Complex coacervates and microgels for emulsions : robust, responsive, reversible

The use of ionic liquids (ILs) as replacement of organic solvents in liquid-liquid extractions has shown great promise due to their low volatility, flammability and toxicity, tunable solvency to a wide variety of extractable compounds and mild- ness to delicate compounds such as biomolecules for pharmaceutical applications. However, the efficiency of extractions using ionic liquids is limited as the inher- ently high viscosity of ILs slows down the mass transfer. Increasing the interfacial area between the immiscible phases is an efficient way to increase the efficiency of liquid extractions; typically done by formulating emulsions, dispersions of fluid droplets suspended in a second immiscible continuous phase. While strategies to formulate stable emulsions from conventional apolar solvents, such as aliphatic or halogenated oils, in water are abundant, the peculiar properties of ionic liquids requires the exploration of new strategies to formulate stable emulsions; for exam- ple, common surfactant stabilization leads to rapid Ostwald ripening due to the inherent water solubility of many ionic liquids. Moreover, while the intended ionic liquid-in-water emulsions must be stable at operating temperatures for prolonged times, it should be possible to break the emulsion on-demand to recover the ex- tracted product. Also, the interfacial layer used for stabilization should not hinder the transfer of the intended product to the droplet phase. To increase the sus- tainability of extraction processes, recovery of both ionic liquid and stabilizer for re-use in a subsequent extraction step is highly desired. Aimed to establish new ways of stabilizing emulsions in general, and ionic liquid emulsions in specific, this thesis describes investigations into two novel stabilizers: interfacial electrostatic complexes and soft colloidal microgels. In Part I, we focussed on how oppositely charged polyelectrolytes interact and form complexes across an oil-water interface. In Chapter 2, we demonstrated a new method for emulsion stabilization, in which electrostatic complexes formed across a liquid interface between two polyelectrolytes, one dissolved in the aqueous phase, the other in the oil phase. Using tensiometry we followed the polyelectrolyte adsorption at the oil-water interface; while the presence of either polyelectrolyte alone leads to interfacial depletion, the presence of both species leads to strong adsorption at the interface. This was further confirmed using confocal fluorescence microscopy where the colocalization of both species at the interface was observed; the strong overlap of peak intensities at the interface suggests a strongly intermixed layer. Using this approach, we prepared stable emulsions, which could be reversibly broken and reformed by simple pH and salt triggers. Interestingly, oil-in-water but also water-in-oil emulsions could be produced. This is the first demonstration of using selective associative phase separation to stabilize a segregating system. The experimental results triggered questions on the nature of the interfacial layer, which was too thin to be ascertained in detail using microscopy. Therefore, we turned to self-consistent field (SCF) modelling to develop a deeper understand- ing of the structure and thermodynamics of this interfacially-templated complex- ation, as presented in Chapter 3. In analogy with our experiments, we use the Scheutjens-Fleer lattice method to consider mixtures of two solvents, an anionic oil- soluble polyelectrolyte, a cationic water-soluble polyelectrolyte, their counterions and additional indifferent monomeric electrolyte. We first considered a two-phase system with only one polyelectrolyte and salt. We found that the polyelectrolyte adsorption depends on its concentration. For polyelectrolyte concentrations lower than the salt concentration, the polyelectrolyte is depleted from the oil-water in- terface while for polyelectrolyte concentrations higher than the salt concentration, the polyelectrolyte adsorbs at this interface. This transition from depletion to ad- sorption originates from a competition between small ion and macroion adsorption, governed by the overall ionic strength. Upon introducing a second polyelectrolyte in the immiscible second solvent, a new phase spontaneously formed at the inter- face between oil and water. Surprisingly, our calculations showed that ion release entropy is not the driving force for complexation, as it often is in bulk complex coacervation; co-assembly is governed by enthalpic contributions. This is due to the solvent-selectivity of the polyelectrolytes in this scenario, which leads to low solvent content in the coacervate layer, hence close approach of the opposite charges resulting in a relatively large Coulombic enthalpy. Finally, we examined systems with asymmetric composition of the two polyelectrolytes within the same theoret- ical approach. This revealed an unusual pseudo-partial wetting scenario, due to interactions occurring at different length scales. When the electrostatic interactions are short ranged, the microscopically thin wetting film transitions to a mesoscopic thin film. However, charges built up on either side of the coacervate layer restrict the growth of the film to macroscopic dimensions. In our experiments we observe that the coacervate layer becomes turbid over time, suggesting structures on op- tical length scales, much larger than the typical dimensions of the polymer coils. This may be explained by the pseudo-partial wetting scenario due to the coexis- tence of a mesoscopic film with interfacial liquid droplets nucleating due to thermal fluctuations. In the second part of this thesis, Part II, we studied the adsorption and or- ganization of colloidal microgels at a variety of liquid interfaces. These soft and deformable hydrogel colloids have gained a lot of interest in recent years due to their excellent ability to stabilize emulsions. As a result of their polymeric nature and osmotic equilibrium with the bulk solution, microgels exhibit an interesting duality between colloidal properties and polymeric behaviour. Microscopic research into their interfacial behaviour is often made difficult as they offer little refractive index contrast to the continuous phase and covalent attachment of fluorophores is known to drastically alter their interactions. To overcome this problem, in Chapter 4 we introduce composite microgels, in which a solid fluorescent core is embedded in the centre of a soft and tunable hydrogel shell, thereby decoupling the imaging features of these microgels with the tunability of their softness, size, solvent-responsivity and interactions. We surprisingly find that while these microgels adsorb sponta- neously, without any energy barrier which is usual for the Pickering adsorption of micron-sized colloids, their anchoring at the liquid interface is irreversibly strong. Due to the high adsorption energy, saturated interfacial layers of these microgels show mild compression of the particles, increasing their packing density at the cost of elastic deformation. Moreover, we showed that these particles are able to stabilize a wide variety of oil-water interfaces and due to their spontaneous adsorp- tion allow the fabrication of Pickering droplets using microfluidics, which is usually hindered by the adsorption barrier for solid particles. In Chapter 5, we arrive at the ultimate aim of this thesis, i.e. to provide proof- of-concept for a fully sustainable extraction process based on IL-in-water emulsions. We first show how microgels are able to create emulsions of a wide variety of ILs in water and prevent their Ostwald ripening, resulting in extended stability at room temperature. Upon heating and applying centrifugal compression, the emulsion can be rapidly broken, with all of the microgels returning the aqueous phase which can then be re-used in a secondary extraction step. Finally, we demonstrated that through the use of a paramagnetic ionic liquid, the concentration and breaking step can be performed without energy input with a simple permanent magnet, rendering the process sustainable from start to end. Finally, in Chapter 6, we studied the adsorption and conformation of these composite microgels at solid-liquid interfaces. We first demonstrate how conven- tional sample preparation for studying microgels at solid interfaces, often involving a drying step, induces strong sample artefacts. We therefore developed a method to study the adsorption and conformation of microgels in-situ using liquid-state confocal and atomic force microscopy. Our results showed how the packing density for particle adsorption is governed by particle-particle repulsion, as adsorption en- ergies are typically very high. Using Quantitative Nanomechanical Mapping, the spatially-resolved mechanical analysis of surfaces using atomic force microscopy, we find that the degree of spreading of microgels during adsorption at a solid interface is governed by adsorption energy and particle softness as expected. This leads us to conclude that the unique properties of microgels at interfaces results from a subtle interplay between adsorption energy and internal elasticity.