The use of chemical dispersants is a well-established approach to oil spill remediation where surfactants in an appropriate solvent are contacted with the oil to reduce the oil–water interfacial tension and create small oil droplets capable of being sustained in the water column. Dispersant formulations typically include organic solvents, and to minimize environmental impacts of dispersant use and avoid surfactant wastage it is beneficial to use water-based systems and target the oil–water interface. The approach here involves the tubular clay minerals known as halloysite nanotubes (HNTs) that serve as nanosized reservoir for surfactants. Such particles generate Pickering emulsions with oil, and the release of surfactant reduces the interfacial tension to extremely low values allowing small droplets to be formed that are colloidally stable in the water column. We report new findings on engineering the surfactant-loaded halloysite nanotubes to be stimuli responsive such that the release of surfactant is triggered by contact with oil. This is achieved by forming a thin coating of wax to stopper the nanotubes to prevent the premature release of surfactant. Surfactant release only occurs when the wax dissolves upon contact with oil. The system thus represents an environmentally benign approach where the wax coated HNTs are dispersed in an aqueous solvent and delivered to an oil spill whereupon they release surfactant to the oil–water interface upon contact with oil.

This work develops the concepts of particle-stabilized emulsions using tubular natural clays known as halloysites to attach to the oil–water interface and stabilize oil-in-water emulsions. Such halloysite nanotubes (HNT) serve as reservoirs for surfactants and can deliver surfactants to the oil–water interface and thus lower the oil–water interfacial tension. This two-step concept of surfactant delivery and droplet stabilization by particles has significant implications to oil spill remediation. However, to deliver surfactant loaded HNTs in a water-based solvent slurry, it is important to stopper the nanotubes to prevent premature release of the surfactant. This work focuses on the use of an environmentally benign two-dimensional metal–organic framework formed by coordinating Fe­(III) with a polyphenolic as a stoppering agent. Such metal–phenolic networks (MPN) form a skin around the HNTs, thus providing a way to effectively sequester surfactant cargo for controlled release. Cryo-scanning electron microscopy (Cryo-SEM) shows that these HNTs and HNT bundles attach to the oil–water interface with side-on orientation. Inverted drop tensiometry was used to characterize the dynamic interfacial tension resulting from the release of a model surfactant (Tween 80) from the HNTs and indicates that the MPN stoppers are effective in sequestering the surfactant cargo for extended periods at neutral pH values. Release triggered by MPN disassembly at acidic pH values can be performed just prior to delivery to oil spills. The concepts and scalability of this process have significant implications for oil spill remediation, enhanced oil recovery, and biomedical and pharmaceutical applications.

Nanoscale capsule-type particles with stimuli-respondent transport of chemical species into and out of the capsule are of significant technological interest. We describe the facile synthesis, properties, and applications of a temperature-responsive silica-poly(N-isopropylacrylamide) (PNIPAM) composite consisting of hollow silica particles with ordered mesoporous shells and a complete PNIPAM coating layer. These composites start with highly monodisperse, hollow mesoporous silica particles fabricated with precision using a template-driven approach. The particles possess a high specific surface area (1771 m 2 /g) and large interior voids that are accessible to the exterior environment through pore channels of the silica shell. An exterior PNIPAM coating provides thermoresponsiveness to the composite, acting as a gate to regulate the uptake and release of functional molecules. Uptake and release of a model compound (rhodamine B) occurs at temperatures below the lower critical solution temperature (LCST) of 32 °C, while the dehydrated hydrophobic polymer layer collapses over the particle at temperatures above the LCST, leading to a shutoff of uptake and release. These transitions are also manifest at an oil−water interface, where the polymer-coated hollow particles stabilize oil-in-water emulsions at temperatures below the LCST and destabilize the emulsions at temperatures above the LCST. Cryogenic scanning electron microscopy indicates patchlike particle structures at the oil−water interface of the stabilized emulsions. The silica-PNIPAM composite therefore couples advantages from both the hollow mesoporous silica structure and the thermoresponsive polymer.

Surfactant adsorption onto reservoir rock surfaces is a major issue in enhanced oil recovery (EOR) applications, decreasing the economic success of an EOR project. A method to minimize loss of surfactant is to encapsulate the surfactant and deliver it directly to the oil–water interface. This can be done through the use of naturally occurring clay nanotubes known as halloysites, where surfactants can be encapsulated in the lumen of the nanotubes. Halloysite nanotubes are about 1 μm in length with an outer diameter of about 70 nm and a lumen diameter of about 50 nm. These natural clay nanotubes are thermally stable, inexpensive, abundantly available, and environmentally friendly. An interesting aspect of the halloysite is that it has a predominantly negatively charged outer silica surface and a positively charged inner alumina surface. The surfactants are loaded into the halloysite nanotubes (HNTs) and coated with a thin layer of paraffin wax through a vacuum suction and solvent evaporation method. A thin paraffin wax coating/skin over the surfactant-loaded halloysites prevents the premature release of surfactants until they are in contact with oil, which promotes dissolution of the wax, releasing the surfactant. Imbibition experiments are carried out by measuring oil recovered after pushing injection fluids containing the HNTs through a capillary packed tightly with fresh and crude oil-saturated crushed shale cores. At 70 °C, the wax-coated surfactant-loaded halloysites system exhibited 40% oil recovery, compared to just 16% for surfactant alone and 3% for wax-coated halloysite (no surfactant). A much lower oil recovery for the surfactant alone (16%) can be attributed to excessive surfactant adsorption to the fresh core (85% adsorption), making these adsorbed surfactants unavailable to be in contact with the oil for enhancing the recovery. The method of surfactant encapsulation in wax-coated halloysites therefore leads to a targeted, stimulus-responsive delivery system to the oil–water interface in shale reservoirs with the potential to enhance oil recovery.