The visible absorption spectra of methylene blue (MB) and thionine (Th) in aqueous solutions and in poly-2-
hydoxyethyl methacrylate hydrogel (PHEMA) matrix were studied at room temperature. The optical spectra
of the dye-loaded hydrogel in presence of surfactants (SDS and CTAB) were also investigated. The transport
and the solute–solute interactions of the ionic dyes in aqueous solutions across the hydrophilic gel were
investigated. The nature of the interacting pairs in these dyes was discussed using the exciton theory.
