The hydration shell of an ion in water

Background :

When an ion is inserted into a water configuration, it changes the structure of the hydrogen bond network. A water molecule tends to rotate (reorient) so that its polarized charge concentration faces the opposite charge of the ion. As the water molecules orient themselves towards the ion, they break the hydrogen bonds to their nearest neighbors. The group of water molecules oriented around an ion is called a hydration shell. The orientation of the molecules in the hydration shell results in a net charge on the outside of this shell, a charge of the same sign as that of the ion in the center. The charge on the outside of the hydration shell tends to orient water molecules in the vicinity, leading to a second hydration shell. The result of forming hydration shells is to weaken the structure of the hydrogen bond network. This explains the observation that salt water has a lower freezing point than pure water: Each ion in the liquid has a hydration shell of oriented water molecules around it, which prevents the water molecules from forming the hexagonal structure of ice.

System settings :

Start with the NVT ensemble; fix the temperature at 298 K and the density at . Insert an ion and view its nearest water molecules.

Questions :

• What relationship is there between the ionic radius and the ease of insertion into the network? (Estimated by viewing the potential energy profile.) The ease of insertion is determined by the potential energy calculated for a choosen position of the insertion--lattice within the simulation box. The potential energy is calculated for each grid spot and presented in a color map. The lower the potential energy (corresponding to a blue color), the better it fits in. If the potential energy is high (corresponding to a red color) the particle cannot be inserted at this position.

• For a given ionic radius, is the ease of insertion the same for positive and negative ions? Explain the difference between the two.

• Try to explain the behavoir of the potential energy after insertion of an ion and the return of the system to equilibrium.

• Discribe how water molecules orient in the first hydration shell, depending on the charge of the inserted ion.

• Explain how water molecules in the second hydration shell orient themselves?