Composition and Charge Compensation in Chitosan/Gum Arabic Complex Coacervates in Dependence on pH and Salt Concentration.

In this study, complex coacervates of the biopolyelectrolytes chitosan and gum arabic were investigated with respect to their composition and charge compensation depending on the pH and salt concentration. Individual polyelectrolyte yields were deduced from thermogravimetric analysis and chitosan quantification via enzymatic hydrolysis/HPLC-ELSD. The polyelectrolyte mass ratio in the complex coacervate is found to remain approximately constant irrespective of the pH, despite the latter's effect on the polyelectrolyte charge ratio. Two regimes are identified, including either chitosan charges in excess (at pH < 6.0) or gum arabic charges in excess (at pH > 6.0). The amount of extrinsic charge compensation in the complex coacervates is discussed in detail. We show for the first time that the doping level, a quantity traditionally used to describe salt-induced changes of the charge compensation in polyelectrolyte complexes, is also suitable for the description of pH-induced extrinsic charge compensation in such systems.

Modelling viscoelastic relaxation mechanisms in thermorheologically complex Fe(III)-poly(acrylic acid) hydrogels.

Mechanical properties of hydrogels with reversible transition metal-polymer crosslinks can be flexibly tuned depending on the dissociation kinetics of the metal bond. We use rheology to investigate the sol-gel transition of a Fe(III)-poly(acrylic acid) network with varying crosslinker content and model the corresponding mechanical relaxation at different stages of gelation. The system transitions from an unentangled chain regime to a crosslink dissociation dominated regime, where the relaxation is governed by two timescales with different activation energies. To account for the interplay of chain and crosslinker dynamics, a time-temperature-superposition procedure is introduced for both processes separately, thus separating the dynamic processes in these thermorheologically complex dynamic networks. The activation energy of chain relaxation remains unchanged whether or not the chain participates in the network. To model contributions to the dynamic modulus of each process, we combine concepts from fractional viscoelasticity with a generalized Maxwell model, which describes the dynamics of an unentangled chain solution with reversible crosslinks. This allows us to quantify the evolution of viscoelastic parameters in the course of gelation, where we find that the terminal relaxation time of the gels increases less than expected at high crosslinker contents. This result is attributed to a facilitated crosslink exchange mechanism and a lower pH of the gel matrix.

Reply to the 'Comment on "Negative effective Li transference numbers in Li salt/ionic liquid mixtures: does Li drift in the "Wrong" direction?"' by K. R. Harris, Phys. Chem. Chem. Phys., 2018, 20, DOI: 10.1039/C8CP02595A.

We have recently directly measured the electrophoretic mobilities of the three species in "Li salt in-ionic liquid" electrolytes with a common anion by electrophoretic NMR. From the mobilities of the three species, we deduced the transference numbers and found a negative value for Li. We attributed this to a vehicle mechanism of 7Li transport, where Li is drifting in net negatively charged Li-anion clusters. In our work, as also in previous eNMR studies, ion drift velocities and thus electrophoretic mobilities are determined in the laboratory frame of reference. In his comment, Harris claims that in this frame conclusions on ion correlations are not deducable, but rather an internal reference frame, defined by the drift of one of the ions, needs to be used and that in such a reference frame no negative transference numbers would occur. While agreeing with Harris on the general relevance of reference frames in eNMR, we argue here that a reference frame of one of the ions leads to loss of information. We propose to use the reference frame of the center of mass of the electrolyte if an internal frame is required. An interesting observation is that the center of mass of an ionic liquid is drifting during the application of the short electric field pulse in eNMR. To account for this drift, we convert our data, determined from drift velocities in the laboratory frame into the center of mass frame. The results show that, independent of the reference frame chosen, using either a laboratory frame as an external frame or a center-of-mass frame as an internal frame, drift velocities of 7Li and anions are identical within error, such that the interpretation of a correlated motion of Li and anions holds.

Quantification of chitosan in aqueous solutions by enzymatic hydrolysis and oligomer analysis via HPLC-ELSD.

A new method for quantitative analysis of chitosan in aqueous solution is introduced, comprising an enzyme-driven cleavage to water-soluble chitooligosaccharides (COS), N-acetylation, separation via UHPLC and detection by use of an evaporative light scattering detector (ELSD). Chitosans with different fractions of acetylation (FA) and molecular weights (Mw) were hydrolyzed using a chitosanase/chitinase mixture. By subsequent N-acetylation with isotopically labelled acetic anhydride, COS mixtures with FA = 1 were obtained allowing for chromatographic separation solely based on their degree of polymerization (DP). ELSD data conversion into molar concentrations was realized using COS-specific external calibration curves, and mass spectrometry (MS) data informed about the chitosan's FA. The overall chitosan concentration was determined by simple addition of the COS concentrations multiplied by their DP. Validity of the method is shown for chitosan in presence of various co-solutes such as the protein BSA, the polysaccharide dextran and the monosaccharide glucosamine.

