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Figure 2. Small-angle X-ray and neutron reflectivity curves from (PSS/myoglobin/deuterated PSS/myoglobin)8 multilayer.

a surface roughness of the polyion film on an order of 1 nm. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) data revealed a surface roughness of 1-2 nm [42]. Polyion films are insoluble in water and in many organic solvents and are stable to 280 °C [38, 95].

Neutron reflectivity analysis of the films composed of alternate layers of deuterated PSS and hydrogen containing PAH has proved that polyanion/polycation films possess not only a high uniform thickness but a multilayer structure, too [19-22]. The interfaces between layers in polyion films are not sharp and partial interpenetration (30-40% of their thickness) between neighboring polymeric layers takes place [21-22]. A distinct spatial component separation may be reached between the first and the third or fourth neighboring polyion layers. In the neutron reflectivity experiments with the selectively deuterated component (usually d-PSS), it was possible to observe 1-3 Bragg reflections in addition to Kiessig fringes. This was not possible in the X-ray reflectivity experiments because of a small scattering contrast of neighboring polycations and polyanions, and because of their large interpenetration. X-ray Bragg reflections from the alternate gold nanoparticle/poly(allylamine) multilayers were observed by Schmitt et al. [55]. They demonstrated that in order to have good spatial separation between gold layers in the film, one needs to make a thicker polyion interlayer (of 3-4 PSS/PAH bilayers). In a similar approach, we formed the four-step unit cell multilayers of myoglobin, deuterated, and "usual" poly(styrenesulfonate): (myoglobin/deuterated-PSS/myoglobin/PSS)9. A Bragg reflection in the neutron reflectivity curve of this four-step unit cell multilayer was observed (Fig. 2). The film's total thickness was calculated at 94.0 nm, and the four-unit cell thickness was 11.1 nm.

The polycation/polyanion bilayer thickness depends on the charge density of the polyions. It was shown that more than 10% of polyion side groups have to be ionized for a stable reproducible multilayer assembly via alternate electrostatic adsorption [34]. High ionization of polyions results in a smaller step of film growth (1-2 nm) and lower ionization gives a larger growth step (3-6 nm). It can be reached either by adding salt to a polyion solution (as previously discussed for strong polyelectrolytes, such as PDDA and PSS), or by varying the pH for weak poly-electrolytes (e.g., polyacrylic acid (PAA) and PAH, as was analyzed by Yoo et al. [30]). Direct zeta-potential measurements confirmed a symmetric positive/negative alternation of the polycation/polyanion multilayer's outermost charge with adsorption cycles [34].

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