Synergistic effects of ion pairs on the dielectric properties of diblock copolymer melts

Solvation Energy of Ions in a Stockmayer Fluid

We calculate the solvation energy of monovalent and divalent ions in various liquids with coarse-grained molecular dynamics simulations. Our theory treats the solvent as a Stockmayer fluid, which accounts for the intrinsic dipole moment of molecules and the rotational dynamics of the dipoles. Despite the simplicity of the model, we obtain qualitative agreement between the simulations and experimental data for the free energy and enthalpy of ion solvation, which indicates that the primary contribution to the solvation energy arises mainly from the first and possibly second solvation shells near the ions. Our results suggest that a Stockmayer fluid can serve as a reference model that enables direct comparison between theory and experiment and may be invoked to scale up electrostatic interactions from the atomic to the molecular length scale.

Recent progress in the science of complex coacervation

Complex coacervation is an associative, liquid-liquid phase separation that can occur in solutions of oppositely-charged macromolecular species, such as proteins, polymers, and colloids. This process results in a coacervate phase, which is a dense mix of the oppositely-charged components, and a supernatant phase, which is primarily devoid of these same species. First observed almost a century ago, coacervates have since found relevance in a wide range of applications; they are used in personal care and food products, cutting edge biotechnology, and as a motif for materials design and self-assembly. There has recently been a renaissance in our understanding of this important class of material phenomena, bringing the science of coacervation to the forefront of polymer and colloid science, biophysics, and industrial materials design. In this review, we describe the emergence of a number of these new research directions, specifically in the context of polymer-polymer complex coacervates, which are inspired by a number of key physical and chemical insights and driven by a diverse range of experimental, theoretical, and computational approaches.

A molecular contact theory for simulating polarization: application to dielectric constant prediction

Microscopic polarization in liquids, which is challenging to account for intuitively and quantitatively, can impact the behavior of liquids in numerous ways and thus is ubiquitous in a broad range of domains and applications. To overcome this challenge, in this work, a molecular contact theory was proposed as a proxy to simulate microscopic polarization in liquids. In particular, molecular surfaces from implicit solvation models were used to predict both the dipole moment of individual molecules and mutual orientations arising from contacts between molecules. Then, the calculated dipole moments and orientations were combined in an analytical coupling, which allowed for the prediction of effective (polarized) dipole moments for all distinct species in the liquid. As a proof-of-concept, the model focused on predicting the dielectric constant and was tested on 420 pure liquids, 269 binary organic mixtures (3792 individual compositions) and 46 aqueous mixtures (704 individual compositions). The model proved to be flexible enough to reach an unprecedented satisfactory mean relative error of about 16-22% and a classification accuracy of 84-90% within four meaningful classes of weak, low average, high average and strong dielectric constants. The method also proved to be computationally very efficient, with calculation times ranging from a few seconds to about ten minutes on a personal computer with a single CPU. This success demonstrates that much of the microscopic polarization concept can be satisfactorily described based on a simple molecular contact theory. Moreover, the new model for dielectric constants provides a useful alternative to computationally expensive molecular dynamics simulations for large scale virtual screenings in chemical engineering and material sciences.

Transfer matrix theory of polymer complex coacervation

Oppositely charged polyelectrolytes can undergo a macroscopic, associative phase separation in solution, via a process known as complex coacervation. Significant recent effort has gone into providing a clear, physical picture of coacervation; most work has focused on improving the field theory picture that emerged from the classical Voorn-Overbeek theory. These methods have persistent issues, however, resolving the molecular features that have been shown to play a major role in coacervate thermodynamics. In this paper, we outline a theoretical approach to coacervation based on a transfer matrix formalism that is an alternative to traditional field-based approaches. We develop theoretical arguments informed by experimental observation and simulation, which serve to establish an analytical expression for polymeric complex coacervation that is consistent with the molecular features of coacervate phases. The analytical expression provided by this theory is in a form that can be incorporated into more complicated theoretical or simulation formalisms, and thus provides a starting point for understanding coacervate-driven self-assembly or biophysics.

Thermodynamics and phase behavior of acid-tethered block copolymers with ionic liquids

We investigate the phase behavior of acid-tethered block copolymers with and without ionic liquids. Two phosphonated block copolymers and their sulfonated analogs were synthesized by fine-tuning the degree of polymerization and the acid content. The block copolymers carrying acid groups with ionic liquids exhibited rich phase sequences, i.e., disorder-lamellae (LAM), gyroid-LAM, gyroid-hexagonal cylinder (HEX), and gyroid-A15 lattice, and the cation/anion ratio in the ionic liquid exerted profound effects on the segregation strength and topology of the self-assembled structures. Additionally, using ionic liquids with excessive cation content was found to enhance the effective Flory-Huggins interaction parameter, χ_eff, of the samples. However, as the anion content of the ionic liquids increased the segregation strength decreased. This is attributed to the packing frustration accompanied by the prevailing repulsive electrostatic interactions of the anions in the ionic liquid and the polymer matrix. As the hydrophobicity of the ionic liquids increased, well-defined ordered phases emerged in the phosphonated block copolymers with increased anion content, contrary to the disordered phases of the sulfonated samples. Thus, the balance between solvation energy of the anions and the electrostatic interactions is a key determinant of the thermodynamics of acid-tethered block copolymers containing ionic liquids.

A new lattice Monte Carlo simulation for dielectric saturation in ion-containing liquids

We develop a new, rapid method for the lattice Monte Carlo simulation of ion-containing liquids that accounts for the effects of the reorganization of solvent dipoles under external electrostatic fields. Our results are in reasonable agreement with the analytical solutions to the dielectric continuum theory of Booth for single ions, ion pairs, and ionic cross-links. We also illustrate the substantial disparity between the dielectric functions for like and unlike charges on the nanometer scale. Our simulation rationalizes the experimental data for the dependence of the bulk dielectric value of water on ion concentrations in terms of saturated dipoles near ions.