tkp4115 - surface and colloid chemistry - folk.ntnu.nofolk.ntnu.no/andersty/3. klasse/tkp4115 -...
TRANSCRIPT
Contents
2012 3Colloidal dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Steric stability of colloidal dispersions . . . . . . . . . . . . . . . . . . . . . . . . . . . 3Critical coagulation concentration (CCC) . . . . . . . . . . . . . . . . . . . . . . . . . 3Aggregation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3The Hamaker constant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Contact angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4Surfactant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2011 5Emulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Osmotic pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Capillary condensation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Surface excess . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Van der Waals forces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Electrostatic stabilisation of dispersions . . . . . . . . . . . . . . . . . . . . . . . . . . 6Critical micelle concentration (CMC) . . . . . . . . . . . . . . . . . . . . . . . . . . . 6Packing parameter for surfactants in solution . . . . . . . . . . . . . . . . . . . . . . 6
2010 7The electrical double layer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Potential-determining ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Brownian motion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7The Marangoni effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Ostwald ripening . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Breakdown mechanisms in an oil-in-water emulsion . . . . . . . . . . . . . . . . . . . 7Isoelectric point . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Electrophoretic mobility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2009 9Micelle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Solubilization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Packing parameter (PP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Zeta potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Emulsion versus microemulsion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Derivation of the sedimentation equation . . . . . . . . . . . . . . . . . . . . . . . . . 10Stabilization of emulsions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2008 11Colloidal size range . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Kinetic stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Surfactants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Critical coagulation concentration (CCC) . . . . . . . . . . . . . . . . . . . . . . . . . 11The Schultze-Hardy rule . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2007 13Krafft temperature - critical micelle temperature (CMT) . . . . . . . . . . . . . . . . 13Critical surface tension . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Flocculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
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The surface tensions dependence on solute concentration . . . . . . . . . . . . . . . . 13
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2012
Colloidal dispersion A system with a dispersed and a continuous phase in which the dis-persed particles are of a colloidal size. E.g. 10−9 m− 10−6 m.
Steric stability of colloidal dispersions Polymers can adsorb onto the particle surfaceand form a layer around the particle. This layer can provide a steric hindrance which preventsflocculation, coagulation and coalescence. The effect is highly dependent on polymer-solventand polymer-particle interactions.
H
Figure 1: Steric hindrance: When H = 2t, ∆G� 0.
Critical coagulation concentration (CCC) The defined concentration of electrolyte neededto induce coagulation. Beyond this threshold value, the colloid will coagulate rapidly. CCCcan be determined experimentally by observing the lowest concentration of electrolyte whichinduces coagulation after a certain time has elapsed. CCC depends on the time allowed toelapse before the evaluation, the polydispersity of the sample, the potential of the surface, theHamaker constant and the valence of the ions.
Aggregation The formation of clusters in a colloidal suspension. E.g. when two or moreparticles adhere on contact to form an aggregate.
Figure 2: Aggregation.
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The Hamaker constant A material property that represents the strength of van der Waalsinteractions between macroscopic bodies.
A = π2Cρ1ρ2
Hamaker constant A, coefficient in particle-particle pair interaction C, number of atoms perunit volume in two interacting bodies ρ1, ρ2.
Contact angle The angle, conventionally measured through the liquid, where a liquid/vapourinterface meets a solid surface.
Figure 3: Contact angle θ.
Surfactant Compounds that lower the surface tension of a liquid, the interfacial tensionbetween two liquids or that between a liquid and a solid.
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2011
Emulsion A colloidal system in which both the dispersed and the continuous phase areliquids.
Osmotic pressure The pressure which needs to be applied to prevent the inward flow of thesolute through a semi-permeable membrane. The osmotic pressure of an diluted solution canbe approximated using the van’t Hoff equation
n2 =πV1RT
n2 is the number of moles of the solute, π is the osmotic pressure, V1 is the volume of thesolvent.
Capillary condensation Capillary condensation occurs due to the vapour pressure’s depen-dence on the surface’s curvature. This is evident by the Kelvin equation which relates curvatureto vapour pressure.
RT · ln pK0
p0= −2γVm
rC
This implicates that vapour condenses more readily with decreasing radius of curvature.
Figure 4: Capillary condensation in a conical pore. The normal vapour pressure increases from 0 top0 from A to D.
