Colloidal lattices of environmentally responsive microgel particles at ionic liquid–water interfaces

Abstract

This work reports new evidence of the versatility of soft and environmentally responsive micron-sized colloidal gel particles as stabilizers at ionic liquid-water droplet interfaces. These particles display a duality with properties ascribed typically to both polymeric and colloidal systems. The utilization of fluorescently labeled composite microgel particles allows in-situ and facile visualization without the necessity of invasive sample preparation. When the prepared particles form monolayers equilibrated at the ionic liquid-water interface on fully covered droplets, the colloidal lattice re-orders itself depending on the surface charge of these particles. Finally, we demonstrate that the spontaneously formed and densely packed layer of microgel particles can be employed for extraction applications, as the interface remains permeable to small active species.

Introduction

In recent years, ionic liquids (ILs) have drawn significant amount of attention as tunable “green solvents” used in the interdisciplinary fields of biotechnology and chemistry due to their high thermal stability [1], [2], [3], non-flammability [4], [5], [6], extensive electrochemical window [7], [8] and negligible vapor pressure properties [9], [10]. They are currently utilized in a vast array of applications including catalysis [11], [12], CO₂ absorption [13], [14], [15], gas storage [16], [17], and as promising solvents for selective extraction processes [18], [19], [20]. While recent work has established the versatility of ILs as novel extraction media, their efficient use in the extraction of chemically sensitive active species still remains largely absent. Thus, the implementation of emulsions with intrinsically large surface area is highly advantageous for the tailored development of IL-based extraction system [21], [22]. These formulated emulsions must be able to remain stable against coalescing via Ostwald ripening while maintaining the ability to demulsify in order to obtain the extracted product absorbed into the IL-phase. The task to form IL-water emulsions is challenging due to low interfacial tension derived from the inherently large miscibility of the two phases along with the highly structured IL-water interfaces resultant of charge hydration and correlations [23], [24].

Traditionally, surfactants have taken the spotlight for the preparation of thermodynamically stable IL-water microemulsions. However, for the design of selective extraction processes, most of the desired requirements are not met due to the inherent water solubility of most ILs leading to Ostwald ripening and ultimately emulsion destabilization [25], [26]. The employment of surfactants may also interact adversely with a variety of biomolecules inducing irreversible changes to the targeted bioproducts, especially for the extraction of compounds such as lipids [27], [28], organic acid [29], [30], and amphiphilic proteins from biological fluids [31], [32]. Meeting these requirements to develop a robust approach to create IL-in-water emulsions is highly sought-after for the advancement of IL-based extraction applications. Particle-stabilized emulsions, also referred to as Pickering emulsions [33], are surfactant-free metastable dispersions of two non-miscible fluids like oil and water. These emulsions display high stability, which makes them very attractive for storage applications. Recently, there has been growing interest in utilizing soft particles like microgels as emulsifiers for the design of responsive Pickering emulsions, which evolve “on demand” via an external stimulus [34], [35]. While previous works have established the adaptability and versatility of microgel particles in stabilizing oil-water interfaces, the details of its adsorption mechanism at these liquid interfaces and their interfacial packing conformation still remains a matter of much debate [36], [37], [38], [39], [40].

In this work, we demonstrate that soft stimuli-responsive composite microgel particles can be effectively utilized to stabilize IL-water interfaces. The microgel particles that we used in this investigation contain a fluorescent polystyrene core embedded in a thermosensitive and pH-responsive gel-shell composed of poly(N-isopropylacrylamide-co-acrylic acid). The rationale behind the design of the particle system is that it allows for non-invasive in-situ visualization of the structure of microgel-laden IL-water interfaces, while preserving their interfacial and multi-responsive properties. We investigate the impact of the microgel particles’ charge and hydrophobicity with their emulsification performance by varying solution temperature and pH. The morphology of the adsorbed soft gel-particles is characterized via direct macroscopic visualization, confocal microscopy observations, and cryo-SEM studies of the droplet interfaces. To determine the main structural parameters of these colloidal lattices, we have constructed Voronoi diagrams and statistically analyzed the defect number, concentration, and configuration in the light of various possible scenarios for the adsorption mechanism of prepared particles at the interface. Moreover, the self-assembled and densely packed layer of composite microgel particles at the IL-water interface does not hinder their potential application in IL-based extraction processes, as the interface remains permeable.