Mobility and solvation of ions in channels

Abstract

In this paper we present some results from a simulation study of the mobility and solvation of ions and uncharged molecules in aqueous solution in smooth cylindrical channels at room temperature. This ideal system provides a reference system with which to compare the behavior of water and ions in real porous materials such as zeolites, bucky tubes, and biological channels. We find that in channels of radii between 2.5 and 5.5 Å the water molecules form a cylindrical solvation shell inside the channel walls with some evidence of a second shell in the center of the largest channel. Not all protons are involved in hydrogen bonding and a number point toward the walls. We attribute this to the concavity of the surface. When a sodium ion is added it tends to lie in the center of the channel where it can form the most complete solvation shell. Its diffusion rate decreases in smaller channels until it moves too slowly in a channel of 2 Å radius to be detected in our simulations. This decrease is only partly due to an increase in the mean square force on the ion. A range of ions of different sizes were studied in a channel with radius 3 Å. While the smaller of these ions (F−, Na+, and Ca++) lie preferentially in the center of the channel, larger ions (Cl−, I−, and Cs+) penetrate some way into the layer of water inside the wall and methane and ions with the charges turned off move next to the wall. Landau free energy analysis shows that this change is due to the balance between entropy and energy. The behavior in smooth channels is quite the opposite of what has been observed in experimental studies and simulations of Gramicidin (pore radius of 2 Å), where Cs+ lies closer to the center of the channel and Na+ lies off the axis. This difference can be attributed to the specific molecular structure of the gramicidin pores (e.g., the presence of carbonyl groups). As in bulk solutions, the mobilities of the ions in smooth channels increase to a maximum with ion size and decrease with increasing magnitude of the charge on the ion, while uncharged species diffuse much more rapidly and show a monotonic decrease with size. This behavior is related to the characteristics of the fluctuations of the forces on the solute molecules.