STRUCTURE OF AN ADSORBED LAYER OF N-DODECYL-N,N-DIMETHYLAMINO ACETATEAT THE AIR SOLUTION INTERFACE AS DETERMINED BY NEUTRON REFLECTION/

The composition and structure of the interfacial layer formed at the a ir/water interface by a zwitterionic surfactant, n-dodecyl-N,N-dimethy lamino acetate, has been determined by neutron reflection. The composi tions determined by neutron reflection and by surface tension measurem ents in conjunction with the Gibbs equation are in excellent agreement if the prefactor in the Gibbs equation is one, i.e., the surfactant b ehaves as though it is completely uncharged. At the critical micelle c oncentration (cmc) the structure most consistent with the neutron data is one where the head group is vertically oriented with respect to th e surface normal, but the chain is strongly tilted away from the surfa ce normal with a value of (cos theta) approximate to 0.45. This is a l arger angle of tilt than observed with other surfactants at a comparab le surface density and is attributed to the vertical orientation of th e head group forcing the chain to be tilted away from the surface norm al. Unlike most other surfactants, there is little change in the tilt of the chain as the coverage is decreased, probably because the tilt i s already nearly a maximum at the cmc, The coverage of the layer incre ases markedly with temperature, and analysis of the thermodynamic para meters of the adsorption suggests the possibility that, at higher cove rages, this is driven by a large gain in entropy from loss of water of hydration from adsorbed surfactant.