Interaction of lodine with nonionic surfactant and polyethylene glycol in aqueous potassium lodide solution

When a nonionic surfactant, such as hexaoxyethylene glycol dodecyl ethers (6ED) is added to an aqueous iodine solution in the presence of potassium iodide, some spectral changes corresponding to the interaction of iodine-potassium iodide mixture with 6ED are found above the critical micelle concentration (CMC) of 6ED. The absorption maximum bands of complexes formed between 6ED and iodine are significantly different from those formed between 6ED and iodine-potassium iodide mixture in aqueous solution. The complexes of iodic compounds (such as iodine, triiodide ion and polyiodide ion) with 6ED show absorption bands at 390 nm, 370 nm and 385 nm which are assigned to an interaction between iodine-6ED, triiodide ion-6ED and polyiodide ion-6ED, respectively. When the concentration of polyethylene glycol (PEG, MW=200–600) becomes high in aqueous triiodide ion solution, the maximum absorption wavelength of triiodide ion solution also shifts toward 370 nm, similar to that of the triiodide ion-6ED complex. If it is assumed that the absorbance between I₃ ^? and the EO chain in 6ED is the same as that between I₃ ^? and the EO chain of PEG, the concept of effective EO number can be applied to explain the behavior in the absorption spectra among I₃ ^?, 6ED and PEG. Here, the effective EO number is defined as the product of EO chain length and concentration of each 6ED and PEG in the complexes. The effective EO number and concentration of PEG decrease in the triiodide ion-PEG complex as the molecular weight of PEG becomes large. Moreover, the effective EO number and concentration of 6ED are lower than those of PEG at all maximum absorption wavelengths.