The structure of the mixed nonionic surfactant monolayer of (6) have used a more fundamental approach, aimed at remonododecyl triethylene glycol and monododecyl octaethylene moving the phenomenological nature of the regular solution glycol adsorbed at the air-water interface has been determined approach. using specular neutron reflection. Using partial isotopic labeling To understand the surface chemical properties in mixtures (deuterium/hydrogen) of the alkyl and ethylene oxide chains of it is important to know the interfacial composition, which each type of molecule, the distribution and relative positions of will in general be different from the bulk composition, and those labeled fragments have been obtained. The frustration to know the factors which control that composition over a caused by the packing of the triethylene and octaethylene glycol headgroups results in a change of the surfactant structure com-wide range of solution compositions and concentrations. We pared to the pure monolayer of either surfactant. Compared to have shown previously that neutron reflectivity, in combinathe pure monolayer the alkyl chain distributions of both surfaction with hydrogen/deuterium (H/D) isotopic substitution, tants are more extended, the triethylene glycol group is less hyhas the selectivity needed to study the adsorption of mixed drated, and the octaethylene glycol group is less extended and surfactants over a wide concentration range (7-11), and for more hydrated. ᭧ 1998 Academic Press giving detailed structural information (12). This provides