Influence of ionic strength on the flexibility of alginate studied by size exclusion chromatography

•We study flexibility of an alginate using SEC and assuming wormlike model.•First scenario: persistence length decreases with ionic strength.•Second scenario: persistence length does not change with ionic strength.•Wormlike model in itself is not sufficient to explain flexibility.•We propose a combination of short-range and long-range effects of the ions. SEC measurements of the [η]–M relationship for alginate from Macrocystis pyrifera and the wormlike model can be used to characterize flexibility through two independent treatments (Bohdanecky's equations and HYDFIT program), both providing the same results. Two different assumptions concerning mass per unit of length lead to different conclusions. First: persistence length decreases with ionic strength (the intrinsic component of the persistence length is 11.3nm and the electrostatic component is 6nm when ionic strength is 0.01). Second: persistence length is independent of ionic strength (12nm). Either of these options shows that the wormlike model in itself is not sufficient to explain flexibility over the whole range of chain lengths for these polyelectrolytes. A plausible explanation could be the presence of a combination of short-range and long-range screening effects of the ions of the solutions. This would also explain some data found in the literature regarding alginate flexibility.