Time-resolved total internal reflection fluorometry study on polarity at a liquid/liquid interface

The polarity of a water/oil (oil: cyclohexane, carbon tetrachloride, toluene, chlorobenzene, o-dichlorobenzene, or 1,2-dichloroethane) interface was investigated by means of time-resolved total-internal-reflection (TIR) fluorescence spectroscopy of a polarity-sensitive probe: sulforhodamine B (SRB), In bulk solutions, the nonradiative decay rate constant of SRB increased with an increase in a solvent polarity parameter [E-T(30)], and this relationship was used to estimate the polarities of water/oil interfaces. For the oil having a relatively low solvent polarity [E-T(30) < 35 kcal/mol], the polarity of the water/oil interface agreed with that of the arithmetic average of the polarities of the two phases [E-T(30)(calc)]. For water/odichlorobenzene and water/1,2-dichloroethane interfaces [E-T(30) of the oil > 35 kcal/mol], on the other hand, the interfacial polarity determined by TIR spectroscopy was lower than the E-T(30)(calc). The results are discussed in terms of thickness/roughness of the water/oil interface.