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.