Molecular Signatures of Chaotropicity and Kosmotropicity of Ions in Water

K. D. Collins (Biophys. J., 1997, 72, 65) has shown that the charge density of ions and their strength of hydration provide a robust basis for understanding the interesting behavior of ions in water.  With increasing size and thus, decreasing charge density, the behavior of ions in water transitions from kosmotropic to chaotropic.    In this work, we use extensive molecular dynamics simulations of solutions of ions to obtain molecular signatures of this transition.  In particular, we simulate aqueous solutions of ions of different sizes and charge.  We study the structure and dynamics of water in the hydration shell.  We find that metrics of the microscopic compressibility of the hydration shell correlate with ionic mobility in water.  In addition, we will examine the theoretical connections between our findings and the ion pairing phenomenon in aqueous solutions.