Effect of Nanoparticle Wettability on the Stability and Coalescence of Pickering Emulsions

Abstract:

Solid-stabilized emulsions, i.e. Pickering emulsions, strongly resist coalescence, even in high-temperature, high-salinity conditions where conventional surfactant-stabilized emulsions would fail. Pickering emulsions are promising candidates for applications in porous media, such as enhanced oil recovery and aquifer treatment. Pickering emulsion stability and coalescence rely on fluid-particle interactions that depend on a variety of particle characteristics, including wettability; however, the role of wettability is not well understood. To determine the effect of wettability, this work examines emulsions stabilized by silica nanoparticles coated with two similarly sized molecules with different hydrophilicities: (3-glycidyloxypropyl)trimethoxysilane (GPTMS) and [3-(2,3-dihydroxypropoxy)propyl]trimethoxysilane (GLYMO). We characterized the particle size and surface (zeta) potential of the coated nanoparticles and generated oil-in-water emulsions with a tip sonicator. Emulsion stability was quantified by observations of microchannel flow and by centrifugation.

GPTMS-coated nanoparticles (GPTMS-NP) exhibited a lower zeta potential (-30 mV) than GLYMO-coated nanoparticles (GLYMO-NP) (-40 mV), indicating greater hydrophobicity. The attached figure displays the two emulsions before and after a representative blockage buildup and breakthrough event at a restriction in a glass microchannel. GPTMS-NP-emulsion blocked flow more easily at the restriction and proved resistant to coalescence; GLYMO-NP-emulsion coalesced more easily at the restriction without significant blockage. These observations match centrifuge measurements showing critical demulsification pressures of 125 and 117 kPa for GPTMS-NP-emulsion and GLYMO-NP-emulsion, respectively. These results demonstrate the influence of high particle attachment energy on emulsion stability. Intermediate-wetting GPTMS-NP are partitioned at the oil-water interface and reduce the interfacial energy between the immiscible fluid phases; more hydrophilic GLYMO-NP favor the aqueous phase and influence a smaller interfacial area, diminishing the associated reduction in interfacial energy. These results confirm the prediction that oil-in-water emulsions stabilized by more hydrophilic particles are weaker.