Anomalous tracer diffusion in film forming colloidal dispersions

Film forming colloidal dispersions can be conceived as a material composed of interpenetrating hydrophobic (polymer) and hydrophilic (partially broken interfaces) phases where the transport properties of one phase are influen ced by the geometric confinement effect imposed by the other. We studied th e transport properties of film forming colloidal dispersions by introducing hydrophobic dye molecules into the colloidal particles and determining the ir motion with forced Rayleigh Scattering as a function of length scale (gr ating distance Lambda) and water content. At water contents between 18 and 3 weight percent we find signatures of anomalous tracer diffusion, namely s tretched exponential decay curves with relaxation times which significantly deviate from the q(2)-dependence (q(2) = 4 pi(2)/Lambda(2)) of Fickian dif fusion. The form of the q-dependence is contrary to what could be expected from a simple confinement model. Analyzing the results in terms of a length scale dependent effective diffusion coefficient we find that diffusion on large length scales proceeds faster than on small length scales by nearly o ne order of magnitude. We attempt to interpret our findings in a simple two -state model with enhanced diffusion on large length scales due to the exis tence of interconnected hydroplasticized regions.