Web Release Date: November 17,
Dynamical Time Scales of Aqueous Solvation at Negatively Charged Lipid/Water
Interfaces
Department of Chemistry, Columbia University, New York, New York 10027
Received: May 24, 2001
In Final Form: August 23, 2001
Abstract:
Time-resolved second harmonic generation spectroscopy of a solvatochromic dye coumarin 314 was used to
investigate how the solvation dynamics at the air/water interface are affected by lipid surfactants with anionic
carboxylate and sulfate headgroups. The surfactants were chosen to mimic common functional groups and
the negative charge of biological aqueous interfaces such as cell membranes and folded proteins. The diffusive
component of the solvation response, associated with rearrangement of the water hydrogen bond network,
exhibits a biexponential decay. The two time scales change as a function of the surface charge density, compared
to the bare air/water interface solvation dynamics, 
1 = 250 ± 50 fs and 2 = 2.0 ± 0.4 ps, which are similar
to the bulk water response, 1 = 130-250 fs and 2 = 0.6-1.2 ps. The faster 1 component is unaffected at
low to intermediate sulfate surface coverage (270 ± 50 fs at 500 Å2/molecule and 225 ± 25 fs at 250 Å2/molecule) but becomes slower at high surface coverage (100 Å2/molecule), 600 ± 70 fs. The slower 2
component changes even at the lowest coverage: 2 = 4.4 ± 0.9 ps at 500 Å2/molecule; 5.2 ± 0.7 ps at 250
Å2/molecule; and 5.4 ± 1.1 ps at 100 Å2/molecule. Different behavior of the two solvation components
indicates that they represent different water motions. The solvation dynamics compared with the structure-sensitive surface vibrational spectroscopy of the same interface [Gragson, D. E.; McCarty, B. M.; Richmond,
G. L. J. Am. Chem. Soc. 1997, 119, 6144. Gragson, D. E.; Richmond, G. L. J. Am. Chem. Soc. 1998, 120,
366.] show a similar dependence on surfactant coverage. For the first time, to our knowledge, a relation is
made between a dynamical relaxation mode, the slower diffusive solvation time scale 2, and a spectral feature,
in this case the strongly hydrogen-bonded water structures induced by the negative surface charge. The
carboxylate and sulfate surfactants induce similar dynamics, suggesting that the solvation is not sensitive to
the chemical composition of the headgroup but rather to the total negative surface charge density.
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