Surface excess For an interface, the adsorption or surface excess of a given component isdefined as the difference between the amount of component actually present in the system, andthat which would be present (in a reference system) if the bulk concentration in the adjoiningphases were maintained up to a chosen geometrical dividing surface. E.g. surface tension is thesurface excess of Gibbs energy, thus for a two-component system the Gibbs-Duhem equationbecomes
ns1 dµ1 + ns
2 dµ2 + A dγ = 0
− dγ =ns1
Adµ1 +
ns2
Adµ2 = Γ1 dµ1 + Γ2 dµ2
where Γi is the surface excess of component i.
Van der Waals forces Caused by molecular-level interactions due to induced or permanentpolarities created. There are three types of vdW forces:
• Keesom: permanent dipole/permanent dipole interactions.
• Debye: permanent dipole/induced dipole interactions.
• London/dispersion: induced dipole/induced dipole interactions.
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These forces are characterized by:
• They are always attractive.
• They are relatively long-ranged.
• The dispersion force (London) is influenced by the presence of other nearby particles.
Electrostatic stabilisation of dispersions The mechanism in which the attractive vander Waals forces are counterbalanced by the repulsive Coulomb forces acting between the(commonly) negatively charged colloidal particles. Thus preventing coagulation, flocculation,aggregation and/or coalescence. A particle’s surface can gain a surface charge by adsorption ofions or dissociation of a chemical surface group.
Figure 5: The electric double layer of a negatively charged particle.
Critical micelle concentration (CMC) The threshold concentration of surfactant at whichmicellization occurs.
Packing parameter for surfactants in solution
P =Vcahlc
c chain, h head.
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2010
The electrical double layer See fig. 5. Particles that are dispersed in an electrolyte, andhave polar surface groups, will attract surrounding ions of opposite charge. For instance, ifa particle with partially negative charged surface groups is emerged in an electrolyte, it willattract the positive ions in the solution. Hence creating an electrical potential gradient, andthus repelling other negatively charged particles which procure an increased stabilization. Theparticle’s surface may also be charged by dissociation of a chemical surface group. The termdouble layer is cast because it includes the charged groups on the surface, and the surroundingcounter-charged ions.
Potential-determining ions The ions that adsorb at the particles’ surface are the potential-determining ions, the counter-ions are the potential-indifferent ions. In systems where surfacecharge arise due to dissociation, the remaining group is the potential-determining ion. In theelectrical double layer approximation, the potential-indifferent ions are considered point-chargeswithout any chemical properties.
Brownian motion The fluctuation of solvent molecules cause colloidal particles’ motion,due to force imbalances on surface impact. The result is a random motion which is dubbedBrownian motion. The Brownian motion decrease with increasing particle mass due to itsmomentum.
The Marangoni effect The mass transfer along an interface between two fluids due to a sur-face tension gradient. Fluids with higher surface tension pull more strongly on the neighbouringfluids, hence in the existence of a surface tension gradient, fluid will flow towards increasingsurface tension. This can be observed by adding a drop of soap (surfactant) to a water film,the water will spontaneously rush away from the soap.
Ostwald ripening According to the Kelvin equation,
RT lnpK0p0
= γVm
(1
R1
+1
R2
)the vapour pressure increase with decreasing drop size, consequently the smaller drops willvaporize faster and their vapour tend to condense on the larger drops. Ergo the larger dropswill ripe at the expense of the smaller drops, a process known as Ostwald ripening.
Breakdown mechanisms in an oil-in-water emulsion
Figure 6: Coalescence, flocculation and creaming.
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Isoelectric point The pH at which a particular molecule carries no net electric charge.
Electrophoretic mobility The velocity of a charged particle per unit field strength.
u = Cεζ
η, C =
{2/3 κR < 101 κR > 10
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2009
Micelle An aggregate of surfactant molecules dispersed in a liquid colloid.
Solubilization Above the critical micelle concentration, a number of solutes that are nor-mally unsolvable/slightly solvable dissolve extensively. This process is called solubilization,and is caused due to the micelles. The solubilizate (solute) is taken up by the micelles, henceresulting in a thermodynamically stable and isotropic solution. The surfactant is then dubbedsolubilizer.
Packing parameter (PP) A dimensionless number used as an indicator of the shapes thatcan be expected for an aggregate (micelle).
P =Vcahlc
Figure 7: Different micelle structures with the corresponding packing parameter range.
