SILICA NANOPARTICLES

Mesoporous silica nanoparticles (MSNs) are defined as those having pore size in the range of 2-50 nm and an ordered arrangement of pores giving an ordered structure. (MSNs) have gained wide popularity over recent years. Their advantages of uniform and tunable pore size; easy, independent functionalization of the surface; internal and external pores; and pore opening gating mechanism make them distinctive and promising as carrier. (Narayan et al., 2018)

Kim et al. (2015) found some important points about membrane separation through their studies on silica nanoparticles. The mesoporous silica with two- or three-dimensional pore structure is one of the most promising types of molecular sieve materials for gas separation membranes applications. Other applications include pervaporation, microfiltration, ultrafiltration, nanofiltration, reverse osmosis, and forward osmosis, depending on the pore range.

Porous membranes with a dense layer of nanoparticles imparts useful functionality and can enhance membrane separation and antifouling properties. Asymmetric membranes facilitate control over membrane flux and selectivity, which enables the formation of stimuli-responsive, hydrogel nanocomposite membranes; they can be easily modified to introduce antifouling features. This approach forms a foundation for the formation of advanced nanocomposite membranes comprising diverse building blocks, with potential applications in water treatment, industrial separations, and as catalytic membrane reactors. (Haase et al., 2017)

Preparation of Functionalized Silica

There are three major techniques for the functionalization of mesoporous: condensation, impregnation, and post synthesis grafting. This approach maintains the substrate structure, and the formed amino oxides remain stable, even after several adsorption/desorption cycles. The supported mesoporous silica membranes with controlled structures, such as silica powder, are synthesized via the well-established sol - gel method, but in the presence of support materials. This technique involves hydrolysis and condensation of respective precursors to form colloidal sols, as reported by Kim et al. (2015). The nanoparticle-functionalized hollow fiber membranes are prepared by STRIPS (solvent transfer-induced phase separation) and photo polymerization have exceptionally high nanoparticle loadings (up to 50 wt..% silica nanoparticles) (Haase et al., 2017) to achieve highly permeable composite membranes (Jin et al., 2012; Yin et al., 2012; Abol- fazli and Rahimpour, 2017). Porous MCM-41 and nonporous spherical silica N Ps were synthesized and used as fillers to fabricate the thin-film nanocomposite (TFN) membrane. (Yin et al., 2012; Abolfazli and Rahimpour, 2017)

Effect on Mechanical Properties

PVDF/Si0₂ hollow fiber membranes prepared by Si0₂ sol - gel method showed increase in the break strength and in Young’s modulus, with increase in concentration of TEOS (tetraethyl orthosilicate) (< 3) then declined with the further increase of TEOS concentration. At higher TEOS concentration, the formed Si0₂ network increased the rigidness of the membrane and confined the crystallization of PVDF, which led to the decrease of the mechanical properties, such as elongation-at-break (Yu et al., 2009b) .

Effect on Morphology

The morphologies, chemical composition, and surface image of membranes were investigated by using FTIR, SEM, AFM, and contact angle analyses. Abolfazli and Rahimpour (2017) studied the modification of a thin-film composite (TFC), hollow fiber polyamide membrane fabricated by interfacial polymerization, using piperazine (PIP) and trimesoyl chloride (TMC) on a porous polysulfone substrate. The effects of triethylene tetra mine (TETA) and silica nanoparticles (Si0₂) contents in the aqueous phase (as the additives) have been investigated. In the case of the Si0₂ nanoparticles, the surface was rougher than when using TETA in the modified membranes. However, the nanoparticle’s size is close to the membrane pore size. As a result, the silica particles block a few pure water channels of the membranes.

Yin et al. (2012) studied the effect of silica NPs onto a thin-film, nanocomposite (TFN) membrane containing porous MCM-41. By increasing the concentration of NPs. hydrophilicity, roughness, and zeta potential increased. (Yu et al. 2009b) reported that by incorporating silica nanoparticles into the PVDF membrane, the cross-sectional morphology experienced a transition from fingerlike macrovoids to a spongelike structure.

