Redox-Responsive Oil-In-Dispersion Emulsions Stabilized by Similarly Charged Ferrocene Surfactants and Alumina Nanoparticles

Abstract

A redox-responsive oil-in-dispersion emulsion was developed by using a cationic ferrocene surfactant (FcCOC 10 N) and Al 2 O 3 nanoparticles, in which the required concentrations of FcCOC 10 N and Al 2 O 3 nanoparticles are as low as 0.001 mM (≈0.005 cmc) and 0.006 wt %, respectively. Rapid demulsification can be successfully achieved through a redox trigger, resulting from the transition of FcCOC 10 N from a normal cationic surfactant form into a strongly hydrophilic Bola type form (Fc + COC 10 N). Moreover, Fc + COC 10 N together with the particles almost resides in the aqueous phase and can be recovered after the reduction reaction. Not only the amount of surfactant and nanoparticles are significantly reduced but also the emulsifier (surfactant and alumina) can be recycled and reused from the aqueous phase, which is a sustainable and economical strategy for various applications.

Conversion of bile salts from inferior emulsifier to efficient smart emulsifier assisted by negatively charged nanoparticles at low concentrations

“…We note that some Pickering emulsions can also be stabilized when the interfacial coverage by particles is quite low. [36,37] Here, we believe that only NaDC molecules adsorb at the oil-water interface with silica particles dispersed in the continuous phase between droplets [31][32][33][34][35] producing an oil-indispersion emulsion [35] as shown by the SEM and fluorescence microscopy images (Figures 2 and S7). It was further observed that this strategy of improving the emulsifying ability of BS by addition of silica particles is universal for other BS.…”

“…The fluorescence image (particles labelled green in Figure S7) also proves the position of the particles is mainly in the aqueous continuous phase. [31][32][33][34][35] This microstructure is different from that of Pickering emulsions stabilized by NaDC and positively charged alumina nanoparticles (Figure 2d), in which particles attach to the oilwater borders forming winkled films on the surfaces of dried oil droplets. We note that some Pickering emulsions can also be stabilized when the interfacial coverage by particles is quite low.…”

“…[31] In contrast, if silica nanoparticles are replaced by positively charged particles such as alumina nanoparticles (Figure S10), [31] the zeta potential was observed to decrease from +34 mV to -10 mV, indicating strong adsorption of NaDC on alumina particle surfaces. The electrostatic repulsion between droplets induced by the negatively charged carboxylate group in NaDC and that between particles and droplets is responsible for the stabilization of the emulsion, [31][32][33][34][35] whereas the hydrogen bonding between NaDC and silica particles if present is not crucial to emulsion stabilization, since BS without hydroxyl groups (sodium dehydrocholate) also yield a stable emulsion in combination with silica particles. However, in contrast to the oil-in-dispersion emulsions stabilized by CTAB in combination with alumina nanoparticles where CTAB adsorbs at the oilwater interface as a good emulsifier, [31] the current system seems to be unusual since NaDC is an inferior emulsifier (Figure 1c) and may desorb from the interface similar to other BS.…”

“…As shown by the SEM images (Figures 2b and 2c), the particles constitute thick lamellae between droplets resulting in an increase of the inter-droplet spacing and thus a reduction of the van der Waals attraction between droplets. [31][32][33][34][35] However, no stable oil-in-dispersion emulsion can be obtained above the cmc of NaDC (> 2 mM, Figure S12). On the one hand, NaDC alone is a poor emulsifier and cannot stabilize an O/W emulsion.…”

pH-Responsive Nanoemulsions Based on a Dynamic Covalent Surfactant

“…Responsive emulsifiers have emerged as a class of smart surfactants that permit regulating the emulsion stability under external stimuli [4][5][6][7][8]. The external stimuli include pH [9][10][11][12][13][14][15], CO 2 [6,16], temperature [17], magnetism [8], light [18,19], and redox [7,20]. Among these external stimulation types, pH has attracted particular interest because it has the advantage of being simple to regulate [11,21].…”