Studies on Ligninsulphonates

Abstract

(Department of Agricultural Chemistry, Faculty of Agrieulture, Kyoto University, Kyoto) Conductivity curves, the equivalent conductance√c or the specific conductivity c plots, show apparent breaks at a certain critical concentration (cM), analogous to a sharp discontinuity around CMG in the curve of colloidal electrolyte solution. This break is more sharp especially in the lower molecular weight fractions of ligninsulphonates. The cm is about 0.20-0.25g/100cc for all fractions, independent of their molecular weights.Dependence of diffusion coefficient on concentration was measured by use of a solution-to-gel (0.595% agar) system. The degree of dependence of diffusion coefficient on concentration changes discontinuously also at cM. In the highest molecular weight (55800) fraction this change is neglisible small, and the diffusion coefficient decreases suddenly with increasing concentration in the region below and also above cM. In the lowest fraction (MW 550), on the other hand, this change is considerably large, and the diffusion coefficient decreases suddenly with increasing concentration in the region below cM but very slowly in the region above cM. These observations indicate that the structure of ligninsulphonate molecules in aqueous solution varies in some unexplained mechanism at cM and that this structural variation occurs remarkably in the low molecular weight fractions than in the high. However, whether this structural variation is due to an association of ligninsulphonate molecules in solution with a manner analogous to micelle formation in colloidal electrolyte, as Gardon pointed out, remains an open question.