Novel Oil-in-Water Emulsions Stabilised by Ionic Surfactant and Similarly Charged Nanoparticles at Very Low Concentrations

A review on recent development of foam Ceramics prepared by particle-stabilized foaming technique

Particle-stabilized technique for fabricating foam ceramics was developed in 2006. Porous ceramics with porosity over 95% can be prepared by this newly developed method. This foaming technique was derived from the principle of Pickering foam to a large extent. The high internal phase volume, narrow distribution of pore size as well as the structural stability of the Pickering system enable the final ceramic products to realize their functionality in a variety of applications. However, the interfacial aspect of the foaming system determines the final product in many ways, which brings this novel method details to explore and possibilities to challenge. The current review introduces the particle-stabilized method combining with colloid and surface science since particles are the building block of ceramic materials. The history of this newly invented method was mentioned at first, followed by foam ceramic products prepared by this foaming technique combining with corresponding mechanism. Some representative applications involving ceramic materials made by particle-stabilized method were discussed. At last, we conclude the overall article and put forward some outlooks and challenges about the future direction of this unique foaming technique.

Emulsions synergistic-stabilized by a hydroxyl sulfobetaine surfactant and SiO2 nanoparticles and their potential application for enhanced oil recovery

The emulsions formed by conventional surfactants have poor stability in high temperature and high salinity reservoirs, which limits the fluidity control ability of emulsion flooding systems. Hydroxyl sulfobetaine surfactants have excellent emulsifying properties and can maintain good activity under high temperature and high salinity conditions. In this study, an emulsion synergistic-stabilized by hydroxyl sulfobetaine surfactant LHSB and SiO2 nanoparticles was reported for the first time, and the feasibility of its enhanced oil recovery was investigated. The results show that the stability, temperature and salt resistance of the emulsion were significantly improved after adding nanoparticles, which positively affected the exploitation of harsh reservoirs. The synergistic-stabilized mechanism between LHSB and SiO2 nanoparticles was revealed by the measurements of zeta potential, surface tension and contact angle. Moreover, core flooding experiments reflect the emulsion synergistic-stabilized by LHSB and SiO2 nanoparticles can effectively enhance oil recovery by 11.41%. This study provides an emulsion flooding system with excellent performance for enhanced oil recovery in harsh reservoirs.

Design of Proanthocyanidins and TiO2 Nanoparticles-Based Novel Emulsions as a Platform for UV Protection

A white oil-in-water novel emulsion stabilized by TiO2 nanoparticles with UVB shielding properties and proanthocyanidins with antioxidant activity was prepared, where the proanthocyanidins aggregated at the oil-water interface to reduce interfacial tension while TiO2 nanoparticles were dispersed in the continuous water phase to hinder droplet coalescence. It was found that the average oil droplet size was less than 10 μm and decreased with the increase of proanthocyanidins concentration, but the increase of the content of TiO2 nanoparticles had little effect on it. The combination of TiO2 nanoparticles and proanthocyanidins was versatile for oil phases with different polarities, and the resulting emulsion exhibited high stability in the face of centrifugation, heating and prolonging storage time. After encapsulating the UVA filter avobenzone in white oil, the emulsion was endowed with the ability to resist UVB and UVA. Further, the emulsion showed great free radical scavenging ability for superoxide anion radical (⋅O2 - ), hydroxyl radical (⋅OH) with the clearance rate of over 70 %, indicating the good antioxidant activity. The ingenious combination of UVB, UVA filter and antioxidant with emulsion as carrier provides a new idea for the preparation of full-band sunscreen emulsion.

