Baofu Qiao

Effect of Anions on Static Orientational Correlations, Hydrogen Bonds, and Dynamics in Ionic Liquids: A Simulational Study

Three different ionic liquids are investigated via atomistic molecular dynamics simulations using the force field of Lopes and PAdua (J. Phys. Chem. B 2006, 110, 19586). In particular, the 1-ethyl-3-methylimidazolium cation EMIM+ is studied in the presence of three different anions, namely, chloride Cl-, tetrafluoroborate BF(4)(-), and bis((trifluoromethyl)sulfonyly)imide TF2N-. In the focus of the present study are the static distributions of anions and cations around a cation as a function of anion size. It is found that the preferred positions of the anions change from being close to the imidazolium hydrogens to being above and below the imidazolium rings. Lifetimes of hydrogen bonds are calculated and found to be of the same order of magnitude as those of pure liquid water and of some small primary alcohols. Three kinds of short-range cation-cation orderings are studied, among which the offset stacking dominates in all of the investigated ionic liquids. The offset stacking becomes weaker from [EMIM][Cl] to [EMIM][BF4] to [EMIM][TF2N]. Further investigation of the dynamical behavior reveals that cations in [EMIM][TF2N] have a slower tumbling motion compared with those in [EMIM][Cl] and [EMIM][BF4] and that pure diffusive behavior can be observed after 1.5 ns for all three systems at temperatures 90 K above the corresponding melting temperatures.

A comparative study of two classical force fields on statics and dynamics of [EMIM][BF4] investigated via molecular dynamics simulations

The influences of two different commonly employed force fields on statical and dynamical properties of ionic liquids are investigated for [EMIM][BF(4)]. The force fields compared in this work are the one of Canongia Lopes and Padua [J. Phys. Chem. B 110, 19586 (2006)] and that of Liu et al. [J. Phys. Chem. B 108, 12978 (2004)]. Differences in the strengths of hydrogen bonds are found, which are also reflected in the static ion distributions around the cation. Moreover, due to the stronger hydrogen bonding in the force field of Liu et al., the diffusive motions of cations and anions and the rotational behavior of the cations are slower compared with those obtained with the force field of Canongia Lopes and Padua. Both force fields underestimate the zero-field electrical conductivity, while the experimental dielectric constant can be reproduced within the expected statistical error boundaries.

Molecular crystallization controlled by pH regulates mesoscopic membrane morphology.

Coassembled molecular structures are known to exhibit a large variety of geometries and morphologies. A grand challenge of self-assembly design is to find techniques to control the crystal symmetries and overall morphologies of multicomponent systems. By mixing +3 and -1 ionic amphiphiles, we assemble crystalline ionic bilayers in a large variety of geometries that resemble polyhedral cellular crystalline shells and archaea wall envelopes. We combine TEM with SAXS and WAXS to characterize the coassembled structures from the mesoscopic to nanometer scale. The degree of ionization of the amphiphiles and their intermolecular electrostatic interactions are controlled by varying pH. At low and high pH values, we observe closed, faceted vesicles with two-dimensional hexagonal molecular arrangements, and at intermediate pH, we observe ribbons with rectangular-C packing. Furthermore, as pH increases, we observe interdigitation of the bilayer leaflets. Accurate atomistic molecular dynamics simulations explain the pH-dependent bilayer thickness changes and also reveal bilayers of hexagonally packed tails at low pH, where only a small fraction of anionic headgroups is charged. Coarse-grained simulations show that the mesoscale geometries at low pH are faceted vesicles where liquid-like edges separate flat crystalline domains. Our simulations indicate that the curved-to-polyhedral shape transition can be controlled by tuning the tail density in regions where sharp bends can form the polyhedral edges. In particular, the pH acts to control the overall morphology of the ionic bilayers by changing the local crystalline order of the amphiphile tails.

