Tuning Surface Chemistry and Ionic Strength to Control Nanoparticle Adsorption and Elastic Dilational Modulus at Air-Brine Interface

Abstract

The relationship between the interfacial rheology of nanoparticle (NP) laden air-brine interfaces and NP adsorption and interparticle interactions is not well understood, particularly as a function of the surface chemistry and salinity. Herein, a nonionic ether diol on the surface of silica NPs provides steric stabilization in bulk brine and at the air-brine interface, whereas a second smaller underlying hydrophobic 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/d ln A. However, the interface is ductile with a relatively slow change in E′ as the interfacial area is varied over a wide range during compression and expansion. In contrast, for silica NPs stabilized with only a single hydrophobic ligand, the interfaces are often more fragile and may fracture with small changes in area. The presence of concentrated divalent cations improves E′ and ductility by screening electrostatic dipolar repulsion and strengthening the attractive forces between nanoparticles. The ability to tune the interfacial rheology with NP surface chemistry is of great interest for designing more stable gas/brine foams.

Tuning Nanoparticle Surface Chemistry and Interfacial Properties for Highly Stable Nitrogen-In-Brine Foams

“…The macroscopic contact angle θ ∞ of the NP at the water–air interface was measured with water droplets on an NP film, as has been done for silica NPs in previous studies (Tables and S8). ,, It ranged from 20° for bare silica NP with DI water droplets, in agreement with the previous literature, to 71° for ED-DM-NPs with API brine droplets indicating much greater hydrophobicity. , It is evident that both salinity ,, and ligand hydrophobicity (explored further in a companion paper) produce a large increase in θ ∞ as ζ becomes less negative. The added DM ligands increased θ ∞ from 25° for ED-NPs in DI water to 71° in API brine (Table S7).…”

“…γ changed little from DI water to 3% NaCl but decreased faster and to a much larger extent in API brine. More extensive data are given versus a range of DM levels on the surface in our companion paper . In API brine, the high interfacial activity was evident in the decrease in γ to 59 mN/m at 1 wt % consistent with the relatively high θ of 71°.…”

“…The amount of secondary “buried” small dimethylsilyl hydrophobic ligands (ED-DM-NPs) is adjusted to further increase the hydrophobicity ( θ ) to increase the NP adsorption and capillary and hydrophobic attraction between NPs at the interface. The stronger attractive interactions, which are further increased with Na + , Ca 2+ , and Cl – ions, are evident in an increase in G s ′, and as shown in a companion paper, an increase in the surface pressure Π up to 20 mN/m and the elastic dilational modulus E ′ to 65 mN/m . All of these properties are shown to increase with the NaCl concentration and then even more with the further addition of Ca 2+ .…”

“…The foam stability is discussed with regard to lamellae drainage, coarsening and coalescence in terms of the key properties, the NP contact angle, bulk G ′, and the interfacial elastic dilational E ′ and shear G s ′ moduli. Highly stable foams are produced with relatively strong elastic interfaces, as characterized by G s ′ and E ′, which are also unusually ductile. The ductile behavior is characterized by changes in E ′ over a large change in the area during expansion and compression .…”

“…Highly stable foams are produced with relatively strong elastic interfaces, as characterized by G s ′ and E ′, which are also unusually ductile. The ductile behavior is characterized by changes in E ′ over a large change in the area during expansion and compression . The combination of a hydrophilic and hydrophobic ligand to modify the adsorption, structure, and interfacial properties of NP-laden air–water (A–W) interfaces offers a way to design many new materials in addition to stable foams in concentrated brines.…”