  In the scientific literature little attention has been given to the use of
dynamic light scattering (DLS) as a tool for extracting the thermodynamic
information contained in the absolute intensity of light scattered by gels. In
this article we show that DLS yields reliable measurements of the intensity of
light scattered by the thermodynamic fluctuations, not only in aqueous polymer
solutions, but also in hydrogels. In hydrogels, light scattered by osmotic
fluctuations is heterodyned by that from static or slowly varying
inhomogeneities. The two components are separable owing to their different time
scales, giving good experimental agreement with macroscopic measurements of the
osmotic pressure. DLS measurements in gels are, however, tributary to
depolarised light scattering from the network as well as to multiple light
scattering. The paper examines these effects, as well as the instrumental
corrections required to determine the osmotic modulus. For guest polymers
trapped in a hydrogel the measured intensity, extrapolated to zero
concentration, is identical to that found by static light scattering from the
same polymers in solution. The gel environment modifies the second and third
virial coefficients, providing a means of evaluating the interaction between
the polymers and the gel.
