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 ₁₄OH); 1-hexadecanol (C ₁₆OH); 1-octadecanol (C ₁₈OH); and 1-eicosanol (C ₂₀OH). 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 ₁₄OH to C ₁₈OH (for a given amphiphile surface density), but the rate constant then increases for C ₂₀OH compared to C ₁₈OH. 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.