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

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Abstract

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 ΔpKaS (pKa in the surfactant minus pKa in water) is given by ΔpKaS = ΔpKai - Ψ/59.16, where pKai represents the intrinsic pKa in the dielectric constant of the micellar interface, modeled as the ΔpKaS 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 ΔpKaS 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.

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