TEMPERATURE EFFECTS ON COUNTERION BINDING TO SPHERICAL POLYELECTROLYTES - THE CHARGE DISCHARGE TRANSITION OF LIGNOSULFONATE

The effect of temperature on the effective charge numbers and diffusio n coefficients of polyelectrolytes has not been dealt with in many stu dies. The present study concerns the temperature behavior of lignosulf onate. Lignosulfonate is a polydisperse polyelectrolyte whose molecule s are compact spheres in aqueous solutions. One of its most remarkable properties is that it loses its charge in 0.1 M NaCl aqueous solution at about 40 degrees C. In order to explain this charge-discharge tran sition, a theory for ion binding to spherical polyelectrolytes based o n the relative population of two hydration states of the charged group s 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 sol utions consider the solvent as a dielectric continuum, and cannot expl ain 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 t heory considered here could also be of utility for describing temperat ure effects on counterion binding in spherical macromolecules other th an lignosulfonate.