Mutual solubilities of methyl diphenyl phosphate and some aliphatic hydrocarbons

Liquid-liquid coexistence curves for binary systems involving methyl diphenyl phosphate and n-alkanes (C₅-C₁₀) and isopentane were determined from -15 to 89 °C. The solubility curves are very unsymmetrical. At a given temperature the solubility of hydrocarbons in methyl diphenyl phosphate can be arranged in the descending series n-C₅ > i-C₅ > C₆ > C₇ > C₈ > C₁₀, while the solubility of the phosphate in the hydrocarbons is C₁₀ > C₈ > C₇ > C₆ > n-C₅ > i-C₅. The analysis of the methyl diphenyl phosphate-rich branch of the solubility curves shows that the excess molar enthalpy is almost the same for all hydrocarbons and the position of a hydrocarbon in the solubility series depends on the entropy term, which can be correlated linearly with the number of CH₂ segments in the chain. The solubility parameters of methyl diphenyl phosphate as well as the upper critical solution temperatures were estimated by use of the theory of regular solutions. Good agreement was found between predicted critical parameters obtained from the solubility data and those evaluated from the vapour pressure measurements, which were performed only for the phosphate-hexane system.