Charge Density Overcomes Steric Hindrance of Ferrocene Surfactant in Switchable Oil-in-Dispersion Emulsions

A ferrocene surfactant can be switched between single and double head form (FcN⁺ C₁₂ /Fc⁺ N⁺ C₁₂ ) triggered by redox reaction. FcN⁺ C₁₂ can neither stabilize an O/W emulsion alone nor an oil-in-dispersion emulsion in combination with alumina nanoparticles due to the steric hindrance of the ferrocene group. However, such steric hindrance can be overcome by increasing the charge density in Fc⁺ N⁺ C₁₂ , so that oil-in-dispersion emulsions can be co-stabilized by Fc⁺ N⁺ C₁₂ and alumina nanoparticles at very low concentrations (1×10^-7  M (≈50 ppb) and 0.001 wt %, respectively). Not only can reversible formation/destabilization of oil-in-dispersion emulsions be achieved by redox reaction, but also reversible transformation between oil-in-dispersion emulsions and Pickering emulsions can be obtained through reversing the charge of alumina particles by adjusting the pH. The results provide a new protocol for the design of surfactants for stabilization of smart oil-in-dispersion emulsions.

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

Bile salts (BS), one of the biological amphiphiles, are usually used as solubilizing/emulsifying agents of lipids or drugs. However, BS such as sodium deoxycholate (NaDC) can't stabilize an oil-in-water (O/W) emulsion alone due to its unusual molecular structure. In this paper we report that these emulsifiers with poor emulsifying ability can be transformed to highly efficient emulsifiers by combining with negatively charged particles (silica or montmorillonite). Both together can synergistically co-stabilize oil-in-water emulsions at extremely low concentrations (minimum 0.01 mM NaDC plus 0.003 wt% particles). Moreover, the emulsions can be reversibly switched between stable and unstable triggered by CO₂/N₂ at room temperature. This strategy is universal for emulsions containing different oils (alkanes, aromatic hydrocarbons and triglycerides) and for different BS and offers a generic model for a variety of BS of different molecular structure, which will extend their applications in more technical fields such as emulsion polymerization, biphasic catalysis and emulsion extraction.

Bile salts can be converted to efficient emulsifiers assisted by a trace amount of similarly charged nanoparticles and the emulsions formed are CO₂/N₂ switchable at room temperature.

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

A novel charge-reversible surfactant, (CH₃ )₂ N-(CH₂ )₁₀ COONa, was designed and synthesized, which together with silica nanoparticles can stabilize a smart n-octane-in-water 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.

Methyl-grafted silica nanoparticle stabilized water-in-oil Pickering emulsions with low-temperature stability

HYPOTHESIS

The viscosity of water-in-oil Pickering emulsions may dramatically increase upon cooling. The solvation of the long-chain alkyl groups grafted on the particles stabilizer is the likely cause of the strong dependence of rheological property on temperature. Thus, we hypothesize that silica nanoparticles (NPs) grafted with short-chain alkyl groups can stabilize Pickering emulsions, yielding weakly temperature-dependent rheological property.

EXPERIMENTS

Using alkyl-grafted (methyl, octyl, and octadecyl) silica NPs as emulsifiers, the rheological properties and microstructure of the water-in-oil Pickering, as well as the solvation of the silica NPs, were studied using diffusing-wave spectroscopy microrheology measurements, confocal laser scanning microscopy, and low-field nuclear magnetic resonance measurements.

FINDINGS

The use of methyl- and octadecyl-grafted silica NPs, which have almost identical optimum contact angles, to stabilize emulsions dramatically reduced the effect of cooling on the viscosity. Moreover, the emulsions stabilized by these methyl-grafted silica NPs exhibited nearly constant rheological properties as the temperature decreased from 75 to 5 °C. The nearly constant rheological properties are attributed to the nearly constant solvation in this temperature range. These materials have potential applications in the cosmetics and petroleum industries.

