Colloidal Electrostatic Interactions Near a Conducting Surface

Charge-stabilized colloidal spheres dispersed in deionized water are supposed to repel each other. Instead, artifact-corrected video microscopy measurements reveal an anomalous long-ranged like-charge attraction in the interparticle pair potential when the spheres are confined to a layer by even a single charged glass surface. These attractions can be masked by electrostatic repulsions at low ionic strengths. Coating the bounding surfaces with a conducting gold layer suppresses the attraction. These observations suggest a possible mechanism for confinement-induced attractions.Comment: 4 pages, 2 figure

Low-surface energy surfactants with branched hydrocarbon architectures

International audienceSurface tensiometry and small-angle neutron scattering have been used to characterize a new class of low-surface energy surfactants (LSESs), "hedgehog" surfactants. These surfactants are based on highly branched hydrocarbon (HC) chains as replacements for environmentally hazardous fluorocarbon surfactants and polymers. Tensiometric analyses indicate that a subtle structural modification in the tails and headgroup results in significant effects on limiting surface tensions γcmc at the critical micelle concentration: a higher level of branching and an increased counterion size promote an effective reduction of surface tension to low values for HC surfactants (γcmc 24 mN m-1). These LSESs present a new class of potentially very important materials, which form lamellar aggregates in aqueous solutions independent of dilution

Measurement of the Interactions between Macroscopic Surfaces Inducing a First-Order Phase Transitions: the Sponge-Lamellar Transformation

The interactions between two macroscopic walls immersed in an isotropic symmetric sponge phase (L$_3$) at different volume fractions, $\Phi$, were studied with a surface force apparatus. At temperatures well below the lamellar (L$_\alpha)/{\rm L}_3$ bulk transition, the force-distance profile is weakly oscillatory with a period that is twice the correlation length ($\xi$) of the L$_3$ phase measured from SAXS. The oscillations are superimposed on an exponentially attractive background with an order correlation length of 2-4 cell sizes of the sponge structure. When the temperature is raised, a first-order phase transition to a lamellar structure can be induced for separations below a threshold. Another oscillatory interaction results which has a period that is twice the reticular distance for an L$_\alpha$ phase of similar $\Phi$. In addition, the thickness of the induced lamellar film increases (reversibly) with temperature. The compressibility modulus extracted from the oscillatory interaction is consistent with that expected for lamellae stabilized by undulation forces

Spiral textures in lyotropic liquid crystals : first order transition between normal hexagonal and lamellar gel phases

The first order transition between the normal hexagonal phase (H$_{\alpha}$) and lamellar gel phase (L$_{\beta}$, L$_{\beta'}$, L$_{\delta}$, ... type) in lyotropic liquid crystals of binary surfactant/water systems is investigated. Structural transformations and epitaxial relations are investigated by small-angle X-ray scattering on powdered and oriented samples. By slow evaporation of water, growth of the gel layered structure from the two-dimensional packing of surfactant cylinders of the hexagonal mesophase in the presence of a solid wall reveals a spectacular new texture composed of interwoven spirals. It is demonstrated that the layers grow from the rods of the hexagonal phase, in planes coplanar with the hexagonal packing and perpendicular to the wall. The configuration is such that line wedge disclinations of strength $s= +~1/2$ of the hexagonal phase are preserved through the phase transition. Estimates of the radii for the developable domain and cores, and also for the bending elastic constant are obtained. A mechanism for the phase transformation is discussed in view of topological structural transformations and a modification of the short-range order associated to the disorder order transition of the configuration of the paraffinic chains