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Using neutron reflectometry we have resolvedto high resolutionthe 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.

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Communications film, resulting in terraces of bilayer steps. The regions of arachidic acid in the mixed LB film can be removed by washing in ethanol to leave behind islands of cadmium arachidate molecules. ExperimentalLangmuir-Blodgett films of arachidic acidicadmium arachidate were prepared using a constant-perimeter barrier trough (purpose-built) located in a Class 10000 microelectronics clean room. The subphase was ultrapure water obtained from a commercial reverse osmosisideionizationiUV sterilization system. The arachidic acid (eicosanoic acid) was obtained from Sigma (99 % purity). For salt formation, CdCIp (BDH, Analar Grade) was added to the suhphase to give an overall concentration of 2.5 x lo4 M. The pH was adjusted to 5.7i0.1 by the addition of HCI (BDH, Aristar Grade) or ammonia solution (BDH, Aristar Grade). Transfer of the floating monolayers onto hydrophilic silicon wafers ((100) orientation) was undertaken at a suhphase temperature of 19i1 "C and a deposition pressure of 30 mN m-'. The dipping speed was 2 mm m i d . Transfer ratios for all the monolayers deposited were 1.0oi0.05.A Digital Instruments Nanoscope Ill atomic force microscope was used to examine the topographical nature of the arachidic acidicadmium arachidate LB film surface following dipping and after washing in ethanol (to remove the free acid). All of the high resolution AFM images were acquired in air at room temperature using the contact mode and a 1 pm x 1 pm piezoelectric scan head. A 200 pm narrow-legged silicon nitride cantilever with a small spring-constant ( k = 0.06 Nm-') was used to minimize film damage due to high contact forces. The lower resolution images were acquired in air using the tapping mode in conjunction with a 10 ym x 10 Fm piezoelectric scan head. This technique employs a stiff silicon cantilever oscillating at a large amplitude near its resonance frequency (several hundred kilohertz) which is detected by an optical beam system. AFM images are presented as unfiltered data in gray-scale and were found to be stable and unchanged over long periods of observation.