Thermodynamically stable pickering emulsions

Liquid-liquid interfacial tension of electrolyte solutions

It is theoretically shown that the excess liquid-liquid interfacial tension between two electrolyte solutions as a function of the ionic strength I behaves asymptotically as O(-sqrt[I]) for small I and as O(+/-I) for large I. The former regime is dominated by the electrostatic potential due to an unequal partitioning of ions between the two liquids whereas the latter regime is related to a finite interfacial thickness. The crossover between the two asymptotic regimes depends sensitively on material parameters suggesting that the experimentally accessible range of ionic strengths can correspond to either the small or the large ionic strength regime. In the limiting case of a liquid-gas surface where ion partitioning is absent, the image charge interaction can dominate the surface tension for small ionic strength I such that an Onsager-Samaras limiting law O(-Iln(I)) is expected.

Fluid-bicontinuous gels stabilized by interfacial colloids: low and high molecular weight fluids

Carefully tuned composite materials can have properties wholly unlike those of their separate constituents. We review the development of one example: colloid-stabilized emulsions with bicontinuous liquid domains. These non-equilibrium structures resemble the sponge mesophase of surfactants; however, in the colloid-stabilized case the interface separating the liquid domains is itself semi-solid. The arrangement of domains is created by arresting liquid-liquid phase separation via spinodal decomposition. Dispersed colloids exhibiting partial wettability become trapped on the newly created interface and jam together as the domains coarsen. Similar structures have been created in polymer blends stabilized using either interfacial nanoparticles or clay platelets. Here it has been possible to create the domain arrangement either by phase separation or by direct mixing of the melt. The low molecular weight liquid and polymer based structures have been developed independently and much can be learnt by comparing the two.

Charged colloidal particles and small mobile ions near the oil-water interface: destruction of colloidal double layer and ionic charge separation

We study suspensions of hydrophobic charged colloids in a demixed oil-water solvent with salt by means of a modified Poisson-Boltzmann theory, taking into account image-charge effects and partitioning of the monovalent ions. We find that the ion's aversion for oil can deform the double layers of the oil-dispersed colloids, which qualitatively affects the colloidal density profiles. The same theory also predicts crystallization of colloid-free micron-sized water-in-oil droplets at water volume fractions as small as approximately 10(-3) in a narrow range of the oil-dielectric constant. These findings explain recent observations by M. E. Leunissen et al. [Proc. Natl. Acad. Sci. U.S.A. 104, 2585 (2007)10.1073/pnas.0610589104].

Spontaneous oil-in-water emulsification induced by charge-stabilized dispersions of various inorganic colloids

Charge-stabilized dispersions of inorganic colloids are shown to induce spontaneous emulsification of hydrophobic (TPM) molecules to stable oil-in-water emulsions, with monodisperse, mesoscopic oil droplet diameters in the range of 30-150 nm, irrespective of the polydispersity of the starting dispersions. The results for cobalt ferrite particles and commercial silica sols extend our first study (Sacanna, S.; Kegel, W. K.; Philipse, A. Phys. Rev. Lett. 2007, 98, 158301) on spontaneous emulsification induced by charged magnetite colloids and show that this type of self-assembly is quite generic with respect to the composition of the nanoparticles adsorbing at the oil-water interface. Moreover, we provide additional experimental evidence for the thermodynamic stability of these mesoemulsions, including spontaneous oil dispersal imaged by confocal microscopy and monitored in situ by time-resolved dynamic light scattering. We discuss the possibility that thermodynamic stability of the emulsions is provided by the negative tension of the three-phase line between oil, water, and adsorbed colloids.

Oil-in-water emulsification induced by ellipsoidal hematite colloids: evidence for hydrolysis-mediated self-assembly

We report on the preparation of a novel type of particle-stabilized oil-in-water emulsions. The emulsification mechanism comprises partial hydrolysis of the oil phase promoted by the alkaline surface of ellipsoidal hematite colloids stabilized by tetramethylammonium hydroxide. This mechanism yields monodisperse oil droplets with embedded single ellipsoidal particles. The emulsions, which are stable for at least several months, can be polymerized by radical initiation, to yield latex-like particles with interesting optical and magnetic properties due to their anisotropic hematite cores. Moreover, we show that complex composite core-shell colloids can be prepared by PMMA growth and silica deposition on polymerized emulsion droplets. Finally, as an example of a possible application for our system, we have measured translational and rotational diffusion coefficients of hematite-stabilized oil droplets by depolarized dynamic light scattering. The latter technique can also be employed to monitor the spontaneous emulsification in time.