The question of reproducibility arises immediately when we draw the analogy
between a chemical reaction and layer-by-layer adsorption. On first sight one may
Figure 1.3 An artists rendition of layer-by-layer
assembly has recently appeared on the cover of
a review published by K. Ariga et al. [45]. It nicely
depicts that the architecture of a
multicomponent film with a defined layer
sequence conveniently depends on the
sequence of immersions in different beakers
containing the different species.
1.5 Reproducibility and Choice of Deposition Conditions j7
say that molecules are unique species and multilayer films are only fuzzy
supramolecular objects. This is essentially the same argument that has downgraded
for years macromolecular chemistry in comparison to organic chemistry. Today it is
generally accepted that ill-defined macromolecules are also unique species that can
indeed be well described by distributions and average properties like polydispersity or
degree of polymerization. The situation is similar for multilayer films as they are
characterized by a sequence of layers in which each layer has its individual structure
and properties (Figure 1.3). While the sequence of layers is as strict as the arrangement of atoms in a molecule, the properties of each layer can only be described as an
average over a certain area or over a certain distance along the layer normal. The most
obvious property of an individual layer is its thickness, which is dependent on the
nature of the underlying surface and on the deposition conditions. Parameters
presumed to be important with respect to the underlying surface are, for example,
nature and density of charged groups, their local mobility (in the case of a polymeric
surface), and the surface roughness. Other important parameters are: solvent,
concentration of adsorbing species, adsorption time, temperature, nature and
concentration of added salt, rinsing time, humidity of the surrounding air, drying,
agitation during adsorption or rinsing, dipping speed, and so forth. Typically,
polymer and salt concentrations and deposition times are well described in the
literature, however, some teams have already seen reproducibility issues when trying
in colder weather to repeat results obtained during a hot summer.