Electron spin resonance study on the sol-gel transition of gellan gum aqueous solution by the addition of paramagnetic metal- ions
Electron spin resonance spectra of the paramagnetic Mn(II) ion were measured as a function of its concentration, [Mn] t , in aqueous solutions of sodium-type gellan gum prepared to have a very low content of divalent metal cations, Ca2+ (0.02 wt%) and Mg2+ (0.001 wt%). In the solution without gellan, the spectrum is composed of six sharp lines and gives the signal intensity directly proportional to [Mn] t . In the presence of gellan at concentrations, [G], of 1–10 mM, the solutions showed three states with increasing of [Mn] t , a fluid sol, a viscous sol and a gel, where the signal intensity increases similarly to the solution without gellan (step A), remains constant (step B) and again increases linearly (step C), respectively. In steps and a remarkable intensity loss of the signal and a distortion of the line shape occurred. It was found that the spectra in steps and are decomposed into two components, one being quite similar to the signal in solution without gellan and the other having a broad component with no fine splittings. Spectral simulation assuming the solvent fluctuation model indicated that in gellan solution there are three types of Mn(II) ions with different mobilities: free, as in pure water, intermediately restricted, showing only a broad signal [Mn]IR, and strongly restricted, with no signal due to the extreme broadening [Mn]SR. [Mn]IR and [Mn]SR do not appear in step A, increase linearly in step B, and remain constant in step C. The ratio [Mn]SR/[Mn]IR is 0.3 and is independent of [G]. These phenomena are discussed in terms of changes in conformation and aggregation states of the molecules.
Key wordsGellan gum Aqueous solution Mn(II)-induced sol-gel transition Electron spin resonance Line shape analysis
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