Effect of organic surfactant on femtosecond solvation dynamics at the air-water interface

Solvation dynamics at an air-water interface covered with a Langmuir monolayer of a neutral surfactant stearic acid CH3(CH2)(16)COOH was measured using femtosecond pump-probe spectroscopy of a molecular probe, coumarin 314. The second harmonic generation was used as a surface-specific ultrafast probe. The results are compared with the solvation dynamics measured for the same probe molecule at the air-water interface without the surfactant. The overall solvation time in the presence of the fatty acid monolayer (tau (s) = 400 +/- 60 fs) differs from that at the air-water interface, where two components tau (1) = 250 +/- 60 fs and tau (2) 1250 +/- 80 fs were obtained, with an amplitude weighted average solvation time of tau (s) = 850 +/- 70 fs. These results are in agreement with bulk studies of coumarins in water. The different solvation dynamics at the surfactant modified interface is attributed to rearrangement of the hydrogen bonding network of water molecules near the interface due to interactions with the hydrophilic carboxyl group of the surfactant.