Rapid transport of polyvinylpyrrolidone (PVP) in dextran solutions accompanied by a 'finger'-like dissipative structure, first reported by Preston et al. [Nature, 1980, 287,4991, was found to be further accelerated when a simple salt was present in both upper and lower compartments of a diffusion cell. The enhancement effect was found on various salts to different extents when compared at a given salt concentration C. The effect depended on C for a given kind of salt. The effect was suggested to be of mechanical origin rather than of thermodynamic one. When the transport rates, after conected for different viscosities, were plotted against the densities of the solutions, most data for different salts at 2 M as well as those for different salt concentrations in the case of CsCl and NaBr approximately fell on a single curve. The curves differed for different PVP concentrations. Effect of the salts were examined on the flow patterns, such as the width of fingers, onset time and the critical PVP concentration for the appearance of fingers. Effect of the molecular weight of PVP was also examined.When an aqueous solution of dextran (a-1,6-glucan), a polysaccharide composed of D-glucose, is placed above another aqueous solution consisting of dextran of the same concentration and polyvinylpyrrolidone (PVP), the diffusion of PVP from the lower solution to the upper one is expected to occur. However, when both dextran and PVP concentrations exceed respective critical values, a rapid transport of PVP has been observed by Preston et a1.l This rapid transport was ascribed to the Occurrence of a finger-like structured flow and was characterized by the linear dependence of the transported amount on time t rather than on square root of t.l,* The molecular mechanism of the phenomenon is not yet fully elucidated in spite of the extensive research by Preston and Camper.'-" Also, extension to systems other than dextran-PVP has been scarcely reported.This phenomenon can be regarded as a kind of mechanochemical system. Hydrodynamic flow is created at the expense of chemical potential not of gravitational potential.We have preliminarily reported12 that the PVP transport in the PVP-dextran system is enhanced in the presence of a simple salt (at the same concentration in both upper and lower solutions), the extent of enhancement depending on the kind of ions. In the preliminary report, we have shown that the enhancement effect of the salts can not be ascribed to their effect on the coil dimension of PVP.12 The intrinsic viscosity of PVP in aqueous solutions considerably decreased in the presence of 2 M CsCl. In the presence of 80 kg m-3 dextran, however, the coil dimension of PVP was decreased so as not to be influenced by the addition of CsCl.The biological significance of the phenomenon has been inferred and extensively examined by Comper.13 The present salt effect may provide another controlling factor of the phenomenon in biological context in terms of changing ionic strength of the media, in addition to that in terms of biosynthesis...

A theory to describe the swelling behavior of polyion gels is presented on the basis of de Gennes theory for the conformational entropy of chains under traction and ideal Donnan approximation combined with the counterion condensation of polyelectrolytes for the osmotic expansion force. The Flory exponent is regarded as a parameter in theory to describe the effect of the intrachain interaction on the fractal nature of the chain conformation. The volume of poly͑acrylic acid͒ gel was measured as a function of the ionization degree, the salt concentration, and the polymerization degree of chains between crosslinks. The analysis of the expansion behavior showed that the values varied from about 0.8 to about 0.6 for the gels having long chains when the salt concentration changed from 10 to 100 mM, while for the gels consisting of short chains the exponent was kept to about 0.8, irrespective of the salt concentration change. The results indicate that the Flory exponent of the expanding polyion gel increases with the strength of the electrostatic interaction between ionized groups. The volume of a copolymer gel of fully neutralized maleic acid and styrene consisting of the short chains was also measured as a function of the salt concentration. The analysis of the expansion behavior gave ϭ0.71 in the salt concentration range between 3 and 320 mM in contrast with a value of 0.8 for the poly͑acrylic acid͒ gel under similar conditions. This result suggests that the hydrophobic interaction between the phenyl groups reduces the value.

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