*2.1.1 Factors affecting the structure of the LbL modified membranes*

One of the most important parameters controlling the thickness, stability, and structure of layers is the salt concentration (ionic strength). Increasing ionic strength leads to thicker layers with a rougher surface [34]. The polyelectrolytes at high salt concentrations turn to coiled and loopy structures (instead of a flat

#### **Figure 1.**

*Schematic representation of different processes used for LbL assembly: (A) dip coating, (B) spin coating and (C) spray coating. [31].*

**Figure 2.**

*Variation of the PEI deposition layer with respect to ionic strength of PEI solution. NaCl concentration in PEI solution (a) 0 M and (b) 0.5 M [35].*

surface) with more charged chain segments, due to the screening effect, which prevents the electrostatic interaction between polyelectrolyte charges. This behavior is represented schematically in **Figure 2** [35]. In the presence of salt, more PEI adsorption occurs as a result of reduced segment/segment repulsions and increased surface/segment attractions. The reduction in repulsive forces between polyelectrolyte segments makes them small coils covering lower surface area per chain, causing to a larger area density of segments. The former is connected to the radius

*Principles of Membrane Surface Modification for Water Applications DOI: http://dx.doi.org/10.5772/intechopen.96366*

of gyration (Rg), directly related to selectivity, while the latter is related to surface charge density. Tekinalp showed that the surface charge groups of PEI deposited layer in the presence of salt was higher compared to the salt free case. It has been also proposed that the first polyelectrolyte solution should contain a high concentration of salt, to improve surface segment interactions and thus higher polyelectrolyte adsorption, where a higher unbound charge can be used to form a stable and thin second layer. On the other hand, there is no need to add salt to the second polyelectrolyte solution in order to prevent the formation of thick and rough layers.

The strength of the interactions between polyelectrolyte layers, hence stability of the deposition is depended on the salt counterions which participate in charge neutralization called extrinsic compensation. The following ion exchange reaction can be used to explain this phenomenon [36]:

$$\text{Pol}^\* \text{Pol}\_m^- \text{Na}\_{aq}^\* + \text{Cl}\_{aq}^- \leftrightarrow \text{Pol}^\* \text{Cl}\_m^- + \text{Pol}^- \text{Na}\_m^\* \tag{1}$$

$$K = \frac{\mathcal{y}^2}{(1-\mathcal{y})\left[NaCl\right]} = \left(\frac{\mathcal{y}^2}{\left[NaCl\right]\_{aq}^2}\right)\_{\mathcal{y}\to 0} \tag{2}$$

where, m, K, and y represent deposited layers, equilibrium constant, and extrinsically compensated polyelectrolyte multilayers, respectively. High salt concentration results in an increased charge screening as the extrinsic charge overcompensation ( *Pol Cl Pol Na m m* +− − + ) is dominant over the intrinsic charge compensation ( *Pol Polm* + − ). At this condition, chains are more mobile due to the poor interactions between polyelectrolytes, which will enhance the possibility of layers detachment [37]. In fact, multilayer formation is thermodynamically favorable as the polyelectrolyte complexation has a small enthalpy change and increase in entropy [38]. The displacement of counterions when polyelectrolytes adsorb on an oppositely charged polyelectrolyte creates an increase in entropy. The sum of three terms determine the Gibbs free energy change of the LbL process. The first one, which is known as intrinsic compensation is the attraction energy carried out between surface charge groups and opposite charges on the polyelectrolyte. The second one, which is not desirable through LbL process is the conformational change caused by entropy loss. The third term is another penalty for deposition and is related to the segment/segment repulsive energy. It is clear that detachment of the deposited layers is prone when the sum of the repulsive and conformation energies exceeds the attraction energy. Therefore, charge density of polyelectrolytes and the ionic strength of the solution predominantly control the stability of the deposited multilayer. However, in some cases, where long-term usage under harsh condition of water purification, stability would be required to be increased, alternatively by using crosslinker between each polyelectrolyte layer [39].

Solution pH is another parameter influencing the morphology of the deposited layer. Thicker layer forms during polyelectrolyte assembly carried out at a high pH. Thin layers with flat chain conformations are attained in the case of highly charged polyelectrolytes. Polyelectrolytes at high pH are partially ionized and incorporating more nonionized chain segments is prone to swell. Therefore, the solution pH determines the charge density of polyelectrolytes, hence surface charge density. At the deposition pH corresponding to the average of the pKa values of the polycation and polyanion, maximum density of ionic cross-links in the assembly is achieved. Unless the charge density is below a minimum value the charge reversal is possible for the formation of multilayers [40].

