STRUCTURE OF MONOLAYERS OF MONODODECYL DODECAETHYLENE GLYCOL AT THE AIR-WATER-INTERFACE STUDIED BY NEUTRON REFLECTION

We have determined the structure of a monolayer of dodecaethylene glyc ol monododecyl ether (C12E12) adsorbed at the air-water interface usin g specular neutron reflection in combination with deuterium labeling. The structure of the adsorbed monolayer was measured at its critical m icellar concentration (cmc, 1.25 x 10(-4) M), 0.3cmc, 0.13cmc, and 0.0 33cmc. The reflection data were analyzed using the optical matrix meth od and the kinematic approximation, both of which gave structures iden tical within tile limitations of the different models used. Fn the opt ical matric; method the monolayer was divided into two layers, the fir st layer consisting of the alkyl chain only, and the second layer cons isting of the ethoxylated head, a fraction of the alkyl chain, and wat er. Significant intermixing of alkyl chain and ethoxylated headgroup w as needed to account for the observed reflectivities. At the cmc about 35% of the alkyl chain seems to be incorporated into The headgroup re gion, and this increases to about 50% at the lowest concentration. The corresponding variation in the thickness of the alkyl chain layer was from 9 to 6 +/- 2 Angstrom and that of the head region from 22 to 20 +/- 2 Angstrom. The distances between the centers of the alkyl chain a nd solvent distributions were determined by the more direct kinematic method and the values were found to be 13, 11, 10, and 7.5 +/- 2 Angst rom for the four concentrations studied. The total thicknesses of the dodecyl chain and the ethoxylate groups were extracted from a simultan eous analysis of three reflectivity profiles from different isotopic c ompositions at a given surfactant concentration. The values of the two thicknesses were found to be 15.0, 13.0, 12.5, and 12.0 +/- 2 Angstro m for the dodecyl chain and 21, 18, 17, and 17 +/- 4 Angstrom for the ethoxylate group at the four concentrations. Comparison of these value s with previous work on C12En with shorter ethylene glycol chains show s that the distributions of the dodecyl chain are similar to those wit h smaller n, but the extent of mixing between the allyl chain and the ethoxylate group increases with n.