A SMALL-ANGLE X-RAY-SCATTERING STUDY OF ALGINATE SOLUTION AND ITS SOL-GEL TRANSITION BY ADDITION OF DIVALENT-CATIONS

The small-angle x-ray scattering (SAXS) technique has been applied to investigate solution and gel structures of alginate in the absence and presence of two divalent cations: Ca(II) and Cu(II). We have observed a broad maximum in the scattering curve, a characteristic of polyelec trolyte, for the purified alginate sample. The scattering maximum disa ppears in excess of added simple salt and shifts toward the higher ang le region with increasing alginate concentration. Concentration depend ence of the position and intensity of the maximum follows power law re lations with exponents close to those predicted by theory. Data analys is shows an increase in correlation length xi and cross-sectional diam eter d(0), of polymer chains upon gelation and suggests that a dimeric structure is adopted in the junction zone, consistent with the ''egg- box'' model previously proposed. In the Ca(II)-alginate system, the mo lecular parameters xi and d(0) are found to have good correlation with the macroscopic properties of gelation, such as gel point determined by viscosity measurements. However, for the Cu(II)-alginate system the re is no clearly transitional behavior observed in xi and d(0), implyi ng that the junction zone may be replaced by a more uniformly distribu ted site binding of Cu(II) ions to the carboxyl groups of both mannuro nate and guluronate residues, in confirmation of previous C-13-nmr res ults. (C) 1995 John Wiley & Sons, Inc.