Layer-by-layer encapsulated nano-emulsion of ionic liquid loaded with functional material for extraction of Cd2+ ions from aqueous solutions
Graphical abstract
Introduction
Ionic liquids, which are salts that are liquid below 100 °C [1], show great potential [1], [2], [3] as environmentally-friendly extraction solvents due to their unique properties, such as negligible vapour pressure, high thermal stability, non-flammability [4]. More specifically, hydrophobic ionic liquids may be used for the separation of metal ions from aqueous solutions and wastewaters, where they can be specifically designed to act as extractors themselves [5] or, more commonly, serve as an extraction medium for active materials, such as crown ethers [6], [7] or any other ion-specific extractants [8]. However, with all their positive characteristics, most ionic liquids are expensive, therefore the necessity of their efficient usage appeals to a design of extraction systems that would be able to provide maximum extraction efficiency at minimum cost. One of the ways to increase the extraction efficiency is to increase the contact area between the ionic liquid and a solution containing ions that need to be extracted. This can be accomplished by creating a nano-emulsion of the ionic liquid, which greatly increases its surface area, minimising the amount of material required for extraction [9], [10], [11]. Moreover, nano-emulsions are kinetically stable, and their small size makes them resistant to sedimentation or creaming [12], providing greater control over the extraction process, and allowing for greater storage times. Several different methods for emulsification of ionic liquids have been reported including dispersing under high temperature [13], ultrasound [14] and vortex-assisted emulsification [15], all of which are used as a part of dispersive liquid-liquid micro-extraction procedure where emulsions are created within a working sample containing extractable ions [16], [17], [18], [19]. Other techniques that can also be used to create emulsions of ionic liquids are high-energy methods such as high-pressure homogenization [20], and low-energy methods such as spontaneous emulsification [21] and the phase inversion temperature method [22].
Since some environments that require ion extraction might not be suitable for in-situ emulsification of the extracting agent within them (large volumes of liquids that would require considerable energy to perform emulsification, or, in the case of low-energy methods, large quantities of chemicals), there is be a necessity for the creation of an extracting emulsion separately that can be further mixed with working solutions. These emulsions therefore must be stable enough for prolonged storage to be available to use on demand. Several methods of stabilization of ionic liquid nano- and micro-emulsions have been reported including micro-gel [10], ionic [23] and non-ionic surfactant stabilization methods [24], [25], [26]. Emulsions fabricated using the aforementioned methods have been reported to be stable, but it is unclear how stable they will be if used for extraction purposes. There is also a question of resistance of emulsions towards mechanical stress which usually accompanies filtration procedures associated with extraction and dilution.
In the current study, we have used a layer-by-layer encapsulated nano-emulsion system based on an ionic liquid as a medium for an active extractant material tested for the extraction of two ionic species, Cd2+ and Ca2+. The layer-by-layer technique involves consecutive deposition of oppositely charged materials onto core surfaces of various shapes. It was originally developed by Iler [27], who deposited alternating layers of silica and boehmite onto flat surfaces. The next study concerning this concept was published by Decher [28], [29] 25 years later who re-invented the idea using polyelectrolytes to layer oppositely charged surfaces. This method has then been successfully used for layering on flat surfaces, particles and emulsions using a variety of techniques [30], [31], [32], [33], [34], [35], [36], [37], [38]. In this study, a flow-based continuous layer-by-layer method for preparation of gram quantities of capsules introduced in our previous paper [39] was used for fabrication of multilayered polyelectrolyte shells formed around nano-emulsion droplets. The layer-by-layer method has been extensively used for encapsulation of emulsions [40], [41], [42], [43], [44], [45] and recently nano-emulsions [46], however, no studies, at least to our knowledge, have been reported regarding layer-by-layer on emulsified ionic liquids acting as a core material. In this paper, we show that it was possible to use an ionic liquid nano-emulsion as a core material for the layer-by-layer process. Two potential problems were anticipated in this work, first being the possibility of reduction of extraction efficiency by the growing thickness of the polyelectrolyte shell impeding the diffusion of ions into capsules, second – post-extraction capsule removal from ionic solutions due to their small size. Another aspect we address is extraction selectivity – the encapsulated nano-emulsion as it was specifically designed to extract Cd2+ ions, so the extraction of Ca2+ was also investigated. Finally, the long-term stability of the multilayered nano-emulsion was assessed.
Section snippets
Materials
Trihexyl(tetradecyl)phosphonium-bis(trifluoromethylsulfonyl)imide ([P14666][Tf2N], further referred to as ionic liquid, ⩾95.0%) loaded with 1-(2-Pyridylazo)-2-naphthol (transitional metal ion extractant) was emulsified in water using benzyldimethylhexadecylammonium chloride (cationic surfactant). Two polyelectrolytes chosen for fabrication of multilayers on the nano-emulsion were poly(diallyldimethylammonium chloride) (PDADMAC, average Mw 100,000–200,000 (low molecular weight), 20 wt% in H2O)
Fabrication of the core nano-emulsion
After preparation of the initial emulsion of extractant-loaded ionic liquid it was mixed with poly(sodium-4-styrenesulfonate) solution (4 g·L−1). The encapsulated emulsion was then centrifuged at 8000 rpm for 10 min. Zeta potential and DLS measurements were taken before and after the centrifugation in order to determine its impact on the stability and the particle size of the capsules. The comparison is presented in Table 1.
Since the size of the capsules has not changed, and even though zeta
Summary and conclusion
The nano-capsule based system for extraction of Cd2+ ions from aqueous solutions was created using emulsified encapsulated ionic liquid (Trihexyl(tetradecyl)phosphonium-bis(trifluoromethylsulfonyl)imide) as a medium for active extractant material (1-(2-Pyridylazo)-2-naphthol). Nano-capsules were created in three steps: emulsification of extractant-loaded ionic liquid also containing cationic surfactant in water using low-energy emulsification phase inversion (EPI) method, initial encapsulation
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