Test of the universality of the critical adsorption profile by neutron reflection

Using neutron reflectivity, we directly and self-consistently obtained all the parameters of the critical adsorption profile of three critical mixtures (n-hexane+perfluorohexane, deuterated methanol+cyclohexane and methanol+deuterated cyclohexane) assuming only a Liu-Fisher form for the profile. The characteristic exponent and the amplitude ratio which can be deduced from the neutron spectra alone are in good agreement with one another, with ellipsometric data and with theoretical values. Owing to auxiliary measurements of coexistence curves and correlation lengths, we could also check the amplitude of the intermediate and distal parts of the profile with good agreements for the methanol cyclohexane mixtures only. The adsorbed amount is much greater in the n-hexane+perfluorohexane mixture where both the power-law amplitude and the characteristic length of the profile are affected. Received 30 May 2001