Molecular structure of carboxymethyl starch in dilute aqueous sodium chloride solutions

The dilute solution behavior of six carboxymethyl starch (CMS) samples was investigated in aqueous NaCl solutions of different concentration (0.01, 0.05, 0.1, and 0.5 M) by dynamic light scattering (DLS) at 25°C. The values of weight average molar mass, Mw, of the investigated CMS samples were in the range from 9 × 105 to 2.1 × 107 g/mol. Three of the CMS samples were obtained from corn starch and the other three from potato starch. They had different degrees of substitution, DS, ranging from 0.4 to 1.1. The translational diffusion coefficient, Dz, and hydrodynamic radius, Rh, were determined for all CMS samples in aqueous NaCl solutions of different concentrations by the DLS method. The obtained results were treated together with previous results obtained by static light scattering and viscometry, which enabled the calculation of the structure-sensitive parameter, ρ, and the dependence of Rh, radius of gyration, Rg, and viscosity radius, Rη, on molar mass. The calculated values of the ρ parameter were independent of molar mass. The parameter ρ did not change significantly with the variation of the salt concentration in water, its values being in the range from 1.10 to 1.19. The values of exponent νi in the log–log dependence of Rg, Rh, and Rη on Mw were in the range between 0.42 and 0.47; 0.36 and 0.49; 0.42 and 0.44, respectively. These values are characteristic of branched structures of macromolecules.