Charge‐Reversible Surfactant‐Induced Transformation Between Oil‐in‐Dispersion Emulsions and Pickering Emulsions

Abstract

A novel charge-reversible surfactant, (CH 3 ) 2 N-(CH 2 ) 10 COONa, was designed and synthesized, which together with silica nanoparticles can stabilize a smart n-octane-inwater emulsion responsive to pH. At high pH (9.3) the surfactant is anionic carboxylate, which together with the negatively charged silica nanoparticles co-stabilize flowable oil-in-dispersion emulsions, whereas at low pH (4.1) it is turned to cationic form by forming amine salt which can hydrophobize in situ the negatively charged silica nanoparticles to stabilize viscous oil-in-water (O/W) Pickering emulsions. At neutral pH (7.5), however, this surfactant is converted to zwitterionic form, which only weakly hydrophobises the silica particles to stabilize O/W Pickering emulsions of large droplet size. Moreover, demulsification can be achieved rapidly triggered by pH. With this strategy particles can be controlled either dispersed in water or adsorbed at the oil-water interface endowing emulsions with the capacity for intelligent and precise control of stability as well as viscosity and droplet size. Scheme 1. Chemical structure of 11-(N,N-dimethylamino) sodium undecanoate (DMUa, left) and charge variation with pH.

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.…”

“…Figure 1d shows that n-octane-in-water emulsions stable to coalescence can be obtained at a NaDC concentration as low as 0.08 mM after addition of 0.1 wt.% silica nanoparticles (diameter = 20 nm, [32] ζ = −29 mV in pure water, Figure S1). The average droplet size in emulsions decreases from 40-50 μm to 8-10 μm with increasing NaDC concentration from 0.01 mM to 0.2 mM (Figures 1e and S2), while it was less affected by changing the silica particle concentration (Figure S3).…”

“…[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.…”