Scattered-light correlation spectroscopy of structural changes in aqueous systems

Abstract

Studies of the properties of aqueous systems have shown water to be an extremely unusual liquid, not readily amenable to experimental investigation or simulation. In view of this, many models of liquid water are used to elucidate various experimental facts [i, 2]. The models proposed for describing various properties of liquid water are customarily divided into two classes, continuous and mixed. In continuous models the main structure of liquid water is assumed to be a tetrahedral network of hydrogen bonds and the differences in the states of the water are attributable to the distortion of the H-bond geometry. The most co~nonly used of the several mixed models is the cluster model, according to which dodecahedral frameworks can be linked together by hydrogen bonds, i.e., can form groups that have an ordered structure. According to [2J, in this case the water contains portions with highly developed hydrogen bands, which alternate with regions where hydrogen bonds are only partial or are absent altogether. We studied the structural rearrangement of water when heated and when ethanol was dissolved in it. The objects studied were twice-distilled water (with electrical conductivity K = 1.2"10 -6 ~-i-cm-i)iandethanol, purified by the standard method [3]. Its purity was checked by comparing the UV absorption spectra with the spectra of standard samples [3]. Information about the dynamic parameters of scatters can, in principle, be obtained by recording the time variations (dynamics) of the amplitude and phase (frequency) of scattered light. The time variations of the parameters of quasielastically scattered radiation manifest themselves in variations of its correlation function. Accordingly, we used methods of scattered-light correlation spectroscopy to study the properties of aqueous systems. The studies were carried out with a monodyne spectrometer. A helium-laser (~ = 632.8 nm) was the light source. Its beam was directed onto the lateral surface of a glass cell with a i0  10-mm cross section, containing the liquid under study. The intensity of the scattered light was recorded at a 90 ~ angle with an F~U-79 photomultiplier. To ensure that all of the light recorded by the detector would be coherent, i.e., that all the waves scattered by different particles of the object under study and entering the detector would be cophasal, the radiation was focused and the solid angle of collection of the scattered radiation was limited by diaphragms. The radiation was recorded in the photon counting regime. The data were processed by the correlation-spectral method [4], which ended with the calculation of the correlation function