Kinetics of bromine transfer across Langmuir monolayers of phosphatidylethanolamines at the water/air interface

Abstract

The kinetics of Br₂ transfer across phosphatidylethanolamine monolayers at the water/air (W/A) interface have been investigated using scanning electrochemical microscopy–double potential step chronoamperometry (SECM-DPSC). An homologous series of three phosphatidylethanolamines was considered: L-α-phosphatidylethanolamine dilauroyl (DLPE), L-α-phosphatidylethanolamine dipalmitoyl (DPPE) and L-α-phosphatidylethanolamine distearoyl (DSPE). The SECM-DPSC approach involved positioning a submarine ultramicroelectrode close to a monolayer assembled at the W/A interface at a defined and controllable surface pressure. Br₂ was produced in an initial (forward) potential step, by the diffusion-controlled oxidation of Br⁻, and then collected by diffusion-controlled reduction in a second (reverse) potential step. The resulting current–time behaviour provided information on both the tip–interface separation (forward step response) and the kinetics of Br₂ transfer (reverse step response). DSPE monolayers were found to diminish the rate of Br₂ transfer across the W/A interface, with the rate constant decreasing as the surface pressure increased. These experimental data were interpreted using a simple energy barrier model. In contrast, monolayers of DLPE and DPPE had no detectable effect on Br₂ transfer kinetics on the SECM time scale.