NEUTRON REFLECTION FROM A LAYER OF MONODODECYL OCTAETHYLENE GLYCOL ADSORBED AT THE AIR-LIQUID INTERFACE - THE STRUCTURE OF THE LAYER AND THE EFFECTS OF TEMPERATURE

We have determined the structure of a monolayer of monododecyl octaeth ylene glycol (C(12)E(8)) adsorbed at the air/water interface at its cr itical micelle concentration and at temperatures of 298 and 323 K usin g neutron specular reflection in combination with isotopic labeling. T here is little effect of temperature on the coverage of the pure mater ial, though a significant variation is observed when the material was slightly contaminated with lower members of the series (C(12)E(m) With m < 8). However, subtle changes in the structure of the adsorbed laye r do occur, the alkyl chain region becoming about 14% thicker at the h igher temperature, whereas the thickness of the ethylene glycol chain region does not change. An analysis of these differences and the chang es in the separation of the different fragments of the surfactant sugg est that the layer is increasingly roughened at the higher temperature and that the ethylene glycol chain region is significantly dehydrated . The structure of C(12)E(8) is compared with other members of the C(1 2)E(m) series at approximately the same area per molecule of 55-65 Ang strom(2). The alkyl chain thickness is constant throughout the series, and the values of the structural parameters indicate a large average tilt of the surfactant molecules away from the surface normal and a si gnificant incidence of gauche conformations in the alkyl chain.