When does like like like? : Microscopic inhomogeneity in homogeneous ionic systems
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概要
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Recent experimental results confirm the previous finding that ionic solute species locally form ordered structures in macroscopically homogeneous, dilute solutions. Ionic dendrimers with univalent counterions are found to show a Bragg diffraction peak in the small-angle X-ray profiles, as has been reported for other ionic polymers and colloidal particles. The Bragg spacing is smaller than the average spacing expected from concentration, suggesting the localized ordered arrangements in solutions. The localization testifies to a weak, but undeniable attraction between like-charged dendrimer ions, which is generated through the intermediary of unlikely charged counterions. The like-like attraction is shown to disappear for bivalent counterions, however; The diffraction peak is not observed, because the charge number of the dendrimers is lowered more strongly by the bivalent ions than the univalent ones. Neutron scattering profiles for polystyrenesulfonate (PSS) solutions demonstrate diffraction peaks, sustaining the existence of the like-like attraction. The transfer from univalent counterions to bivalent ones increases the Bragg spacing, suggesting weakening of the attraction. Dynamic light scattering indicates the presence of two diffusive (fast and slow) modes for homogeneous PSS solutions, which correspond to the Brownian motion of free macroions and the motion of the localized structures, respectively. With increasing counterion valency, the fast mode becomes smaller while the slow mode is increased. A strong attraction is detected by a direct measurement of the interaction potential using colloidal particles of a high charge density while no attraction but only repulsion is found for low charge particles. This is reasonable in light of the nature of the counterion -mediated attraction. Recent computer simulation works substantiate qualitatively the existence of the like-like attraction. Quantitatively, however, they fail to reproduce the observed fact that the attraction is more intense for univalent counterions than for bivalent ones. The size of the local structure is found to depend on the diameter of filter pores employed in purification process, while the Bragg spacing is not influenced. It is concluded that the structures are ruptured by filtration and thereafter regenerated rather rapidly, suggesting that they are not filtrable aggregates but loose assemblies of macroions containing solvent. The likewise local structure is inferred to exist in simple ionic solution and in“dust”plasma as well, albeit with largely different time and length scales. The structural inhomogeneity (and hence the like-like attraction) thus appears to be one of basic features of dilute ionic systems in general.
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