Micelles in the Physical/Analytical Chemistry Laboratory. Acid Dissociation of Neutral Red Indicator

This paper describes the third in a series of laboratory experiments demonstrating the effects of micelles on physicochemical properties. The acid dissociation constant for neutral red indicator is measured spectrophotometrically in water and in solutions of Tween-80, a neutral surfactant, and sodium dodecyl sulfate (SDS), an anionic surfactant. The Δp K a S (p K a in the surfactant minus p K a in water) is given by Δp K a S = Δp K a i - Ψ/59.16, where p K a i represents the intrinsic p K a in the dielectric constant of the micellar interface, modeled as the Δp K a S in the neutral Tween-80 micelles, and Ψ is an electrostatic potential, equal to -100 + 5 mV for SDS at the experimental ionic strength of 0.100 M. Students observe that the measured Δp K a S for SDS is much larger than the value calculated from the equation. This discrepancy is due to a specific molecular interaction between the cationic indicator and the anionic headgroups of SDS.