In-Plane Molecular Rotational Dynamics at a Negatively Charged Surfactant/Aqueous Interface

The in-plane rotational dynamics of the molecular probe coumarin 314 (C314) at the negatively charged surfactant sodium dodecyl sulfate (SDS)/aqueous interface was measured using femtosecond time-resolved second-harmonic spectroscopy. The in-plane orientational time constant at an SDS surface coverage of 100 Å2 per SDS molecule is 348 ( 12 ps, which is comparable to the out-of-plane reorientational time of 383 ( 9 ps at the same SDS density. At 100 Å2, the SDS surfactant forms a homogeneous monolayer because it is in the liquid-condensed region of the surface pressure phase diagram. The rotation dynamics is slower than at the neat air/water interface, where the in-plane orientational time constant is 304 ( 8 ps and the out-ofplane reorientational time constant is 336 ( 6 ps. For purposes of comparison, we found that the orientational relaxation time of C314 in bulk water is 262 ( 10 ps. There is clear evidence for strong C314-SDS interfacial interactions based on the significant dependence of equilibrium properties on the SDS density. For example, large changes are seen in the C314 interfacial molecular orientation, marked changes in the SHG surface spectra, and changes in the surfactant phase diagram. Unlike these equilibrium properties, neither the inplane nor the out-of-plane rotations are strongly affected by interfacial SDS. The relative insensitivity of interfacial molecular rotations to the presence of anionic surfactants has implications to molecular rotations at biological cell/membrane aqueous interfaces, most of which contain anionic phospholipids.