In order to overcompensate surface charges, charge density and polyelectrolyte concentration in dipping solution should be high. Temperature selected during polyelectrolyte assembly influences the final structure of the film. Higher temperature results in thicker layer due to the chain mobility leading to an increase in the number of loops and tails adsorbed to the membrane surface. The molecular weight of polyelectrolyte influences the stability of the deposited layer. Wang et al. studied the impact of molecular weight of cationic PEI on the permeability and rejection performance of the hydrolyzed PAN membrane [41]. 3.5 bilayers were established by keeping the molecular weight of sulfonated poly (ether ether ketone) constant, which was selected as an anionic polyelectrolyte. Water fluxes with the increase in layers were decreased for both high (25,000) and low (800) molecular weight PEI, however, no salt rejected in the case of low molecular weight PEI was observed. This was explained by the lower structure of the selective layer. Consequently, when using low molecular weight PEI, a higher number of layers could be necessary to achieve salt rejection. The support membrane is, on the other hand, limited to the first few bilayer depositions. Surface charge density and the relative dielectric permittivity of the support may alter the morphology of the multilayer assembly up to a thickness in the micrometer range [42]. In general, a good support membrane for the LbL assembly is expected to have a low surface roughness and a high surface charge density.

#### **2.2 Surface modification containing nanomaterials (TFN)**

The purpose of the surface modification of membranes used in water treatments is to reduce or eliminate fouling, which is the main problem in any membrane separation process. In the NF processes, the starting material is usually selected as UF membrane. The polyamide thin film composite (TFC-PA) membranes have been successfully used in water treatment for the purpose of desalination and decolorization, however, membrane fouling and chlorine intolerance cause to decline the permeation flux, shorten the service life, and increase the operating cost, and hence reduce the long term process efficiency. Therefore, researches based on nanoparticle decorations in a skin layer of the asymmetric membrane for NF applications have been rapidly increased.

In TFC-PA approach, nanoparticles are introduced either in organic or aqueous phase. However, the hydrophilic nature of the mostly inorganic nanomaterials necessitates their use in aqueous amine solution. Interfacial polymerization (IP) occurs as soon as acyl and amine monomers interact with each other, in which nanoparticles are either embedded within the polymer matrix or dispersed on top of the polymer film depending on the approach of introducing nanoparticles. The so-called membrane is typically rinsed with hexane or water followed by heat treatment to complete polymerization.

Carbon nanotube (CNT) has intensively attracted attention due to high aspect ratio, low density, mechanical strength, and stiffness. However, its hydrophobic nature makes dispersion problems in various solvents (NMP, DMAc, DMSO, DMF) as well as within a polymer matrix. Therefore, many efforts have been focused on introducing hydrophilic/functional moieties or macromolecules on a CNT surface [43]. Various methods, including acid treatments, plasma oxidation, chemical grafting, in situ polymerization, amination, hydrothermal treatment, and TiCl4 precipitation on the acid treated multi-walled carbon nanotube (MWCNT) have been successfully investigated for the addition of functional groups such as carboxylic, amine, hydroxyl etc.

MWCNT-NH2 has been embedded in PA layer to improve separation performance of the NF membrane. Dispersion of 0.001 to 0.01 w% MWCNT-NH2 in

#### *Principles of Membrane Surface Modification for Water Applications DOI: http://dx.doi.org/10.5772/intechopen.96366*

piperazine monomer solution followed by the interfacial polymerization revealed that the MWCNT-NH2 was successfully dispersed within the PA layer, and the modified NF membranes had high hydrophilicity, smoothness, enhanced separation performance, and antifouling property [44]. Similarly, Xue et al. studied the effect of MWCNTs with different functional groups (MWCNT–COOH, MWCNT–OH, or MWCNT–NH) on the NF performances [45]. Piperazine solution consisting of 1 w% functionalized CNT was cast on PSf UF membrane. The coated membranes were than immersed into an organic acyl solution to initiate interfacial polymerization. The remarkable results for thin film nanocomposite NF membranes obtained from different fabrication approaches are summarized in **Table 1**.

Wu et al. used MF membrane to fabricate NF by vacuum filtration of functionalized CNT suspension followed by IP process [55]. The thickness and roughness of the intermediate layer determined the PA active layer morphology. Authors observed that the thickness of the active layer was increased with an increase of CNT layer which associated with the absorbed monomer on the coated membrane. Remarkable results from CNT loaded UF and NF membranes have been summarized in literature [43].

