Neutron reflectivity studies of critical adsorption: The correspondence between a critical adsorption profile and specular neutron reflection

For interfacial order parameter profiles which decay as Az(-mu), such as th e composition profile of the noncritical interface of a binary liquid mixtu re at a critical end point, there is a reported one-to-one correspondence b etween the profile and the reflectivity which can be described by an analyt ical theory [S. Dietrich and R. Schack, Phys. Rev. Lett. 58, 140 (1987)]. E vidence for mu=0.53+/-0.02 for adsorption at the hydrophilic silicon/liquid surface of the mixture (2-butoxyethanol+deuterium oxide) near its lower cr itical end point, determined from neutron reflectivity measurements, is pre sented. 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 P(0)approxim ate to 0.11, which is not in agreement with the theoretical value P-0=0.94/-0.05 and values determined by other experimental methods. [S1063-651X(99) 01704-3].