Colloidal particles act in many ways like surfactant molecules, particularly if adsorbed to a fluid–fluid interface. Just as the water or oil-liking tendency of a surfactant is quantified in terms of the hydrophile–lipophile balance (HLB) number, so can that of a spherical particle be described in terms of its wettability via contact angle. Important differences exist, however, between the two types of surface-active material, due in part to the fact that particles are strongly held at interfaces. This review attempts to correlate the behaviour observed in systems containing either particles or surfactant molecules in the areas of adsorption to interfaces, partitioning between phases and solid-stabilised emulsions and foams.

Particles as surfactants—similarities and differences

Emulsions stabilised solely by colloidal particles

We present an approach to fabricate solid capsules with precise control of size, permeability, mechanical strength, and compatibility. The capsules are fabricated by the self-assembly of colloidal particles onto the interface of emulsion droplets. After the particles are locked together to form elastic shells, the emulsion droplets are transferred to a fresh continuous-phase fluid that is the same as that inside the droplets. The resultant structures, which we call "colloidosomes," are hollow, elastic shells whose permeability and elasticity can be precisely controlled. The generality and robustness of these structures and their potential for cellular immunoisolation are demonstrated by the use of a variety of solvents, particles, and contents.

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Colloidosomes: Selectively Permeable Capsules Composed of Colloidal Particles

1: Magnetic Particles: Preparation, Properties and Applications: M. Ozaki. 2: Maghemite (gamma-Fe2O3): A Versatile Magnetic Colloidal Material C.J. Serna, M.P. Morales. 3: Dynamics of Adsorption and Oxidation of Organic Molecules on Illuminated Titanium Dioxide Particles Immersed in Water M.A. Blesa, R.J. Candal, S.A. Bilmes. 4: Colloidal Aggregation in Two-Dimensions A. Moncho-Jorda, F. Martinez-Lopez, M.A. Cabrerizo-Vilchez, R. Hidalgo Alvarez, M. Quesada-PMerez. 5: Kinetics of Particle and Protein Adsorption Z. Adamczyk.

Surface and Colloid Science

Graphite oxide sheet, now called graphene oxide (GO), is the product of chemical exfoliation of graphite and has been known for more than a century. GO has been largely viewed as hydrophilic, presumably due to its excellent colloidal stability in water. Here we report that GO is an amphiphile with hydrophilic edges and a more hydrophobic basal plane. GO can act like a surfactant, as measured by its ability to adsorb on interfaces and lower the surface or interfacial tension. Since the degree of ionization of the edge −COOH groups is affected by pH, GO’s amphiphilicity can be tuned by pH. In addition, size-dependent amphiphilicity of GO sheets is observed. Since each GO sheet is a single molecule as well as a colloidal particle, the molecule−colloid duality makes it behave like both a molecular and a colloidal surfactant. For example, GO is capable of creating highly stable Pickering emulsions of organic solvents like solid particles. It can also act as a molecular dispersing agent to process insoluble mat...

Graphene Oxide Sheets at Interfaces

A theoretical treatment is developed for the wetting of a solid particle at an oil−water interface in terms of the components of the surface energies of all three phases. Calculated oil−water contact angles for a solid of given hydrophobicity with a range of oils of different polarity show good agreement with experimental data. For a hydrophobically modified silica surface, oils of increasing polarity give higher oil−water contact angles measured through water. The results are in line with the effect of oil type on Pickering emulsion type where more polar oils preferentially give water-in-oil emulsions. The approach is also used to predict the type of solid particles that would be required to stabilize emulsions of two immiscible oils. For hydrocarbon−fluorocarbon pairs, it is predicted that perfluorinated particles would stabilize fluorocarbon-external emulsions while silica particles with a high coverage of hydrocarbon groups should stabilize hydrocarbon-external emulsions.

Solid Wettability from Surface Energy Components: Relevance to Pickering Emulsions

We have studied the compression and structure of compressed monolayers of sulfate polystyrene latex particles on air/water and octane/water interfaces. If compressed sufficiently (on a Langmuir trough) the monolayers at air/water surfaces give rafts of hexagonally packed particles, while those at oil/water interfaces undergo a transition from the originally hexagonal to a rhombohedral structure. We have found that beyond collapse the particle monolayers on both air/water and octane/water interfaces fold and corrugate, and there is no expulsion of individual particles or particle aggregates from the interface. In the case of air/water interfaces, the structuring of particle monolayers (below collapse) was found to be very sensitive to the electrolyte concentration in the aqueous phase. At low electrolyte concentration, a fairly ordered structure resulting from the interparticle repulsion was observed, while at high electrolyte concentration, the particles form 2D clusters. In marked contrast, particle mono...

Compression and Structure of Monolayers of Charged Latex Particles at Air/Water and Octane/Water Interfaces

Using a laser tweezers method, we have determined the long-range repulsive force as a function of separation between two charged, spherical polystyrene particles ( $2.7\ensuremath{\mu}\mathrm{m}$ diameter) present at a nonpolar oil-water interface. At large separations (6 to $12\ensuremath{\mu}\mathrm{m}$ between particle centers) the force is found to decay with distance to the power $\ensuremath{-}4$ and is insensitive to the ionic strength of the aqueous phase. The results are consistent with a model in which the repulsion arises primarily from the presence of a very small residual electric charge at the particle-oil interface. This charge corresponds to a fractional dissociation of the total ionizable (sulfate) groups present at the particle-oil surface of approximately $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$.

Measurement of long-range repulsive forces between charged particles at an oil-water interface.

We have studied the effect of particle wettability on the structure of planar monolayers of micron size monodisperse silica particles at the octane−water interface. The particles' wettability was adjusted by silanization of their surfaces with dichlorodimethylsilane; thus their contact angle at the octane−water interface was in the range 70−150°. The modified particles were spread at the interface, and the monolayer structure was studied by direct microscope observation. A very distinct difference in the structure of monolayers depending on the particle contact angle has been observed. Very hydrophobic particles with large contact angles gave well-ordered monolayers at interparticle distances larger than 3 particle diameters. Such well-ordered monolayers in the case of silica particles are reported for the first time. The monolayers of less hydrophobic particles (with contact angles smaller than 115°) were totally disordered and aggregated. The disorder−order transition in the monolayer structure occurred...

John H. Clint

Papers

Emulsions stabilised solely by colloidal particles

Solid Wettability from Surface Energy Components: Relevance to Pickering Emulsions

Compression and Structure of Monolayers of Charged Latex Particles at Air/Water and Octane/Water Interfaces

Measurement of long-range repulsive forces between charged particles at an oil-water interface.

Order−Disorder Transition in Monolayers of Modified Monodisperse Silica Particles at the Octane−Water Interface