Invertible Micelles Based on Ion-Specific Interactions of Sr2+ and Ba2+ with Double Anionic Block Copolyelectrolytes

pH-Dependent Solution Micellar Structure of Amphoteric Polypeptoid Block Copolymers with Positionally Controlled Ionizable Sites

While solution micellization of ionic block copolymers (BCP) with randomly distributed ionization sites along the hydrophilic segments has been extensively studied, the roles of positionally controlled ionization sites along the BCP chains in their micellization and resulting micellar structure remain comparatively less understood. Herein, three amphoteric polypeptoid block copolymers carrying two oppositely charged ionizable sites, with one fixed at the hydrophobic terminus and the other varyingly positioned along the hydrophilic segment, have been synthesized by sequential ring-opening polymerization method. The presence of the ionizable site at the hydrophobic segment terminus is expected to promote polymer association toward equilibrium micellar structures in an aqueous solution. The concurrent presence of oppositely charged ionizable sites on the polymer chains allows the polymer association to be electrostatically modulated in a broad pH range (ca. 2-12). Micellization of the amphoteric polypeptoid BCP in dilute aqueous solution and the resulting micellar structure at different solution pHs was investigated by a combination of scattering and microscopic methods. Negative-stain transmission-electron microscopy (TEM), small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS) analyses revealed the dominant presence of core-shell-type spherical micelles and occasional rod-like micelles with liquid crystalline (LC) domains in the micellar core. The micellar structures (e.g., aggregation number, radius of gyration, chain packing in the micelle) were found to be dependent on the solution pH and the position of the ionizable site along the chain. This study has highlighted the potential of controlling the position of ionizable sites along the BCP polymer to modulate the electrostatic and LC interactions, thus tailoring the micellar structure at different solution pH values in water.

Double-hydrophilic block copolymer-metal ion associations: Structures, properties and applications

Hybrid polyionic complexes (HPICs), constructed from double-hydrophilic block copolymers and metal ions, have been largely developed with increasing interest in the past decade in the fields of catalysis, materials science and biological applications. The chemical natures of both blocks are very versatile, but one block should be able to interact with ions, and the second one should be neutral. Many metals have been used to form HPICs, which have, in their simplest architectural form, a core-shell structure of a few tens of nanometers in radius with an external shell made of the neutral block of the copolymer. In this review, we focus our discussion on the stability, shape, size and inner structure of these hybrid micelles. We then describe the most recent applications of HPICs, as reported in the literature, and point out the current challenges, missing structural information and future perspectives for this class of organized structures.

Hydration and Charge-Transfer Effects of Alkaline Earth Metal Ions Binding to a Carboxylate Anion, Phosphate Anion, and Guanine Nucleobase

To investigate the ability of alkaline earth metal ions to tune ion-mediated DNA adsorption, hydrated Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ ions bound to a carboxylate anion, phosphate anion, and guanine nucleobase were modeled using density functional theory (DFT) and a combined explicit and continuum solvent model. The large first solvation shell of Ba²⁺ requires a larger solute cavity defined by a solvent-accessible surface, which is used to model all hydrated ions. Alkaline earth metal ions bind indirectly or directly to each binding site. DFT binding energies decrease with increasing ion size, which is likely due to ion size and hydration structure, rather than quantum effects such as charge transfer. However, charge transfer explains weaker ion binding to guanine compared to phosphate or carboxylate. Overall, carboxylate and phosphate anions are expected to compete equally for hydrated Mg²⁺, Ca²⁺, Sr²⁺, and Ba²⁺ ions and larger alkaline earth metal ions may induce weaker ion-mediated adsorption. The ion size and hydration structure of alkaline earth metal ions may effectively tune ion-mediated adsorption processes, such as DNA adsorption to functionalized surfaces.

100th Anniversary of Macromolecular Science Viewpoint: Attractive Soft Matter: Association Kinetics, Dynamics, and Pathway Complexity in Electrostatically Coassembled Micelles

Electrostatically coassembled micelles constitute a versatile class of functional soft materials with broad application potential as, for example, encapsulation agents for nanomedicine and nanoreactors for gels and inorganic particles. The nanostructures that form upon the mixing of selected oppositely charged (block co)polymers and other ionic species greatly depend on the chemical structure and physicochemical properties of the micellar building blocks, such as charge density, block length (ratio), and hydrophobicity. Nearly three decades of research since the introduction of this new class of polymer micelles shed significant light on the structure and properties of the steady-state association colloids. Dynamics and out-of-equilibrium processes, such as (dis)assembly pathways, exchange kinetics of the micellar constituents, and reaction-assembly networks, have steadily gained more attention. We foresee that the broadened scope will contribute toward the design and preparation of otherwise unattainable structures with emergent functionalities and properties. This Viewpoint focuses on current efforts to study such dynamic and out-of-equilibrium processes with greater spatiotemporal detail. We highlight different approaches and discuss how they reveal and rationalize similarities and differences in the behavior of mixed micelles prepared under various conditions and from different polymeric building blocks.

Screening lengths and osmotic compressibility of flexible polyelectrolytes in excess salt solutions

We report results of small angle neutron scattering measurements made on sodium polystyrene sulfonate in aqueous salt solutions. The correlation length (ξ) and osmotic compressibility are measured as a function of polymer (c) and added salt (cS) concentrations, and the results are compared with scaling predictions and the random-phase approximation (RPA). In Dobrynin et al.'s scaling model the osmotic pressure consists of a counter-ion contribution and a polymer contribution. The polymer contribution is found to be two orders of magnitude smaller than expected from the scaling model, in agreement with earlier observations made on neutral polymers in good solvent condition. RPA allows the determination of single-chain dimensions in semidilute solutions at high polymer and added salt concentrations, but fails for cS≤ 2 M. The χ parameter can be modelled as the sum of an intrinsic contribution (χ0) and an electrostatic term: χ∼χ0 + K'/√cS, where χ0 > 0.5 is consistent with the hydrophobic nature of the backbone of NaPSS. The dependence of χelec∼ 1/√cS disagrees with the random-phase approximation (χelec∼ 1/cs), but agrees with the light scattering results in dilute solution and Dobrynin et al.'s scaling treatment of electrostatic excluded volume.