Headgroup Immersion Depth and Its Effect on the Lateral Diffusion of Amphiphiles at the Air/Water Interface

Two-dimensional electrochemical measurements were carried out with line microelectrodes positioned in the plane of the air/water interface to characterize the lateral mobilities of several long alkyl chain ferrocene amphiphiles possessing headgroups of different polarity. In the liquid range of mean molecular areas (MMA) of 55-95 Å2/molecule, the diffusion constants increase linearly with MMA. The slopes of these linear plots depend strongly on the headgroup polarity, demonstrating that the immersion depth of the amphiphiles is the key factor in determining their lateral mobility. Investigations of the alkane chain length dependence revealed an unusual inverse effect on D. This suggests that for this class of amphiphiles the immersion depth depends not only on the headgroup polarity but also on the length of chains. Thus, the longer alkane chain amphiphiles exhibit smaller immersion depth and larger diffusion constants of lateral mobility. The effects of the immersion depth and chain length were analyzed in terms of a modified hydrodynamic model of Saffman and Delbrück. Quantitative analysis of the experimental data yielded the average values of the headgroup immersion depth for the dodecaneand hexadecaneferrocenecarboxamides of 6.4 and 4.0 Å, respectively.