NEUTRON REFLECTION FROM TRIETHYLENE GLYCOL MONODODECYL ETHER ADSORBED AT THE AIR-LIQUID INTERFACE - THE VARIATION OF THE HYDROCARBON CHAIN DISTRIBUTION WITH SURFACE CONCENTRATION

The structure of layers of triethylene glycol monododecyl ether (C-12E3) adsorbed at the air/water interface at four concentrations from the critical micelle concentration (cmc = 5.5 x 10(-5) M) down to 1 x 10(-6) M have been determined using neutron specular reflection together with isotopic substitution. The distribution of any part of the layer is described in terms of the full width of the distribution at half height, sigma*; i.e. sigma* is a measure of the thickness of that part of the layer. The width, sigma*, of the alkyl chain part of the layer varied from 14 +/- 1 angstrom at an area per molecule (A) of 36 angstrom2 down to 10 +/- 1.5 angstrom at an A of 89 angstrom2. The corresponding widths of the ethylene glycol region varied from 13 +/- 1 to 5 +/- 2 angstrom, and the separation of the two chain regions changed from 8 +/- 0.5 to 4.5 +/- 1 angstrom. The separation of the alkyl chains from the midpoint of the water distribution was found to be 2 angstrom greater than from the ethylene glycol chains at the cmc but exactly the same at 1 x 10(-6) M. Isotopic labeling of the two halves of the alkyl chain was used to establish that the alkyl chain is substantially tilted away from the surface normal at all concentrations and that at least one gauche conformation occurs on average in the alkyl chain at the highest surface concentration. The shape of the overall chain distribution is better described by a Gaussian distribution than by a uniform layer. The static displacement of the surfactant molecules in the direction normal to the surface is shown to be strongly disordered at the cmc, the mean displacement being about 9 angstrom, which is of the order of half the projected length of the molecule in the vertical direction. The ethylene glycol chain is oriented close to the normal direction when A = 36 angstrom2 but seems to lie nearly flat in the surface plane when A = 89 angstrom2.