Aqueous solvation dynamics at the anionic surfactant air/water interface

Effect of the negatively charged carboxylate group of the stearic acid surfactant CH3(CH2)(16)COO- on the solvation at the surfactant air/water interface was investigated. A direct comparison is made with the previously studied interfacial aqueous solvation at a monolayer of the neutral stearic acid CH3(CH2)(16)COOH, as well as the surfactant-free air/water interface. Static and time-resolved femtosecond second harmonic generation (SHG) spectroscopy of a solvatochromic dye coumarin 314 (C314) adsorbed at the interface was employed as a surface-selective molecular probe of the interfacial solvation environment. The SI-So electronic transition frequency of C314, which reflects static solvation at the interface, is relatively insensitive to the surface charge. On the contrary, the characteristic time of the solvation dynamics at the anionic surfactant monolayer (200 Angstrom (2)/molecule), tau (-)(s) = 1.6 +/- 0.3 ps, is significantly slower than at the neutral monolayer of undissociated stearic acid at the same surface coverage, tau (s)0 = 400 +/- 60 fs. It is also slower than the air/water interfacial solvation, with the amplitude-averaged time tau (s) = 850 +/- 70 fs measured without the lipid surfactant. The observed effect is consistent with the alignment of the water molecules near the interface by the electric field of the charged surfactant, which results in an increased hydrogen bond order and loss of diffusional (rotational and translational) mobility.