Enhanced Transport Behavior of Small Molecules in Polymer Solutions

The transport of small molecules in crowded polymeric or biological systems is complex and has extensive implications for drug delivery, imaging, tracer diffusion, and other processes. In this study, we looked at an interesting 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 a clear signature of two distinct local densities near polymer-rich and polymer-lean zones. Also, importantly, it shows that near the dilute to semidilute transition, the volume fraction of the polymer-rich zone decreases, increasing the low-density volume fraction. We show that 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 semidilute polymer solutions and helps identify the concentration regime at which a non-sticky molecule can exhibit enhanced transport behavior.