Vapor pressure osmometry was used to measure osmotic coefficients for tributylphosphate (TBP), tricresylphosphate (TCP), and triethylhexylphosphate (TEHP) in n-octane at 30, 40, 50, and 60 °C and at molalities up to 0.3 mol/kg. Activity coefficients and excess thermodynamic properties (unsymmetrical definition) were calculated from these osmotic coefficients. At 30 °C, the excess Gibbs free energies for 0.1 mol of solute in 1.0 kg n-octane were −42 J, −66 J, and −20 J for TBP, TCP, and TEHP, respectively. The more ideal behavior of the TEHP-octane system is attributed to the increasing importance of hydrocarbon–hydrocarbon interactions as the chain length is increased. The excess enthalpies for 0.1 mol of solute in 1.0 kg of solvent were −100 J, −300 J, and −150 J for TBP, TCP, and TEHP, respectively. Thus, association of these solutes arises primarily from entropie effects.Our data could generally be accommodated adequately by postulating association of monomers into dimers. The exception was TCP at lower temperatures, where more complex models were required.
