MOLECULAR DYNAMICS SIMULATION OF ION DISTRIBUTION IN NANOCHANNELS

摘要

Ion distribution in nanochannels with overlapped electric double layers is important for understanding many interesting phenomena in nature and designing novel nanofluidic devices for different applications. Molecular Dynamics has been proved to be a powerful tool to study the ion distribution and electroosmotic flow inside nanochannels. However, a big problem in molecular dynamics simulation is the assignment of the number of ions in the simulation domain since no theory is available to determine the number of ions in the nanochannel, which is directly related to the chemical potential of the bulk electrolyte. In the literature, attention has been paid mainly to meet the requirement of the overall neutrality among surface charges and mobile ions. However, since both positive and negative ions may exist in the solution, the exact number of positive and negative ions corresponding to certain bulk concentration was unknown and assigned somewhat arbitrarily. We believe that this arbitrary assignment may lead to artificial results and tries to attack this fundamental problem in molecular dynamics simulation by extending the simulation domain to include two bulk regions sandwiching the nanochannel of interest. This way, when the system reaches equilibrium, the concentration of the electrolyte in the bulk region and the number of ions in the center region will emerge naturally instead of artificially assigned. It was shown that both cation and anion concentrations in the nanochannel could be significantly different from the ion concentration in the bulk region.