Compressibility in multi-phases system

GROMACS version: 2018
GROMACS modification: No

Dear all,
I wonder how the compressibility parameter should be treated for a multi-phases system likes emulsion (molecules solvated in water) on top of solid slab? The compressibility is 4.5e-5 bar^-1 for water at 1 atm and 300k. The what should be the compressibility for solid part?

Any comment, please?