Effect on Performance

Abolfazli and Rahimpour (2017) showed that the water flux increased with increasing the silica contents in the aqueous phase. The hollow fiber composite membrane depicted a salt (NaCl) rejection of about 26 %, and flux of about 31 LMH. On preparing TFC, the presence of silica NPs in the aqueous solution affected the IP reaction and the formation of the polyamide layer leading to the increase of membrane hydrophilicity. Increased hydrophilicity caused the water molecules to pass through the membrane at higher speeds (Liu et al., 2011). Yin et al. (2012) found the same effect for silica NPs, where the permeate water flux increased from 28.5±1.0 to 46.611.1 LMH with the incorporation of MCM-41 NPs, while maintaining high rejections of NaCl and Na₂S0₄ (97.9% and 98.5%, respectively). The internal pores of MCM-41 NPs contributed significantly to the increase of water permeability than the nonporous. Others (Jin et al., 2012) found that permeation performance for PA-Si0₂ membranes increased nearly 50% without loss of salt rejection rate by adding 1.0% (wt..) Si0₂ NPs in aqueous solution. The order of rejection to inorganic salts Na₂S0₄> MgS0₄> MgCl₂> NaCl revealed that both PA and PA-Si0₂ membranes were negatively charged. The zeta potentials testing results indicated that addition of Si0₂ increases the negative charge quantities on the surface of PA-Si0₂ membrane for negatively charged hydroxyl groups and silanol-covered nano-Si0₂ surfaces. The value of molecular weight cutoff (MWCO) for PA-Si0₂ membranes was about 1000 g/mole, and the additive of Si0₂ NPs to PA membranes enlarges the pore size slightly. The PA- Si0₂ membrane had a higher stable flux, and could remove nearly 50% salts when treated with oily wastewater in one-cycle filtration as a type of application.

Mechanism of Action

Studying the mechanism of action through the fabrication of a highly hydrophilic PVDF UF membrane as in Liang et al. (2013) via post fabrication tethering of superhydrophilic silica NPs to the membrane surface. The authors found that by plasma-induced graft copolymerization and by providing sufficient carboxyl groups as anchor sites, the binding of silica NPs were surface-tailored with amine- terminated cationic ligands. The NP binding was achieved through dip-coating, which improved the wettability of the membrane and converted the membrane surface from hydrophobic to highly hydrophilic. The irreversibly bound layer of superhydrophilic silica NPs endowed the membranes with strong antifouling performance as demonstrated by three sequential fouling filtration runs. Kim et al.

(2015) studied the mechanism for the synthesis of mesoporous silica in the presence of a cationic surfactant. When dissolved in water, the cationic surfactant forms micelle structures. In this process, the cationic heads of the surfactant molecules are arranged to the outer side, while their hydrophobic tails collect in the center of each micelle. The silica source then covers the micelle surfaces. Once the surfactant is removed via calcination or extraction, the pores are activated.

Table 2.4 summarizes the effect of nanosilica on membrane characteristics and performance.

Effect of functionalized silica NPs on membrane characteristics and performance

TABLE 2.4

TABLE 2.4 (Cont.)

METAL OXIDES NANOPARTICLES

Preparation

Coprecipitation Method

In the coprecipitation method, the synthesis reaction is generally carried out at room temperature. The prepared NPs' properties are determined by initial reaction parameters such as pH and reaction temperature. This method is very easy and simple for the synthesis of NPs in aqueous media in presence of different surfactants (lida et ah, 2007).

Sol-Gel Method

The method involves preparation of metal oxides by hydrolysis of precursors, usually alcoxides in alcoholic solution, resulting in the corresponding oxohydroxide. Condensation of molecules by giving off water leads to the formation of metal hydroxide network. Hydroxyl species undergo polymerization by condensation and form a dense porous gel. Appropriate drying and calcinations lead to ultrafine porous oxides (Hampden-Smith and Interrante, 1998)

Thermal Decomposition

The thermal decomposition method is based on the decomposition and oxidation of several types of precursors in an organic medium by using high temperature. Reaction is usually endothermic and heat is important to break chemical bonds in the compound enduring decomposition (Navaladian et al., 2007).

Microemulsion Process

Microemulsion process is based on creating a thermodynamically stable and homogeneous dispersion of two immiscible liquids (usually water and oil solvent), using a surfactant (Vidal-Vidal et al., 2006).