Zeta potential The potential at the surface of shear (slipping plane), at which the the fluidlayer surrounding the particle cease to be stationary. The zeta potential is thereby able to givean indication of the stability of the colloid.
Figure 8: Zeta potential.
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Emulsion versus microemulsion Coarse emulsions are in the range of micrometers, theyusually contain a mixture of two or more immiscible liquids, and a surfactant. Microemulsionsare commonly prepared by adding a cosurfactant to an emulsion.
• Microemulsions contain particles at least an order of magnitude smaller than those incoarse emulsions.
• Microemulsions are clear, coarse emulsions are cloudy.
• Microemulsions form spontaneously; coarse emulsions ordinarily require vigorous stirring.
• Microemulsions are stable with respect to separation into their components; coarse emul-sions may inhibit a degree of kinetic stability, but will ultimately separate.
Derivation of the sedimentation equation Under stationary conditions, the velocity v isconstant, which results in
∑∀i Fi = 0
Figure 9: The forces acting upon a sphere emerged in liquid.
FB = Vsρ1gFG = −Vsρ2gFv = 6πηRsv
Vsg (ρ1 − ρ2) + 6πηRsv = 0
v =m
6πηRs
(1− ρ1
ρ2
)g =
m
f0
(1− ρ1
ρ2
)g
Stabilization of emulsions
• Ionic surfactant at surface yields electrostatic stabilization.
• Non-ionic surfactant or polymer at interface yields steric stabilization.
• Polyelectrolytes yields electrosteric stabilization.
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2008
Colloidal size range Particles with some linear dimension in the range of 10−9 m− 10−6 mis considered a colloidal particle.
Kinetic stability Some colloidal dispersion are thermodynamically unstable, but their ki-netics of breakdown are so slow that they can be considered as stable. Thus the system iskinetically stable.
Surfactants Compounds that lower the surface tension of a liquid, the interfacial tensionbetween two liquids or that between a liquid and a solid. The surfactants can be divided intofour main categories.
• Anionic surfactants; surfactants that contain an anionic functional group at their head.
• Cationic surfactants; surfactants that contain a cationic functional group at their head.
• Zwitterionic (amphoteric) surfactants; surfactants with head groups that contain bothcationic and anionic functional groups.
• Non-ionic surfactants; electrically neutral surfactants, for instance long chain alcohols.
Figure 10: The different types of surfactants. From top to bottom; non-ionic, anionic, cationic andzwitterionic.
Critical coagulation concentration (CCC) See earlier paragraph.
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Figure 11: The total interaction energy as a function of the distance between two identical colloidalparticles, for different values of κ (concentration). As the concentration increases towards CCC, theenergy barrier becomes smaller until it is completely diminished, resulting in rapid coagulation.
The Schultze-Hardy rule States that it is the valence of the ion of opposite charge tothe colloid that has the principal effect on the stability of the colloid. The CCC value for aparticular electrolyte is essentially determined by the valence of the counter-ion regardless ofthe nature of the ion with the same charge as the surface.
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2007
Krafft temperature - critical micelle temperature (CMT) The minimum temperatureat which surfactants form micelles. Below CMT there is no CMC, and the surfactant willremain in its crystalline form.
Critical surface tension The surface tension that yields cos θ = 1
γLV = γC ⇒ cos θ = 1
Figure 12: Zisman plot
Flocculation A process of contact and adhesion whereby the particles of a dispersion formlarger-size clusters.
The surface tensions dependence on solute concentration The surface tension can bedescribed as γ = f(C). The solute can then be divided into three groups, depending on howthe surface tension varies with their concentration.
• Increase surface tension, primarily salts in aqueous solutions. This effect arises becausethe ions attract the polar water molecules which leads to surface depletion of the solute,thereby increasing the cohesive forces of the water molecules, and thus increases thesurface tension.
• Continuously decrease surface tension, primarily hydrophobic components in aqueoussolutions. These components escape to the surface and thereby decrease the cohesiveforces of the water molecules at the surface, thus lower the surface tension.
• Sharply decrease surface tension up to a certain concentration, surfactants. Surfactantsescape to the solvents surface and binds at the gas-liquid interface, thereby decreasing thesurface tension strongly. When a certain concentration of surfactant is obtained (CMC),the surfactant will form micelles and further increase in solute concentration has no effecton the surface tension.
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Figure 13: γ as a function of the concentration for the three groups.
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