Carbon nanotubes exhibit very unique properties in biphasic systems. Their interparticle attraction leads to reduced droplet coalescence rates and corresponding improvements in emulsion stability. Here we use covalent and noncovalent techniques to modify the hydrophilicity of multiwalled carbon nanotubes (MWCNTs) and study their resulting behavior at an oil-water interface. By using both paraffin wax/water and dodecane/water systems, the thickness of the layer of MWNTs at the interface and resulting emulsion stability are shown to vary significantly with the approach used to modify the MWNTs. Increased hydrophilicity of the MWNTs shifts the emulsions from water-in-oil to oil-in-water. The stability of the emulsion is found to correlate with the thickness of nanotubes populating the oil-water interface and relative strength of the carbon nanotube network. The addition of a surfactant decreases the thickness of nanotubes at the interface and enhances the overall interfacial area stabilized at the expense of increased droplet coalescence rates. To the best of our knowledge, this is the first time the interfacial thickness of modified carbon nanotubes has been quantified and correlated to emulsion stability.

Recent interest in the use of nanoparticles in emulsion stabilization has driven increased efforts to understand how the characteristics of the particles influence the emulsion properties. While it is clear that the contact angle and wettability must play significant roles in determining the type of emulsion formed, it is not straightforward to measure the contact angle of a nanoparticle. In this paper, we compare multiple techniques for characterizing the water–air contact angle of silica nanoparticles while systematically varying the hydrophobicity of the nanoparticles using silanization. We then compare the performance of the particles in decane/water emulsions. While the heat of immersion measured by microcalorimetry is found to provide the best method for discriminating between the wettability of the particles, the fraction of surface covered by the silane groups was observed to affect the structure of the emulsion more profoundly than the differences in the contact angles of the particles. Furthermore, we find that the phase of initial dispersion is extremely influential in determining the resulting emulsion type and droplet size.

The adsorption of anionic, cationic, and nonionic surfactants was measured on high-surface area silica and alumina nanoparticles when in the presence of the proposed polyelectrolyte sacrificial agents. Surfactant adsorption was characterized using two types of adsorption isotherms: one with constant polymer concentration and varying surfactant concentration, and another with a varying polymer concentration and constant surfactant concentration. Polystyrenesulfonate and Polydiallyl dimethylammonium chloride were tested as potential sacrificial agents on alumina and silica, respectively. Each surfactant/polymer system was allowed to reach equilibrium and supernatant surfactant concentrations were measured. This information was then plotted in order to determine what, if any, effect the proposed sacrificial agent had on the equilibrium adsorption. Results indicate that both of these polymers can have a large effect on total surfactant adsorption at a variety of surfactant concentrations.

SummarySurfactant flooding is an effective enhanced oil recovery method in which the oil/water interfacial tension (IFT) is reduced to ultralow values (<0.01 mN/m). The microscopic fluid-fluid displacement has been extensively studied at high IFT (>10 mN/m). However, the microscopic displacement dynamics can be significantly different when the IFT is ultralow because the dynamic contact angle increases with the increase of the capillary number. In this study, surfactant flooding was performed and visualized in micromodels to investigate the dynamics of multiphase displacement at ultralow IFT. Although the micromodels used were strongly water-wet, the displacements of oil by surfactant solutions at ultralow IFT appeared as drainage. Furthermore, a macroscopic oil film was left behind on the surface, which indicates that a contact line instability occurred during displacements. The shape of the oil/water meniscus was determined by the balance between viscous forces and capillary forces. The meniscus can be significantly distorted by viscous forces at ultralow IFT. Therefore, the water-wet micromodel exhibits an oil-wet behavior at ultralow IFT, and the displacements of oil by surfactant solutions at ultralow IFT manifested as drainage rather than imbibition. The flow behavior is further complicated by the spontaneous formation of microemulsion during displacement. The microemulsion is mainly formed from the residual oil. The formation of a microemulsion bank made the surfactant solution discontinuous, with transport in the form of droplets in the microemulsion bank and displacement front. The novelty of this work is to reveal the effects of dynamic contact angle on the ultralow IFT displacement.

Capillary rheometry is combined with small-angle neutron scattering to simultaneously measure the viscosity and nanostructure of complex fluids containing proteins, surfactants, polymers, and inorganic nanoparticles at shear rates up to 106 s−1.