The techniques of neutron diffraction and x-ray diffraction, as applied to structural studies of liquids and glasses, are reviewed. Emphasis is placed on the explanation and discussion of the experimental techniques and data analysis methods, as illustrated by the results of representative experiments. The disordered, isotropic nature of the structure of liquids and glasses leads to special considerations and certain difficulties when neutron and x-ray diffraction techniques are applied, especially when used in combination on the same system. Recent progress in experimental technique, as well as in data analysis and computer simulation, has motivated the writing of this review.

Neutron diffraction experiments were carried out on aqueous solutions containing either guanidinium or thiocyanate ions. The first-order difference method of neutron diffraction and isotopic substitution was applied, and the hydration structures of two of nature's strongest denaturant ions were determined. Each ion is shown to interact weakly with water: Guanidinium has no recognizable hydration shell and is one of the most weakly hydrated cations yet characterized. Hydration of thiocyanate is characterized by a low coordination number involving around one hydrogenbonded water molecule and approximately two water molecules weakly interacting through ''hydration bonds.'' The weak hydration of these denaturant ions strongly supports suggestions that a major contribution to the denaturant effect is the preferential interaction of the denaturant with the protein surface. By contrast, solute species such as many sugars and related polyols that stabilize proteins are strongly hydrated and are thus preferentially retained in the bulk solvent and excluded from the protein surface. J ust over a century ago, Hofmeister (1) arranged ions into a series based on their ability to salt out proteins. Since then, others have extended this approach and have used it to characterize the ability of particular ions to nature or denature biological material (2-5). In this article we provide convincing evidence from neutron diffraction and isotopic substitution experiments that, we believe, goes a significant way to rationalizing the differing behavior of certain ions in biological solution. We came to this conclusion by focusing on two of nature's strongest denaturant ions [guanidinium (Gdm ϩ ) and thiocyanate (SCN Ϫ )] and by referring to previously published results derived from neutron diffraction and isotopic substitution. The method was first introduced in the 1970s and has been used to obtain structural information at the atomic level around specific atoms and ions in solution (6). In contrast to other methods, it is formally exact and does not require sophisticated modeling procedures for the determination of properties, such as nearestneighbor coordination numbers and ion-water geometry.Specifically, the article addresses the question of what the differences are, if any, in the hydration structure of ions whose denaturation properties have been arranged in a series according to their ability to denature or stabilize proteins. The results we present below on the hydration structure of Gdm ϩ and SCN Ϫ have relevance to the mechanics of ion denaturation of biologically active material in aqueous electrolyte solution. The Gdm ϩ cation is the most powerful protein denaturant commonly used in studies of protein stability and folding (5), despite the fact that the detailed mechanism of its denaturation properties remains unresolved. Gdm ϩ ions bind to polypeptides, and this binding is expected to provide a significant contribution to protein denaturation because, on unfolding, many buried peptide backbone amides, hydrogen-bonded with...

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27The structure of liquid alumina at a temperature ≈2400 K near to its melting point was measured using 28 neutron and high-energy x-ray diffraction by employing containerless aerodynamic-levitation and laser-29 heating techniques. The measured diffraction patterns were compared to those calculated from molecular 30 dynamics simulations using a variety of pair potentials, and the model found to be in best agreement with 31 experiment was refined by using the reverse Monte Carlo (

Acoustic levitation has the potential to enable novel studies due to its ability to hold a wide variety of substances against gravity under container-less conditions. It has found application in spectroscopy, chemistry, and the study of organisms in microgravity. Current levitators are constructed using Langevin horns that need to be manufactured to high tolerance with carefully matched resonant frequencies. This resonance condition is hard to maintain as their temperature changes due to transduction heating. In addition, Langevin horns are required to operate at high voltages (>100 V) which may cause problems in challenging experimental environments. Here, we design, build, and evaluate a single-axis levitator based on multiple, low-voltage (ca. 20 V), well-matched, and commercially available ultrasonic transducers. The levitator operates at 40 kHz in air and can trap objects above 2.2 g/cm density and 4 mm in diameter whilst consuming 10 W of input power. Levitation of water, fused-silica spheres, small insects, and electronic components is demonstrated. The device is constructed from low-cost off-the-shelf components and is easily assembled using 3D printed sections. Complete instructions and a part list are provided on how to assemble the levitator.