Structure and nanostructure in ionic liquids.

Ionic liquids (ILs) offer a wide range of promising applications because of their much enhanced properties. However, further development of such materials depends on the fundamental understanding of their hierarchical structures and behaviors, which requires multiscale strategies to provide coupling among various length scales. In this review, we first introduce the structures and properties of these typical ILs. Then, we introduce the multiscale modeling methods that have been applied to the ILs, covering from molecular scale (QM/MM), to mesoscale (CG, DPD), to macroscale (CFD for unit scale and thermodynamics COSMO-RS model and environmental assessment GD method for process scale). In the following section, we discuss in some detail their applications to the four scales of ILs, including molecular scale structures, mesoscale aggregates and dynamics, and unit scale reactor design and process design and optimization of typical IL applications. Finally, we address the concluding remarks of multiscale strat...

Multiscale Studies on Ionic Liquids

Bromine–oxygen radial distribution functions [g(r)] have been calculated by means of molecular dynamics simulations for aqueous solutions of rubidium bromide, 2‐bromopropane and bromoethane. X‐ray absorption spectra at the bromine K edge have been recorded for these solutions. The water contribution to the extended x‐ray absorption fine structure spectra has been calculated starting from the gBr,O(r) distribution function. Fits of the x‐ray absorption spectra have been performed directly on the raw experimental data, allowing the reliability of the g(r) distribution functions to be verified. The agreement between theoretical and experimental spectra is satisfactory. A procedure to improve model g(r) functions on the basis of the short‐range structural information provided by extended x‐ray absorption fine structure data is proposed.

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An extended x‐ray absorption fine structure study of aqueous solutions by employing molecular dynamics simulations

The vast majority of the molecular, crystalline or liquid structures of groups V, VI and VII of the periodic table and their compounds obey the octet rule (Z = 8 - N). The structure and stability of those structures are discussed in a simple tight-binding approximation. In this framework we show that the Peierls electronic instability of a simple cubic structure leads to the octet rule. This instability does not rely upon the periodicity and consequently may occur in crystalline, amorphous or liquid matter. In a general discussion on the stability of covalent structures, we show that the existence of a Peierls distortion is governed by the balance between the attractive (band) term and the empirical repulsive term. As the hardness of the latter increases when going down the periodic table, this explains why the Peierls distortion is stronger for the light elements.

Peierls instabilities in covalent structures I. Electronic structure, cohesion and the Z = 8 – N rule

Chapter 2. Low-frequency spetroscopic studies of interactions in liquids

The study of spatial features in diatomic liquids and gases is reviewed with a specific emphasis on experimental measurements using neutron and X-ray diffraction. The formalism for defining the pair-correlation function is described and related to the liquid structure factor which can be extracted from the diffraction measurements. Different molecular systems can be classified in terms of the intermolecular potential which depends on various factors related to geometrical conformation, electron density distribution and anisotropy. The increased complexity over monoatomic systems gives rise to new phenomena associated with the orientational correlation of molecular axes. The techniques of neutron and X-ray diffraction are critically reviewed and a full list of published work (plus some unpublished results) is presented. The experimental measurements are analysed and compared with the results of computer simulations using model potentials. It is noted that several systems (e.g. N2, O2, F2, CO) have a relatively simple behaviour but others (e.g. Cl2, Br2, I2) are only just on the point of being properly understood, while the hydrogen-bonded liquids (e.g. HF, HCl) still pose many problems that currently remain unresolved. The review ends with a brief consideration of likely developments in the foreseeable future.

https://iopscience.iop.org/article/10.1088/0034-4885/54/5/002/pdf

F P Ricci

Papers

Structural characterization of diatomic fluids by diffraction studies