Voltammetric study of absorption and reactivity of metal complexes in cotton immersed in aqueous buffer solutions

Cotton is an excellent cellulose-based absorbent mainly due to amorphous cellulose regions in the nanocrystalline (cellulose-I) natural fiber structure. It is shown here for the case of a woven cotton textile with ca. 20 μm fiber diameter and ca. 270 μm thickness that cation and anion absorption and diffusion processes occur within cotton fibers. An electrochemical approach based on voltammetry is developed to investigate and quantify absorption and leaching processes as well as the chemical reactivity within cotton fibers.The metal complexes Fe(CN)63-, Ru(NH3)63+, [Fe(II)(N,N-bis(pyridin-2-yl-methyl)-1,1-bis(pyridin-2-yl)-1-aminoethane)Cl]+ or Fe(BBA)Cl+, and [Fe(III)(protoporphyrinato-IX)(H2O)(OH)]2− or hemin are readily absorbed into cotton in the electrochemically active form. The ability of Fe(CN)63- and Ru(NH3)63+ to absorb appears to be predominantly due to “entrapment” without specific interaction with the amorphous cellulose. The approximate rate of diffusion of these ions in cellulose is estimated to be a factor 4 slower when compared to the rate of diffusion in solution. In contrast, Fe(BBA)Cl+ and hemin diffuse much slower (by a factor 24 and 70, respectively) within the cotton matrix. The slower diffusion is correlated with “specific” binding of the metal complexes to sites within the amorphous cellulose. In addition, Fe(BBA)Cl+ exhibits a new redox process within the cotton matrix indicating a change in chemical reactivity when compared to that observed in aqueous solution environments.