Aqueous solvation dynamics at the anionic surfactant air/water interface

Effect of the negatively charged carboxylate group of the stearic acid surf actant CH3(CH2)(16)COO- on the solvation at the surfactant air/water interf ace was investigated. A direct comparison is made with the previously studi ed interfacial aqueous solvation at a monolayer of the neutral stearic acid CH3(CH2)(16)COOH, as well as the surfactant-free air/water interface. Stat ic and time-resolved femtosecond second harmonic generation (SHG) spectrosc opy of a solvatochromic dye coumarin 314 (C314) adsorbed at the interface w as employed as a surface-selective molecular probe of the interfacial solva tion environment. The SI-So electronic transition frequency of C314, which reflects static solvation at the interface, is relatively insensitive to th e 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 mo nolayer of undissociated stearic acid at the same surface coverage, tau (s) 0 = 400 +/- 60 fs. It is also slower than the air/water interfacial solvati on, with the amplitude-averaged time tau (s) = 850 +/- 70 fs measured witho ut the lipid surfactant. The observed effect is consistent with the alignme nt of the water molecules near the interface by the electric field of the c harged surfactant, which results in an increased hydrogen bond order and lo ss of diffusional (rotational and translational) mobility.