Aggregates and other particles as the origin of the “extraordinary” diffusional phase in polyelectrolyte solutions

Abstract

SYNOPSISThe "extraordinary" diffusional phase ( E P ) at low ionic strength, and the conditions for its removability by filtration were investigated for dilute solutions of the following linear polyelectrolytes: poly ( L-lysine ) , heparin, chondroitin-6-sulfate, hyaluronate, polystyrene sulfonate, and variably ionized polyacrylamide. The E P was not present for all the different types studied, and for heparin, for example, the phase was present only for samples from certain sources. In all cases the phase was removable by filtration through sufficiently small pore-size membranes. Once filtered, the EP remained absent for over one week. It is concluded that the extraordinary diffusional phase consists of fairly stable polyelectrolyte aggregates, and sometimes also includes other very small particulate impurities. These aggregates and other small particles are thought to be present, or at least nascent, in the dry polyelectrolyte material, so that their properties may depend critically on the manner in which such dry material is produced. Tests for "reversibility" of the EP by cycling between high and low C, by dialysis further confirm these conclusions. The evidence is thus against the E P representing any type of temporal aggregates or local ordering, at least for the linear polyelectrolytes studied in this work. Rather, due to the extremely feeble scattering of ordinary polyelectrolytes at low ionic strength, the weak scattering from residual aggregates and other particles, not removed by ordinary filtration and centrifugation procedures, give autocorrelable scattering signals with long decay times. The "loss" of the extraordinary phase as ionic strength increases appears to be due simply to the weak E P scattering signal getting buried in the sharply increasing scattering from the ordinary polyelectrolyte phase. Model calculations based on experimental data support this latter conclusion.

The slow relaxation mode: from solutions to gel networks

“…It has been attributed to dynamics of large multichain domains (transient aggregates or clusters) formed due to electrostatic interaction [76][77][78][79][80] or some insoluble chain clusters, or even a trace amount of large particles introduced during the imperfect preparation of solution. [89][90][91][92][93] The transient cluster interpretation was supported by studies of poly(styrenesulfonate) with sodium counterions in aqueous solutions under dialysis. For a long time, this transient cluster has been attributed to the effective interaction between similarly charged segments, namely, the overlapping of the ion clouds of neighboring polyions, so that the more loosely associated small ions are 'shared' by two or more polyions chains, resulting in a fluctuating dipole field that tends to retard the relative motions of those participating polyions.…”

“…The interpretation of the slow mode, especially for those very slow relaxation modes observed in salt-free or low-salt polyelectrolyte solutions, is very controversial. [76][77][78][79][80][81][82][83][84][85][86][87][88][89][90][91][92][93][94][95] It has been attributed to large multichain domains formed because of electrostatic interaction or some insoluble clusters or even a trace amount of dust particles introduced during the imperfect preparation of polymer solution. Actually, not everyone accepts or recognized such a slow mode, even though it has been repeatedly observed in many dynamic LLS experiments for more than three decades, because of some problems or questions related to previous light-scattering experiments, such as some earlier premature data analysis methods and the preparation of dust-free viscous solutions.…”

An Overview of Polyelectrolytes

“…Thus D a pp for [NaBr] < 10'3 M was not associated with the mutual diffusion coefficient, but rather is a manifestation of an extraordinary behavior of the dynamics of the PLL955 solution. This very sharp drop in the value of Dapp for a 1 mg/mL solution of PLL at 10~3 M univalent salt for a has been verified for PLL4O6 (77), PLL946 (77), PLL2273 (77), PLL616 (72), PLL2500 (73), PLL38OO (74), and PLL952-PLL138O (75). Representative data are shown in Figure 1, where the location of the ordinary-extraordinary (o-e) transition is indicated by the solid line.…”

“…The colorful metaphor for this model is that of "stars" (the aggregates, denoted by EP for "extraordinary phase") and the "sun" (the monomer particles, denoted by OP for "ordinary phase"). These authors thus envision the o-e transition as follows (75), The weak EP scattering from the aggregates (the "stars") is detectable against the dark (extremely weakly scattering) background of the ordinary polyelectrolyte populations at low ionic strength. As C a increases, the scattering from the ordinary polyelectrolytes rises dramatically (the sun comes up and the atmosphere scatters brightly), and overwhelms the feeble scattering from the aggregates of the EP phase.…”

Continuous Monitoring of the Effect of Changing Solvent Conditions on Polyelectrolyte Conformations and Interactions

“…Both neutron and static light scattering have revealed broad scattering maxima for such solutions [16][17][18][19] . Dynamic light scattering of low ionic strength polyelectrolyte solutions has frequently revealed a slow diffusional mode [20][21][22] , although this has been attributed by some to the presence of incompletely dissolved aggregates, which can be removed by filtration [23][24][25][26] .…”