The Experimental Study of Liquid Structure

The structure of pure liquids and liquid solutions is conveniently studied using diffraction techniques [5]. The most common of these is X-ray diffraction. Two other useful techniques are neutron and electron diffraction. In these experiments, radiation, which is usually monoenergetic, penetrates the liquid sample and is scattered through an angle 0 (see fig. 2.6). The analytical information is obtained by studying the intensity of the scattered radiation as a function of this angle. In 

X-ray diffracLion, Lhe X-rays are scattered by the electron clouds around individual atoms. Since the atoms and molecules of the liquid sample are not fixed in space, the information resulting from the diffraction experiment must be interpreted in terms of statistical averages. The neutrons used in a neutron diffraction experiment are scattered by the nuclei of the atoms in the liquid sample so that the scattering pattern is quite different from that for X-rays. In electron diffraction, the electrical potential, which depends on the spatial configuration of the nuclei and electronic density distribution, determines the diffraction pattern. Early experiments involved simple monoatomic liquids such as the inert gases and liquid metals. However, many molecular liquids have also been studied, including polar liquids such as water, the alcohols, and amides [5]. In this section, attention is focused on two of these techniques, namely. X-ray and neutron diffraction. 

Consider first of all the scattering of neutrons by the nuclei in a monoatomic liquid. This process is characterized by a scattering cross-section, Qq, which, in turn, is related to Z d, the bound atom scattering length. For slow neutrons. 

The relationship between the scattered intensity and the structure factor for a monoatomic liquid is 

At very large angles, the structure factor goes to unity (see fig. 2.6) 

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Chemists are obviously concerned mainly with liquids in the last three groups. However, they are the most difficult to model from the point of view of theory. Much of the theoretical effort has been directed to understanding the properties of the simplest liquids, namely, the inert gases. In the following sections, the statistical mechanical approach developed to understand liquid properties is outlined. The purpose of this subject is to establish a connection between the properties of the individual atoms or molecules in the liquid and the bulk properties of the system. An important part of this development is the experimental study of liquid structure which is also outlined in the following discussion. 

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