Atomic force microscopy studies on cation-induced network formation of gellan

Effects of cations on the network formation of the microbial polysaccharide gellan were investigated using atomic force microscopy (AFM). Aqueous dispersions of gellan with various cations were spread onto freshly cleaved mica surfaces and visualized in air using the alternating current cyclic contact mode. AFM images of tetramethylammonium salt of gellan revealed branched rod-like structures, the heights of which are uniformly ca. 0.5 nm, suggesting the absence of the side-by-side aggregation of double helices. Adding KCl, CsCl, and CaCl2 promoted interhelical aggregation, evident from increases in the heights of fibrous aggregates of gellan double helices up to 2–3 nm, resulting in the formation of local network structures in the presence of CsCl and macroscopically percolated network structures in the presence of CaCl2. The surfaces of bulk gellan gels containing KCl and CsCl were imaged under respective salt solutions using the direct current contact mode, revealing fairly straight and stationary network strands.