Why do we need solvation thermodynamics

Consider a general chemical reaction in the gaseous phase, written symbolically as 

the chemical potential of each species is written as 

As a simple example, suppose two monomers (M) form a dimer (D). The reaction is 

The equilibrium constant, and the corresponding standard Gibbs energy of this reaction, are 

Gibbs energy in the liquid phase, we have to transfer each of the species from the ideal-gas phase into the liquid. This transfer involves the solvation Gibbs energies of all the species participating in the reaction. 

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