Surfactant (in situ)-Surfactant (Synthetic) Interaction in Na₂CO₃/Surfactant/Acidic Oil Systems for Enhanced Oil Recovery: Its Contribution to Dynamic Interfacial Tension Behavior

The effect of synthetic surfactant molecular structure on the dynamic interfacial tension (DIFT) behavior in Na₂CO₃/surfactant/crude oil was investigated. Three surfactants, a nonionic (iC₁₇(EO)₁₃), an alcohol propoxy sulfate (C_14-15(PO)₈SO₄), and sodium dodecyl sulfate (SDS) were considered in this study. Sodium tripolyphosphate (STPP) was added to ensure complete compatibility between brine and Na₂CO₃. In Na₂CO₃/iC₁₇(EO)₁₃/oil and Na₂CO₃/C_14-15(PO)₈SO₄/oil systems, a strong synergistic effect for lowering the dynamic interfacial tension was observed, in which the dynamic IFT are initially reduced to ultralow transient minima in the range 1.1 × 10^-3-6.6 × 10^-3 mNm^-1 followed by an increment to a practically similar equilibrium value of 0.22 mNm^-1 independent of Na₂CO₃ concentration (for iC₁₇(EO)₁₃) and to decreasing equilibrium values with increasing alkali concentrations (for C_14-15(PO)₈SO₄). The observed difference in the equilibrium IFT for the two systems suggest that in both systems, the mixed interfacial film is efficient in reducing the dynamic interfacial tension to ultralow transient minima (∼10^-3 mNm^-1) but the mixed film soap-iC₁₇(EO)₁₃ is much less efficient than the mixed film soap-C_14-15(PO)₈SO₄ in resisting soap diffusion from the interface to the bulk phases. In both systems, the synergism was attributed, in part, to the intermolecular and intramolecular ion-dipole interactions between the soap molecules and the synthetic surfactant as well as to some shielding effect of the electrostatic repulsion between the carboxylate groups by the nearby ethylene oxide (13 EO) and propylene oxide (8 PO) groups in the mixed interfacial monolayer. SDS surfactant showed a much lower synergism relative to iC₁₇(EO)₁₃ and C_14-15(PO)₈SO₄, probably due to the absence of ion-dipole interactions and shielding effect in the mixed interfacial layer at the oil-water interface.