Neutron reflection study of butanol and hexanol adsorbed at the surface of their aqueous solutions

Neutron-reflectivity measurements have been made on aqueous butanol and hexanol solutions over a range of concentrations up to the saturation limit of the alcohol. The surface excesses are in good agreement with previous surface tension measurements. The thicknesses of the alcohol layers have been determined as a function of concentration and the separation of their surface distributions from that of water has been determined. The thicknesses of both butanol and hexanol layers are generally larger than the fully extended length of the molecule except at very low surface concentrations. A large part of this is caused by thermal roughness but there appears to be some additional 'structural' roughness. The separation between water and alcohol distributions is surprisingly low and indicates that the molecules are on average tilted away from the surface normal.