Associative behaviour of κ-carrageenan in aqueous solutions and its modification by different monovalent salts as reflected by viscometric parameters

Abstract

The viscometric behaviour of κ-carrageenan in aqueous solutions and in the presence of monovalent salts was investigated at 25 °C. Coil, helix or double helix conformations were induced by cooling hot κ-carrageenan solutions under appropriate ionic conditions. A new viscometric approach was used for modeling the behaviour of κ-carrageenan solutions. The intrinsic viscosity, [η], is markedly changed by the presence of different monovalent salts (NaCl, NaI and CsI). In pure water, the intrinsic viscosity amounts to 48 dL·g−1. In 0.1 M NaCl solutions (single helix state) [η] is 6.2 dL·g−1, whereas in 0.1 M NaI (double helix conformation) it is approximately twice as large. In 0.1 M CsI (dissimilar cation and counter-ion) the intrinsic viscosity is three times larger, suggesting the formation of the associated κ-carrageenan helices. Stepwise association of κ-carrageenan helices was followed in presence of NaI/CsI mixtures of different compositions. The value of Smidsrød-Haug stiffness parameter (B) measured for κ-carrageenan in NaCl solutions is 4.47 × 10−2, higher than that of DNA (5.5 × 10−3), but lower than those reported for carboxymethyl cellulose (6.3 × 10−2), indicating that the chain conformation is moderately rigid.