Environmental Engineering Reference
In-Depth Information
dipole..An.example.would.be.the.interactions.between.water.and.unsaturated.organ-
ics,.where.the.water's.dipole.can.induce.asymmetric.displacement.of.π-electrons..The.
London.force.is.the.interaction.of.two.induced.dipoles.that.result.from.the.interaction.
of.the.electromagnetic.ields.of.two.molecules..While.this.force.is.universal,.it.tends.to.
be.weaker.than.the.Keesom.and.Debye.forces.under.typical.environmental.conditions..
Refer.to.Ackler.et.al..[4].for.examples.of.application.
The. van. der. Waals. forces. cause. nanoparticles. to. be. attracted. to. each. other. as.
well. as. to. certain. other. environmental. constituents.. As. a. result,. nanoparticles. can.
form.larger.agglomerates..These.agglomerates.generally.tend.to.be.less.buoyant.and.
therefore.more.readily.settle.out.of.suspension.
...
solency Force
The.solvency.force.(
F
s
).differs.from.the.electrostatic.and.van.der.Waals.forces.in.that.
it.is.not.a.point-to-point.interaction..Rather,.it.is.a.free.energy.gradient.resulting.from.
the.differential.energy.levels.of.the.pure.solvent.and.the.solvent.plus.the.nanopar-
ticle..For.example,.dispersion.of.a.nanomaterial.
X
.in.water.(hydrosol).with.two.water.
binding.sites.on.each.nanoparticle.requires.that.the.water.molecules.go.from.being.
associated.with.other.water.molecules.to.being.associated.with.the.nanoparticles:
.
G
i i
.
(6.8)
X
+
H O H O
i
H O X H O
2
2
2
2
The. net. free. energy. difference. (∆
G
). between.
X
. +.
H
2
O•H
2
O
. and.
H
2
O•X•H
2
O
. is.
referred.to.as.the.free.energy.of.solvation..If.the.free.energy.of.solvation.is.thermo-
dynamically.advantageous.(∆
G
.<.0),.then.the.material.will.spontaneously.disperse.in.
water..The.force.component.of.this.energy.gradient.therefore.is.the.force.of.solvency..
In.practice,.one.can.quantify.the.solvency.force.by.the.dispersibility.of.the.material,.
one.of.the.critical.properties.of.nanomaterials.identiied.in.Table.2.2.
6.2.3 i
mpliCatiOns
OF
p
OlymOrphism
The. degree. of. polymorphism. also. affects. the. physical. and. chemical. properties. of.
nanomaterials..Polymorphism.is.the.ability.of.a.material.to.manifest.more.than.one.
form..As.discussed.previously,.the.base.molecular.structures.of.almost.all.nanoma-
terials.are.crystalline.in.nature..Most.nanomaterial.preparations.comprise.a.distribu-
tion.of.particle.sizes.as.a.function.of.the.material's.mode.of.synthesis..This.often.is.
referred.to.as.single-component.polymorphism.
Another. signiicant. form. of. polymorphism. is. the. interparticle. structure. of. the.
materials. that. can. form. multi-component. crystalline. phases.. For. example,. carbon.
nanotubes.can.form.either.aligned.bundles.or.tangles.referred.to.as.nanoropes..Each.
form.has.differing.surface.properties.and.electrical.densities.[5].
A.third.type.of.polymorphism.occurs.when.the.host.nanoparticles.condense.with.
guest.molecules.in.heterogeneous.structures..Such.guest.molecules.may.include.sol-
vents,.respective.counter-valent.ions.(salts),.or.other.solids.(co-crystals)..This.form.of.
polymorphism.often.is.seen.when.nanoparticles.condense.while.still.in.association.with.
their.Stern.layer.constituents.as.guest.molecules..In.practice,.polymorphism.can.result.
in.signiicantly.different.properties.for.nanoparticles.of.the.same.material..Rudalevige.
