Web Release Date: May 31,
Aqueous Solvation Dynamics at the Anionic Surfactant Air/Water Interface
Department of Chemistry, Columbia University, New York, New York 10027
Received: January 31, 2001
Abstract:
Effect of the negatively charged carboxylate group of the stearic acid surfactant CH3(CH2)16COO- 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)16COOH, 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 S1
S0 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 Å2/molecule), 
s- = 1.6 ± 0.3 ps, is significantly slower than at the neutral monolayer of undissociated
stearic acid at the same surface coverage, s0 = 400 ± 60 fs. It is also slower than the air/water interfacial
solvation, with the amplitude-averaged time 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.
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