The enthalpies of mixing of organophosphate esters with hydrocarbons

Heats of mixing were obtained calorimetrically at 298°K for each of the phosphate esters tri-n-butylphosphate (TBP), bis(2-ethylhexyl)phosphate [(H)DE-HP], and di-n-butylphosphate [(H)DBP] with n-dodecane and with 1,3-diethylbenzene. These data and data from the literature on Gibbs free energies and heats of mixing of the phosphate esters with decane, hexane, and benzene are interpreted according to a nonideal associated mixture model. This takes into account the size effect, nonspecific (regular-type) interactions, and association to dimers, which is partial for TBP and complete for the acidic esters, which in turn associate further to trimers and possibly higher oligomers. The enthalpies of mixing with the aliphatic hydrocarbons involve two parameters, one describing the association and the other the nonspecific interaction, and independently obtained quantities. The enthalpies of mixing with the aromatic hydrocarbons require further parameters, involving the mutual interactions, which could not be calculated.