Modelling of molecular light scattering

The scattering of light originates from the fluctuation in the permittivity that gives rise to a turbidity of the fluid. The turbidity of an otherwise homogeneous fluid is, according to Einstein's theory, proportional to the mean-square fluctuation in the relative permittivity. A general expression for the mean-square fluctuation or variance of a fluctuating property is derived. It shows that the turbidity depends on the second derivatives of an appropriately chosen state function – the choice is dependent on which properties that fluctuate. The paper provides a general derivation and outlines the criteria for the choice of state function.Data for light-scattering are modelled using the Clausius–Mossotti equation and an excess Gibbs energy function. The excess Gibbs energy applied consists of three terms: Flory–Huggins (accounting for the size of the components), Debye–Hückel (accounting for the charges of electrolytes) and Kirkwood (accounting for the dipolar nature of proteins). The present investigation shows that the model parameters determined by light-scattering experiments have a well-defined physical significance and light-scattering data can thus be used in parallel with other data to provide information from which model parameters can be estimated.

► A general expression for the mean-square fluctuation or variance of a fluctuating property is derived.
► Data for light-scattering are modelled using the Clausius–Mossotti equation and an excess Gibbs energy function.
► The model parameters determined by light-scattering experiments have a well-defined physical significance.
► Light-scattering data can be used in parallel with other data to provide information from which model parameters can be estimated.