Dynamical properties across different coarse-grained models for ionic liquids

Abstract

Room-temperature ionic liquids (RTILs) stand out among molecular liquids for their rich physicochemical characteristics, including structural and dynamic heterogeneity. The significance of electrostatic interactions in RTILs results in long characteristic length-and timescales, and has motivated the development of a number of coarse-grained (CG) simulation models. In this study, we aim to better understand the connection between certain CG parameterization strategies and the dynamical properties and transferability of the resulting models. We systematically compare five CG models: a model largely parameterized from experimental thermodynamic observables; a refinement of this model to increase its structural accuracy; and three models that reproduce a given set of structural distribution functions by construction, with varying intramolecular parameterizations and reference temperatures. All five CG models display limited structural transferability over temperature, and also result in various effective dynamical speedup factors, relative to a reference atomistic model. On the other hand, the structure-based CG models tend to result in more consistent cation-anion relative diffusion than the thermodynamic-based models, for a single thermodynamic state point. By linking short-and long-timescale dynamical behaviors, we demonstrate that the varying dynamical properties of the different CG models can be largely collapsed onto a single curve, which provides evidence for a route to constructing dynamically-consistent CG models of RTILs.

Understanding and Modeling Polymers: The Challenge of Multiple Scales

“…As already discussed in the previous section, one key to reducing the dynamic consistency problem is accurate structural coarse-graining, as it helps to recover consistent dynamics even in standard MD simulations, i.e., consistent barrier crossing dynamics and consistent relative speedup. , In fact, using kinetic information as additional input for the parametrization of coarse-grained force fields may improve their quality, , because it gives more weight to transitional conformations, which are typically not well sampled in standard coarse-graining approaches. For instance, Xia, Keten and co-workers have recently proposed an “energy renormalization” method for adjusting the activation barriers in CG models of polymer glasses, which allows to recover the dynamical and rheological properties obtained from all-atom reference simulations over a wide temperature range. − …”

“…Since forces drive the dynamics, it becomes more difficult to restore the correct dynamical properties without further adjustments. Indeed, recent studies on ionic liquids 374 have suggested that structure-based CG models tend to have a more consistent dynamical behavior than thermodynamically CG models, e.g., regarding the relative mobility of anions and cations. We will specifically discuss issues of dynamic coarsegraining in the next section.…”

Broad chemical transferability in structure-based coarse-graining

“…Previous work has suggested that the inclusion of the bond interactions, even after subtracting their contribution to the mean force, can provide numerical stability for determining optimal nonbonded parameters. [65][66][67] All pairwise interactions were represented with radially-isotropic fourth-order basis splines with control points spaced every 0.01 nm ranging from 0.0 to 1.4 nm. In this fashion, a set of CG pairwise potentials was generated for each mapping at each state point.…”

Ionic liquid-coated lipid nanoparticles increase siRNA uptake into CNS targets

“…31 ILs exist as viscous liquids below 100 °C and are composed of bulky, asymmetric cations and anions. [32][33][34] Coordinating interactions between the cation and anion direct the biophysical and physicochemical properties within the ILs and the resulting interactions with the external microenvironment. Thus, structural alterations to the cation or anion, (e.g., alkyl chain length, functionalization with chemical moieties, branching, or steric organization) [35][36][37][38][39][40][41][42] can cause radical changes in bulk properties of ILs and their interplay with other biomolecules.…”