LATERAL DIFFUSION OF AMPHIPHILES IN FATTY-ACID MONOLAYERS AT THE AIR-WATER-INTERFACE - A STEADY-STATE AND TIME-RESOLVED FLUORESCENCE QUENCHING STUDY

Lateral diffusion of amphiphilic molecules has been investigated in oleic acid (OA) monolayers at the air-water interface by studying the fluorescence quenching of N-(1-pyrenylsulfonyl)dipalmitoyl-L-alpha-phosphatidylethanolamine (pyrenyl-DPPE) by 4-(N,N-dimethyl-N-hexadecylammonium)-2,2,6,6-tetramethylpiperidine-1-oxyl iodide (CAT-16). Both steady-state and time-resolved techniques were employed to yield data which show clear evidence for the features predicted for diffusion-controlled reactions in two dimensions. Analysis of the steady-state quenching behavior as a function of quencher concentration produced nonlinear Stern-Volmer plots. The time-resolved measurements showed single-exponential behavior for the pyrene chromophore in the absence of quencher, providing evidence that it is not aggregated in the monolayer film. In the presence of quencher the decays were nonexponential. The lateral diffusion coefficients were found to decrease with increasing surface pressure. This result reflects an expected decrease of monolayer fluidity upon compression.