  Diffusive transport of small molecules within the internal structures of
biological and synthetic material systems is complex because the crowded
environment presents chemical and physical barriers to mobility. We explored
this mobility using a synthetic experimental system of small dye molecules
diffusing within a polymer network at short time scales. We find that the
diffusion of inert molecules is inhibited by the presence of the polymers.
Counter-intuitively, small, hydrophobic molecules display smaller reduction in
mobility and also able to diffuse faster through the system by leveraging
crowding specific parameters. We explained this phenomenon by developing a de
novo model and using these results, we hypothesized that non-specific
hydrophobic interactions between the molecules and polymer chains could
localize the molecules into compartments of overlapped and entangled chains
where they experience microviscosity, rather than macroviscosity. We introduced
a characteristic interaction time parameter to quantitatively explain
experimental results in the light of frictional effects and molecular
interactions. Our model is in good agreement with the experimental results and
allowed us to classify molecules into two different mobility categories solely
based on interaction. By changing the surface group, polymer molecular weight,
and by adding salt to the medium, we could further modulate the mobility and
mean square displacements of interacting molecules. Our work has implications
in understanding intracellular diffusive transport in microtubule networks and
other systems with macromolecular crowding and could lead to transport
enhancement in synthetic polymer systems.
