(627g) Highly Stable Gas-in-Water Foams at High Salinity Stabilized with Nanoparticles and like-Charged Surfactants

The design of surface chemistries on nanoparticles (NPs) to stabilize gas/brine foams with concentrated electrolytes, especially with divalent ions, has been elusive. At these conditions, the relationship between the interfacial rheology of NP laden gas-brine interfaces and NP adsorption and interparticle interactions is not well understood. Herein, we design binary ligands on the surface of silica NPs whereby the primary ligand provides steric stabilization in bulk brine and at the air-brine interface, whereas a second ligand raises the hydrophobicity to promote NP adsorption. The level of NPs adsorption at steady state is sufficient to produce an interface with a relatively strong elastic dilational modulus E’ = dγ/dln A. With these favorable properties, highly stable nitrogen/water (N₂/brine) foams are formed with CaCl₂ concentrations up to 2% from 25°C to 90^°C. The viscoelastic gas-brine interface arrests coarsening (Ostwald ripening) with no observable change in foam bubble size over 48 hours. With the addition of a liked-charged surfactant to the NP solution, the foams are also highly stable, but may be formed at a much lower shear rate than in the case of NPs alone. Unlike the case for most previous studies, the NP amphiphilicity was essentially independent of the surfactant concentration given the very low adsorption of the surfactant on the like-charged NP surfaces.

The concept of tuning of nanoparticle surfaces with binary ligands to achieve both colloidal stability and high interfacial activity was also found to be successful for designing highly stable CO₂-in-brine foams. Unlike the case for the N₂-in-brine foams, the intermolecular interactions between CO₂ and the surface ligands influenced the interfacial properties. The ability to design NP laden viscoelastic interfaces for highly stable foams, even with high divalent ion concentrations, is of fundamental mechanistic interest for a broad range of foam applications including mobility control in CO₂ sequestration and enhanced oil recovery.