Direct observation of critical adsorption on colloidal particles

Abstract

Despite the fundamental role adsorbed solvent layers play in generating critical Casimir forces between colloidal particles, the structure of these layers has yet to be directly determined. Using small-angle neutron scattering, we have measured critical adsorption on the surface of small spherical silica particles suspended in a binary mixture of lutidine and water. The surface concentration profile and excess adsorption Γ were studied as functions of temperature at the critical concentration and three off-critical concentrations. We are able to differentiate three distinct contributions to the excess adsorption including the intrinsic shape of the concentration profile. The adsorption associated with the profile shape is found to increase monotonically with increasing 2,6-lutidine concentration and to decrease with increasing temperature, this later observation is consistent with expectations for curvature induced corrections to planar adsorption and leads to Γ∼(T-Tc)/Tc (-0.52), where Tc is the critical temperature. This scaling relation corresponds to a stronger divergence than found in the planar case, but a substantially weaker divergence than found previously.

Tunable thermo-reversible bicontinuous nanoparticle gel driven by the binary solvent segregation

“…The thickness of this preferentially adsorbed solvent layer keeps growing when the binary solvent moves closer to the phase-separation temperature. In our case, ramping up the temperature increases the thickness of water rich adsorption layer on silica particle surface, which has been well studied in literature 18,23 . A recent experiment shows that at large particle concentrations, the attraction is driven by the local solvent segregation (or capillary condensation) of the binary solvent confined between neighboring particles (shown schematically in Supplementary Fig.…”

“…1(b), spherical silica nanoparticles (diameter~27 nm) dispersed in a mixture of water and 2,6-lutidine are used as a model system. The nanoparticle has high surface charge with its surface preferentially wetted by water 18 . As a result, the particle has a net repulsion between them.…”

“…It is well known that an effective attraction between particles can be generated in a binary solvent when the solvent condition is moving towards its phase-separation point. This solvent introduced attraction has been widely termed as the Critical Casmir force 18,28,29 . Despite the current debate on the exact physical origin of this attraction 24,29,30 , the consensus of the community is that when the surface of particles is preferred to be wetted by one individual component in a binary solvent, it can develop an adsorption layer on particle surface.…”

Numerically Exact Approach to Ultra-Small Particles in Near-Critical Mixtures

“…By tuning temperature, the solvent correlation length can be changed, affecting the magnitude and range of interactions. The nature of the observed critical behavior significantly depends on the surface properties of solute particles [18]. Generally, these forces are attractive if the boundary conditions are symmetric, and repulsive if the boundary conditions are asymmetric [2].…”

Critical adsorption profiles around a sphere and a cylinder in a fluid at criticality: Local functional theory

“…In experiments, it has been studied intensively in near-critical binary fluid mixtures on a solid surface or a noncrtical vapor-liquid interface at constant pressure [8][9][10][11][12][13][14][15]. Much attention has also been paid on preferential adsorption on colloidal particles [16][17][18], which is known to give rise to colloid aggregation [16,19,20]. We mention a few experiments on critical adsorption of supercritical pure fluids [21][22][23].…”