Volume 73, Issue 11

A dilute suspension of uniform, non-Brownian spheres settles slowly in a viscous solvent. The initially well-mixed system showing Poisson or random occupancy statistics evolves to a system having reduced number fluctuations, but otherwise appearing random. The reduced number fluctuations are consistent with recent measurements of velocity fluctuations in settling suspensions. These experimental results test the assumptions leading to the theoretical predictions by Calflisch and Luke that the velocity fluctuations increase without limit with increasing sample dimension. The theoretical prediction assumes Poisson occupation statistics contrary to our observations.

The molecular Ornstein­Zernike (MOZ) formalism used to compute the structure of a liquid solution is briefly presented. Its ability to describe the equilibrium properties of aprotic solvents and of their electrolyte solutions is demonstrated from selected examples. The potential of mean force and the relative motion of ions in water are studied by the powerful method of intermolecular nuclear magnetic relaxation dispersion (NMRD) in paramagnetic solutions. The interest of the ion­ion dynamics in medical magnetic resonance imaging (MRI) is shown by a typical NMRD study involving paramagnetic gadolinium Gd3+ complexes.

The Poisson­Boltzmann (PB) equation is used to investigate effective colloid-interface interactions and the phase behavior of charge-stabilized colloidal suspensions. When a colloidal particle, immersed in an electrolyte, approaches an interface, which may be neutral (such as air) or charged (like electrodes, glass, etc.), image charge effects plus the deformation of the colloidal ion atmosphere by the interface lead to an effective interaction which can be attractive or repulsive, depending on the surface charge density and the dielectric constants of the interface and the electrolyte. Two cases are considered: i) a spherical particle near a like-charged interface, and ii) a rod-like particle in the vicinity of an oppositely charged interface. The latter serves as a model for the adsorption of (anionic) DNA on a cationic membrane, and it is shown that the effective attraction, induced by the release of counterions on approach of the DNA to the membrane, makes up an essential contribution to the total DNA-membrane effective interaction. To understand the phase behavior of charge-stabilized colloidal suspensions, we study a PB cell model of a bulk suspension and investigate how the PB equation can best be linearized. It is found that the previously predicted gas­liquid phase coexistence results when the PB equation is linearized about the Donnan potential. No indication of such a spinodal instability could, however, be found, when the free energy is evaluated using the numerical solution of the full PB equation. This suggests that the predicted gas­liquid phase coexistence is an artifact of the linearization.

Simulations of simple solutes (charged and uncharged spheres) in model water have been performed in order to elucidate aspects of solvation in water at ambient and supercritical states. The variation of solvation entropy as a function of solute charge has been used to investigate hydrophobic and hydrophilic ordering and the structure-making and structure-breaking effects of ions. Simulations with model solvents, which differ from water in certain features, have been used to try to identify the particular properties of water that are associated with these phenomena.

The equilibrium properties of electrolyte solutions over wide ranges of concentration, temperature, and solvent dielectric constant are discussed on a corresponding-states basis. If low-melting salts are used, these properties can be studied up to the pure fused salt. We mainly focus on systems at low reduced temperature, where the depth of the interaction potential is large compared with the thermal energy. Examples are singly charged ions in solvents of low dielectric constant and of highly charged ions in water. The state of the ions is discussed on the basis of thermodynamic, electrical conductance and dielectric constant data. Special attention is given to the transition to the fused salt, where ion clusters have to redissociate to form the dissociated structure of the salt. This transition can lead to liquid­liquid phase separations. The resulting critical points serve as important targets for testing theories. Examples are given for large deviations from corresponding-states behavior caused by specific short-range interactions.

In drawing up the specifications for a standard for multidimensional nuclear magnetic resonance spectroscopy (NMR) it became clear that the spectroscopic data content needed to be qualified by experimental condition information especially pertaining to the pulse sequences used to obtain the free induced decays or spectra. Failure to include this information not only severely inhibits the ability of subsequent data handling packages to work with the experimental data, but also makes interpretation of the final results virtually impossible. This paper has been produced in collaboration with the NMR spectrometer manufacturers in an attempt to get agreement on a definitive list of the most frequently used pulse sequence programs. The list includes entries where common agreement has been reached as to the acronym to name the experiment and the key instrument independent parameters needed to report concisely. It is not intended to restrict in any way the freedom of manufacturers or users to develop new and novel experimental pulse sequences, but should aid reporting of experimental data where the more common sequences are in use.

The relatively young field of ion mobility spectrometry has now advanced to the stage where the need to reliably exchange the spectroscopic data obtained worldwide by this technique has become extremely urgent. To assist in the validation of the various new spectrometer designs and to assist in inter-comparisons between different laboratories reference data collections are being established for which an internationally recognized electronic data exchange format is essential. To make the data exchange between users and system administration possible, it is important to define a file format specially made for the requirements of ion mobility spectrometry. The format should be computer readable and flexible enough for extensive comments to be included. In this document, we define a data exchange format, agreed on by a working group of the International Society for Ion Mobility Spectrometry at Hilton Head Island, USA (1998) and Buxton, UK (1999). This definition of this format is based on the IUPAC JCAMP-DX protocols, which were developed for the exchange of infrared spectra [1] and extended to chemical structures [2], nuclear magnetic resonance data [3], and mass spectra [4]. This standard of the Joint Committee on Atomic and Molecular Physical Data is of a flexible design. The International Union of Pure and Applied Chemistry have taken over the support and development of these standards and recently brought out an extension to cover year 2000 compatible date strings and good laboratory practice [5]. The aim of this paper is to adapt JCAMP-DX to the special requirements of ion mobility spectra [6].

New values of electrolytic conductivity were determined for aqueous KCl solutions with molalities of 0.01, 0.1, and 1.0 mol/kg in the temperature range 0 to 50 °C, at 5 K intervals. Expanded uncertainties, 2 u c , were also calculated in accordance with the presently accepted protocol for the treatment of uncertainty. The new conductivity values are recommended as primary standards of electrolytic conductivity based on molality. They replace the previous values, based on the nonstandard demal scale, which were determined only at 0, 18, and 25 °C. The accuracy of the technique used was evaluated by repeating the determination of the previously recommended demal-based IUPAC standards of electrolytic conductivity and through comparison with other absolute measurements.

A unified scale is recommended for reporting the NMR chemical shifts of all nuclei relative to the 1 H resonance of tetramethylsilane (TMS). The unified scale is designed to provide a precise ratio, Ξ , of the resonance frequency of a given nuclide to that of the primary reference, the 1H resonance of TMS in dilute solution (volume fraction, φ < 1%) in chloroform. Referencing procedures are discussed, including matters of practical application of the unified scale. Special attention is paid to recommended reference samples, and values of Ξ for secondary references on the unified scale are listed, many of which are the results of new measurements. Some earlier recommendations relating to the reporting of chemical shifts are endorsed. The chemical shift, δ , is redefined to avoid previous ambiguities but to leave practical usage unchanged. Relations between the unified scale and recently published recommendations for referencing in aqueous solutions (for specific use in biochemical work) are discussed, as well as the special effects of working in the solid state with magic-angle spinning. In all, nine new recommendations relating to chemical shifts are made. Standardized nuclear spin data are also presented in tabular form for the stable (and some unstable) isotopes of all elements with nonzero quantum numbers. The information given includes quantum numbers, isotopic abundances, magnetic moments, magnetogyric ratios and receptivities, together with quadrupole moments and line-width factors where appropriate.