Effect of fatty alcohol monolayers on the rate of bromine transfer across the water/air interface: Assessment of candidate models using scanning electrochemical microscopy

The effect of fatty alcohol monolayers on the rate constant for Br-2 transfer across the water/air (W/A) interface has been investigated using scanning electrochemical microscopy-double potential step chronoamperometry (SECM-DPSC). An homologous series of four fatty alcohols was considered: 1-tetradecanol (C14OH); 1-hexadecanol (C16OH); 1-octadecanol (C18OH); and 1-eicosanol (C20OH). For these measurements, a submarine ultramicroelectrode (UME) was positioned in the aqueous subphase in a Langmuir trough at a typical distance of 1-2 mum from the W/A interface where the monolayer was assembled at a defined and controllable surface pressure. The SECM-DPSC approach involved electrogenerating Br-2 in an initial (forward) potential step by the diffusion-controlled oxidation of Br- in aqueous sulfuric acid solution. Electrogenerated Br-2 was subsequently collected by diffusion-controlled reduction in a second (reverse) potential step. The resulting current-time behavior provided information on both the tip-interface separation (forward step) and the kinetics of Br-2 transfer (reverse step). Fatty alcohol monolayers diminish the rate constant of Br-2 transfer across the W/A interface, with increasing carbon chain length from C14OH to C18OH (for a given amphiphile surface density), but the rate constant then increases for C20OH compared to C18OH. Reasons for this behavior are discussed, and the experimental data are used to examine four models proposed for the kinetics of molecular transfer across monolayer or bilayer membranes