Structure of 1-Butylpyridinium Tetrafluoroborate Ionic Liquid: Quantum Chemistry and Molecular Dynamic Simulation Studies†

Density functional theory (DFT) calculations combined with molecular dynamic (MD) simulations have been performed to show in detail the structure characteristic of 1-butylpyridinium tetrafluoroborate ([BPy(+)][BF(4)(-)]), a representative of pyridinium-based ionic liquids (ILs). It is found that the relative stability for ion pair configurations is synergically determined by the electrostatic attractions and the H-bond interactions between the ions of opposite charge. [BPy(+)][BF(4)(-)] IL possesses strong long-range ordered structure with cations and anions alternately arranging. The spatial distributions of anions and cations around the given cations are clearly shown, and T-shaped orientation is indicated to play a key role in the interaction between two pyridine rings. DFT calculations and MD simulations uniformly suggest that the H-bonds of the fluorine atoms with the hydrogen atoms on the pyridine rings are stronger than those of the fluorine atoms with the butyl chain hydrogens. The present results can offer useful information for understanding the physicochemical properties of [BPy(+)][BF(4)(-)] IL and further designing new pyridinium-based ILs.

How Hydrogen Bonds Affect the Growth of Reverse Micelles around Coordinating Metal Ions

Extensive research on hydrogen bonds (H-bonds) have illustrated their critical role in various biological, chemical and physical processes. Given that existing studies are predominantly performed in aqueous conditions, how H-bonds affect both the structure and function of aggregates in organic phase is poorly understood. Herein, we investigate the role of H-bonds on the hierarchical structure of an aggregating amphiphile-oil solution containing a coordinating metal complex by means of atomistic molecular dynamics simulations and X-ray techniques. For the first time, we show that H-bonds not only stabilize the metal complex in the hydrophobic environment by coordinating between the Eu(NO3)3 outer-sphere and aggregating amphiphiles, but also affect the growth of such reverse micellar aggregates. The formation of swollen, elongated reverse micelles elevates the extraction of metal ions with increased H-bonds under acidic condition. These new insights into H-bonds are of broad interest to nanosynthesis and biological applications, in addition to metal ion separations.

Study of 1,3-dimethylimidazolium chloride with electronic structure methods and force field approaches.

The 1,3-dimethyl imidazolium chloride [MMIM]Cl is an example of ionic liquid and frequently studied in literature. In this article [MMIM]Cl is studied using an ab initio method [second order Moller-Plesset perturbation theory (MP2), density functional theory (DFT)] and classical force field approach with the aim of looking at some properties on different scales. Selected properties are studied with the different methods and compared to each other. The comparison between the results obtained with MP2 and the DFT approach allows us to comment on the validity of this latter and thus on its employment in larger systems. On the other hand, the comparison between the DFT results and those of the classical approach allows us to test the reproducibility of electrostatic properties by this latter approach. As the results show the used DFT setup is rather satisfactory, while the classical force fields are describing the electrostatic properties in an insufficient way. A revision (improvement) of the classical force fields is at this stage necessary in order to capture the electrostatic properties in a proper way.

Molecular Origins of Mesoscale Ordering in a Metalloamphiphile Phase

Controlling the assembly of soft and deformable molecular aggregates into mesoscale structures is essential for understanding and developing a broad range of processes including rare earth extraction and cleaning of water, as well as for developing materials with unique properties. By combined synchrotron small- and wide-angle X-ray scattering with large-scale atomistic molecular dynamics simulations we analyze here a metalloamphiphile–oil solution that organizes on multiple length scales. The molecules associate into aggregates, and aggregates flocculate into meso-ordered phases. Our study demonstrates that dipolar interactions, centered on the amphiphile headgroup, bridge ionic aggregate cores and drive aggregate flocculation. By identifying specific intermolecular interactions that drive mesoscale ordering in solution, we bridge two different length scales that are classically addressed separately. Our results highlight the importance of individual intermolecular interactions in driving mesoscale ordering.

