Nanoscale wetting of individual colloids at liquid interfaces
Measuring the wetting and position of a colloidal particle relative to a liquid interface as well as the interface microstructure are key elements to disclose the properties of materials self-assembled at liquid interfaces, including particle-stabilized emulsions. Up to now, in situ investigations of the wetting properties of individual colloids and structural characterization of liquid interfaces at the single-particle level were limited to particles larger than 500nm. We have recently developed a new technique, which exploiting freeze-fracturing, shadow-casting (FreSCa) cryo-SEM now pushes this limit down to the 10nm range.
With this method mm-sized planar particle-laden oil-water interfaces are exposed upon fracture and imaged with a cryo-SEM after ultrafast freezing. 3D information on the particle position relative to the interface is obtained by coating the interface with a thin tungsten layer at a 30° angle relative to the interface so that particles trapped at and protruding from the interface leave a shadow behind them. By measuring directly the particle size at the interface and the shadow length, one can calculate the vertical position of individual nanoparticles at the interface with sub-nanometer accuracy and thus their individual contact angles θ =cos-1(|h-r|/r), where h, is the protrusion height of the particle from the interface into the oil and r is its radius. The method applies both to hydrophobic and hydrophilic particles, has no intrinsic restrictions on the choice of the liquid phases, does not require the use of spreading solvents, allows for changing pH and ionic strength of the aqueous phase and can be used to investigate directly the effect of surfactant addition.
This methodology has been applied to investigate diverse systems, both of fundamental and applied relevance, as listed below.
Single-particle Contact Angle Measurements
FreSCa (Freeze-fracture, Shadow-Casting) Cryo-SEM, has been be applied to hydrophilic and hydrophobic spherical colloids, of various sizes, materials and surface chemistry trapped at the interface between water and a variety of non-polar phases. If particles have a contact angle below the shadowing angle, then they do not cast any shadow and their contact angle can be measured by measuring their cross-section at the interface and assuming their diameter (measured by another technique).
Wetting of Soft Particles (Microgels)
Microgel particles are colloids consisting of highly hydrated, cross-linked polymers. Often they are prepared using temperature or pH-responsive polymers to obtain stimulus-responsive particles, which can swell or contract under external triggers. They are also used as emulsion stabilizers, to form compliant, responsive shells on emulsion droplets. Our work highlighted that upon adsorbing at a liquid-liquid interface, the microgels deform, flatten and occupy a cross-section at the interface much larger than their diameter in bulk. Additionally they protrude very little in the oil phase and their size at the interface is independent of the pH of the water phase. The combination of these observations suggests that they assume the shape sketched in the figure.
Effect of Surfactants in Emulsion Double-inversion
A practical rule says that adding hydrophilic particles to oil-water systems yields oil in water emulsions, while, conversely, adding hydrophobic particles gives water in oil emulsions. This fact is based on the maximization of the portion of the particle surface exposed to the preferred liquid phase as a consequence of droplet curvature. Direct visualization of this phenomenon with small colloidal particles had to date not been demonstrated. In our work we have directly correlated emulsion inversion (a transition between o/w to w/o emulsions or viceversa) upon addition of surfactants to a transition of the wettability of silica nanoparticles.
Effect of particle shape and chemical composition on their wetting
Anisotropic particles present very interesting properties upon adsorption at liquid interfaces beyond the features of isotropic, spherical objects. In particular, ellipsoidal particles are of interest because they deform the interface in a well known fashion, giving rise to capillary interactions with quadrupolar symmetry, which can be used to direct the assembly of complex structures at the interface. In order to know more about their wetting behavior we have systematically investigated the wetting of microellipsoids obtained by the progressive stretching of polystyrene spheres. In theory, the contact angle, which is exclusively a material property, should not depend on the particle geometry, but we instead observe that the particles become apparently increasingly more hydrophilic for larger aspect ratios. We have rationalized this observations by invoking the presence of surface heterogeneities whose impratnce scales with the length of the contact line, i.e. with the particle aspect ratio.
In addition to shape anisotropy, colloids can also exhibit surface chemistry anisotropy. We have also investigated the effect of this parameter by studying the adsorption behavior of Janus colloids, i.e. particles with two regions of different wettability on their surfaces. In agreement with energy minimization calculations, we found that the particles are adsorbed at a position corresponding to the Janus boundary if the wettability contrasts between the tow lobes are high and if their relative size is similar. In this case the particles behave as colloidal surfactants. In the case of small wettability differences and large differences of the two lobes, the particles instead behave as uniform colloids and adsorb at the interface with the contact angle of one of the two lobes.
Finally, there is also the case where the particles show both shape and chemical anisotropy, as in the case of Janus dumbbells. Here energy minimization predicts different orientations depending on particle geometry and wettability contrasts. For the particles we investiagted, an oblique confirguration is expected, with both lobes crossing the interface. In reality we obserevd also two other configurations, where either only the hydrophobic or the hydrophilic lobe were wetted by the interface. These are metastable configurations, which depend on the particle orientation upon adsorption. These observations demonstrate that kinetic effects are indeed important druing wetting, a topic which we now investigate extensively in our group.