A linearized hydrodynamic description of concentration fluctuations in ionic solutions is formulated and is used to provide a phenomenological theory of light scattering. Our formulation takes ionic effects into account and using model calculations we are able to show that Rayleigh scattering from charge fluctuations in simple 1–1 ionic solutions should be negligible in the usual light scattering experiments. This then justifies the previous treatment of an electrolyte as a single nonionic solute species in comparisons with light scattering and explains the failure to detect Rayleigh scattering from charge fluctuations in these experiments. This approach is extended to calculate Rayleigh-to-Brillouin intensity ratios and we find that for NaCl the neglect of charge fluctuations is within experimental error. We expect this to be less satisfactory for LiCl. For 2–2 salts the neglect of ionic effects is clearly inadequate and our study indicates the appropriate theoretical description. Our estimates of the ionic contributions exhibit some improvement in comparisons to experiment but more detailed calculations await theoretical characterization of appropriate model electrolyte systems.
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Pailthorpe et al. (1977) studied this question.
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