Neutron reflection from a layer of monododecyl hexaethylene glycol adsorbed at the air-liquid interface: the configuration of the ethylene glycol chain

Abstract

We have determined the structure of a layer of monododecyl hexaethylene glycol (C₁₂E₆) adsorbed at the air/water interface at its critical micelle concentration using neutron specular reflection together with isotopic substitution. For both the C₁₂ hydrocarbon chain and the E₆ ethylene glycol chain distributions the full width at half height, σ*, was found to be 13.5 ± 1 Å, to be compared with fully extended chain lengths of 16.7 and 21.6 Å, respectively. The separation of the alkyl and ethylene glycol chain regions was found to be 9 ± 0.5 Å, 1 Å less than the alkyl chain water separation of 10 ± 0.5 Å. Labeling experiments were also used to establish the widths of the two halves of the E₆ chain. The E₃ at the free end was found to be 14 ± 2 Å thick (σ*) and at the "anchored" end 12 ± 2 Å, and the separation between the distributions of the two fragments was found to be 3.5 ± 1 Å. The complete set of measurements on the labeled E₆ fragment, the first measurements of their kind, indicate that the E₆ distribution is characteristic of a significant incidence of gauche conformations in the ethylene glycol chain. The separate labeling experiments also show that the layer is roughened, probably by capillary waves. When the contribution of this roughness is removed from the overall thickness of the alkyl chain the chain is found to be significantly tilted away from the surface normal.