Small-angle X-ray scattering of kappa- and iota-carrageenan in aqueous and in salt solutions

Aqueous and saline (0.03-0.08 M NaCl) solutions of the Na(+)-salt of kappa- and iota-carrageenan (3-17 mg/ml) have been investigated using small-angle X-ray scattering (SAXS). Investigation of the SAXS curves normalized for concentration allows to detect the transition from the dilute to the semidilute regime. Some of the experimental curves exhibit a clear maximum. In the case of kappa-carrageenan, the position of this maximum is proportional to the cube root of polysaccharide concentration. The fluctuation theory of Borue and Erukhimovich (BE theory), which predicts a maximum in the scattering curves for low ionic strength, gives a good fit for all our experiments. An increase of the polymer concentration or the ionic strength causes the appearance of two subsystems in solution exhibiting a different characteristic screening scale of Coulombic interaction for ideal Gaussian chains (and two different segment lengths). The condition for an increase of the relative contribution of the subsystem with the larger segment length correlates with the one for the disorder-order transition of carrageenan. In the most concentrated (17.1 mg/ml) solution of Na(+)-iota-carrageenan, the increase of NaCl molarity above 0.06 M gives rise to additional scattered intensity at the smallest angles, indicating molecular association.

Viscosity and scattering of very weakly charged polyelectrolytes

Careful examination of the concentration range where both intrinsic viscosity and light scattering show a polyelectrolyte effect, even for singly charged halato-telechelic ionomers in DMF, together with the neutron scattering results at higher concentration show that weakly charged polymers may be a very useful tool to understand the complicated effects of coulombic interaction in polyelectrolyte solutions. A theoretical framework is given for a systematic study of such weakly charged polymers. The current state of understanding is presented of the properties of solutions of strong polyelectrolytes and of charged rigid particles. Finally it is shown how the transposition to weak polyelectrolytes solutions sheds light on the respective contributions of intra- and intermolecular interactions.