Molecular rotation at negatively charged surfactant/aqueous interfaces.

The effect of charge on the rotational dynamics of the molecular probe coumarin 314 (C314) at air/water interfaces covered with the negatively charged surfactant sodium dodecyl sulfate (SDS) was investigated using femtosecond time-resolved second harmonic spectroscopy. The out-of-plane orientational time constant at the highest SDS surface coverage of 100 A2 per molecule is 383 +/- 9 ps. The rotational dynamics is slower than at the air/water interface where the out-of-plane reorientational time constant is 336 +/- 6 ps. At the air/water interface the rotational dynamics is over three times slower than the bulk orientational diffusion time of 100 ps. The relatively small effect of the surfactant charge density on the C314 rotation time constant is surprising, considering the marked dependence of the C314 orientation, spectra, and surfactant phase diagram on surfactant density.