Paul Ander

Heterogeneity of gum arabic and its salts

Abstract Moving-boundary electrophoresis experiments with the alkali-metal salts of gum arabic have shown the existence of a previously unreported, minor component. This component arises when gum arabic is kept in solution for several days, and it appears to be a hydrolysis product. From the frictional coefficients, calculated from the electrophoretic mobilities at infinite ionic strength, it was deduced that the hydrolysis proceeds with the removal of specific branches from the main backbone of the gum arabic molecule. Approximately 35 percent of the branches are more readily hydrolysed than the others and, thus, are different from the rest of the branches. The labile branches appear to be made more susceptible to hydrolysis by a physical change, possibly dehydration, that occurs in crude gum arabic when it is stored in the dry state. The equivalent weight of the hydrolysis product is 1490, compared to 1190 for gum arabic. The electrophoretic data, combined with light-scattering data, indicate that its molecular weight is approximately 20 percent less than that of gum arabic, but that, in solution, its molecules are larger than those of gum arabic. The high molecular weights observed in light-scattering experiments have been shown to be due to the presence of the hydrolysis product, which appears to cause aggregation through hydrogen bonding. The aggregates are very loosely held together, however, and can break apart in an electric field and in a shear field. At high ionic strengths, both gum arabic and its hydrolysis product are free-draining, and both have very large frictional coefficients, indicative of the large size of the ionic segments in these molecules.

Size and shape of potassium and sodium arabate aggregates in aqueous salt solutions

Abstract The average molecular weights obtained by light-scattering for several different preparations of sodium and potassium arabate covered a wide range of values; this variation was attributed to aggregation. The aggregates were found to be stable in solutions for simple salts. The extent of aggregation is apparently determined during the initial stages of preparation of the pure arabate salts from the crude gum. Analysis of the light-scattering data suggests a rod-like shape for the aggregated arabate particles. In 20m M potassium chloride, the relationship between the molecular weight of the arabates and their intrinsic viscosities is described by the relationship [η] = 0.103 M 0.42 ml.g −1 . The exponent of this equation is consistent with a highly branched, macromolecular model.

The determination of the molecular weight and interactions of a polyelectrolyte in aqueous salt solutions by light scattering

SummaryThe light scattering results for a polyelectrolyte in a binary solvent are experimentally evaluated in terms of the theory for multicomponent systems developped independently by Kirkwood and Stockmayer. Two proven experimental methods for molecular weight determination are discussed.ZusammenfassungDie Ergebnisse der Lichtstreuung für Polyelektrolyte in einem zweiwertigen Lösungsmittel werden experimentell mit der Theorie für Mehrkomponentensysteme ausgewertet, wie sie unabhängig voneinander Kirkwood und Stockmayer entwickelt haben. Zwei experimentelle Methoden für Molekulargewichtsbestimmung werden diskutiert.