Letters A New Approach for Measuring the Effect of a Monolayer on Molecular Transfer across an Air/Water Interface Using Scanning Electrochemical Microscopy Christopher J. Slevin, † Steve Ryley, ‡ David J. Walton, ‡ and Patrick R. Unwin* ,† Department of Chemistry, University of Warwick, Coventry, United Kingdom CV4 7AL, and Centre for Molecular and Biomolecular Electronics, School of Natural and Environmental Sciences, Coventry University, Priory Street, Coventry, United Kingdom CV1 5FB Received March 20, 1998. In Final Form: July 13, 1998 A new approach is described for the quantitative study of the effect of a monolayer on solute transfer rates across an air/water interface. The technique is illustrated through measurements of oxygen transfer across a monolayer of 1-octadecanol as a function of compression. The experimental approach uses an inverted ultramicroelectrode (UME), positioned in the water phase in a Langmuir trough close to the air/water interface to induce the transfer of oxygen from air to water via the reduction (and depletion) of oxygen. The steady-state current response, measured as a function of UME-interface separation at various monolayer compressions, demonstrates that the rate of oxygen transfer is governed primarily by the accessible free area of the interface. The transfer of molecules and ions between phases is a fundamental and general phenomenon involved in a range of chemical 1 and biological 2 processes. Under- standing the effects of monolayers on the mechanisms and rates of interfacial transfer is of particular importance, with practical implications ranging from membrane transport in biological systems 2 to the exchange of chemical species across phase boundaries in natural environments. 3 For monolayers at air/water interfaces, the effects of amphiphile, chain length, and packing on water evapora- tion have been widely studied 4 following the first dem- onstration by Rideal 5 that monolayers were effective in reducing evaporation rates. However, much less work has considered the permeability of monolayers at air/water interfaces to solute molecules. Of those studies that have been carried out in the latter area, several report that thin organic films have little effect on solute transfer across air/water interfaces, 3,6 but this may be attributed to the intrinsically low liquid-phase mass transfer rates utilized. A gas absorption method introduced to overcome this problem 7 showed that various long chain alcohol mono- layers reduced the initial uptake rates of several gases, * E-mail: P.R.Unwin@warwick.ac.uk. † University of Warwick. ‡ Coventry University. (1) See, for example: (a) Adamson, A. W. Physical Chemistry of Surfaces; Wiley: New York, 1997. (b) Atherton, J. H. Res. Chem. Kinet. 1994, 2, 193. (c) Benjamin, I. Chem. Rev. 1996, 96, 1449. (2) See, for example: (a) Volkov, A. G.; Deamer, D. W.; Tanelian, D. L.; Markin, V. S. Liquid Interfaces in Chemistry and Biology; Wiley: New York, 1998. (b) Oxygen Transport in Biological Systems; Egginton, S., Ross, H. F., Eds.; Cambridge University Press: New York, 1993. (c) Gennis, R. B. Biomembranes; Springer: New York, 1995. (3) For overviews, see: (a) Thibodeaux, L. J. Environmental Chemo- dynamics: Movement of Chemicals in Air, Water and Soil, 2nd ed.; Wiley: New York, 1996. (b) Schwarzenbach, R. P.; Gschwend, P. M.; Imboden, D. M. Environmental Organic Chemistry; Wiley: New York, 1993. (4) (a) Barnes, G. T.; Hunter, D. S. J. Colloid Interface Sci. 1990, 136, 198. (b) Seaver, M.; Peele, J. R.; Manuccia, T. J.; Rubel, G. O.; Ritchie, G. J. Phys. Chem. 1992, 96, 6389. (c) Drummond, C. J.; Elliot, P.; Furlong, D. N.; Barnes, G. T. J. Colloid Interface Sci. 1992, 151, 189. (d) Retardation of Evaporation by Monolayers; LaMer, V. K., Ed.; Academic Press: New York, 1962. (5) Rideal, E. K. J. Phys. Chem. 1925, 29, 1585. (6) Linton, M.; Sutherland, K. L. Aust. J. Appl. Sci. 1958, 9, 18. © Copyright 1998 American Chemical Society SEPTEMBER 15, 1998 VOLUME 14, NUMBER 19 S0743-7463(98)00320-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/21/1998