Using neutron reflectometry we have resolvedto high resolutionthe internal structure
of self-assembled polyelectrolyte multilayer films and have developed a detailed molecular picture of
such systems by analyzing the data with a composition-space refinement technique. We show that such
surface films consist of stratified structures in which polyanions and polycations of individual layers
interdigitate one another intimately. Nevertheless, the deposition technique leads to results that are
predictable, if well-defined and constant environmental conditions are maintained during the preparation.
For alternating layers of poly(styrenesulfonate) (PSS) and poly(allylamine hydrochloride) (PAH), adsorbed
onto atomically flat surfaces, a roughening of successively deposited layers leads to a progressively larger
number of adsorption sites for consecutive generations of adsorbed polymer, and thus to an increase in
layer thicknesses with an increasing number of deposited layers. Because of the interpenetration of
adjacent polyelectrolyte species, however, this increase settles quickly into an equilibrium thickness. In
fully hydrated films (100% relative humidity), water occupies ≥40% of the volume within the films. About
twice as much water (by volume) is associated with PSS as with PAH. Incorporated inorganic salt plays
a minor role only, if any. The equilibrium thickness of the deposited layer structure may be fine-tuned
via the ionic strength, I, of the solutions used for the preparation. We show that the dependence of the
thickness d
lp per layer pair on I is linear, with a sensitivity, Δd
lp/ΔI = 16 Å × L/mol. Concurrently with
the layer thickness the interface roughness σ between adjacent layers increases: σ ∼ 0.4 × d
lp. In contrast
to the ionic strength of the deposition solutions, the degree of polymerization of the polyanions used in
the preparation plays a minor role only in determining the overall structure of the deposited films. The
results reported here are quantitatively consistent with those of a recent study (Tarabia et al. J. Appl.
Phys.
1998, 83, 725−732), if one assumes that the hydration of the polyelectrolyte molecules in the sample
films investigated in the two studies is similar.
X-ray reflectivity measurements reveal a significant dewetting of a large hydrophobic paraffin surface floating on water. The dewetting phenomenon extends less than 15 A into the bulk water phase and results in an integrated density deficit of about one water molecule per 25-30 A(2) of water in contact with the paraffin surface. The results are supported by molecular dynamics simulations and related to the hydrophobic effect.
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