Ion Solvation in Polymer Blends and Block Copolymer Melts: Effects of Chain Length and Connectivity on the Reorganization of Dipoles

Weakening of Solvation-Induced Ordering by Composition Fluctuation in Salt-Doped Block Polymers

The spontaneous ordering of block polymers doped with ions is affected by both selective solvation and long-range Coulombic interaction. The mean-field treatment was recently shown to overestimate the solvation-induced ordering, requiring a large solvation radius to fit experimental phase diagrams, which may be relieved by including composition fluctuations. Treating the composition fluctuations in such systems is challenging because of the need of resolving heterogeneous dielectric profile that couples with the ordering itself. Starting from a minimal model, we develop a Landau-Brazovskiĭ expansion for the free energy of salt-doped block polymer near the ordering transition. It is found that the wavelength for typical composition fluctuations first decreases with salt doping, due to Coulombic interaction, then increases due to ionic solvation. Two mechanisms that weaken the solvation-enhanced ordering are identified: the Brazovskiĭ-type composition fluctuation that stabilizes disordered phase, and the coupling between mismatch in dispersion interaction and the dielectric permittivity through monomeric polarizability.

Salt-Induced Liquid-Liquid Phase Separation: Combined Experimental and Theoretical Investigation of Water-Acetonitrile-Salt Mixtures

Salt-induced liquid-liquid phase separation in liquid mixtures is a common phenomenon in nature and in various applications, such as in separation and extraction of chemicals. Here, we present results of a systematic investigation of the phase behaviors in water-acetonitrile-salt mixtures using a combination of experiment and theory. We obtain complete ternary phase diagrams for nine representative salts in water-acetonitrile mixtures by cloud point and component analysis. We construct a thermodynamic free energy model by accounting for the nonideal mixing of the liquids, ion hydration, electrostatic interactions, and Born energy. Our theory yields phase diagrams in good agreement with the experimental data. By comparing the contributions due to the electrostatic interaction, Born energy, and hydration, we find that hydration is the main driving force for the liquid-liquid separation and is a major contributor to the specific ion effects. Our theory highlights the important role of entropy in the hydration driving force. We discuss the implications of our findings in the context of salting-out assisted liquid-liquid extraction and make suggestions for selecting salt ions to optimize the separation performance.

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.

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.

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

We study the solvation of ion pairs in diblock copolymer melts. Our theory accounts for the size of the ions, the permanent dipole moment and the molecular polarizability of the monomers, the Kuhn length, the compressibility of the liquid mixtures, and the degrees of polymerization. We demonstrate that the electrostatic field near an ion pair causes marked, synergistic effects on the volume fractions of the two blocks and hence the dielectric function. In particular, we illustrate the oscillatory behavior of the dielectric function near an ion pair and the disparity of the dielectric functions between like and unlike charges. These results depend significantly on the chain length and Kuhn length of the diblock copolymers on the nanometer scale.

Thermodynamics of Salt-Doped Block Copolymers

We provide a perspective on the thermodynamics of salt-doped block copolymer electrolytes consisting of ion-conducting and inert blocks, taking poly(ethylene oxide)-b-polystyrene and lithium salts as an example. We focus on the origin for enhanced immiscibility between the constituent blocks upon addition of lithium salts and discuss issues from selected experiments and from our recent self-consistent field study.