Effect of Surfactants on In-Plane and Out-of-Plane Rotational Dynamics of Octadecylrhodamine B at Toluene−Water Interface

Abstract

We measured the time-resolved fluorescence anisotropy of N-octadecylrhodamine B (C18RB) at a toluene−water interface under the total internal reflection conditions (an incident angle of 70°), in the absence and presence of surfactants (Triton X-100 (TX-100), sodium dodecyl sulfate (SDS), or dihexadecyl hydrogen phosphate (DHP, pKa ≈ 2)). Without surfactants, the in-plane fluorescence anisotropy, rΦ, was almost 0 independent of time, and this implied that C18RB molecules rotated very fast (rotational correlation time, τΦ ≤ 1 ns) at the interface. In the nonionic TX-100 system, C18RB molecules rotated fast (τΦ ≤ 1 ns), but their rotational regions were restricted by the surfactant molecules. In the cases of SDS and deprotonated DHP systems, the interfacial concentration of the protonated C18RB, C18RBH+, increased with an increase in the surfactant concentration (1.0 × 10-10 − 5.0 × 10-8 M) by electrostatic attractive force. In these anionic surfactant systems, rΦ decreased exponentially with time and τΦ cou...