Fluctuation-induced extensive molecular transport via modulation of interaction strength

Abstract

In our previous work, we studied the thermodynamics of two cases of intercompartmental transport through a carbon nanotube: one involving water molecules and the other involving nonpolar molecules. Free energy calculations indicate that transporting water molecules from one compartment to another via a narrow channel is impossible, whereas for nonpolar molecules, only approximately half can be transported. Therefore, the interaction strength between transported molecules significantly affects molecular transport. In this study, we examined the effect of interaction strength on molecular transport both kinetically and thermodynamically via simple models and molecular simulation methods. The results revealed that, depending on the interaction strength, the transport behavior can be categorized into three regimes: water-like, nonpolar-like, and transition regimes. Interestingly, the molecular fluctuations in the transition regime are so large that a significant number of molecules are transported between the compartments in an oscillatory manner, exceeding the transport of half of the molecules in the nonpolar-like regime. Thus, to induce molecular transport driven by large fluctuations, the interaction strength should remain within a moderate range. Moreover, potential of mean force (PMF) analysis supports this large fluctuating behavior, as the PMF profile exhibits a flat region that allows significant variation with no free energy cost. We elucidate the role of interaction strength in molecular transport, as well as the deep connection between molecular fluctuations and molecular transport.