150
6.3 Effect of Nanoparticles on Water Permeability
The inherent properties of the added nanoparticles could be utilized to overcome the
limitations of polymeric membrane trade-off between selectivity and permeability.
The opposite behavior of the permeability and selectivity is a well-known phenomenon of polymeric membranes. Incorporation of hydrophilic nanoparticles into
a membrane or depositing them on a membrane surface as a separate thin layer (thin
film nanocomposite) has brought a new perspective to the transport properties of
membranes. Nanoparticles show a considerable effect on the flux improvement.
Because it is well agreed that accumulation of hydrophilic nanoparticles occurs
preferentially at the membrane water interface due to the nonsolvent-induced phase
separation process that involves a large gradient in water concentration. For example, membranes prepared by the incorporations of finely dispersed metal oxide
nanoparticles for a wide range of applications including ultrafiltration (Bottino et al.
2001), nanofiltration, desalination, and water and wastewater treatment (Ng et al.
2013) enhanced the membrane properties in terms of water flux.
Generally, membrane water permeability depends on the porosity, surface pore
size, pore interconnection, hydrophilicity, and skin thickness of the membrane.
Incorporation of nanoparticles characterized by their unique properties such as small
size, large surface area, high reactivity, hydrophilicity, and a large number of active
sites greatly alters the resultant membrane properties with regard to their morphology and surface properties. Nanoparticles randomly distributed in a polymer matrix
restricts the mobility and spreading of the polymer chains and eventually increases
the average distance between the polymer chains, thereby causing an enhancement
in membrane permeability. It is well known that during immersion precipitation, the
presence of nanoparticles decreases the thermodynamic stability of the polymer
solution, which results in a rapid liquid–liquid demixing with more porous structures
across the membrane thickness. Furthermore, they act as nucleating agents giving
rise to nucleation growth rate during the phase inversion process. The composition
of the system crosses from one phase to two, upon addition of these nanoparticles.
Microvoids formed by the interfacial stresses and defects between the two phases
increase the pore connectivity in the bottom and sublayer (Zhang et al. 2013a, b).
These fingerlike microvoids elongate across the sublayer by the addition of nanoparticles and gradually turn into spherical macrovoids, which could be suppressed at the
maximum concentration of nanoparticles. At this point, viscosity is increased drastically causing to migrate nanoparticles difficult, and they settle in the bottom layer.
This is the reverse of the polymer shrinkage during the demixing process (Rahimpour
et al. 2012). In the case of a slow precipitation process, the polymer-rich phase
occurs at the polymer-water interface, and the resultant membrane is denser at the
top layer and less porous and has less finger-like macrovoids in the sublayer.
In the study of Ganpat et al. (2012), the effect of nanoparticles on the ultrafiltration membrane pore characteristics has been discussed by dispersing zeolite
nanoparticles from 0.01 to 1 wt% in N-methyl pyrrolidone with additive D-αtocopheryl polyethylene glycol succinate to prepare a nanocomposite of polysulfone/zeolite hollow fiber membrane. They found an increase in water permeability
Y. Yurekli
6.3 Effect of Nanoparticles on Water Permeability
The inherent properties of the added nanoparticles could be utilized to overcome the
limitations of polymeric membrane trade-off between selectivity and permeability.
The opposite behavior of the permeability and selectivity is a well-known phenomenon of polymeric membranes. Incorporation of hydrophilic nanoparticles into
a membrane or depositing them on a membrane surface as a separate thin layer (thin
film nanocomposite) has brought a new perspective to the transport properties of
membranes. Nanoparticles show a considerable effect on the flux improvement.
Because it is well agreed that accumulation of hydrophilic nanoparticles occurs
preferentially at the membrane water interface due to the nonsolvent-induced phase
separation process that involves a large gradient in water concentration. For example, membranes prepared by the incorporations of finely dispersed metal oxide
nanoparticles for a wide range of applications including ultrafiltration (Bottino et al.
2001), nanofiltration, desalination, and water and wastewater treatment (Ng et al.
2013) enhanced the membrane properties in terms of water flux.
Generally, membrane water permeability depends on the porosity, surface pore
size, pore interconnection, hydrophilicity, and skin thickness of the membrane.
Incorporation of nanoparticles characterized by their unique properties such as small
size, large surface area, high reactivity, hydrophilicity, and a large number of active
sites greatly alters the resultant membrane properties with regard to their morphology and surface properties. Nanoparticles randomly distributed in a polymer matrix
restricts the mobility and spreading of the polymer chains and eventually increases
the average distance between the polymer chains, thereby causing an enhancement
in membrane permeability. It is well known that during immersion precipitation, the
presence of nanoparticles decreases the thermodynamic stability of the polymer
solution, which results in a rapid liquid–liquid demixing with more porous structures
across the membrane thickness. Furthermore, they act as nucleating agents giving
rise to nucleation growth rate during the phase inversion process. The composition
of the system crosses from one phase to two, upon addition of these nanoparticles.
Microvoids formed by the interfacial stresses and defects between the two phases
increase the pore connectivity in the bottom and sublayer (Zhang et al. 2013a, b).
These fingerlike microvoids elongate across the sublayer by the addition of nanoparticles and gradually turn into spherical macrovoids, which could be suppressed at the
maximum concentration of nanoparticles. At this point, viscosity is increased drastically causing to migrate nanoparticles difficult, and they settle in the bottom layer.
This is the reverse of the polymer shrinkage during the demixing process (Rahimpour
et al. 2012). In the case of a slow precipitation process, the polymer-rich phase
occurs at the polymer-water interface, and the resultant membrane is denser at the
top layer and less porous and has less finger-like macrovoids in the sublayer.
In the study of Ganpat et al. (2012), the effect of nanoparticles on the ultrafiltration membrane pore characteristics has been discussed by dispersing zeolite
nanoparticles from 0.01 to 1 wt% in N-methyl pyrrolidone with additive D-αtocopheryl polyethylene glycol succinate to prepare a nanocomposite of polysulfone/zeolite hollow fiber membrane. They found an increase in water permeability
Y. Yurekli