Interaction between Heptaethylene Glycol Dodecyl Ether and Sodium Dodecyl Sulfate

The absorption spectra of iodine dissolved in aqueous solutions of heptaethyleneglycol dodecyl ether (C12E7) exhibit two extra bands at about 295 nm and 386 nm, when the concentration of C12E7 exceeds than its critical micelle concentration (CMC). These bands are attributable to the charge transfer interaction between iodine (an electron acceptor)and micellar C12E7 (electron donor), and can be utilized to determine the CMC of C12E7.The absorbance at 295 nm (and at 386 nm, too) plotted against C12E7 concentration showed an abrupt increase at 7 X 10-5 mol/l which agreed with the cmc determined by other methods. A similar behavior was observed also in the case of sodium dodecyl sulfate (SDS). Thus, the con- centration at which the absorption shows an abrupt increase may be called the apparent CMC of C12E7.Addition of a small amount of SDS (2 x 10-5 mol/l) effected a slight increase in the apparent CMC, together with a slight decrease in the absorbance. Upon further addition of SDS (1 X 10-4 mol/l) the apparent CMC increased a little, but the absorbance decreased significantly.When the concentration of SDS added (1 X 10-2 mol/l) exceeded the CMC, the absorbance increased monotonouslly with the concentration of C12E7, showing no apparent CMC. The SDS added probably formed mixed micelles with C12E7, and consequently, the charge-transfer complex between iodine and C12E7 was mostly destroyed.