Crystalline polymorphism induced by charge regulation in ionic membranes

Significance The crystallization of molecules with polar and hydrophobic groups, such as ionic amphiphiles and proteins, is of paramount importance in biology and biotechnology. This combined X-ray scattering and theoretical study demonstrates how crystalline order within membranes formed by coassembled cationic and anionic amphiphiles can be controlled by varying pH and molecular tail length. Our work suggests design of bilayer membranes with specific crystalline arrangements at ambient temperature and physiologically relevant pH environments with suitable choice of headgroup and tail. Changes in crystallinity are likely to affect molecular diffusion rates across membranes and may enable control over the encapsulation and release of molecules within the membrane. Moreover, pH-induced crystalline transformations are likely used by organisms to control metabolic flow in harsh environments. The crystallization of molecules with polar and hydrophobic groups, such as ionic amphiphiles and proteins, is of paramount importance in biology and biotechnology. By coassembling dilysine (+2) and carboxylate (–1) amphiphiles of various tail lengths into bilayer membranes at different pH values, we show that the 2D crystallization process in amphiphile membranes can be controlled by modifying the competition of long-range and short-range interactions among the polar and the hydrophobic groups. The pH and the hydrophobic tail length modify the intermolecular packing and the symmetry of their crystalline phase. For hydrophobic tail lengths of 14 carbons (C14), we observe the coassembly into crystalline bilayers with hexagonal molecular ordering via in situ small- and wide-angle X-ray scattering. As the tail length increases, the hexagonal lattice spacing decreases due to an increase in van der Waals interactions, as demonstrated by atomistic molecular dynamics simulations. For C16 and C18 we observe a reentrant crystalline phase transition sequence, hexagonal–rectangular-C–rectangular-P–rectangular-C–hexagonal, as the solution pH is increased from 3 to 10.5. The stability of the rectangular phases, which maximize tail packing, increases with increasing tail length. As a result, for very long tails (C22), the possibility of observing packing symmetries other than rectangular-C phases diminishes. Our work demonstrates that it is possible to systematically exchange chemical and mechanical energy by changing the solution pH value within a range of physiological conditions at room temperature in bilayers of molecules with ionizable groups.

Description of ionic surfactant/water system by adjusting mesoscopic parameters

Dissipative particle dynamics simulations were utilized to simulate a model surfactant solution-air system. Amphiphilic surfactant molecules were modeled as dimers composed of a hydrophilic head and a hydrophobic tail. With a simple model, the influence of conservative interaction parameters on the surfactants properties, including surfactant efficiency and critical micelle concentration (CMC), was investigated in the present research. It is not the surfactant total concentration, but the bulk concentration, that should be employed to achieve the right surfactant properties. It is found that the adjustment of interaction between water and head or air and tail (a(WH) or a(AT)) will result in the obvious change in surfactant efficiency. The parameter that affects CMC the most significantly is the interaction between water and tail (a(WT)). On the basis of the findings about the relationship between conservative interaction parameters and surfactant behaviors, we varied the interaction parameters and simulated a real ionic surfactant system with different tail lengths.

Driving force for crystallization of anionic lipid membranes revealed by atomistic simulations.

Crystalline vesicles are promising nanomaterials due to their mechanical stability in various environments. To control their fabrication, it is essential to understand the effects of different experimental conditions on crystallization. Here we perform atomistic molecular dynamics simulations of anionic lipid membranes of 1,2-dilauroyl-sn-glycero-3-phosphol-L-serine. In the presence of Na(+) monovalent counterions, we access the phase transition from the liquid-like disordered liquid-crystalline phase to the ordered gel phase by lowering the temperature of the system. The phase transition is conclusively evidenced by the scattering structure factor. Quantitative calculations show that the enhancement of the intertail van der Waals interaction (about -6 k(B)T) plays a dominant role in driving the phase transition rather than the increase in the cohesive interaction (-0.5 k(B)T) between lipids and counterions. Meanwhile, in the presence of multivalent counterions of Zn(2+) or La(3+) the gel phase is found throughout the temperature range investigated. Moreover, the van der Waals interaction per hydrocarbon group is ∼20% stronger in the gel phase (∼ -1.8 k(B)T regardless of the counterions) than in the liquid-crystalline phase (-1.5 k(B)T).