Stimuli responsive gold nanoparticles of size smaller than 30 nm are able to cross toluene-water interfaces in both directions. The crossing was observed by finely tuning electrostatic, hydrophobic and interfacial interactions. A model accounting for the latter interactions was proposed to explain the crossing experiments.

Colloid and interface science has been developed in many aspects and now many phenomena taking place in the colloidal scale and/or at the interface are well described by theory and fully addressed by experiments. However, some fundamental and "old" problems have remained unsolved up to now. Here, I would like to address the "simple" case of contact angles (CAs) of submicron particles at the water interface. Is the macroscopic CA, measured by the wetting angle of a water drop on a macroscopic substrate, the same as the CA of a submicron particle at the water interface? In this presentation, I will explain how to measure in-situ CAs of colloidal particles at the fluid interface using ellipsometry, compare different CAs measurements and discuss the role of electrostatics at the water surface.

What happens at the interface of particle dispersions ? Do particles lie at the wetting angle and fully pack at the water surface ? In the first part of this talk, I will introduce some issues on the attachment of (nano)particles at the water interface. Interfacial energy landscapes will be discussed after presenting in-situ contact angle measurements of 100 nm particles at the air-water. Interfaces populated by silica (nano)particles and particle-stabilised foams will be described in second part of the talk. Here, indirect and in-situ experiments on planar interfaces , free standing films , bubbles and foams will help understanding the stabilization mechanisms in particle-stabilised aqueous foams.

Swap transactions: Bidirectional spontaneous transfer of gold nanoparticles coated with stimuli-responsive polymer brushes across oil-water interfaces has been implemented. The water-to-oil transfer of the gold nanoparticles is dictated by the ionic strength in water, while the nanoparticle oil-to-water transfer occurs only when the environmental temperature is reduced below 5 °C.

The conformation and the dilatational properties of three non-ionic triblock PEO-PPO-PEO (where PEO is polyethyleneoxide and PPO is polypropyleneoxide) copolymers of different hydrophobicity and molecular weight were investigated at the water-hexane interface. The interfacial behavior of the copolymers was studied by combining dilatational rheology using the oscillating drop method and ellipsometry. From the dilatational rheology measurements the limiting elasticity values, E_0, of the Pluronics as function of surface pressure, PI, and adsorption time were obtained, i.e. E_0(t) and E-0(PI). Here, it is shown that E_0(t) depends on the number of PEO units and on the bulk concentration, showing maximum and minimum surface elasticity values which indicate conformational changes in the interfacial layer. Furthermore, in the framework of the polymer scaling law theory, conformational transitions were discussed in E_0 vs. PI plots. In a dilute regime (PI < 14 mN m−1) at the water-hexane interface, E_0 = 2PI fits well all the data, which indicates a two-dimensional "stretched chain" conformation. Increasing PI, two other interfacial transitions could take place. The different behavior of Pluronic copolymers could be also described by the local minima of E_0, which depends on the hydrophobicity of the copolymers. Conformational transitions observed by interfacial rheology were compared to ellipsometric data. Experimental results were discussed and explained on the basis of two- and three-dimensional copolymer structure taking into account that PPO chains could be partially immersed in hexane and water.

In this paper the mechanism behind the stabilization of Pickering emulsions by stacked catanionic micro-crystals is described. A temperature-quench of mixtures of oppositely charged surfactants (catanionics) and tetradecane from above the chain melting temperature to room temperature produces stable oil-in-water (o/w) Pickering emulsions in the absence of Ostwald ripening. The oil droplets are decorated by stacks of crystalline discs. The stacking of these discs is controlled by charge regulation as derived from conductivity, scattering and zeta potential measurements. Catanionic nanodiscs are ideal solid particles to stabilize Pickering emulsions since they present no density difference and a structural surface charge which is controlled by the molar ratio between anionic and cationic components. The contact angle of catanionic nanodiscs at a water/oil interface is also controlled by the non-stoichiometry of the components. The resulting energy of adhesion and the repulsion between droplets is much larger than kT. As a consequence of these unique properties of nanodiscs, this type of emulsions presents an extremely high resistance towards coalescence and creaming, even in the presence of salt.