**2.1 Surface modification based on LbL self-assembly**

LbL self-assembly is a noninvasive method that does not impact the bulk properties of the supporting membranes. The superiority of this technique lies in the well-controlled of thickness, roughness, and charge of the layer. LbL uses polyelectrolytes which are normally hydrophilic and exhibit a charge. Chitosan, Polyethyleneimine (PEI), Poly(diallyldimethylammonium chloride) (PDADMAC), Poly(allylamine hydrochloride) (PAH) are extensively preferred cationic polyelectrolytes, while alginate, Poly(sodium 4-styrenesulfonate) (PSS), Sodium carboxymethyl cellulose are preferred as anionic polyelectrolytes [27, 28]. Deposition can be accomplished by alternating oppositely charged polyelectrolytes on support followed by rinsing after each step, which is used to remove excess and weakly adsorbed polyelectrolytes, hence defect-free ultra-thin layer around 2 nm is formed. A binary layer formed by two opposite polyelectrolyte deposits can be reproduced up to 60–100 multilayers [29, 30]. One of the coating processes including dip coating, spin coating or spray coating, which are schematically represented in **Figure 1** can be applied for LbL assembly. The main drawback of dip coating is the long process time due to diffusional resistance, and rinsing for the elimination of polyelectrolyte complex formation and hence flocculation on the surface. Successive deposition of highly ordered polyelectrolyte multilayers as a result of rapid rearrangement of polymer chains on the substrate is performed by a spin coating within a short time, however, the surface area of the material to be coated is limited. In the case of spray coating process, the polyelectrolyte solution is sprayed over a support membrane and the excess solution is drained by gravity. The processing time to finish deposition is almost two orders of magnitude lower than the dip-coating process and large surfaces can be easily coated with an automated spray coater.

The thickness and the morphology of the layers are influenced by the polyelectrolyte type, number of bilayers and deposition conditions (pH, salt concentration, polyelectrolyte concentration, and deposition temperature and time). For example, linear growth in the multilayer thicknesses was achieved for the PSS/PDADMAC system at 25 °C, while, the increase was reported exponential at 55 °C [32]. The number of sequential layered pairs is commonly known as to produce the thicker film, which corresponds to a lower permeability. However, according to Lajimi's results, the maximum flux, charge density, and hydrophilicity were observed when the number of CHI/ALG bilayers attained 15 [33]. This was explained by the transition of polyelectrolyte layers from a loose stratified structure to a tightened interpenetrating granular structure. In the following subsection, various parameters affecting the structure of the deposited layers will be discussed.
