Femtosecond Aqueous Solvation at a Positively Charged Surfactant/Water Interface.

The effect of a positively charged dodecyl trimethylammonium bromide (DTAB) surfactant on the aqueous solvation of coumarin 314 (C314), a solvatochromic molecule, at the air/water interface was investigated. Steady-state and femtosecond time-resolved second harmonic generation (SHG) spectroscopy was employed as a surface-selective tool to characterize the solvation of C314. The molecular orientation and the static and dynamic solvation of the probe C314 at the positively charged surfactant interface were found to be significantly different from that at the previously studied neat air/water interface and at a negatively charged surfactant air/water interface. Steady-state SHG spectra show a red shift and therefore increased local polarity at the positively charged interface (λmax = 436 nm) compared to the negatively charged interfaces (λmax = 431−432 nm) and the surfactant-free air/water interface (λmax = 423 nm), though less polar than bulk water (448 nm). Two diffusive dynamical time scales τ1 and τ2 of aqueous solvation were obtained at the DTAB interfaces, as is the case at air/water, at negatively charged surfactant interfaces, and in bulk water. Both the solvation times and their relative amplitudes change as a function of the positively charged surfactant surface coverage. As the DTAB surface density increases from 500 Å2/molecule to 100 Å2/molecule, the fast component τ1 slows from 380 to 550 fs, whereas the amplitude of the slower solvation component τ2 is reduced to almost zero. This is significantly different from the behavior observed at the negatively charged surfactant interfaces. [ABSTRACT FROM AUTHOR]