Pickering emulsions stabilized by hydrophilic nanoparticles: in situ surface modification by oil

We propose a novel route for the stabilization of oil-in-water Pickering emulsions using inherently hydrophilic nanoparticles. In the case of dialkyl adipate oils, in situ hydrophobisation of the particles by dissolved oil molecules in the aqueous phase enables stable emulsions to be formed. Emulsion stability is enhanced upon decreasing the chain length of the oil due to its increased solubility in the precursor aqueous phase. The oil thus acts like a surfactant in this respect in which hydrogen bonds form between the carbonyl group of the ester oil and the hydroxyl group on particle surfaces. The particles chosen include both fumed and precipitated anionic silica and cationic zirconia. Complementary experiments including relevant oil–water–solid contact angles and infra-red analysis of dried particles after contact with oil support the proposed mechanism

Oil-in-oil emulsions stabilised solely by solid particles

A brief review of the stabilisation of emulsions of two immiscible oils is given. We then describe the use of fumed silica particles coated with either hydrocarbon or fluorocarbon groups in acting as sole stabilisers of emulsions of various vegetable oils with linear silicone oils (PDMS) of different viscosity. Transitional phase inversion of emulsions, containing equal volumes of the two oils, from silicone-invegetable (S/V) to vegetable-in-silicone (V/S) occurs upon increasing the hydrophobicity of the particles. Close to inversion, emulsions are stable to coalescence and gravity-induced separation for at least one year. Increasing the viscosity of the silicone oil enables stable S/V emulsions to be prepared even with relatively hydrophilic particles. Predictions of emulsion type from calculated contact angles of a silica particle at the oil–oil interface are in agreement with experiment provided a small polar contribution to the surface energy of the oils is included. We also show that stable multiple emulsions of V/S/V can be prepared in a two-step procedure using two particle types of different hydrophobicity. At fixed particle concentration, catastrophic phase inversion of emulsions from V/S to S/V can be effected by increasing the volume fraction of vegetable oil. Finally, in the case of sunflower oil + 20 cS PDMS, the study is extended to particles other than silica which differ in chemical type, particle size and particle shape. Consistent with the above findings, we find that only sufficiently hydrophobic particles (clay, zinc oxide, silicone, calcium carbonate) can act as efficient V/S emulsion stabilisers

Collodial particles at a range of fluid-fluid particles

The study of solid particles residing at fluid-fluid interfaces has become an established area in surface and colloid science recently experiencing a renaissance since around 2000. Particles at interfaces arise in many industrial products and processes like anti-foam formulations, crude oil emulsions, aerated foodstuffs and flotation. Although they act in many ways like traditional surfactant molecules, they offer distinct advantages also and the area is now multi-disciplinary involving research in the fundamental science and potential applications. In this Feature Article, a flavour of some of this interest is given based on recent work from our own group and includes the behaviour of particles at oil-water, air-water, oil-oil, air-oil and water-water interfaces. The materials capable of being prepared by assembling various kinds of particles at fluid interfaces include particle-stabilised emulsions, particle-stabilised aqueous and oil foams, dry liquids, liquid marbles and powdered emulsions

Whipped oil stabilised by surfactant crystals

We describe a protocol for preparing very stable air-in-oil foams starting with a one-phase oil solution of a fatty acid (myristic acid) in high oleic sunflower oil at high temperature. Upon cooling below the solubility limit, a two-phase mixture consisting of fatty acid crystals (length around 50 μm) dispersed in an oil solution at its solubility is formed which, after whipping, coat air bubbles in the foam. Foams which do not drain, coalesce or coarsen may be produced either by increasing the fatty acid concentration at fixed temperature or aerating the mixtures at different temperatures at constant concentration. We prove that molecular fatty acid is not surface-active as no foam is possible in the one-phase region. Once the two-phase region is reached, fatty acid crystals are shown to be surface-active enabling foam formation, and excess crystals serve to gel the continuous oil phase enhancing foam stability. A combination of rheology, X-ray diffraction and pulsed nuclear magnetic resonance is used to characterise the crystals and oil gels formed before aeration. The crystal-stabilised foams are temperature-sensitive, being rendered completely unstable on heating around the melting temperature of the crystals. The findings are extended to a range of vegetable oil foams stabilised by a combination of adsorbed crystals and gelling of the oil phase, which destabilise at different temperatures depending on the composition and type of fatty acid chains in the triglyceride molecules

