Understanding the mechanism of gelation and stimuli-responsive nature of a class of metallo-supramolecular gels.

Utilizing metal-ligand binding as the driving force for self-assembly of a ditopic ligand, which consists of a 2,6-bis-(1'-methylbenzimidazolyl)-4-oxypyridine moiety attached to either end of a penta(ethylene glycol) core, in the presence of a transition metal ion (Zn(II)) and a lanthanide metal ion (La(III)), we have achieved formation of stimuli-responsive metallo-supramolecular gels. We describe herein a series of experimental studies, including optical and confocal microscopy, dynamic light scattering, wide-angle X-ray diffraction, and rheology, to explore the properties of such gels, as well as the nature of the gelation mechanism. Morphological and X-ray diffraction observations suggest gelation occurs via the flocculation of semicrystalline colloidal particles, which results in the gels exhibiting pronounced yielding and thixotropic behavior. Application of mechanical stress results in a decrease in the particle size, which is accompanied by an increase in gel strength after removal of the stress. Moreover, studies show that the presence of lanthanide(III) perchlorate increases the mechano-responsiveness of the gels, as a consequence of reduced crystallinity of the colloidal particles, presumably due to the different coordination ability of lanthanide(III) and zinc(II), which changes the nature of the self-assembly in these materials.

The effect of surfactant on the efficiency of shear-induced drop coalescence.

The volume-averaged shear-induced drop-coalescence efficiency epsilonv is measured by in situ videomicroscopy of blends of poly(propylene glycol) and poly(ethylene glycol), emulsified with poly(ethyleneglycol-b-propyleneoxide-b-ethyleneglycol) block copolymer surfactant. Adsorption of copolymer to the immiscible blend interface is indicated by a reduction in the interfacial tension, measured by the drop retraction method. The effects of temperature, copolymer molecular weight, copolymer concentration, and capillary number Ca are explored. At small Ca, epsilonv is essentially independent of shear rate and drop size, and depends mainly on the solubility, diffusivity, and surface pressure of the surfactant, indicating that drop trajectories during flow are perturbed by surfactant Marangoni stresses that are controlled by the diffusion-limited sorption of surfactant. At larger Ca, epsilonv approaches zero. This arrest of coalescence is associated with the onset of slight deformation of the drops during their collision, and drainage of a film of continuous fluid between them. The effect of the surfactant, though significant, saturates even while the amount of surfactant adsorbed to the interface is quite small. Governing dimensionless parameters, associated material parameters and the behavior of more insoluble surfactants are discussed.