Neutron reflectivity studies of critical adsorption: the correspondence between a critical adsorption profile and...

For interfacial order parameter profiles which decay as Az(-mu), such as the composition profile of the noncritical interface of a binary liquid mixture at a critical end point, there is a reported one-to-one correspondence between the profile and the reflectivity which can be described by an analytical theory [S. Dietrich and R. Schack, Phys. Rev. Lett. 58, 140 (1987)]. Evidence for mu=0.53+/-0.02 for adsorption at the hydrophilic silicon/liquid surface of the mixture (2-butoxyethanol+deuterium oxide) near its lower critical end point, determined from neutron reflectivity measurements, is presented. This value is in good agreement with the theoretical prediction of mu=0.516+/-0.004. Further examination of the data permits the determination of the asymptotic surface enrichment scaling factor amplitude P0 approximately 0.11, which is not in agreement with the theoretical value P(0) = 0.94+/-0.05 and values determined by other experimental methods.