DIRECT DETERMINATION BY NEUTRON REFLECTION OF THE STRUCTURE OF TRIETHYLENE GLYCOL MONODODECYL ETHER LAYERS AT THE AIR-WATER-INTERFACE

Neutron reflectivity with isotopic substitution has been used to deter mine directly the structure of triethylene glycol monododecyl ether ad sorbed at the air-water interface. The surface excess has been measure d from concentrations ranging from twice the critical micelle concentr ation (cmc) (=5.5 X 10(-5) M) to one fiftieth cmc. The results are in good agreement with those from surface tension measurements and show t hat the area per molecule tends to a limiting value of 36 angstrom2. T he structure of the adsorbed layer has been determined at four concent rations: cmc, 0.2 cmc, 0.0545 cmc, and 0.0182 cmc. Two different metho ds of analysis of the data have been employed. One is the optical matr ix method which fits a single structural model to the reflectivity pro files from a set of isotopic species at a given concentration and the other is a more direct approach based on the kinematic theory. The two methods give structures identical within experimental error. The vari ation of the thicknesses of the hydrocarbon chain region of the adsorb ed layer and the region where solvent has a density less than its bulk value (approximately described as the head group region) and the rela tive locations of chain and water distributions across the surface wit h bulk concentration have been determined. For the saturated monolayer at the cmc the thickness of the chain region is found to be 20 +/- 1 angstrom, that of the head group region to be 11 +/- 1 angstrom, and t hat of the mean center to center distribution of chains and water to b e 10 +/- 1 angstrom. The number of water molecules associated with eac h surfactant molecule is 6. At the lowest concentration (0.0182 cmc) w here the monolayer is at its most dilute, it is found that the thickne ss of the whole layer is 15 +/- 1 angstrom, that of the head regions i s 7 +/- 1 angstrom, and the separation of the two distributions is 5 /- 1 angstrom, with the number of water molecules per surfactant now a bout 10. Comparison of these values with the width of the chain and so lvent distributions shows that the immersion of the hydrocarbon chains in the water varies from about 25 % at the cmc to 40 % at 0.0182 cmc.