Osmosis-Driven Water Transport through a Nanochannel: A Molecular Dynamics Simulation Study

Free Energy Profile for the Complete Transport of Nonpolar Molecules through a Carbon Nanotube

Gas molecules or weakly interacting molecules are commonly observed to diffuse through and fill space. Therefore, when the molecules initially confined in one compartment are allowed to move through a channel into another empty compartment, we expect that some molecules will be transported into the initially empty compartment. In this work, we thermodynamically analyze this transport process using a simple model consisting of graphene plates, a carbon nanotube (CNT), and nonpolar molecules that are weakly interacting with each other. Specifically, we calculate the free energy change, or the potential of mean force (PMF), as the molecules are transported from one compartment to another compartment. The PMF profile clearly exhibits a global minimum, or a free energy well, at the state wherein the molecules are evenly distributed over the two compartments. To better understand the thermodynamic origin of the well, we calculate the energetic and entropic contributions to the formation of the well, and we show that the entropic change is responsible for it and is the driving force for transport. Our work not only enables a fundamental understanding of the thermodynamic nature of the transport of weakly interacting molecules with molecular details, but also provides a method for calculating the free energy change during transport between two separate spaces connected by a nanochannel.

Free energy change in the complete transport of all water molecules through a carbon nanotube

In this work, we investigated the free energy barrier for transporting water molecules from one place to another. To properly address this issue, we considered a simple model system in which two separate compartments were connected via a subnanometer channel; all water molecules were initially in one compartment, and the other compartment was empty. Using umbrella sampling in molecular dynamics simulations, we calculated the free energy change for transporting all water molecules to the initially empty compartment. The free energy profile clearly indicated the presence of a free energy barrier, and the magnitude and shape of the barrier were dependent on the number of water molecules to be transported. To better understand the nature of the profile, we performed additional analyses on the potential energy of the system and hydrogen bonding between water molecules. Our study sheds light on a method for calculating the free energy of a transport system as well as the fundamental aspects of water transport.

The Physical Chemistry and Chemical Physics (PCCP) Section of the International Journal of Molecular Sciences in Its Publications: The First 300 Thematic Articles in the First 3 Years

The Physical Chemistry and Chemical Physics Section (PCCP Section) is one of the youngest among the sections of the International Journal of Molecular Sciences (IJMS)—the year 2021 will only mark three years since its inception [...]