Rachel Yerushalmi-Rozen

Weak polyelectrolytes (WPE) are used to prepare responsive materials in a variety ofapplications. When dissolved in highly crowded environments, of biorelevant media or synthetic complex fluids, that contain substantial concentrations of macromolecules, molecular assemblies, or nanoparticles, WPE exhibit deviations from the acid–base equilibrium observed in single‐component solutions. In this mini review, studies of charge regulation (CR) are described in simple yet relevant model systems comprising poly (acrylic acid) (PAA) and high concentrations of non‐charged micelles (Pluronic) or nonadsorbing, short chains of poly (vinyl alcohol) (PVA). Titration experiments reveal in the first mixture significant reduction in the degree of ionization of the PAA. In PAA–PVA mixtures, an unexpected increase in the degree of ionization of PAA is observed. Structural characterization of the solutions, via transmission electron microscopy and small‐angle X‐ray scattering, reveals that the local organization is affected by the interaction between PAA chains and the additives, leading to crowding of the micelles in one case, and crowding of the PAA chains in the other. These systems provide insights into the molecular mechanisms leading to the responsiveness of WPE and the effect of crowding on CR.

The self-assembly of like-(negatively) charged gold nanoparticles (GNPs) into hexagonal structures was observed in salt-free, dilute solutions of negatively charged branched polysaccharide, Gum Arabic (GA).

Entropy-driven assembly of nematic liquid-crystal phases of cellulose nanocrystals (SCNCs) in aqueous suspensions results in the emergence of a cholesteric liquid crystalline phase (N* phase).

The chiral environment of enantiomerically pure d-alanine solutions is observed to disrupt and modify the entropy-driven assembly of cellulose nanocrystals (CNCs) into a chiral nematic mesophase.

Weak polyelectrolytes (WPEs) are widely used as pH-responsive materials, pH modulators and charge regulators in biomedical and technological applications that involve multi-component fluid environments. In these complex fluids, coupling between (often weak) interactions induced by micelles, nanoparticles and molecular aggregates modify the pKa as compared to that measured in single component solutions. Here we investigated the effect of coupling between hydrogen bonding and excluded volume interactions on the titration curves and pKa of polyacrylic acid (PAA) in solutions comprising PEO-based micelles (Pluronics and Brij-S20) of different size and volume fraction. Titration experiments of dilute, salt-free solutions of PAA (5 kDa, 30 kDa and 100 kDa) at low degree of polymer ionization (α < 0.25) drive spatial re-organization of the system, reduce the degree of ionization and consequentially increase the pKa by up to ~0.7 units. These findings indicate that the actual degree of ionization of WPEs measured in complex fluids is significantly lower (at a given pH) than that measured in single-component solutions.

Hybrids comprising cellulose nanocrystals (CNCs) and percolated networks of single-walled carbon nanotubes (SWNTs) may serve for the casting of hybrid materials with improved optical, mechanical, electrical, and thermal properties. However, CNC-dispersed SWNTs are depleted from the chiral nematic (N*) phase and enrich the isotropic phase. Herein, we report that SWNTs dispersed by non-ionic surfactant or triblock copolymers are incorporated within the surfactant-mediated CNC mesophases. Small-angle X-ray measurements indicate that the nanostructure of the hybrid phases is only slightly modified by the presence of the surfactants, and the chiral nature of the N* phase is preserved. Cryo-TEM and Raman spectroscopy show that SWNTs networks with typical mesh size from hundreds of nanometers to microns are distributed equally between the two phases. We suggest that the adsorption of the surfactants or polymers mediates the interfacial interaction between the CNCs and SWNTs, enhancing the formation of co-existing meso-structures in the hybrid phases.

The self-assembly and phase behavior of cellulose nanocrystals (CNCs) in binary liquid mixtures of ethylene-glycol (EG):water was investigated. Our findings indicate that a small fraction of water delays the onset of colloidal jammed states previously reported in water-free organic solvents. Here the full phase diagram of CNCs evolves, including the chiral nematic phase (N*), characterized by long-range orientational order and non-isotropic macroscopic properties. Furthermore, the effect of the solvent-mixture composition on the properties of the CNC mesophases is found to be scale-dependent: the micron-size pitch of the N* phase decreases as the dielectric constant (ε_r) of the solvent mixture is reduced (higher EG content). Yet the nanometric inter-particle spacing of the CNC rods (measured using SAXS and cryo-TEM) is almost independent on the EG content. Also, unlike theoretical predictions, the transition to the biphasic regime is not sensitive to ε_r of the solvent mixtures and takes place at a higher CNC volume fraction than in aqueous suspensions. These observations may be rationalized by hypothesizing that vicinal water, adsorbed at the CNC surface, prevents kinetic arrest, and dictates the local dielectric constant and thus the effective diameter of the rods (via the Debye length), while ε_r of the liquid-mixture dominates the pitch length (micron scale) and the optical properties. These findings indicate that the water content of EG:water mixtures may be used for engineering colloidal inks where delayed kinetic arrest and jamming of the CNCs enable printing and casting of tunable, optically-active thin films and coatings.