Aggregates of alginates binding with surfactants of single and twin alkyl chains in aqueous solutions: Fluorescence and dynamic light scattering studies

The natural polyelectrolytes, especially polysaccharides, are intensively used in foods, cosmetics, and pharmaceuticals with many amphiphiles. Alginate chain contains three types of dyad sequential blocks as MM, GG, and MG, and these residue sequences endow the alginate chains with different stiffness. It is therefore interesting to illustrate the effect of chemical composition and sequence of alginate on the aggregation with positively charged surfactants. The single-tail cationic surfactant dodecyltrimethyl ammonium bromide (DTAB) and twin-tail (Gemini) surfactant ethanediyl-1,2-bis (dodecyldimethylammonium bromide) (12-2-12) were allowed to bind on the natural polyanion of sodium alginate with different M/G ratios (0.6 and 1.85, respectively). The interaction and aggregation were investigated using fluorescence emission of pyrene probes and dynamic light scattering to reveal effects of the polyelectrolyte composition and surfactant structure. The results indicated that the cooperative aggregation was much stronger for the Gemini surfactant 12-2-12 than that for the single-tail DTAB with the same tail length and the binding strength of alginate to the both 12-2-12 and DTAB with the same tail length was almost identical. Also, the cac was almost independent of the alginate composition and M and G sequence. Dynamic light scattering illustrated that the hydrodynamic radius Rh for a surfactant-bound alginate chain was always smaller than that without binding. Particularly, the Rh exhibits two very different change trends with the surfactant concentration. Binding with 12-2-12 produced a deep minimum Rh at quite a lower surfactant concentration than its cac, while Rh monotonically decreased with increasing DTAB concentration. The appearance of the minimum Rh could be attributed to the hairpin-like conformation of the alginate chain formed due to binding with 12-2-12.