Dry oil powders and oil foams stabilised by fluorinated clay platelet particles

A series of platelet sericite particles coated to different extents with a fluorinating agent has been characterised and their behaviour in mixtures with air and oil studied. The material which forms by vigorous shaking depends on both the surface tension of the oil and the surface energy of the particles which control their degree of wetting. Oil dispersions are formed in liquids of relatively low tension (<22 mN m−1), e.g. hexane and cyclomethicone, for all particles. Particle-stabilised air-in-oil foams form in liquids of higher tension, e.g. dodecane and phenyl silicone, where the advancing three-phase contact angle θ, measured on a planar substrate composed of the particles into the liquid, lies between ca. 65° and 120°. For oils of tension above 27 mN m−1 like squalane and liquid paraffin with particles for which θ > 70°, we have discovered that dry oil powders in which oil drops stabilised by particles dispersed in air (oil-in-air) can be prepared by gentle mixing up to a critical oil : particle ratio (COPR) and do not leak oil. These powders, containing up to 80 wt% oil, release the encapsulated oil when sheared on a substrate. For many of the systems forming oil powders, stable liquid oil marbles can also be prepared. Above the COPR, catastrophic phase inversion occurs yielding an ultra-stable air-in-oil foam. We thus demonstrate the ability to disperse oil drops or air bubbles coated with particles within novel materials

Pickering emulsions responsive to CO₂/N₂ and light dual stimuli at ambient temperature

A dual stimulus-responsive n-octane-in-water Pickering emulsion with CO₂/N₂ and light triggers is prepared using negatively charged silica nanoparticles in combination with a trace amount of dual switchable surfactant, 4-butyl-4-(4-N,N-dimethylbutoxyamine) azobenzene bicarbonate (AZO-B₄) as stabilizers. On one hand, the emulsion can be transformed between stable and unstable at ambient temperature rapidly via the N₂/CO₂ trigger, and on the other hand a change in droplet size of the emulsion can occur upon light irradiation/re-homogenization cycles without changing the particle/surfactant concentration. The dual responsiveness thus allows for a precise control of emulsion properties. Compared with emulsions stabilised by specially synthesized stimuli-responsive particles or by stimuli-responsive surfactants, the method reported here is much easier and requires relatively low concentration of surfactant (≈1/10 cmc), which is important for potential applications

A lithium depletion boundary age of 21 Myr for the Beta Pictoris moving group

Optical spectroscopy is used to confirm membership for 8 low-mass candidates in the young Beta Pic moving group (BPMG) via their radial velocities, chromospheric activity and kinematic parallaxes. We searched for the presence of the Li I 6708A resonance feature and combined the results with literature measurements of other BPMG members to find the age-dependent lithium depletion boundary (LDB) -- the luminosity at which Li remains unburned in a coeval group. The LDB age of the BPMG is 21 +/- 4 Myr and insensitive to the choice of low-mass evolutionary models. This age is more precise, likely to be more accurate, and much older than that commonly assumed for the BPMG. As a result, substellar and planetary companions of BPMG members will be more massive than previously thought.Comment: Accepted for MNRAS Letter

Mechanical compression to characterize the robustness of liquid marbles

In this work, we have devised a new approach to measure the critical pressure that a liquid marble can withstand. A liquid marble is gradually squeezed under a mechanical compression applied by two parallel plates. It ruptures at a sufficiently large applied pressure. Combining the force measurement and the high-speed imaging, we can determine the critical pressure that ruptures the liquid marble. This critical pressure, which reflects the mechanical robustness of liquid marbles, depends on the type and size of the stabilizing particles as well as the chemical nature of the liquid droplet. By investigating the surface of the liquid marble, we attribute its rupture under the critical pressure to the low surface coverage of particles when highly stretched. Moreover, the applied pressure can be reflected by the inner Laplace pressure of the liquid marble considering the squeezing test is a quasi-static process. By analyzing the Laplace pressure upon rupture of the liquid marble, we predict the dependence of the critical pressure on the size of the liquid marble, which agrees well with experimental results