Precipitation of polyelectrolytes by salts

Abstract

Abstract Precipitation of poly(acrylic acid) by added salts has been investigated under various conditions. The PAA was neutralized to various degrees of ionization α by tetra‐ n ‐butyl ammonium hydroxide. The minimum salt concentration C̄ s at which precipitation occurs was determined at various polymer concentration C p , temperature T , and α. The amount of bound cations to polyions, the change in ion hydration, and the polymer expansion near C̄ s were estimated by conductance, refractive index, and viscosity measurements, respectively. It is concluded that there are two kinds of precipitation; The L type which satisfies the relation C̄ s ≐ α C p and occurs at high α due to cation binding to the polyions, and the H type in which C̄ s is independent of C p and which appears at low α with salts other than those having large ions. This classification of the precipitation is justified also by the C̄ s – T relation; in the L type, C̄ s is nearly independent of T , and in the H type C̄ s increases with T . At intermediate α‐values pronounced specificities of salt ions are found in the precipitation; at α = 0.25, with Mg 2+ the H type appears, while with Ba 2+ or Ca 2+ the L type appears, although all of these divalent cations are bound to polyions to the same degree. This is due to the difference in the hydration of the polyion‐cation complexes. In the L ‐type the formation of cation bindings, such as salt bridge, makes the polymer hydrophobic, while in the H type highly concentrated salt ions render the polymer solution unstable by changing the arrangement of the water molecule.