Environmental Engineering Reference
In-Depth Information
the. system. delays. the. return. to. ground. state. by. the. activation. energy. involved. in.
reversing.the.charge.separation..An.example.of.this.would.be.a.material.with.a.low.
dielectric.constant,.such.as.polystyrene,.whose.electrons.are.removed.from.the.sur-
face.as.the.result.of.an.implied.electromagnetic.ield.resulting.in.a.net.static.charge..
The.resistive.nature.of.the.material.slows.electron.movement.to.ill.the.charge.hole,.
thereby.returning.to.the.ground.state.
The. development. of. a. net. charge. on. the. surface. of. a. nanoparticle. affects. the.
ion/dipole.distribution.of.the.constituents.of.the.solvent.(in.this.case,.air.or.water).
immediately.adjacent.to.the.nanoparticle..Speciically,.a.collection.of.counter.ions.
immediately. adjoins. the. charged. surface.. The. layer. of. counter. ions. and. the. asso-
ciated. net. charge,. which. moves. with. the. Brownian. motion. of. the. nanoparticle,. is.
referred.to.as.the.Stern.layer..If.the.ions.in.this.layer.do.not.balance.the.particle's.
surface.charge,.the.net.difference.(the.Stern.potential).then.acts.upon.the.rest.of.the.
suspension's. constituents.. The. differential. movement. of. the. Stern. potential. within.
the. luid. medium. produces. an. electromagnetic. shear. force. referred. to. as. the. zeta.
potential.(ξ)..For.considerations.here,.the.zeta.potential.can.be.generalized.to.be.the.
net.charge.of.the.nanoparticle.as.presented.to.the.environment..In.modeling.particle.
stability.or.kinetics.for.larger.particles,.the.displacement.of.the.Stern.layer.can.be.
ignored..However,.for.nanoparticles,.the.presence.of.the.Stern.layer.may.have.a.sig-
niicant.effect.and.should.be.considered.integral.in.the.derivation.of.particle.density.
and.volume.
Electrostatic. or. Coulomb. forces. generally. cause. like. particles,. which. tend. to.
acquire.like.charges,.to.repel.each.other..These.forces.oppose.van.der.Waals.force-
mediated.agglomeration.into.larger.clusters.(as.described.below)..While.the.applica-
tion.of.this.theory.to.engineered.nanoparticles.may.be.new,.engineers.have.applied.
the.underlying.science.to.water.and.wastewater.treatment.processes.since.at.least.the.
1800s.[3]..In.the.water.treatment.process.of.coagulation,.operators.add.chemicals.to.
destabilize.colloidal.suspensions.of.naturally.occurring.nanoparticles..These.addi-
tives.suppress.the.double-layer.charge.described.above,.enabling.particles.to.contact.
one.another.and.adhere.by.van.der.Waals.forces..Chapter.7.provides.further.informa-
tion.on.this.form.of.treatment.
...
an der waals Forces
The. van. der. Waals. forces. (
F
vdw
). also. represent. a. point-to-point. interaction. between.
molecular. moieties.. They. differ. from. electrorepulsive. force. in. that. the. charge. sepa-
ration.is.intramolecular,.and.therefore.the.force.potential.is.a.fraction.of.charge.per.
moiety..At.the.scale.of.nanoparticles,.van.der.Waals.forces.are.always.attractive..They.
are.principally.the.sum.of.three.component.forces:.(1).the.Keesom.force,.(2).the.Debye.
force,.and.(3).the.London.dispersion.force..The.Keesom.force.results.from.interactions.
between.two.permanent.dipoles..An.example.would.be.the.interactions.between.water.
molecules.or.between.ionized.salts.and.water.molecules..The.Debye.force.represents.
the.interaction.between.a.permanent.dipole.and.an.inducible.dipole,.which.results.from.
the.electromagnetic.ield.associated.with.the.permanent.dipole.inducing.a.charge.sepa-
ration.in.the.transient.dipole..In.luid.systems,.the.magnitude.of.this.induction.tends.
to.vary.in.the.infrared.frequency.as.the.result.of.molecular.vibration.of.the.permanent.
