The determination of segment density profiles of polyethylene oxide layers adsorbed at the air-water interface

The adsorption of polyethylene oxide of molecular weight 17 800 and 87 000 at the air/water interface has been studied by neutron reflection and surface tension measurements. Over the concentration range 10^-4-10^-1 wt% of polymer the surface excesses determined by neutron reflection were consistent with the results derived from the surface tension using the Gibbs equation. Reflectivity measurements were made on three samples of different isotopic composition at a fixed concentration of 0.1 wt%. The adsorbed polymer layers were found to be essentially completely immersed in the water, contrary to earlier suggestions that a significant fraction of the polymer protrudes into the vapour phase. The distribution profiles of the polymers with respect to the surface normal direction are composed of two major blocks, one constituting the layer at the top surface and the other forming a diffuse region extending into the bulk solution. Over 90% of the homopolymer is in the first region with a thickness of 18 ± 2 Å, and the remaining diffuse region has a thickness of 35 ± 5 Å. Alternatively, the volume fraction profile of the polymer can be described as a 1 : 1 sum of two half Gaussians of widths 10 and 35 Å. Change of molecular weight and addition of salt do not change either the surface coverage or the segment density profiles significantly.