Thermodynamics of solution of homologous series of solutes in water

Abstract

Values of ΔG^⊖_s(g → aq) and ΔG^⊖_s(liq → aq) at 298 K are documented for 14 homologous series of gaseous and liquid solutes, and corresponding enthalpies of solution listed for 7 homologous series. It is shown by a thermodynamic argument that only parameters for the process g → aq can be used to assess solute–water interactions and that the standard state of pure liquid solute includes a different solute–solute interaction term for each solute standard state. For most of the homologous series, parameters for the process g → aq are linear in the number of carbon atoms in the solute; from such linear equations, methylene and group contributions are obtained. It is shown that the methylene increments to ΔG^⊖_s(g → aq) and to ΔH^⊖_s(g → aq) are not constant but vary from one homologous series to another. In a few homologous series the methylene increment is not constant, the most outstanding examples being the alkan-1-ols and n-alkanes. Above dodecan-1-ol, ΔG^⊖_s(g → aq) becomes gradually more negative than expected, so that octadecan-1-ol is 16 times as soluble as calculated from results on the low alkan-1-ols. A similar, but much larger, effect is observed for the n-alkanes: n-octadecane is more soluble than expected by a factor of 5 × 10³(the factor for n-hexatriacontane is 2 × 10¹⁸) and it is deduced that the n-alkane C₅₁H₁₀₄ will be as soluble in water as in the non-aqueous solvents ethanol and phenol.