Effect on Mechanicaf Properties

Nanoparticles have the potential to improve membrane mechanical properties in UF applications as shown by tensile testing. PVDF Nanocomposite membranes incorporated with ZnO NPs exhibited increased tensile strength and elongation-at-break (Liang et al., 2012). The strength was enhanced especially at high ZnO loading, where the tensile strength was twice that of the unmodified membrane when the ZnO to PVDF ratio was at 3:15 and 4:15. Likewise, PVDF with АЬОз increased tensile strength and elongation-at-break with increased АЬОз-particle concentration to 2% and, then declined as the АЬОз- particle concentration was further increased (Yan et al., 2006). PVDF/Ti0₂ hollow fiber membranes prepared by either Ti0₂ sol - gel or blending methods showed 30% increase in tensile strength (Yu et al., 2009a). Han et al. (2010) studied the effect of using multiple types of NPs (Ti0₂ and A120₃) in PVDF hollow fiber membranes. It was noted that all NP membranes had higher tensile strength, and the best improvement was from 1.71 MPa to 3.74 MPa, with a combination of 2 wt. % ТЮ₂ and 1 wt.. % Al₂0₃.The improvement could be attributed to the reduced macrovoid formation observed in the NP membrane.

Effect on Performance and Hydrophilicity

Increase in water flux has been observed with PVDF and PS/CS membranes incorporated with ТЮ₂ (Kumar et al., 2013), (Ong et ah, 2015), (Yu et ah, 2009a), (Efome et ah. 2015), CuO, (Baghbanzadeh et ah, 2016), and A1₂0₃ (Yan et ah, 2006). This was attributed to the interplay between the hydrophilicity and viscosity of the casting solution, which resulted in the maximum pore size. Also, at high particle concentrations, sometimes pores are plugged causing a further reduction in the flux. Yan et ah (2006) studied the effect of AL0₃ NP concentration for PVDF UF membrane on pure water fluxes and rejection. The authors observed significant improvement in water flux by the addition of nanosized A1₂0₃ particles, which have some favorable characteristics, such as hydrophilicity and higher ratio surface areas. Although increasing the Al₂0₃-particle concentration caused a decrease of the contact angle, the porosity and rejection, were not affected.

Effect on Morphology

Reports on the effect of addition of NPs on porosity and mean pore size are often contradictory. In some cases, it has led to a higher porosity and mean pore size (Baghbanzadeh et ah, 2016), (Kumar et ah, 2013), (Ong et ah, 2015), (Yu et ah, 2009a), (Liang et ah, 2012), (Han et ah, 2010), This may be attributed to the hindrance effect of nanomaterials that reduces the interactions between solvent and polymer molecules. As a result, easier and faster diffusion of the solvent molecules from the polymer matrix to the coagulant takes place, which further increases the porosity and mean pore size (Kim et ah, 2001). However, others have reported that the addition of nanomaterials led to a reduction in both porosity and mean pore size (Yu et ah, 2009b), (Yan et ah, 2006), which might result from the deposition of nanomaterials in the membrane pores. On reaching higher concentrations of NPs added to the membrane, owing to the irregular standing of the Nanomaterials increase in the viscosity of casting solution occurs, which results in a delayed phase separation, thus creating a dense structure in the sub-layer (Yu et ah, 2009b). At the low concentrations, nanomaterials act as the nucleating agents that increase the rate of nucleation of the polymer lean phase (Wang et ah, 2012) and enhance the phase separation, mostly owing to an increment in the hydrophilicity of the casting solution, which leads to larger mean pore size. The effect of Ti0₂ NPs has been also investigated. It has been reported that surface images revealed neat PES membranes consisting of cellular pores, but with the addition of small amounts of ТЮ₂ NPs, within 1-2 wt. %, surface pores switched to a lacy structure. By further increasing the Ti0₂ loading, the lacy structure returned into a cellular structure accompanied by the agglomerated NPs, resulting in a poor skin layer arrangement (Kumar et ah, 2013). On using Ab0₃ NPs, increasing the concentration caused an increase of the surface roughness of the membrane, which is attributed to the accumulation of hydrophilic A1₂0₃ particles on the membrane surface, This improves the membrane surface hydrophilicity significantly and reduces the interaction between the contaminants and the membrane surface (Yan et al., 2006). Table 2.5 summarizes the effect on metal oxide NPs on the membranes for selected applications.