The hydration structure and thermodynamics associated with the ion pairing between a halide anion and the tetramethyl ammonium cation in water are investigated by molecular dynamics simulations. Correlating the potential of mean force and different energy terms with the structure of the ion pair as a function of the interionic distance provides molecular level insight into recent experiments that shows increased affinity between a larger anion and the hydrophobic cation. (C) 2015 Elsevier B.V. All rights reserved.