Structure and Dynamics in Liquids

Structure and Dynamics in Liquids

Pure and Applied Chemistry

Vol. 76, No. 1, pp. 1–261 (2004)

reviewed by Roberto Fernández-Prini

A joint meeting of the European and Japanese Molecular Liquid Groups (EMLG and JMLG) was held 7 to 15 September 2002 in Rhodes, Greece, under the auspices of IUPAC. The theme of the meeting was Novel Approaches to the Structure and Dynamics in Liquids: Experiments, Theories, and Simulation. It was organized in parallel with a NATO Advanced Study Institute, which meant that scientists as well as students attended.

Twenty-nine papers from the meeting have been published in Pure and Applied Chemistry, 76 (1), 1–261 (2004). The main objective of these contributions was to obtain descriptions of the behavior of liquids and their solutions at the molecular level; however, more classical information (e.g., thermodynamic properties) was also reported in many of them. It is interesting to note that the majority of the published works employed molecular simulation tools to back their experimental or theoretical results. This is a reasonable and healthy trend in physics and chemistry.

The papers cover many different molecular liquids and their solutions, new techniques, and theoretical methods. They discuss very simple systems, like mixtures of isotopomers, to more complicated ones including polymeric liquids and interfacial structure, as well as ionic liquids and supercritical fluids.

The two latter types of solvents are of great practical importance at present because of their potential use in many chemical and other industrial processes due to their benign effect on the environment. Looking at the set of published conferences, the evolution of the field of molecular liquids becomes quite clear; the present scenario is quite different from that observed one or two decades ago. Now molecules and their interactions are the main actors and understanding their detailed behavior is an important goal. In turn this will lead to knowledge about how their properties can be controlled to serve more closely the purpose of solvents in different processes.

A paper by G. Jancsó analyzes the possibility of nonideal behavior in isotopomer mixtures. Jancsó’s study yielded information about intermolecular forces as well as the effect of the different volumes (or sizes) of the isotopomers. Excess thermodynamic properties of the mixtures, including vapor pressure, were used as a basis to estimate the nonideality of the mixture.

V. A. Durov’s paper describes the performance of quasichemical models to describe associative processes (supramolecular ordering). In his study, interactions were separated into different types and the formation of different kinds of aggregates was considered. In his paper, he shows that this model can be used to account for the dielectric behavior of mixtures, as well as light scattering, and that it can describe dynamic properties.

Four articles analyzed the effect of long-range fluctuations, which are typical of critical phenomena, on the behavior of different systems. W. Schröer and H. Weingärtner discussed, within the restricted primitive model, the features of the critical point for ionic systems that interact through long-range coulombic forces. They showed that solutions of ionic liquids in relatively low dielectric constant solvents behave similarly to more conventional electrolytes. M. Besnard and coworkers contributed two other papers referring to critical phenomena in fluids. They used spectroscopic techniques to study the behavior of the fluids under near-critical conditions that are relevant to the use of supercritical fluids in many processes. Using IR spectroscopy and incoherent neutron scattering they suggested the presence of aggregates, up to trimers, in water at 380 ºC at different densities. In the second paper they studied local density inhomogeneities in pure hexafluorbenzene employing Raman spectroscopy; they reported the existence of these inhomogeneities in the pure fluid. The contribution of M. Musso et al. reported the effect of the critical line broadening of Raman bands for fluid N₂. This study showed the high sensitivity of the Raman band to changes in intermolecular interactions. Long-range fluctuations of fluid density affect more the line shape than the line width.

Clusters of water were the subjects of two contributions. A. Vegiri used molecular simulation to analyze the enhanced relaxation of some water clusters in the presence of weak electric fields. The effect was attributed to an increase in the vibrational amplitude of individual water molecules. J. M. Bowman and S. S. Xantheas described the details of the vibrational modes of Cl-(H₂O). Their approach suggests it is necessary to revise interacting potentials that had been developed previously.

H. Krienke’s paper discussed his studies of thermodynamic and structural properties of molecular liquids and electrolyte solutions using integral equations and simulation experiments. The paper describes ion solvation and association in several solvents of different polarity, underlining the important role of polarizability. N. Nishiyama et al. reported the results of the study of the solvation dynamics of ions dissolved in acetonitrile using the reference interaction-site model together with mode-coupling theory. Molecular dynamics was also employed to study structure of ionic liquids and, in another contribution, the same simulation procedure was used to calculate time correlation functions for methanol.

A number of papers refer to the use of modern experimental techniques. The contributions of M. G. Giorgini and H. Torii describe the use of the Raman non-coincidence effect. This is a novel spectroscopic method that provides a very sensitive experimental tool to discriminate between dipolar and hydrogen bonding intermolecular interactions. In another paper, E. Pontecorvo et al. present another new spectroscopic technique, which consist of X-ray inelastic scattering to study vibrational dynamics in molecular liquids. This method was observed to be sensitive to vibrational dynamics at length scales typical of intermolecular distances; the liquid studied was glycerol at ambient temperature. Mizuno et al. reported the development of an external reference method of NMR that obtained values of the chemical shifts on a unified scale; results for water and methanol are presented.

Two other papers describe the use of modern techniques to study the structure in solutions. EXAFS was used to describe the solvation of ions dissolved in water and alcohols and ultrasonic birefringence was used to study polymer mixtures.

Several papers describe the use of the molecular dynamic simulation technique, either classical or hybrid, to describe structural and dynamic features of solutions. Other papers report the results of using spectroscopic techniques, light-scattering, transport of ions in dense fluids, interfacial proton transfer, and structure and dynamics in molten zinc and manganese halides. Cu phthalocyanines have been studied by resonance Raman spectroscopy to evaluate the effect of substituents and the aqueous medium on the aggregation and photochemical properties of these molecules.

In summary, the EMLG-JMLG meeting was successful having fulfilled to a large extent their ambitious goal. Many modern theoretical, experimental, and simulation results were presented, showing that scientists in this field, which is extremely active, are able nowadays to describe very subtle effects that are important for the structure and the dynamics in liquid phase, either for pure liquids or for mixtures.

Roberto Fernández-Prini <rfprini@cnea.gov.ar> is from the Comisión Nacional de Energía Atómica, in Buenos Aires, Argentina, and is a member of the IUPAC Physical and Biophysical Chemistry Division Committee.

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