External-reflection Fourier-transform infrared spectroscopy is applied at the air-water interface to determine molecular information about the dynamics of surfactant adsorption on flowing liquid surfaces. An overflowing cylinder is the experimental platform chosen to carry out this research, as the steady-state continually-expanding liquid surface is well-characterised. It is a non-trivial task to glean molecular information about surfactant sub-monolayers with a single specular-reflectance bounce of the infrared beam off the flowing water surface; however this aim was fulfilled by the careful design of an optical bench and the optimisation of the throughput of radiation to the infrared detector. A spectrum representing the surfactant monolayer is derived from the subtraction of a reflectance spectrum of water from a reflectance spectrum of surfactant solution. The experimental conditions for optimising the carbon-hydrogen stretching peaks absorbances, whilst minimising the background noise, were investigated: the chosen optimum conditions were s-polarised light with an angle of incidence of 40°. In preliminary experiments under these experimental conditions, the integrated peak area of the carbon-hydrogen stretching region was monitored with changing bulk concentration of a cationic surfactant, hexadecyltrimethylammonium bromide (CTAB). A correlation with neutron reflection data allowed a comparison of the integrated peak area of the carbon-hydrogen stretching region with surface excess of the adsorbed surfactant monolayer. The results confirmed the surface sensitivity of the technique, and showed that there was a non-linear relationship between the integrated peak area of the carbon-hydrogen stretching region and surface excess of the surfactant below the critical micelle concentration.

Submitted by: Richard Campbell - phone: 01865 275433

e-mail: richard.campbell@chem.ox.ac.uk