Structural and thermodynamic properties of ammonium chloride clusters

COMP 239

Robert Q. Topper, rtopper@monmouth.edu, Department of Chemistry, Medical Technology and Physics, Monmouth University, Thomas A. Edison Hall of Science, West Long Branch, NJ 07764-1898
Ammonium chloride exhibits interesting phase phenomena across a wide range of temperatures, pressures and preparation methods, including the orientational order-disorder phase transition exhibited at 243K. Its structural and thermodynamic properties continue to attract experimental and theoretical attention. In the present study, the thermodynamic and structural properties of bulk NH4Cl and small NH4Cl clusters are investigated using Monte Carlo methods. A basic electrostatic model used in previous work correctly predicts the order-disorder transition temperature, but does not include the possibility of proton transfer between NH4+ and Cl- to form HCl, An extended model fit to ab initio data developed by Guillot and Guissani, and which includes terms to describe the proton transfer reaction, is used to predict the order-disorder transition temperature of the bulk phase as well as disorder and melting transition temperatures in small NH4Cl clusters. Comparisons are presented between the predictions of the two models and to available experimental data.