GO is another carbon-based nanomaterial, which has charged oxygen-containing functional groups. Owing to its laminar structures with high surface area, GO nanosheet is mostly preferred for MF, UF membrane surface modification via vacuum filtration or LbL assembly methods. The number of deposition cycles can adjust the thickness of GO layer at a molecular level. Thin layer formation occurs based on alternatively depositing polyelectrolytes and GO nanosheets through


#### **Table 1.**

*Thin film nanocomposite NF membranes fabricated by using different approaches and their performance summary.*

electrostatic attractions. Zhao et al. fabricated ultrathin hybrid membranes via LbL self-assembly using gelatin (GE) and GO on hydrolyzed PAN membrane [56]. The positively charged GE interacted with negatively charged GO in the self-assembly process, leading to efficient multilayers.

Song et al. functionalized and anchored GO nanosheets with polyelectrolyte to further enhance the separation performance of the GO membranes [57]. GO modification was carried out with ethylenediamine (EDA) molecules, followed by poly (allylamine hydrochloride) (PAH) anchoring to amplify the surface charge density. Amine reduced GO (ArGO) anchored by PAH (PAH@ArGO) nanosheets with positive charge and PSS@GO nanosheets with negative charge were alternately deposited on the polycarbonate support via LbL assemblyThe selective layer thickness of the PE@ArGO membrane was about 160 nm, possessed high density positive/negative charge gated ion transport nanochannels and superior salt rejection by means of Donnan charge exclusion.

However, instability of GO (disintegration or re-dispersion) in water is one biggest block that limits its practical usage. Covalent crosslinking is a promising strategy for the solution of this problem. The functional groups on the GO nanosheets such as hydroxyl and carboxyl groups are convenient sites for the cross-linking reaction with different crosslinker. Mie et al. fabricated a GO membrane covalently cross-linked by 1,3,5- benzenetricarbonyl trichloride between acyl chloride and carboxyl groups [58]. Results revealed that cross-linking effectively ensured the GO membrane with necessary stability to prevent its inherent dispensability in an aqueous environment.

MOFs are porous crystalline materials possess superior compatibility in polymer matrix, apart from other inorganic nanomaterials. By their unique features including size, shape, and polarity, MOFs provide preferential passage for certain molecules, simultaneously rejected undesired substances, when embedded in membrane phase. Gong et al. prepared positively charged NF membrane by incorporating NH2-MIL-125(Ti) porous titanium based MOFs material into PEI and trimesic acid (TMA) crosslinking system [59]. The structure and the experimental procedure are illustrated in **Figure 3(1a-c** and **2)**. The effect of MOFs loading amount on the NF

#### **Figure 3.**

*(1) (a, b) Schematic of NH2-MIL-125(Ti) structure (H, white; C, black; O, red; N, blue; Ti, yellow polyhedral) and (c) the structural formula of NH2-MIL-125(Ti), (2) Schematic of the preparation of NH2- MIL-125(Ti)/PEI/TMA composite membranes, (3) The effect of NH2-MIL-125(Ti) loading on heavy metal removal performance of NH2-MIL-125(Ti)/PEI/TMA composite membranes, (4) Selectivity of different PEG solutions (pressure: 4 bar; solute concentration: 200 mg/L) by the MPT-0.010 composite membrane, (5) Heavy metal (Ni2+) removal performance of the MPT-0.010 composite membrane at different (a) feed concentrations (test pressure: 4 bar) and (b) test pressures (salt concentration: 1000 mg/L) [59].*

*Principles of Membrane Surface Modification for Water Applications DOI: http://dx.doi.org/10.5772/intechopen.96366*

performance, metal rejection, and stability were studied. The surface roughness of the membrane increased from 6.9 to 92.4 nm when the MOFs loading increased from 0.0 to 0.02 w%. The rejection of Ni2+, Mn2+, Zn2+ and permeability optima was obtained, at 0.01 w% of MOFs (**Figure 3 (3)**). Beyond this value, metal rejections reduced seriously. The MWCO of the membrane based on 90% or above PEG rejection (**Figure 3 (4)**) showed that the composite membrane could be considered as a loose NF membrane having an effective pore radius and PEG rejection of 1.5–2.2 nm and 1000–2000 Da, respectively. Furthermore, the resultant composite membrane was found to be positively charged over a large pH interval (3–11) that could be ascribed to the protonation of amine groups. The increase in hydraulic permeability, while maintaining with similar rejection by the introduction of MOFs was attributed to the preferential water channels and suitable window size, that can selectively cut off heavy metal ions allowing water molecules to pass through. In addition, the positive surface charge density of the nanocomposite membrane contributed to the rejection of heavy metal cations by electrostatic repulsive forces.
