Among different deposition techniques, the layer-by-layer (LbL) method has focused the attention of a large number of research groups. The pioneering work of Iler in 1966 did not become public until it was rediscovered by Decher in the beginning of 1990s as a simple and automatable method to fabricate films at the nanometer scale[1, 2]. Compared to LbL, other deposition techniques are limited to flat substrates and require expensive and delicate instrumentation. On the contrary, LbL does not depend neither on the substrate shape or size and a wide range of different materials can be deposited on different substrates such as windows or small optical fibers[5–7]. Additionally, this method can be also used to attach analytes of different chemical nature[8, 9]. As a consequence of these features, LbL has been used to functionalize surfaces with different goals such as antibacterial applications, the fabrication of hydrophobic or hydrophilic films[11, 12], or to develop sensors[13, 14]. The main idea of LbL method consists of the assembly of oppositely electrically charged polyelectrolytes (polycation and polyanion respectively) which form a bilayer; the process can be repeated as many times as the design requires. The chemical properties of the polyelectrolytes, such as the average molecular weight, the ionization degree, the concentration or the ionic strength[16, 17], just to mention some of the most important ones, define the morphology of the final film and, hence, its features.
The polyelectrolytes that can be used are divided in two categories, the strong and weak ones: in the first group, the ionization degree is not adjustable, whereas in the second one, it is adjustable by the pH of the solution. Depending on the ionization degree, the polymers get adsorbed on the substrate in a different manner: highly ionized solutions would yield to flat polyelectrolytes and very thin films; meanwhile, low ionization levels produce curled chains and rough layers. As the pH can be used to set the ionization degree, typically at least one of the polymers is weak, although in most times both of them belong to this category. In the case of polyelectrolytes whose ionization degree is not adjustable, the ionic strength of the solution can be varied by adding salts, and in this manner, altering the morphology of the polymer chains by electrostatical interactions. Another important factors are temperature, which defines the kinetics of the process, as well as the way the substrates is exposed to the polyelectrolytes solutions, for example, by dipping or spray.
Some of the ideas that were established about LbL, as the ones mentioned above, have been set under consideration. It was supposed the Z potential of the last deposited layer should always show the opposite sign of the following one; on the other hand, the roughness of the film was accepted to be reduced as the films grows. A recent work has revealed that when using certain polymers, these rules are not satisfied: With a 10-4 M concentration of poly(sodium phosphate) (PSP) and poly(allylamine hydrochloride) (PAH), the Z potential is not alternated between one layer and the next one; moreover, the roughness of the film increases with the number of bilayers when the substrate is sprayed with the polymeric solutions. This behavior seems to be a consequence of using PSP, an inorganic short chain polymer with interesting properties; the use of this kind of polymers establishes a new researching line and raises again some questions about the fundamentals of LbL, taking into account other non-electrostatic interactions such as hydrogen bonds during the growing process of the film. In the light of these results, some works have focused in the study of the key parameters of LbL in order to revise the effect of polymers as PSP in detail and redefine the rules of this technique.
In this work, nanofilms were prepared onto glass slides using PSP and PAH. Two different concentrations were used for the experiments, 10-3 and 10-4 M, because these are the same concentration values reported in the sprayed films studied by Decher et al.. Moreover, the substrates were dipped or sprayed with the solutions to check also how these alternatives affect the features of the film. The growing process was evaluated by preparing substrates with different number of bilayers so that their thickness, roughness, contact angle, and optical transmittance spectra were measured. To our knowledge, this is the first time that a comparative study of the properties of PSP/PAH films fabricated by dip-coating LbL and spray-assisted LbL is presented in the literature.