Enhanced transport behavior of small molecules in polymer solutions

The transport of small molecules in crowded polymeric or biological systems is a complex process with extensive implications for drug delivery, imaging, tracer diffusion, and other biological processes. In this study, we examined an intriguing case where a methylated small molecule, rhodamine 6G (R6G), diffused faster than a similar-sized non-methylated molecule (6-HEX) in an aqueous polyethylene oxide solution, and dramatically enhanced its diffusivity near the dilute-to-semidilute transition. The commonly used universal scaling model cannot explain such phenomena. The experimental diffusivity measurement was performed using fluorescence correlation spectroscopy, and an all-atom molecular dynamics simulation was conducted to estimate theoretical diffusivity. We demonstrate that the degree of hydrophobicity of the dye molecule directly influences the level of non-sticky behavior exhibited by the dye. Using both experiment and simulation, we show that the hydrophilic dye (6-HEX) shows a stronger affinity (sticky molecule) to the polymer chains and moves along with them. Our simulations show two different interconnected local densities of polymer-rich and polymer-lean zones, observed at a length scale of the polymer's radius of gyration. Additionally, near the dilute to semi-dilute transition, the volume fraction of the polymer-rich zone decreases, thereby increasing the volume fraction of the polymer-lean zone. We show that the methylated, hydrophobic dye interacts less with the polymer and traverses through the low-density region, which enhances its diffusivity. This study aids in understanding the transport behavior of small molecules in dilute and semi-dilute polymer solutions and helps identify the concentration regime at which a non-sticky molecule can exhibit enhanced transport behavior.