Molecular Dynamics Behavior And Salt Solution Mechanism In Electrolyte Solutions

Dynamics of ammonium and ammonia in solutions is closely related to the metabolism of ammoniac compounds,therefore plays an important role in various biological processes.Nuclear Magnetic Resonance?NMR?measurements indicated that the reorientation dynamics of NH₄⁺is faster in its aqueous solution than in methanol,which deviates from the Stokes-Einstein-Debye rule since water has higher viscosity than methanol.To address this intriguing issue,we herein study the reorientation dynamics of ammonium ion in both solutions using numerical simulation and an extended cyclic Markov chain model.An evident decoupling between translation and rotation of methanol is observed in simulation,which results in the deviation of reorientation from the Stokes-Einstein-Debye rule.Slower hydrogen bond?HB?switchings of ammonium with methanol comparing to that with water,due to the steric effect of the methyl group,remarkably retards the jump rotation of ammonium.The observations herein provide useful insights into the dynamic behavior of ammonium in the heterogeneous environments including the protein surface or protein channels.In addition,the solvation of carbon dioxide in sea water plays an important role in the carbon circle and the world climate.The salting-out/salting-in mechanism of CO₂ in electrolyte solutions still remains elusive at molecule level.The ability of ion salting-out/salting-in CO₂ in electrolyte solution follows Hofmeister Series and the change of water mobility induced by salts can be predicted by the viscosity B-coefficients.In this work,the chemical potential of carbon dioxide and the dynamic properties of water in aqueous NaCl,KF and NaClO₄ solutions are calculated and analyzed.According to the viscosity B-coefficients,NaClO₄?Viscosity B-coefficient is 0.012?should salt out the carbon dioxide relative to in pure water,but the opposite effect is observed for it.Our simulation results suggest that the salting-in effect of NaClO₄ is due to the strongly direct anion-CO₂ interaction.The inconsistency between Hofmeister Series and the viscosity B-coefficient suggests that it is not always right to indicate whether a salt belongs to salting-in or salting-out just from these properties of the salt solution in the absence of solute.