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RESEARCH PRODUCT

Temperature effects on counterion binding to spherical polyelectrolytes: the charge-discharge transition of lignosulfonate

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Abstract The effect of temperature on the effective charge numbers and diffusion coefficients of polyelectrolytes has not nobee dealth with in many studies. The present study concerns the temperature behavior of lignosulfonate. Lignosulfonate is a polydisperse polyelectrolyte whose molecules are compact spheres in aqueous solutions. One of its most remarkable properties is that is loses its charge in 0.1 M NaCl aqueous solution at about 40°C. In order to explain this charge-discharge transition, a theory for ion binding to spherical polyelectrolytes based on the relative population of two hydration states of the charged groups is presented. The water molecules adjacent to the charged groups are assumed to be arranged in two hydration shells, a tightly bound inner shell and an outer shell which is necessary for the group to keep its charge. The theory incorporates the ideas of the so-called “n-states” models employed in the study of biopolymers and membrane ionomers. The classical approaches describing ion association in electrolyte solutions consider the solvent as a dielectric continuum, and cannot explain the sharp transition of the charge number with temperature. Since many macromolecules of biological importance (e.g. globular proteins) behave as spherical polyelectrolytes, and since their effective charge numbers determine their physicochemical properties in solution, the theory considered here could also be of utility for describing temperature effects on counterion binding in spherical macromolecules other than lignosulfonate.