THEORY OF QUASIELASTIC LIGHT SCATTERING FROM CHEMICALLY REACTIVE IONIC SOLUTIONS

The intensity I(ks,ω) as a function of direction ks and frequency ω of scattered light from a solution containing dilute ionic reactants in equilibrium (e.g., Zn2+ + SO42- ⇄ ZnSO)4 is derived for the circumstance where fluctuations in polarizability arise predominantly from fluctuations in reactant concentrations. The reaction is assumed to be coupled to temperature and pressure fluctuations in the usual way through its standard enthalpy and volume changes. The irreversible processes of chemical reaction, thermal conduction, viscosity, particle diffusion, and ohmic conduction are assumed to be responsible for relaxing the various fluctuations. Expressions for both absolute magnitude and frequency dependence of I(ks,ω) are given in appropriate limits. It is found that particle diffusion dominates the power spectrum for the case of fast reactions observed at low frequencies. Coupling of the chemical reaction to fluctuations in T and P gives rise to Brillouin doublets at high frequencies in the fast reaction limit. The fluctuations in charge density in the solution are damped much more rapidly by ohmic conduction than by particle diffusion, and this relaxation gives a contribution to the Rayleigh line which has a dispersion at very high frequencies ∼109-1010 cps in 0.1 M solutions.