The biological synthesis of ethanol has been extensively studied as a sustainable route to biofuel production. However, separating ethanol from the aqueous fermentation mixture is often achieved through an energy intensive distillation, followed by water adsorption with molecular sieves. Liquid/liquid extraction followed by flash separation is a promising alternative route to achieve this separation with a lower cost and energy consumption. In fact, Offeman et al. [1] proposed the use of castor oil, as a renewable, low volatility and non-toxic solvent to achieve this separation, and compared it to other vegetable oils and extraction solvents. However, these systems often form relatively stable emulsions that need to be broken to complete the separation, as such industrial liquid/liquid extraction equipment have large settler regions with coalescence plates needed to obtain decantable immiscible liquid phases; thus, there is a practical need to increase the rate of demulsification of these systems in a controllable manner. This paper focuses on the stability of Pickering emulsions stabilized with nanocomposite magnetic nanoparticles and the accelerated demulsification by an external magnetic field, as relevant to the extraction of ethanol from aqueous solutions.
Pickering emulsions are stabilized by adsorbed solid particles rather than emulsifiers [2], [3], [4], [5], [6], [7], [8]. An additional advantage of Pickering emulsions over traditional emulsions stabilized by emulsifiers is their improved stability [9], [10]. Solid particle covering of droplets creates a formidable barrier against coalescence, stabilizing concentrated emulsions efficiently. Adsorbed solid particles are stabilized in the oil-water interface by being partially wetted by both immiscible liquid phases. In addition, magnetic nanoparticles (NPs) have been utilized to stabilize Pickering emulsions for a wide range of applications, including magnetically controllable emulsification and demulsification, oil recovery, wastewater treatment, magnetic separation, and liquid-liquid extraction [11], [12], [13], [14], [15], [16], [17], [18].
Iron and iron oxide-based NPs are among the most common types of magnetic NPs utilized to stabilize Pickering emulsions [19], [20], [21], [22]. Pavia-Sanders et al. reported that polyvinylpyrrolidone-coated Fe3O4 NPs had an exceptional capacity to remove oil in a short period of time (40 min) [23]. Similarly, Fe3O4/PS nanocomposites and superparamagnetic Fe3O4/β-cyclodextrin have been shown to remove oil from water [24]. Bare Fe3O4 NPs are hydrophilic and can be used to stabilize o/w emulsions [19]; however, the surface of the Fe3O4 NPs can be adjusted to become hydrophobic depending on the coating materials used for stabilizing the particles.
Cellulose and lignin are two of the most abundant natural biopolymers on earth. Cellulose and lignin-based NPs have been investigated for many decades because they are low-cost, non-toxic, renewable, and biocompatible [25], [26], [27]. Both cellulose nanocrystals and lignin nanoparticles are amphiphilic [28], [29] and they have been employed in Pickering emulsions by various authors [4], [10], [30], [31], [32], [33]. Moreover, binding iron oxide nanoparticles to cellulose or lignin NPs make the hybrid nanoparticles suitable for magnetically controllable Pickering emulsions and magnetically induced oil-water separations [11], [13], [22], [34], [35], [36]. Recently, our group developed castor oil/water Pickering emulsions by utilizing CNC@Fe3O4 NPs [13]. Mikhaylov et al. developed magnetite/cellulose nanocrystal-stabilized liquid paraffin oil-in-water Pickering emulsions and investigated the adsorption of chromium ions on the emulsions [22]. Low et al. synthesized Fe3O4-cellulose nanocrystals (MCNC) via ultrasound assisted co-precipitation method to stabilize palm olein-in-water Pickering emulsion [35]. Later, Low et al. reported using Fe3O4@cellulose nanocrystals-stabilized Pickering emulsions containing curcumin to magnetically trigger drug release [34]. On the other hand, the synthesis and applications of multifunctional magnetic-lignin nanoparticles in Pickering emulsions are less studied. Recently, Hasan et al. utilized lignin@Fe3O4 nanoparticles in magnetically controllable castor oil/water Pickering emulsion and oil herding [11]. Although, there are few examples of magnetic nanoparticles and lignin-based hybrid materials being used in Pickering emulsions and separation processes [37], [38], [39], [40].
Demulsification is the process of separating an emulsion into its component phases, often by separating the dispersed droplets from the continuous phase. Magnetic NPs have been used in stabilizing Pickering emulsions and demulsified on-demand with various types of magnetic fields in literature [11], [13], [14], [15]. In our previous studies, we successfully demulsified castor oil-water Pickering emulsions, stabilized by CNC@Fe3O4 NPs and lignin@ Fe3O4 NPs, using a permanent magnet, although in the case of CNC@Fe3O4, only the water/oil emulsions could be broken, while the oil/water emulsions remained emulsified after 24 h on a permanent magnet [11], [13]. Previously, Yang et al. [15] successfully studied demulsification of carbonyl iron particle (CIP) stabilized Pickering emulsions by using alternating magnetic field. In another study, water-in-oil (W/O) type commercially-available crude oil emulsion was successfully demulsified by using a permanent magnet and an electromagnetic field [16].
A number of parameters influence emulsion stability, including emulsifier type and concentration, droplet size and dispersion, processing conditions, electrolyte concentration, etc. [9], [10]. Hydrophilic particles are known to promote the formation of oil-in-water (o/w) emulsions, whereas hydrophobic particles typically tend to enable water-in-oil (w/o) emulsions [41]. However, amphiphilic particles with both hydrophilic and hydrophobic groups on their surface are known to produce either o/w or w/o emulsions depending on the interaction between the particles and the oil-water interface [42], [43].
In this paper, various castor oil/ethanol/water emulsions were stabilized by CNC@Fe3O4 NPs and lignin@ Fe3O4 NPs and were then broken on-demand by using a permanent magnet. Although, there are other examples of research articles that reported Pickering emulsions’ stability and magnetically controlled demulsification, most of the research was conducted for conventional two-component Pickering emulsions [2], [3], [4], [5], [6], [7], [8]. Here, we concentrated our efforts on conducting a comprehensive investigation into the stability of two-phase Pickering emulsions involving three components relevant to the liquid/liquid extraction of ethanol from water. It investigates a variety of factors affecting the stability of the castor oil/water/ethanol emulsions such as NP concentrations, oil to water ratios, ethanol concentrations, NP film wettability, and interfacial tension of the ternary system at equilibrium conditions. Additionally, we explored the application of magnetically controlled demulsification in the presence of ethanol, and the recyclability of the nanoparticles. Notably, this paper investigated the change in magnetic properties of the nanoparticles after three cycles of emulsification/demulsification/decantation steps to demonstrate their limited degradation after recycling.
