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 (C 14 OH); 1-hexadecanol (C 16 OH); 1-octadecanol (C 18 OH); and 1-eicosanol (C 20 OH). For these measurements, a submarine ultramicroelectrode (UME) was positioned in the aqueous subphase in a Langmuir trough at a typical distance of 1−2 μm 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 C 14 OH to C 18 OH (for a given amphiphile surface density), but the rate constant then increases for C 20 OH compared to C 18 OH. 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.