18.2 Polymeric Surface Modifi cation/
18.2.2 Nanoparticle Surface Modifi cation Approaches
Surface modifi cation/functionalization of nanoparticles is typically achieved by: (1) grafting polyelectrolytes from the surface of pre-synthesized nanoparticles; (2) physisorption of polyelectrolytes onto the surface of pre- synthesized nanoparticles; or (3) incorporating polyelectrolytes into the
Table 18.1 Polymeric Modifi ers Used to Modify Nanoparticles for
Environmental Remediation and their Reported Ability to Enhance Colloidal Stability and Transport in Porous Media
Surface modifi er Charge/stabilization type Performance
Stabilization Transport Polymers
Polyethylene glycol (PEG) Nonionic/steric Good a Poor a,b Polyvinyl alcohol (PVA) Nonionic/steric Poor a Suspected to
be poor
Guargum Nonionic/steric Poor a Suspected to
be poor Polyelectrolytes
Triblock copolymers c PMAA 48 –PMMA 17 –PSS 650 [14,15,21]
Anionic/electrosteric Excellent Excellent PMAA 42 –PMMA 26 –PSS 462
[14,15,21]
Anionic/electrosteric Good Excellent Polystyrene sulfonate (PSS) [22] Anionic/electrosteric Excellent Good Polyaspartate (PAP) [22] Anionic/electrosteric Excellent Good Carboxymethyl cellulose(CMC)
22,23] d
Anionic/electrosteric Fair [22], Excellent [23]
Poor, a excellent(23) Poly acrylic acid (PAA) [24,25] Anionic/electrosteric Excellent Good Mixture of PAA–PSS–bentonite
[26]
Anionic/electrosteric — Excellent
a
Unpublished data from our laboratory.
b The poor transportability of PEG modifi ed nanoparticles is attributed to the strong specifi c interaction between PEG and silica sand.
c Triblock copolymer-functionalized nanoscale zero-valent iron (NZVI) can form stable picking emulsions of dense nonaqueous phase liquid (DNAPL) (TCE) water.
d The diff erence in the performance of CMC between [22] and [23] is presumably due to diff erent modifi cation approaches.
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nanoparticles during the particle synthesis. The fi rst approach uses an advanced synthesis method such as atom transfer radical polymerization (ATRP) [ 20 ] to produce a dense polymer brush layer from initiators that are covalently bound to the nanoparticle surface. The dense polymer brush is theoretically predicted to enhance colloidal stability and transport of nanoparticles as will be further discussed. Adsorbing polymers onto the nanoparticles (the second approach) is a less time- and material-intensive procedure than growing polymers from the particles and is desirable for the application of a large quantity such as environmental remediation (if the resulting surface modifi ed nanoparticles can provide suffi cient adsorbed polymer layer thickness, density, and appropriate confi guration to achieve the intended task). The last approach is a single-step synthesis procedure where a polymeric modifi er interacts with the particle surface during nanoparticle synthesis. The physicochemical properties and structures of nanoparticles modifi ed in this way are likely to be diff erent from the reactive nanoparticles obtained by grafting or physisorption of polymeric modifi ers on to the surface of pre-synthesized nanoparticles. For example, Fe–Pd nanoparticles synthesized in the presence of sodium carboxymethyl cellulose (CMC) are reported to be colloidally stable, have excellent transportability through a loamy-sand soil, and are more reactive than bare Fe–Pd nanoparticles [ 23 ]. In contrast, NZVI particles modifi ed by physisorption of CMC become less reactive [ 27 ], have moderate colloidal stability [ 22 ], but poor transportability through porous media.
