We present the results of a neutron-reflectivity study of the universal critical-adsorption profile near the liquid-vapor interface of a binary-liquid mixture (methanol $+$ deuterated cyclohexane) near criticality. As the critical temperature is approached from the one-phase region, we observed an abrupt decrease in the intensity of the specularly reflected neutron beam near the total-reflection edge. By interpreting the observation as the expected neutron-reflectivity discontinuity associated with a slowly decaying power-law profile, we evaluated the power-law exponent ( $\beta /\nu =0.50\mathrm{}\pm 0.05$).

• Received 3 March 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.1977