Behavior of Smart Surfactants in Stabilizing pH-Responsive Emulsions

Newly structured pH-responsive smart surfactants (N⁺ -(n)-N, n=14, 16) from alkyl trimethylammonium bromides are reported. In neutral and alkaline media N⁺ -(n)-N behaves as a normal cationic surfactant and stabilizes conventional emulsions alone, as well as Pickering emulsions and oil-in-dispersion emulsions together with oppositely and similarly charged nanoparticles, respectively. In acidic media N⁺ -(n)-N becomes a hydrophilic Bola-type surfactant, N⁺ -(n)-NH⁺ , and is an inferior emulsifier either when used alone or together with charged nanoparticles, resulting in demulsification. N⁺ -(n)-NH⁺ returns to the aqueous phase alone or together with nanoparticles after demulsification without contaminating the oil phase, and the aqueous phase can be recycled when triggered by pH change. This protocol is a green process and leads to preparation of various temporarily stable emulsions which are often used in emulsion polymerization, heterogeneous catalysis, and oil transportation.

Transition between a Pickering Emulsion and an Oil-in-Dispersion Emulsion Costabilized by Alumina Nanoparticles and a Cationic Surfactant

The transition between a novel oil-in-dispersion emulsion and an oil-in-water (O/W) Pickering emulsion triggered by pH was achieved using alumina nanoparticles in combination with a cationic surfactant. In acidic and neutral aqueous media, positively charged particles and the surfactant both at very low concentrations costabilize an oil-in-dispersion emulsion with the surfactant adsorbed at droplet interfaces and particles dispersed in the aqueous phase between the droplets. In alkaline media, however, particles become negatively charged and are hydrophobized in situ by adsorption of the surfactant to become surface-active and stabilize an O/W Pickering emulsion. The transition between the two is also possible by lowering the pH. The transformation can be achieved several times in a mixture of 0.1 wt % nanoparticles and 0.01 mM surfactant. This transition is significant, since particles can be made to either adsorb at the oil-water interface, which is beneficial for applications like biphasic catalysis, or remain dispersed in the aqueous phase, which is favorable for their recovery and reuse.

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

A redox-responsive oil-in-dispersion emulsion was developed by using a cationic ferrocene surfactant (FcCOC₁₀N) and Al₂O₃ nanoparticles, in which the required concentrations of FcCOC₁₀N and Al₂O₃ 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 FcCOC₁₀N from a normal cationic surfactant form into a strongly hydrophilic Bola type form (Fc⁺COC₁₀N). Moreover, Fc⁺COC₁₀N 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.

Redox and Doubly pH-Switchable Pickering Emulsion

In this work, a novel Pickering emulsion is stabilized by silica nanoparticles functioned with a redox and pH-responsive surfactant FA-DMDA-Ox that is prepared simply by direct neutralization of ferrocenecarboxylic acid (FA) and N,N-dimethyldodecylamine (DMDA) and exhibits redox and doubly pH-switchable behavior. Here, the Pickering emulsion can be stabilized easily by combining hydrophilic silica nanoparticles with less than 0.1 wt % FA-DMDA-Ox. After adding Na₂SO₃ and H₂O₂ alternately, the demulsification and emulsification of this Pickering emulsion are controlled reversibly. Moreover, the emulsion is switched "off" upon the addition of HCl and switched "on" upon the addition of NaOH and is also switched off upon the addition of NaOH and switched on upon the addition of HCl, which demonstrate the doubly pH-switchable behavior. Based on the analysis of ζ-potential, contact angle, and adsorbed amount of silica nanoparticles, the pH and redox-switchable mechanism of the Pickering emulsion are analyzed. Here, the redox-switchable behavior is induced by the reversible adsorption and desorption of FA-DMDA-Ox on the surface of silica nanoparticles. The pH-switchable behavior is driven by the controllable dispersion systems of silica nanoparticles and FA-DMDA-Ox because of the doubly pH switchability of FA-DMDA-Ox. More importantly, upon adding fresh oil after removing the original oil, the Pickering emulsion is recycled three times. Hence, the multiswitchable Pickering emulsion can be expected to be treated as a multifunctional material in practical applications, such as oil or wax removal in the petroleum industry.