Lipoid pH indicators as probes of electrical potential and polarity in micelles

Abstract

Two types of fluorescent pH indicators, a hydroxycoumarin and an aminocoumarin dye, are incorporated by means of long paraffinic chain substituents to neutral, anionic, and cationic micelles. The apparent pK values for the dyes in the micelles are compared to the pK values in aqueous solution. Shifts of the apparent pK are found for charged as well as for uncharged micelles, being different for the two indicators. A comparison of the pK shifts of the indicators in neutral micelles to pK shifts in a series of mixed solvents of different dielectric constant (e) leads us to attribute the shifts in the neutral system to a reduced polarity at the micelle surface for which a value of e 32 is estimated. With respect to charged micelles this polarity effect is responsible for part of the apparent pK shift. It may be revealed by comparing the results for the two indicator types. The value of the effective dielectric constant is found to be similar to that in the neutral micelle. The remaining part of the apparent pK shift may be taken as a measure of the electrical potential at the surface of the charged micelles. Some examples of potential measurements for various counterions are presented. A thermodynamical section attempts to present clear definitions of (and relations between) terms as “apparent pK”, “interfacial pK”, “interfacial pH”, and “acidity function”.