Coarse-Grained Model of Entropy-Driven Demixing

Modeling and Elucidating the Behavior of a Thermoresponsive LCST Ionic Liquid

Desalination of seawater by forward osmosis is a technology potentially able to address the global water scarcity problem. The major challenge limiting its widespread practical application is the design of a draw solute that can be separated from water by an energetically efficient process and then reused for the next cycle. Recent experiments demonstrate that a promising draw solute for forward-osmosis desalination is tetrabutylphosphonium 2,4,6-trimethylbenzenesulfonate ([P4444][TMBS]). When mixed with water, this ionic liquid (IL) is thermoresponsive and exhibits a lower critical solution temperature (LCST), above which it phase-separates into an IL-rich phase and a water-rich phase. Elucidating the physical mechanism of the liquid-liquid phase separation, as well as rationally designing optimized derivatives, necessitates an accurate model to describe this and related ILs. In this paper, we resort to explicit-solvent all-atom molecular dynamics simulations and adopt AMBER-based force-field parameters for the cation whose partial charges were assigned by the RESP fitting procedure. Utilizing the same methodology, we parametrize the anion. The simulations' results indicate the IL/water mixture, at the experimental critical composition, can unambiguously phase-separate only when the partial charges of the ions are scaled down. Nevertheless, the best-performing charge scaling factor is found to be 0.95, a value much milder than those reported for ILs in neat phases. This can be explained by a diminished charge transfer, or induced dipoles, within the ions when the IL is in a mixture with water. With this charge scaling, the simulations reproduce well the LCST composition-temperature phase diagram, albeit overestimation of the critical temperature by 10 K. In particular, very good agreement is obtained for the composition of the two segregated phases. Estimation of viscosity points to IL/water mixture that is almost twice as viscous in simulations than that reported experimentally. Furthermore, we analyze changes in energy between different components in the mixture and find that the driving force for phase separation is, at least, enthalpic. Structural analyses of the ions and their interactions with water molecules corroborate the importance of the latter in mediating structural organizations of the anions, as well as in strengthening the interactions between the cations.

Thermoresponsive Ionic Liquid/Water Mixtures: From Nanostructuring to Phase Separation

The thermodynamics, structures, and applications of thermoresponsive systems, consisting primarily of water solutions of organic salts, are reviewed. The focus is on organic salts of low melting temperatures, belonging to the ionic liquid (IL) family. The thermo-responsiveness is represented by a temperature driven transition between a homogeneous liquid state and a biphasic state, comprising an IL-rich phase and a solvent-rich phase, divided by a relatively sharp interface. Demixing occurs either with decreasing temperatures, developing from an upper critical solution temperature (UCST), or, less often, with increasing temperatures, arising from a lower critical solution temperature (LCST). In the former case, the enthalpy and entropy of mixing are both positive, and enthalpy prevails at low T. In the latter case, the enthalpy and entropy of mixing are both negative, and entropy drives the demixing with increasing T. Experiments and computer simulations highlight the contiguity of these phase separations with the nanoscale inhomogeneity (nanostructuring), displayed by several ILs and IL solutions. Current applications in extraction, separation, and catalysis are briefly reviewed. Moreover, future applications in forward osmosis desalination, low-enthalpy thermal storage, and water harvesting from the atmosphere are discussed in more detail.

Structure and dynamics of responsive colloids with dynamical polydispersity

Dynamical polydispersity in single-particle properties, for example a fluctuating particle size, shape, charge density, etc, is intrinsic to responsive colloids (RCs), such as biomacromolecules or microgels, but is typically not resolved in coarse-grained mesoscale simulations. Here, we present Brownian dynamics simulations of suspensions of RCs modeling soft hydrogel colloids, for which the size of the individual particles is an explicitly resolved (Gaussian) degree of freedom and dynamically responds to the local interacting environment. We calculate the liquid structure, emergent size distributions, long-time diffusion, and property (size) relaxation kinetics for a wide range of densities and intrinsic property relaxation times in the canonical ensemble. Comparison to interesting reference cases, such as conventional polydisperse suspensions with a frozen parent distribution, or conventional monodisperse systems interacting with an effective pair potential for one fixed size, shows a significant spread in the structure and dynamics. The differences, most apparent in the high density regimes, are due to many-body correlations and the dynamical coupling between property and translation in RC systems, not explicitly accounted for in the conventional treatments. In particular, the translational diffusion in the RC systems is surprisingly close to the free (single RC) diffusion, mainly due to a cancellation of crowding and size compression effects. We show that an effective monodisperse pair potential can be constructed that describes the many-body correlations reasonably well by convoluting the RC pair potential with the density-dependent emergent size distributions and using a mean effective diffusion constant.