Fabrication of solvent transfer-induced phase separation bijels with mixtures of hydrophilic and hydrophobic nanoparticles.

Bicontinuous interfacially jammed emulsion gels (bijels), in which the oil and water phases are co-continuous throughout the structure, have potential for applications in separation, catalysis, tissue engineering and energy devices. Among the possible fabrication paths, the solvent transfer-induced phase separation (STRIPS) method has proven to be a powerful approach to produce bijels in a continuous fashion with a broad selection of liquids and nanoparticles. The successful formation of bicontinuous domains requires the use of neutrally wetting particles which was achieved by in situ modification of silica nanoparticles with an oppositely charged surfactant. This approach, however, is not ideal for applications that are adversely affected by the presence of surfactant. In this work, we use a pair of nanoparticles, one hydrophilic, and the other hydrophobic, to stabilize STRIPS bijels without any surfactants and show that the ratio of the hydrophilic to hydrophobic nanoparticles required to form stable bijels changes with the polarity of the oil phase. Highly non-polar oils require a smaller ratio than moderately polar oils. Furthermore, if a sufficiently polar oil is selected, STRIPS bijels can be stabilized using only the hydrophilic nanoparticle. Our results demonstrate the potential to imbue the interface of biphasic liquid mixtures such as bijels with multifunctionality by using two functional nanoparticles of opposite polarity.

Robust Bijels for Reactive Separation via Silica-Reinforced Nanoparticle Layers.

Pickering emulsions have been successfully used as media for catalysis and separation. However, simultaneous reaction and separation cannot be performed in a continuous mode in these systems, because reagents cannot be readily loaded into or recovered from the dispersed phase. Bicontinuous interfacially jammed emulsion gels (bijels), in which the oil and water phases are continuous throughout the structure, have potential as media for simultaneous reaction and separation in a continuous mode. In this work, we take a major step toward realizing this vision by demonstrating the ability of bijels to be used in reactive separation performed in a batch fashion. To perform effectively, bijels must maintain their morphology and interfacial mass transfer properties during reaction. To strengthen the bijels, we modify the solvent transfer-induced phase separation (STRIPS) method to make bijels resistant to mechanical stresses and prevent detachment of nanoparticles from the oil/water interface due to pH changes by chemically fusing the interfacial nanoparticles. The reinforced bijel is successfully tested in base-catalyzed hydrolysis of esters and remains robust under these challenging conditions.