Solvation free energy

Dear All
I want to calculate the solvation free energy of an ion into water with Forcite. My system has solid phase in contact with liquid solution,some ions adsorb on the surface of solid phase, and some ions stay in the bulk solution. If I want to calculate the solvation free energy of the ion, should I ONLY select the ion in the bulk solution, or I can also select the ion adsorbed on the surface? Or should I built a aqueous box with the ions and calculate the solvation free energy, in this case, there is no solid phase in the system?
Thank you!

Regards,
Jingjing