4.6 Pore Functionalization
Pore functionalization in a nanoporous membrane determines the efficacy of the
process by influencing the desalination rate and salt rejection. The type of functional
groups (either hydroxyl or hydrogen) used for activation induces the selectivity and
determines the orientation of water in the vicinity of the pore which alters the rate of
desalination (Sint et al. 2008; Killingsworth 2012; Xue et al. 2013). The principle
involved in surface modification techniques served as the base for the development
of advanced pore functionalization techniques like chemical treatment, graft polymerization, and copolymer templating which could impart desired characteristics
like selectivity by manipulating the pore properties.
4.6.1 Chemical Treatment
Pores of nanoporous membranes can be functionalized by using chemical treatment
techniques which include reactions like oxidation, addition, substitution, adsorption,
and hydrolysis, respectively. The membrane surface containing pores is exposed to
various types of functional groups during the chemical treatment that aids in the
manipulation of pore properties. Among these techniques, oxidation plays a dominant role. By implementation of corona discharge and flame treatment methods,
oxygen-containing groups like hydroxyl and carboxyl groups are introduced onto
the surface of the membrane. This method results in the gain of modified membranes
with antifouling property. Jesse et al. reported the functionalization of different
metal-organic frameworks by low-pressure hydrogen adsorption to enhance their
properties (Rowsell and Yaghi 2006). However chemical treatment is a conventional
technique that is now employed as a step in advanced modification processes like
copolymer templating for selective functionalization. Ordered mesoporous carbon
C-FDU-18 s synthesized by diblock copolymer templating was selectively
functionalized by chemical treatment. During oxidative treatment numerous hydrophilic groups are created in the pore channels without destroying the ordered
structure of the C-FDU-18 s (Deng et al. 2010). Few research works reported that
hydroxylated pores exhibit higher water permeability and hydrogenated pores are
more effective in rejection of salt (Killingsworth 2012).
4.6.2 Plasma Treatment
Plasma implantation is a technique in which oxygen or nitrogen groups are attached
to the membrane pores by C–C bond breaking to form alcoholic or carbonyl
functional groups on its surface. As a result of the functionalization, the wetting
properties of the membrane at the pore vicinity can be manipulated to control the
4 Functional Properties of Nanoporous Membranes for the Desalination of Water
145
Pore functionalization in a nanoporous membrane determines the efficacy of the
process by influencing the desalination rate and salt rejection. The type of functional
groups (either hydroxyl or hydrogen) used for activation induces the selectivity and
determines the orientation of water in the vicinity of the pore which alters the rate of
desalination (Sint et al. 2008; Killingsworth 2012; Xue et al. 2013). The principle
involved in surface modification techniques served as the base for the development
of advanced pore functionalization techniques like chemical treatment, graft polymerization, and copolymer templating which could impart desired characteristics
like selectivity by manipulating the pore properties.
4.6.1 Chemical Treatment
Pores of nanoporous membranes can be functionalized by using chemical treatment
techniques which include reactions like oxidation, addition, substitution, adsorption,
and hydrolysis, respectively. The membrane surface containing pores is exposed to
various types of functional groups during the chemical treatment that aids in the
manipulation of pore properties. Among these techniques, oxidation plays a dominant role. By implementation of corona discharge and flame treatment methods,
oxygen-containing groups like hydroxyl and carboxyl groups are introduced onto
the surface of the membrane. This method results in the gain of modified membranes
with antifouling property. Jesse et al. reported the functionalization of different
metal-organic frameworks by low-pressure hydrogen adsorption to enhance their
properties (Rowsell and Yaghi 2006). However chemical treatment is a conventional
technique that is now employed as a step in advanced modification processes like
copolymer templating for selective functionalization. Ordered mesoporous carbon
C-FDU-18 s synthesized by diblock copolymer templating was selectively
functionalized by chemical treatment. During oxidative treatment numerous hydrophilic groups are created in the pore channels without destroying the ordered
structure of the C-FDU-18 s (Deng et al. 2010). Few research works reported that
hydroxylated pores exhibit higher water permeability and hydrogenated pores are
more effective in rejection of salt (Killingsworth 2012).
4.6.2 Plasma Treatment
Plasma implantation is a technique in which oxygen or nitrogen groups are attached
to the membrane pores by C–C bond breaking to form alcoholic or carbonyl
functional groups on its surface. As a result of the functionalization, the wetting
properties of the membrane at the pore vicinity can be manipulated to control the
4 Functional Properties of Nanoporous Membranes for the Desalination of Water
145