Interfacial tension (IFT) data were measured and correlated by a theoretical model for the system CO2/polyethylene glycol (PEG) (40 wt.%)+water (60 wt.%). The effect of both bare SiO2 nanoparticles (NPs) as well as their modified surface counterparts on the IFT alteration was investigated. SiO2 NPs were synthesized followed by coating with polyvinyl alcohol (PVA). IFT measurements based on a well-established pendant droplet technique were carried out in the presence of 0.00–0.05 wt.% NPs over the temperatures and pressures of 298–323 K and 0.1–4.0 MPa, respectively. The obtained results depict that IFT decreases with increasing pressure and increasing NPs concentration, and increases with increasing temperature. It was shown that increasing the SiO2 NPs concentration from 0.00 to 0.05 wt.% causes a 35.8% decrease at constant pressure and temperature of 3.0 MPa and 323 K, respectively. Increasing pressure from 2.0 to 4.0 MPa in the presence of 0.03 wt.% SiO2 NPs decreases the IFT of the system by 27% at a constant temperature of 313 K. Lower IFTs were observed (11.50–18.90%) when PVA-coated NPs were added to the fluid. Finally, the parameters of the employed theoretical model were determined using global and local parameters, the latter of which showed higher precisions in the representation of data.
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