Kinetics of Polyion Adsorption

For the time-dependent control of adsorption and monitoring of the assembly in-situ, the quartz crystal microbalance method is quite suitable [23, 46, 86]. The kinetics of

the adsorption process could be delineated by the QCM-technique, which is indispensable for establishing proper assembly conditions (e.g., a saturation adsorption time).

The multilayer assemblies are characterized by means of a quartz crystal microbalance technique in two ways:

1. After drying a sample in a nitrogen stream, we measured the resonance frequency shift and calculated an adsorbed mass by the Sauerbrey equation; or

2. By monitoring the resonator frequency during the adsorption process onto one side of the resonator, which was in permanent contact with polyion solutions. While performing experiments in permanent contact with the polyion solution, we touched the surface of solutions with one side of the resonator, while the upper electrode was kept open to air and the upper contact wire was insulated from the solution by a sili-cone paint covering.

The fitting of adsorption to an exponential law yields a first-order rate of adsorption for PSS t = 2.5 ± 0.2 min and for PAH t = 2.1 ± 0.2 min. This means that during the first 5 min ca 87% of the material is adsorbed onto the charged support and t = 8 min (t = 3t) gives 95% full coverage. Typically, in most publications on polyion assembly, adsorption times of 5 to 20 min are used. One does not need to maintain an adsorption time with great precision: a minute more or less does not influence the layer thickness if we are at the saturation region. For other species, PDDA, PEI, mont-morillonite clay, myoglobin, lysozyme, and glucose oxidase, the first-order rate of adsorption onto an oppositely charged surface was found to be 2, 3, 1.8, 3, 4, and 5 min, respectively. Interestingly, 5-20 min is essentially greater than the diffusion-limited time (mass transport limitation), which is necessary for complete surface covering (for the used linear polyion concentrations it is a few seconds). Only for 45-nm silica/PDDA assembly do we have an example when 2 s time corresponds to the diffusion limited time for the SiO2 mono-layer adsorption.

One could suppose that linear polyion adsorption occurs in two stages: quick anchoring to a surface and slow relaxation. To reach a surface-charge reversion during linear polyion adsorption, one needs a concentration greater than 10-5 M [23]. The dependence of polyion layer thickness on concentration is not great: thus, in the concentration range of 0.1-5 mg/mL PSS/PAH pair yielded a similar bilayer thickness. A further decrease in polyion concentration (using 0.01 mg/mL) decreases the layer thickness of the adsorbed polyion. An increase in the component concentrations to 20-30 mg/ml may result in the nonlinear (exponential) enlargement of the growth rate with adsorption steps, especially if an intermediate sample rinsing is not long enough [47].

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