Electrostatic interactions in soft particle systems: mesoscale simulations of ionic liquids

Abstract

Computer simulations provide a unique insight into the microscopic details, molecular interactions and dynamic behavior responsible for many distinct physicochemical properties of ionic liquids. Due to the sluggish and heterogeneous dynamics and the long-ranged nanostructured nature of ionic liquids, coarse-grained meso-scale simulations provide an indispensable complement to detailed first-principles calculations and atomistic simulations allowing studies over extended length and time scales with a modest computational cost. Here, we present extensive coarse-grained simulations on a series of ionic liquids of the 1-alkyl-3-methylimidazolium (alkyl = butyl, heptyl-, and decyl-) family with Cl, [BF4], and [PF6] counterions. Liquid densities, microstructures, translational diffusion coefficients, and re-orientational motion of these model ionic liquid systems have been systematically studied over a wide temperature range. The addition of neutral beads in cationic models leads to a transition of liquid morphologies from dispersed apolar beads in a polar framework to that characterized by bi-continuous sponge-like interpenetrating networks in liquid matrices. Translational diffusion coefficients of both cations and anions decrease upon lengthening of the neutral chains in the cationic models and by enlarging molecular sizes of the anionic groups. Similar features are observed in re-orientational motion and time scales of different cationic models within the studied temperature range. The comparison of the liquid properties of the ionic systems with their neutral counterparts indicates that the distinctive microstructures and dynamical quantities of the model ionic liquid systems are intrinsically related to Coulombic interactions. Finally, we compared the computational efficiencies of three linearly scaling O(N log N) Ewald summation methods, the particle-particle particle-mesh method, the particle-mesh Ewald summation method, and the Ewald summation method based on a non-uniform fast Fourier transform technique, to calculate electrostatic interactions. Coarse-grained simulations were performed using the GALAMOST and the GROMACS packages and hardware efficiently utilizing graphics processing units on a set of extended [1-decyl-3-methylimidazolium][BF4] ionic liquid systems of up to 131 072 ion pairs.

4. Multigranular modeling of ionic liquids

“…There is an inevitable trade-off between computational cost and physical accuracy inherent to any CG model development. Additionally, CG models can reveal essential structural properties and qualitatively describe transport properties of model IL systems at mesoscopic level by integrating over less important degrees of freedom at atomic scale [31,35,36,55,56,133,134].…”

“…In order to evaluate the different treatment of electrostatic interactions in CGMD simulations, we proposed a CG model for [BMIM][PF 6 ] IL based on following principles [37]: (i) Three methylene units in aliphatic chains are treated as single CG bead. This is one of the most popular coarse-graining schemes widely used in other CG simulations [55,134]; (ii) Imidazolium ring and two remaining methyl units are symmetrically divided into two CG beads; (iii) The anionic group is described by one CG bead, which is the normal scheme in other CG models for IL system [35,55,133,134]. With these guiding principles, the CG prototype for [BMIM] [PF 6 ] IL was constructed and is illustrated in Figure 4.15.…”

The influence of hydrophilicity on the orientational dynamics and structures of imidazolium-based ionic liquid/water binary mixtures

“…It is noteworthy that the asymptotic decay of these reorientational correlation functions can be approximated by a biexponential decay function with the form of C(t) = c 0 + c 1 e −t/τ 1 + c 2 e −t/τ 2 . 54,57,58 The accessible fitting parameters τ 1 and τ 2 of the imidazolium ring planes in varied RTIL/water mixtures are provided in tables of the supplementary material. The slowest component time constants can be compared to the experimental values given in Table I.…”

TheENUFmethod—Ewald summation based onnonuniformfast Fourier transform: Implementation, parallelization, and application

“…[1,2] Electrostatic interactions between charged particles play a prominent role in determining structures, dynamics, and states of these physical systems, leading to many important applications in academia and industrial communities. [3][4][5][6][7][8] An accurate description of electrostatic interactions in model systems is a nontrivial task in computer simulations. The slow decay feature of electrostatic interactions with respect to particle distance poses a significant challenge to model charged simulation systems as the computation of electrostatic interactions between charged particles is extremely time consuming.…”

“…As simulation system size expands, the calculation of electrostatic interactions becomes the major computational bottleneck for a thorough understanding of phase behaviors of charged physical systems at extended spatiotemporal scales. [1,2,4,8] A traditional way to sum electrostatic interactions between charged particles and all their infinite periodic images is the Ewald summation method. [9] By introducing a differentiable and localized function, the Ewald summation method recast the total electrostatic interaction, a single slowly and conditionally convergent series, into a short range particle-particle interaction part that can be calculated using spherical cutoff treatment in real space and a long range interaction part for smeared charges that can be computed by solving the Poisson's equation in reciprocal space.…”