Free energy and enthalpy changes for the process of transfer from gas and from dilute aqueous solutions of some alkanes and monofunctional saturated organic compounds

Abstract

Standard free energies, Δ_solvG°(oct), and enthalpies, Δ_solvH°(oct), of solvation in octan-1-ol of some alkanes (heptane, octane), ketones (propan-2-one, butan-2-one, hexan-2-one, heptan-4-one), ethers (dipropyl ether, dibutyl ether, tetrahydropyran), alkanols (butan-1-ol, butan-2-ol, 2-methylpropan-2-ol) and amines (pyrrolidine, N-methylpyrrolidine, and piperidine) with open-chain and cyclic structure have been determined at 298.15 K. The Δ_solvG°(oct) values were obtained from measurements of partial vapour pressures of dilute solutions and the Δ_solvH°(oct) values, by adding the heats of solution, determined by calorimetry, to known values of enthalpy of vaporization.

These data are compared with the standard free energies and enthalpies of solvation in water, and the standard thermodynamic functions for the ideal transfer process of the solutes from pure water to pure octan-1-ol, δ_trX°(w–oct)(X=G, H) are calculated.

For the examined solutes, hydrocarbons and monofunctional saturated organic compounds, the thermodynamic functions of solvation in octanol and in water are closely correlated to the position of the functional group in the molecular skeleton. The values of the enthalpy of transfer from water to octan-1-ol are also related more to the topologic characteristics of the solute molecules than to their size or to the nature of their functional group. In contrast, the size of the molecules, as well as the presence of a functional group, are important with regard to Δ_trG°(w–oct). An increase of 1 cm³ mol^–1 in the intrinsic volumes corresponds to an increase in the value of the partition coefficients of ca. 15%. The substitution of a part of the hydrocarbon surface with a polar surface produces a very large increase in the values of the partition coefficients.

The values of the enthalpy of transfer from water to octan-1-ol are always positive, in contrast to the standard free energies of transfer. The entropic term operates by always favouring the transfer towards the alcoholic phase.

The potential ability of anhydrous octanol to extract organic solutes of various molecular structure from water-saturated octanol and from hexadecane is also evaluated and discussed.