factor S(q) and [Formula: see text] is the correlation length, the relaxation time obtained from fits to stretched exponential was [Formula: see text], which describes unscreened Zimm-like dynamics. This is in contrast to implicit-solvent simulations using a Langevin thermostat where [Formula: see text]. At [Formula: see text], a crossover region was observed that eventually transitions to another inflection point [Formula: see text] at length scales larger than [Formula: see text] for both implicit- and explicit-solvent simulations. The simulation results were also compared to scaling predictions for correlation length, [Formula: see text], specific viscosity, [Formula: see text], and diffusion coefficient, [Formula: see text], where [Formula: see text] is the polyelectrolyte concentration. The scaling prediction for [Formula: see text] holds; however, deviations from the predictions for [Formula: see text] and D were observed for systems at higher [Formula: see text], which are in qualitative agreements with recent experimental results. This study highlights the importance of explicit-solvent effects in molecular dynamics simulations, particularly in semidilute solutions, for a better understanding of polyelectrolyte solution behavior.

1H DOSY analysis of high molecular weight acrylamide-based copolymer electrolytes using an inverse-geometry diffusion probe

Copolymers of [2-(acryloyloxy)ethyl]trimethylammonium chloride (AETAC) and acrylamide (AAm) (AETAC-co-AAm) are polyelectrolytes used as flocculants in wastewater purification. Diffusion-ordered two-dimensional NMR spectroscopy (DOSY) experiments for AETAC-co-AAm samples with Mw ranging from 1.9 to 3.9 million and a polyacrylamide sample with Mw of 1.3 million were carried out in pure D2O and in D2O containing 0.1 or 1 M NaCl using an inverse-geometry diffusion probe system. Projections of the DOSY contour plots onto the diffusion coefficient (D) dimension gave distributions of D for the AETAC and AAm units in the samples. The D values at the maximum point of the distribution (Dp) agreed fairly well with those determined by dynamic light scattering.