The mass and confi guration of charged macromolecules adsorbed onto the particle surface is governed by the molecular weight, ionization, and charge density of the macromolecule, the charge density and polarity of the solid surface, the solvent quality, and ionic strength [ 19 , 28 ]. The mass adsorbed and the confi guration of the adsorbed layer is dictated by a balance between electrostatic attraction to the surface and repulsions among neighboring ionized monomer units, a loss of chain entropy upon adsorption, and also nonspecifi c dipolar interactions among the macromolecule, the solvent, and the surface [ 19 , 28 ]. Homopolymers are normally sorbed onto the surface in the train–loop–tail orientation ( Fig. 18.1 ). Trains are sequences in contact with the sorbed surface. Loops and tails are attached but extend from the surface [ 19,29, 30 ]. Block copolymers adsorb similarly, but they can be designed to anchor to the surface and theoretically can control the adsorbed layer confi guration [ 19 ]. For example, poly(methacrylic acid) (PMAA) (Table 18.1), which has specifi c sorption affi nity on the oxide surface, is used as an anchoring block of the trick block copolymer for NZVI surface modifi cation [ 14 ]. The adsorbed layer properties, including adsorbed layer thickness ( d ) ( Fig. 18.1 ) and adsorbed polymer mass per specifi c surface area of nanoparticles (surface excess, Γ ), play an important role in stabilizing and enhancing the mobility of nanoparticles in the subsurface as will be discussed next.
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18.3 Eff ect of Surface Modifi ers on the Mobility of Nanomaterials in the Subsurface
The sizes of pores in a groundwater aquifer are typically of the order of the sizes of aquifer materials themselves (tens to hundreds of micrometers), which are several orders of magnitude larger than the sizes of nanomaterials (10–500 nm) used for environmental remediation. Intuitively the transport of nanomaterials in the subsurface should be facile, however, recent studies have reported limited mobility of nanoparticles for environmental remediation in saturated porous media, that is, practical transport distances of only a few centimeters or less for bare nanoparticles [ 13 , 15 , 26 ]. In situ remediation with reactive nanomaterials typically involves injection of relatively high concentration particle dispersions being injected into the subsurface.
Given this scenario, there are two physical phenomena limiting the transport
Figure 18.1 A schematic diagram illustrating train–loop–tail of orientation of the adsorbed homopolymers on the surface of nanoscale zero-valent iron (NZVI) and the site blocking eff ect and the mass transfer limitation on trichloroethylene (TCE) dechlorination due to trains, loops, and tails of adsorbed polyelectrolytes.
Cl Cl
Cl H
C C
Cl Cl
Cl H
C
Tail
Loop d
Train C
Cl Cl
Cl H
C C
Cl Cl
Cl H
Cl Cl C C
Cl H
C C
Cl Cl
Cl H
C C Cl Cl
Cl H
C C Cl Cl
Cl H
C C
Cl Cl
Cl H
C C
Cl Cl
Cl H
C C
Cl Cl
Cl H
C C
Cl Cl
Cl H
C C Cl Cl
Cl H
C C
Cl Cl
Cl H
C C
NZVI
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of nanomaterials in water-saturated porous media. First, nanomaterials can be fi ltered from solution by deposition onto aquifer materials [ 31 ]. Second, aggregation or agglomeration can cause pore plugging that limits transport [ 15 , 32 ]. Both aggregation and deposition can be considered as two- step process, transport and attachment. Whether the particles will aggregate (or agglomerate, i.e., loose aggregation) or deposit onto a collector is controlled by attachment, which is governed by colloidal forces between two particles (aggregation) or particles and collectors (deposition) acting at a separation distance on the order of nanometers [ 31 ]. For bare nanoparticles, these colloidal forces are electrical double-layer repulsion, van der Waals attraction, magnetic attraction for magnetic nanoparticles (e.g., NZVI), hydration forces, and hydrophobic interactions [ 31 , 33 ]. For polyelectrolyte-coated nanomaterials (a common approach to inhibit aggregation) elastic and osmotic repulsive forces may also occur. Because of the importance of these forces on aggregation, deposition, and thus the mobility of nanomaterials in the subsurface environment, a brief overview of relevant physical chemistry of colloidal forces is provided here. More thorough reviews of this topic can be found elsewhere [ 31 , 33 ].