It is considered that, in the amylose-iodine complex, the helical amylose moiety polarizes the trapped iodine molecules, generating blue color in the product'. The stability of the complex depends2 on such factors as the temperature, the pH, and the additives. The standard free-energy, enthalpy, and entropy of formation of the complex were evaluated by us2 to be -10 kJ.mol-', -65 kJ.mol-', and -165 J.mol-'.K-', respectively. The large-negative entropy supported ordering of the system, that is, rigidity in the conformation, as well as enhancement of the water structure. The stability of the complex is, however, least in the presence of Na2S203, which reacts with iodine at the molecular level with immediate destruction of the complex. Such additives as nonaqueous solvents and urea destabilize the complex, essentially through conformational alterations. Temperature acts similarly, the effect being brought about by the increased kinetic energy of the solvent molecules, as well as of the segments of the biopolymer, reversing the decreased entropy of complex-formation.
Whatever might be the factors that could affect the complex, there is no doubt that its stability under normal conditions is quite high. The "vitality" (strength) of the amylose-iodine interaction at the molecular level was realized from the following demonstrations. We selected an amylose sample of viscosity average molecular weight 14,090, whose maximum iodine-binding capacity in 0.01~ KI at 303 K, mol/mol was 12: 1 as 1,:amylose.
At compositions of iodine and amylose (a) below saturation (5:l) and (6) near saturation (lO:l), the samples of the complex were mixed with large proportions of benzene, shaken thoroughly, and allowed sufficient time (4 h) to attain equilibrium. It was observed that the iodine-poor sample retained all of the iodine, the nonaqueous phase being free from it. The iodine-rich sample, on the other hand, lost 20% of the total iodine present. Progressive addition of urea to the aqueous phase, up to 4M, progressively enhanced liberation of iodine into the benzene