Modeling of the Interfacial Behavior of $$\hbox {CO}_{2}$$ + $$\hbox {H}_{2}$$O and $$\hbox {H}_{2}$$S + $$\hbox {H}_{2}$$O with CPA EOS and Gradient Theory

In this work, the combination of the gradient theory and cubic plus association equation of state has been applied to describing the interfacial density profiles and surface tensions of $$\hbox {CO}_{2}$$ + $$\hbox {H}_{2}$$ O and $$\hbox {H}_{2}$$ S + $$\hbox {H}_{2}$$ O systems. The ranges of temperature and pressure are (297.9–469.4) K and (1–691.4) bar, respectively. Different cross-association schemes are applied to the phase equilibrium of $$\hbox {CO}_{2}$$ + $$\hbox {H}_{2}$$ O and $$\hbox {H}_{2}$$ S + $$\hbox {H}_{2}$$ O systems and the experimental cross-association energies have been used for the model resulting in more accurate phase equilibrium calculations. Moreover, two forms of influence parameter in terms of bulk densities of phase and temperature are used for the present model. Firstly, the coefficients of influence parameters are regressed based on the surface tensions of pure components (carbon dioxide, hydrogen sulfide and water). The binary interaction parameter of the cross-influence parameter is set equal to zero making this model predictive. Subsequently, the interfacial density profiles of the components have been determined. The predictions of the bulk-density-dependent influence parameter are in good agreement with the experimental surface tensions (overall AAD of 14.75 % and 8.87 % for temperature and bulk-density-dependent influence parameters, respectively).