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 θ〉 ≈ 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.
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