Structure and Stability of Bicontinuous Colloid-Stabilized Emulsions: Insights from X-ray Scattering and Microscopy

Abstract

Manufacturing industries often depend on solvents and energy-intensive separations. Sustainable alternatives are biphasic liquid-based reactions, which facilitate simultaneous reactions and separations without using solvents. Promising biphasic reactors are bicontinuous particle-stabilized emulsions (bijels), consisting of two interpenetrating liquid networks stabilized by an interfacial layer of colloidal particles. Their architecture enlarges the interfacial area between immiscible liquids and facilitates continuous mass transfer. Bijels are synthesized as fibers via solvent transfer induced phase separation (STrIPS), where a homogeneous precursor mixture of oil, water, solvent and surface-active nanoparticles undergoes spinodal decomposition upon solvent extraction. Stabilization of the resulting bicontinuous structure requires particles that are equally wetted by both liquids, achieved through in-situ adsorption of oppositely charged surfactants. The rapidly changing liquid composition during STrIPS, however, alters dynamic surfactant adsorption and interfacial behavior of particles, complicating reproducible bijel fabrication. Moreover, the formed bijel scaffold is mechanically fragile and prone to gradual destabilization. In this thesis, we explore how surfactant-particle interactions determine bijel formation and structure (Chapters 2-3), how bicontinuity can be preserved after synthesis (Chapter 4) and how mechanical strength can be regulated through covalent reinforcements (Chapter 5). Together, these insights provide strategies to transform STrIPS-bijels into robust, stable materials. Chapter 2 introduces a synchrotron-based microfluidic setup for in-situ monitoring the STrIPS-bijel formation via time-resolved (ultra-)small angle X-ray scattering ((U)SAXS). The liquid networks form through three consecutive stages: initial fluid dynamics, an induction period and spinodal decomposition. Notably, nanoparticles already begin to self-assemble into dense structures before phase separation. Chapter 3 investigates how surfactant concentration, particle loading and ionic strength affect the surfactant-particle interactions relevant for bijel formation. Time-resolved (U)SAXS and ex-situ confocal imaging reveal that increasing surfactant concentrations accelerate phase separation and particle self-assembly, and can halve the bijel formation time, although higher surfactant concentrations can disrupt assembled structures. These effects are attributed to the surfactant adsorption equilibrium, which can modulate interfacial tension, coarsening, interparticle attractions and interfacial rigidity. Furthermore, we find that bicontinuous structures are formed when particles weakly aggregate before phase separation. Chapter 4 then addresses transient stability of STrIPS-bijels after synthesis. Degradation is primarily driven by dissolution of the aqueous phase into the surrounding oil, followed by an oil inflow that disrupts liquid networks. This stability can be extended from hours to several weeks by suppressing the phase exchange by enriching the oil phase with water and glycerol, reducing air exposure, using high-viscosity oils and storing in hydrophobic containers. Lastly, Chapter 5 focuses on strengthening bijels through covalent reinforcing the particle scaffold using tetraalkoxysilanes with different alkyl-chain lengths. Short-chain silanes deposit silica more rapidly, shortening the reinforcement time from one day to 1.5 h while maintaining bicontinuity. We discover that silica grows preferentially towards aqueous channels, and that the deposition can be varied from selectively coating the particle scaffold to complete filling of aqueous channels by adjusting silane concentration and treatment time. By integrating insights into formation dynamics, stabilization and reinforcement, we establish a framework for reproducibly designing bijels with tunable structures, essential to apply bijels for industrially-relevant applications.