Eur. Phys. J. E 5, 117-126

Collapse of sodium polyacrylate chains in calcium salt solutions

R. Schweins and K. Huber

Universität-GH Paderborn, Fachbereich 13 Chemie und Chemietechnik, Warburger Str. 100, 33098 Paderborn FRG, Germany

rsl@chemie.uni-paderborn.de
huber@chemie.uni-paderborn.de

(Received 18 July 2000 and Received in final form 24 August 2000)

Abstract
The sodium salt of polyacrylic acid (NaPA) precipitates in the presence of Ca²⁺-ions. This phase behaviour can be represented by a phase diagram where the critical NaPA concentration is plotted versus the critical Ca²⁺ concentration resulting in a straight line as a phase boundary. The location of this phase boundary is influenced by the presence of an inert monovalent salt like NaCl. The present contribution focuses on the coil dimensions of NaPA chains in dilute aqueous solution corresponding to the one phase region of such a phase diagram. A variety of parameters with which the size and shape of the polyelectrolyte chains can be modulated are revealed. Approaching the phase boundary by decreasing the NaPA concentration at a constant Ca²⁺ content leads to a collapse of the NaPA chains. Combined static and dynamic light scattering suggests a compact spherical shape as the final state of this transition, both in 0.1 M NaCl and in 0.01 M NaCl. In the lower NaCl concentration, indication is presented for the existence of a cigar or pearl necklace like intermediate. Most strikingly, the collapsed chains can be reexpanded by increasing the concentration of inert NaCl at constant content of NaPA and Ca²⁺. Clearly, excessive Na⁺-ions displace the Ca²⁺-ions from the NaPA chains.

PACS
78.35.+c - Brillouin and Rayleigh scattering; other light scattering.
61.25.Hq - Macromolecular and polymer solutions; polymer melts; swelling.
64.75.+g - Solubility, segregation, and mixing; phase separation.


© EDP Sciences, Società Italiana di Fisica, Springer-Verlag 2001