Monomer–eximer dynamics in spread monolayers. I. Lateral diffusion of pyrene dodecanoic acid at the air–water interface

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Abstract
In this paper, we present a combined experimental and theoretical study of monomer–eximer dynamics in spread monolayers. The system studied here is a spread monolayer of 12‐(1‐pyrene) dodecanoic acid (PDA) and oleic acid at the air–water interface of a neutral aqueous solution. The microscopic kinetic behavior of the PDA probe in the monolayer was monitored through the eximer–monomer, steady‐state photoexcitation of the pyrene moiety. The steady‐state ratio of eximer to monomer intensities, I2/I1, was found to depend linearly on the mole fraction of PDA in oleic acid over the concentration range studied. These data were interpreted theoretically by constructing a reaction‐dynamic model to describe the lateral diffusion and subsequent interaction of the pyrene species. Using analytical results from the theory of random walks on lattices with traps, and from data from Monte Carlo simulations, an estimate is obtained for the two‐dimensional diffusion coefficient of the pyrene probe in the monolayer; this estimate is 1.7×10−6 cm2/sec.