Correction: Supramolecular ionogels prepared with bis(amino alcohol)oxamides as gelators: ionic transport and mechanical properties.

[This corrects the article DOI: 10.1039/D0RA01249A.].

Conductivity spectra of polyphosphazene-based polyelectrolyte multilayers.

Polyelectrolyte multilayers are built up from ionically modified polyphosphazenes by layer-by-layer assembly of a cationic (poly[bis(3-amino-N,N,N-trimethyl-1-propanaminium iodide)phosphazene] (PAZ+) and an anionic poly[bis(lithium carboxylatophenoxy)phosphazene] (PAZ-). In comparison, multilayers of poly(sodium 4-styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH) are investigated. Frequency-dependent conductivity spectra are taken in sandwich geometry at controlled relative humidity. Conductivity spectra of ion-conducting materials generally display a dc plateau at low frequencies and a dispersive regime at higher frequencies. In the present case, the dispersive regime shows a frequency dependence, which is deviating from the typical behavior found in most ion-conducting materials. Dc conductivity values, which can be attributed to long-range ionic transport, are on the order of sigmadc = 10-10-10-7 S.cm-1 and strongly depend on relative humidity. For PAZ+/PAZ- multilayers sigmadc is consistently larger by one decade as compared to PSS/PAH layers, while the humidity dependence is similar, pointing at general mechanisms. A general law of a linear dependence of log(sigmadc) on relative humidity is found over a wide range of humidity and holds for both multilayer systems. This very strong dependence was attributed to variations of the ion mobility with water content, since the water content itself is not drastically dependent on humidity.

Ion Conduction and Its Activation in Hydrated Solid Polyelectrolyte Complexes.

For the first time, temperature-dependent conductivities at constant water content for a series of solid polyelectrolyte complexes with varying mixing ratios of anionic poly(sodium 4-styrene sulfonate) and poly(diallyldimethylammonium chloride) are presented. For water absorption, the samples are first equilibrated at an ambient temperature and at fixed relative humidity (RH). During the conductivity measurements, the so achieved water content of the samples is kept constant. At all of the hydration levels, the dc conductivities of the hydrated polyelectrolyte complexes (PEC) display Arrhenius behavior with activation enthalpies that are significantly lower than those of dry complexes. The activation enthalpy decreases linearly with water content. The lower activation enthalpies in case of hydrated as compared to dried complexes are attributed to a lowering of the energy barriers for ion motion. Finally, it is shown that the temperature-dependent conductivity spectra at constant water content obey the time-temperature superposition principle. Additionally, temperature-dependent conductivities at constant water content are compared to data sets determined in a separate study with constant RH at all of the temperatures. For the latter case, the influence of the type of alkali ion is also considered. Using the broad variety of data sets, the influences of water content and temperature on the conductivity mechanism can be separated from each other.

A conductivity study and calorimetric analysis of dried poly(sodium 4-styrene sulfonate)/poly(diallyldimethylammonium chloride) polyelectrolyte complexes.

Ionically cross-linked polyelectrolyte complexes (PECs) of anionic poly(sodium 4-styrene sulfonate) (PSS) and cationic poly(diallyldimethylammonium chloride) (PDADMAC), xPSS.(1-x)PDADMAC, with molar fractions x ranging from 0.30 to 0.70, were prepared and subsequently dried. The PEC samples were analyzed by differential scanning calorimetry, and the ionic conductivity sigmadc of the samples was measured as a function of temperature by means of impedance spectroscopy. The thermograms display an endothermic peak in the temperature range of 90-143 degrees C, which is attributed to a glass transition of the PEC. The glass transition temperature Tg has a symmetric x dependence with a minimum at x=0.50. The temperature dependence of sigmadcT is not affected by the glass transition. The ionic conductivity of the samples before drying is three orders of magnitude larger than sigmadc after drying; nevertheless, their activation enthalpies are identical. Arrhenius parameters obtained from the systematic study of several PEC compositions are discussed. The ionic conductivity of the PSS-rich samples is significantly higher than sigmadc of PDADMAC-rich samples. This implies a relatively high Na+ mobility as compared to Cl(-) mobility in PEC. In contrast to the symmetric x dependence of Tg, the conductivity of PEC increases and the activation enthalpy decreases with increasing x in the investigated composition range. A strong x dependence of sigmadc is observed for PSS-rich PEC, which is attributed to a significant variation in the mobility of the charge carriers.

Free volume anomalies in mixed-cation glasses revealed by positron annihilation lifetime spectroscopy (PALS).

PALS experiments reveal a minimum in ortho-positronium (o-Ps) lifetimes and a maximum in the corresponding intensities that emerge when mixed-cation (Li/Na) borate glasses are heated from ambient temperatures up to 473 K. These free volume 'anomalies' appear to be a true manifestation of the mixed alkali effect (MAE). They are consistent with a mechanism of ion transport involving cooperation between hops of unlike cations, resulting in increased disturbance of the glass network. The result lends support to the dynamic structure model.