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The consideration of polar interactions is of vital importance for the development of predictive and accurate thermodynamic models for polar fluids, as they govern most of their thermodynamic properties, making them highly non-ideal fluids.

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In this paper, interfacial tension (IFT) of two ternary water-rich hydrocarbon systems were investigated at a representative reservoir temperature of 150°C and equilibrium pressures up to 140 MPa. Two-phase and three-phase IFT measurements of water-methane-hexadecane and water-methane-toluene mixtures were carried out in our previously validated Pendant Drop facility and the equilibrium phase densities, required to determine pertinent IFT values, were determined by measuring the volume and mass of samples separately. The results showed a high pressure dependency of IFT values below the hydrocarbon dew point (i.e., three-phase region) whereas a slight increase on the interfacial tension was observed in the two-phase region.Aiming at developing a general model for describing this property in multiphase systems, the generated and literature IFT data were used to develop and validate our model based on the Linear Gradient Theory (LGT). The LGT has been proven capable of describing vapor-liquid and liquid-liquid interfaces in systems containing polar and non-polar compounds when coupled with an appropriate thermodynamic model. In this work the LGT was coupled with the Cubic-Plus-Association equation of state (CPA EoS) for a correct description of the equilibrium properties of the phases involved. The modelling results confirmed the superiority of the LGT over classical models in that one single model can be used for describing the IFT of multiphase systems with greater accuracy.