Fluorescent dye N,N'-dioctadecylrhodamine as a new interfacial acid-base indicator.

This paper reports the spectral properties and protolytic behavior of the fluorescent dye N,N'-dioctadecylrhodamine on the micelle/water and microdroplet/water interfaces as well as in Langmuir-Blodgett films soaked into aqueous media. Long hydrocarbon chains provide similar orientation of its cation and zwitterion, with the dissociating group (COOH-->COO(-)) directed toward the bulk (aqueous) phase. Both absorption and fluorescence of the dye can be used for monitoring electrical surface potentials and for determination of bulk pH.