Structure of an adsorbed layer of n-Dodecyl-N,N-dimethylamino acetate at the air/solution interface as determined by neutron reflection

The composition and structure of the interfacial layer formed at the air/water interface by a zwitterionic surfactant, n-dodecyl-N,N-dimethylamino acetate, has been determined by neutron reflection. The compositions determined by neutron reflection and by surface tension measurements in conjunction with the Gibbs equation are in excellent agreement if the prefactor in the Gibbs equation is one, i.e., the surfactant behaves as though it is completely uncharged. At the critical micelle concentration (cmc) the structure most consistent with the neutron data is one where the head group is vertically oriented with respect to the surface normal, but the chain is strongly tilted away from the surface normal with a value of (cos theta) approximate to 0.45. This is a larger angle of tilt than observed with other surfactants at a comparable surface density and is attributed to the vertical orientation of the head group forcing the chain to be tilted away from the surface normal. Unlike most other surfactants, there is little change in the tilt of the chain as the coverage is decreased, probably because the tilt is already nearly a maximum at the cmc, The coverage of the layer increases markedly with temperature, and analysis of the thermodynamic parameters of the adsorption suggests the possibility that, at higher coverages, this is driven by a large gain in entropy from loss of water of hydration from adsorbed surfactant.