Impact of Ions on Individual Water Entropy

Solutes determine the properties of a solution. In this study, we probe ionic solutions through the entropy of individual water molecules in the solvation shells around different cations and anions. Using a method recently developed by our group, we show the solvation shell entropy stemming from the individual contributions correlates extremely well with experimental values for both polarizable and nonpolarizable force fields. The behavior of water entropy as a function of distance reveals significant (∼20%) contributions from the second solvation shell even for the low concentration considered here. While for the cations, contributions from both translational and rotational entropy loss are similar in different solvation shells, water around anions loses much more rotational entropy due to their ability to accept hydrogen bonds. Most importantly, while charge density of cations or anions correlates with the translational entropy loss, anions with similar charge density as that of cations has a much stronger and long-range effect on water. We also show how the modulation of water entropy by ions is correlated to the structural modifications of hydration shell. This study thus provides a step toward understanding the entropic behavior of water in molecular recognition processes between proteins and drug molecules.