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Distribution of Counterions around Lignosulfonate Macromolecules in Different Polar Solvent Mixtures

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posted on 07.02.2012, 00:00 by Ulla Vainio, Rolf A. Lauten, Sylvio Haas, Kirsi Svedström, Larissa S. I. Veiga, Armin Hoell, Ritva Serimaa
Lignosulfonate is a colloidal polyelectrolyte that is obtained as a side product in sulfite pulping. In this work we wanted to study the noncovalent association of the colloids in different solvents, as well as to find out how the charged sulfonate groups are organized on the colloid surface. We studied sodium and rubidium lignosulfonate in water–methanol mixtures and in dimethyl formamide. The number average molecular weights of the Na- and Rb-lignosulfonate fractions were 7600 g/mol and 9100 g/mol, respectively, and the polydispersity index for both was 2. Anomalous small-angle X-ray scattering (ASAXS) was used for determining the distribution of counterions around the Rb-lignosulfonate macromolecules. The scattering curves were fitted with a model constructed from ellipsoids of revolution of different sizes. Counterions were taken into account by deriving an approximative formula for the scattering intensity of the Poisson–Boltzmann diffuse double layer model. The interaction term between the spheroidal particles was estimated using the local monodisperse approximation and the improved Hayter–Penfold structure factor given by the rescaled mean spherical approximation. Effective charge of the polyelectrolyte and the local dielectric constant of the solvent close to the globular polyelectrolyte were followed as a function of the methanol content in the solvent and lignosulfonate concentration. The lignosulfonate macromolecules were found to aggregate noncovalently in water–methanol mixtures with increasing methanol or lignosulfonate content in a specific directional manner. The flat macromolecule aggregates had a nearly constant thickness of 1–1.4 nm, while their diameter grew when counterion association onto the polyelectrolyte increased. These results indicate that the charged groups in lignosulfonate are mostly at the flat surfaces of the colloid, allowing the associated lignosulfonate complexes to grow further at the edges of the complex.