Preliminary Communication Ion Pairing in Confined Electrolytes D. NICHOLSON and N. QUIRKE* Department of Chemistry, Imperial College, London SW7 2AY, UK (Received November 2002; In final form December 2002) Equilibrium molecular dynamics has been used to investigate ion and water distributions of 0.5 M sodium chloride solutions of approximate density, 1000 kg m 23 , confined in model carbon nanotubes having radii of 0.65, 1.33 and 1.97 nm. It is observed that ion pairing can occur; its nature and location being highly dependent on pore radius. Especially notable in the smallest cylinder, where hydration is frustrated by confinement, is the formation of tightly bound ion pairs at the cylinder axis. Keywords: Electrolyte solutions; Nanotubes; Sodium chloride; Water INTRODUCTION Water and electrolyte solutions in confined spaces are of great importance in biological systems (e.g. ion transport through membranes), and industrial membrane processes (e.g. desalination). In the past two decades, there has been intense and increasing research activity [1–9] in these areas. Simulation studies have contributed significantly to this effort. Much of this work has focused on very dilute electrolytes where single ions are surrounded only by water molecules, including studies of single ion transport through model cell membrane pores and studies of the hydration structure around isolated ions. Another group of simulations has used primitive models, in which there is no explicit description of the water molecules. Both approaches avoid a major difficulty for electrolytes at realistic dilutions: namely the many water molecules required for each ionic species (55:1 in a 1M solution). Thus, a fully explicit electrolyte simulation at normal dilutions requires a very large number of particles in order to ensure a statistically sufficient number of ionic particles. However, it is arguable that, especially in confined spaces, packing and other effects may disrupt hydration shells leading to significant modifications of transport and equilibrium properties. In this note, we report some results from a simulation study in which dilute (0.5 M) sodium chloride solutions, where both cations and anions were present simultaneously, are confined in cylindrical spaces. We wish to draw attention to some of the interesting features of equilibrium structure that can be conferred on electrolytes subjected to confinement; especially the possibility of strong ion-pairing that may occur in certain situations. MODEL AND SIMULATIONS We chose graphitic nanotubes as the basic pore model type. The nanotubes were modelled as cylinders of continuum graphitic material. Physical radii are measured from the pore axis to a position at the centre of a first layer atom in the adsorbent. Three values of 0.65, 1.33 and 1.97 nm were chosen. An internal density for the electrolyte was based on a volume, estimated by choosing an internal pore radius as the position where the single particle distribution (obtained in preliminary runs) goes to zero close to the pore wall. Most runs were made with an internal density of 1000 kg m 23 . The electrolyte concentration was 0.5 M NaCl and ISSN 0892-7022 print/ISSN 1029-0435 online q 2003 Taylor & Francis Ltd DOI: 10.1080/0892702031000078427 *Corresponding author. Molecular Simulation, 2003 Vol. 29 (4), pp. 287–290