= Solvation free energy
{title2=$\Delta G_{\mathrm{solv}}^{\circ}$}
For transfer of the same species $X$ from a gaseous reference state into a <solvent> $S$, define the molar <solvation free energy> by
$$
X(g)\longrightarrow X(S),\qquad
\Delta G_{\mathrm{solv}}^{\circ}(S)=\mu_X^{\circ}(S)-\mu_X^{\circ}(g).
$$
Here $\mu^{\circ}$ is a standard <chemical potential>. Negative <solvation free energy> means that transfer into the stated <solvent> lowers the <Gibbs free energy> relative to the stated gaseous reference. Include <solute>-<solvent> interactions, making room in the <solvent>, and its reorganization; counting only a favorable pair interaction omits part of the transfer.<IUPAC Compendium of Chemical Terminology (Gold Book) by International Union of Pure and Applied Chemistry/solvation energy>{ref}
State the <temperature>, gaseous reference <pressure>, dissolved reference <concentration> or other <standard state> convention, and species identity. Reference-state changes alter the quoted <solvation free energy>. For example, with an ideal dilute <thermodynamic activity> $a=c/c^{\circ}$, replacing $c^{\circ}$ by $\alpha c^{\circ}$ while preserving the physical <chemical potential> requires $\mu^{\circ}$ to increase by $RT\ln\alpha$. Thus the corresponding <solvation free energy> increases by $RT\ln\alpha$ if the gaseous reference stays fixed. This follows directly from $\mu=\mu^{\circ}+RT\ln a$, with $R$ the <molar gas constant>.
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