SALT EFFECTS ON THE CRITICAL MICELLAR CONCENTRATION, IODIDE COUNTERION BINDING, AND SURFACE MICROPOLARITY OF 1-METHYL-4-DODECYLPYRIDINIUM IODIDE MICELLES

Electrolyte effects on the cmc and iodide counterion binding of micelles of 1-methyl-4-dodecylpyridinium iodide (1) have been investigated by conductance measurements and by UV spectroscopy. The intramolecular charge-transfer (CT) absorption band of the ionic head group of 1 was successfully employed as an intrinsic microscopic medium polarity reporter for the innermost part of the electrical double layer. This micropolarity can be expressed in terms of Kosower's Z values which demonstrate the reduced polarity near the micellar surface. The applicability of Mukerjee's band-match method reveals that the Stern layer is quite homogeneous. The presence of electrolytes only modestly affects the micropolarity in the Stern layer. Sodium salts decrease the cmc in the order Cl- < Br- < NO3- < I- < OTs- (which parallels the lyotropic series for the inorganic anions) and the effect on the cmc follows the Shinoda equation. The above order also applies for the ability of the anions to reduce the iodide counterion binding (B) as shown by the decrease in the molar extinction coefficient of the intramolecular CT band. The association constant for binding of iodide ions to the long-chain pyridinium cations within the micelles was also derived from the optical absorption data. The deviant salt effect of sodium p-toluenesulfonate is briefly discussed. © 1979 American Chemical Society.