Droplet–Bijel–Droplet Transition in Aqueous Two-Phase Systems Stabilized by Oppositely Charged Nanoparticles: A Simple Pathway to Fabricate Bijels

Abstract

We demonstrate a novel yet straightforward methodology of stabilizing aqueous two-phase systems (ATPS) using oppositely charged nanoparticles (OCNPs). We employ commercialgrade, Ludox, OCNPs to induce self-assembly. This self-assembly route promotes the stronger adsorption of nanoparticles at the water− water interface by triggering the formation of 2D and 3D aggregates of varying sizes and shapes. The interplay of this size and shape promotes stability due to increased Gibbs detachment energy and modulates the resulting cluster adsorption at the interface, thereby the structural state of emulsions. We demonstrate the influence of polymers' and particles' composition on the structural transformation from droplet−bijel− droplet using a phase diagram. For the first time, such a structural transition and the single pathway are reported within the domain of ATPS to produce stable bijels or colloidal capsules. It is asserted that the essential condition of three-phase contact angle (θ) = 90°t o favor the formation of bijels can be established by selecting a suitable experimental condition using a phase diagram without employing any complicated surface modification procedures reported in the literature. Further, the mechanistic route favoring the formation of bijels and emulsion droplets at different experimental regimes is presented based on the empirical study using turbidity and zeta potential measurements. These studies reveal that the formation of bijels will be most favored when the parameter M (ratio of weight fraction of positively charged nanoparticles to negatively charged nanoparticles) is chosen between 0.7 and 4. It is intriguing to note the fact that, while the droplets stabilized by OCNPs have shown good resilience under high centrifugal action, the bijels produced in this way continued to remain stable for a long time, offering a facile route to prepare the bijels with a hierarchical bicontinuous network structure.

Multicompartment Hydrogels

“…A transition of W/W emulsion‐based compartment hydrogels and bijels was introduced by Sabapathy and co‐workers. [ 181 ] The authors employed a new way of stabilizing the phases via assembly of oppositely charged LUDOX nanoparticles. Depending on mixture composition (ratio of charged particles) and respective polymer concentration (PEG and dextran), a bicontinuous bijel phase or droplets were generated.…”

Particle–Polymer Union with Changeable Wettability for Constructing Bijels Using a Simple Mixing Method

“…The wetting of particles plays an essential role in the construction of bijels, needing the appropriate wettability of stabilizers to generate the zero curvature and the bicontinuous liquid regions . The common strategy to adjust the surface properties of particles is in situ modification by using surfactants or polymers, which is more gentle compared with chemical modification.…”

Exploiting Heteroaggregation to Quantify the Contact Angle of Charged Colloids at Interfaces

“…The heteroaggregation of colloidal particles with different compositions, sizes, or charges is a fundamentally important phenomenon that caters to the need for technological developments in ceramics, core–shell composites, oil recovery, and pharmaceuticals. Similarly, the aggregation of oppositely charged macro-ions such as polyelectrolytes and other biological systems have been widely studied in the context of technological applications such as microencapsulation and the layer-by-layer deposition of oppositely charged polyelectrolytes. , The numerous studies on different aspects of fundamentals and applications of heteroaggregation is a testament to the broad interest vested by researchers working in this field. ,− Recently, there has been renowned interest in the stabilization of oil–water and water–water interfaces using oppositely charged particle–particle and particle–polyelectrolyte mixtures. − …”