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.

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). We report that a solvated, non-adsorbing, highly branched natural polysaccharide, Gum Arabic (GA), strongly affects the assembly of the SCNCs and modifies the phase diagram: GA leads to significant crowding of the SCNC rods and induces a new liquid-liquid phase transition, where SCNC-rich and GA-rich droplets coexist. The solvated GA does not induce coagulation or gelation of the suspended SCNCs (at low concentrations of 1-3 wt% of GA). In the SCNC-rich droplets, finite-sized nematic nano-islands assemble and further evolve into cholesteric tactoids and nucleate the formation of the N* phase at significantly lower concentration (about 1.5 wt%) than in GA-free suspensions. We observe that the inter-particle distance and the chiral pitch of the N* phase are determined by the concentration of GA (for a given SCNC concentration). The resulting mesophases are characterized via transmission electron microscopy at cryogenic temperatures (cryo-TEM), small-angle X-ray scattering (SAXS), and polarized optical microscopy (POM). Our findings indicate that GA can be used to tune the phase diagram and optical properties of SCNC suspensions, and overcome kinetic barriers that lead to gelation or kinetic arrest.

The stability of colloidal and nanoparticle (NPs) dispersions is a prerequisite for their utilization in advanced technologies. Yet, disposal of large volumes of NPs-rich fluid is challenging for waste management and would benefit from the ability to induce controlled aggregation and sedimentation of the NPs. Here, we demonstrate that sterically stabilized dispersions of carbonaceous NPs (Carbon Black and Single-Walled Carbon Nanotubes) in aqueous solutions of Arabinogalactan (AG) are pH responsive: the dispersions are stable at pH = 7 but coagulate at pH = 2 and separate into NPs-poor fluid and NPs-rich sediment. The sediment can be easily re-dispersed at pH = 7. The molecular and colloidal origins of the unexpected pH responsiveness of the sterically stabilized AG dispersions are investigated using surface tension measurements and Small-Angle X-ray Scattering experiments. While the pH responsiveness, along with the re-disposability, opens new pathways for reducing the ecological footprint of the NPs dispersions, the molecular origins of the observed behavior are not resolved and would require further investigation.

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. The effect is specific to D-alanine and cannot be attributed to the adsorption of alanine molecules (neither D- nor L-alanine) onto the CNC particles.

Weak polyelectrolytes (WPEs) are responsive materials used as active charge regulators in a variety of applications, including controlled release and drug delivery in crowded bio-related and synthetic environments. In these environments, high concentrations of solvated molecules, nanostructures, and molecular assemblies are ubiquitous. Here, we investigated the effect of high concentrations of non-adsorbing, short chains of poly(vinyl alcohol), PVA, and colloids dispersed by the very same polymers on charge regulation (CR) of poly(acrylic acid), PAA. PVA does not interact with PAA (throughout the full pH range) and thus can be used to examine the role of non-specific (entropic) interactions in polymer-rich environments. Titration experiments of PAA (mainly 100 kDa in dilute solutions, no added salt) were carried out in high concentrations of PVA (13–23 kDa, 5–15 wt%) and dispersions of carbon black (CB) decorated by the same PVA (CB-PVA, 0.2–1 wt%). The calculated equilibrium constant (and pKa) was up-shifted in PVA solutions by up to ~0.9 units and down-shifted in CB-PVA dispersions by ~0.4 units. Thus, while solvated PVA chains increase the charging of the PAA chains, as compared to PAA in water, CB-PVA particles reduce PAA charging. To investigate the origins of the effect, we analyzed the mixtures using small-angle X-ray scattering (SAXS) and cryo-TEM imaging. The scattering experiments revealed re-organization of the PAA chains in the presence of the solvated PVA but not in the CB-PVA dispersions. These observations clearly indicate that the acid–base equilibrium and the degree of ionization of PAA in crowded liquid environments is affected by the concentration, size, and geometry of seemingly non-interacting additives, probably due to depletion and excluded volume interactions. Thus, entropic effects that do not depend on specific interactions should be taken into consideration when designing functional materials in complex fluid environments.

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.

Mixtures of nanometric rods and spherical micelles that are dominated by excluded volume (entropic) interactions tend to de-mix into sphere- and rod-rich mesophases. Here we report the observation of hybrid mesophases formed by mixtures of polymer-decorated sulfuric-acid-hydrolyzed cellulose nanocrystals (CNCs, rod-like particles), at high CNCs concentrations (7 wt%) where the native CNCs suspensions are at the bi-phasic (isotropic-chiral nematic) regime, and spherical micelles of the tri-block-copolymers Pluronic F127 (and F108) in aqueous suspension. The polymers adsorb onto the CNCs particles, and at F127 concentrations above (10-18 wt%) the macroscopic phases of the hybrid mixture are transformed into mesoscopic droplets with a nanostructure that differs from that of the CNCs or the native Pluronics, as indicated by small-angle X-ray scattering. The observed phases preserve the chiral nematic nature of the CNCs phases with a shortened pitch, suggesting that co-assembly of CNCs and self-assembling polymers may be used for engineering of emergent phases with emergent structures and properties.

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 (εr) 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 εr 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 εr 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.

Dispersion of carbon nanotubes in solutions of polyvinyl-alcohol is required for solution casting of composite materials with improved interfacial adhesion where chains adsorbed on the nanotubes serve in the dual role of dispersant and compatible "connector" to the polyvinyl-alcohol matrix. Yet polyvinyl-alcohol is known to induce coagulation of nanotubes in aqueous solutions and thus far, it has not been used for dispersing pristine nanotubes. Here, we report that non-fully hydrolyzed (80-90%) polyvinyl-alcohol can be used for the preparation of stable, surfactant-free, dispersions of multi-wall carbon nanotubes in ethanol-water mixtures (of at least 50 vol% ethanol). Cryo-TEM imaging and rheological measurements of stable, long-lived dispersions reveal the formation of random networks of suspended tubes, with an averaged mesh size of ∼500 nm, indicating that the individual tubes do not aggregate or coagulate. We hypothesize that the polyvinyl-acetate sequences found in non-fully hydrolyzed polymers swell in the presence of ethanol, leading to the formation of a long-ranged steric (entropic) repulsion among polymer-decorated nanotubes. The unexpected role of the polyvinyl-acetate sequences along with a detailed dispersion mechanism are described.