trattamento delle sospensioni acquose
TRANSCRIPT
Trattamento sospensioni colloidali.
• Proprietà e classificazione colloidi.• (Filtrazione )• Aggregazione.
– Coagulazione (Coalescenza)– Flocculazione
LONG-RANGE VAN DER WAALS ATTRACTION
s0 a
s00
Hamaker constant, A,proportional tointermolecularattraction constant
Potential energyof interaction
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! = "Aa
12s0
Attractive forceof interaction
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Fatt
=d!
ds0
=Aa
12s0
2
ELECTROSTATIC REPULSION DEPENDS ON….
1. The Stern potential, ψδ
2. The thickness of the diffuse double layer, κ-1
Electrical Repulsion
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+++++ +
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+++++ +
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+Electrical Repulsion
strato dell'acqua legata" in quanto si muove con essa; il secondostrato, di spessore relativamente molto maggiore è detto "strato
diffuso" o "strato dell'acqua libera"
Forces between particles (surfaces)
s
Attractive forces• van der Waals forces
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kT
s
DLVO Theory
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kT
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Repulsive forces• electrostatic forces (aqueous or high ε media)
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When (Φ/kT) ≥ 10, → “stability”
DerjaguinLandauVerweyOverbeek
Controlling DLVO interactions
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12kTs
water, 25ºC
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##0RT $ 0.329z C Å-1
Stabilizing colloids against vander Waals
1. Charge Stabilization
DLVO potential (1940’s)
Derjaguin
Landau
Verwij
Overbeekpicture1968
Stabilizing colloids against vander Waals
1. Charge Stabilization
2. StericStabilization
3. Nano-halo Stabilization?
Classification of Colloidal Systems
Classification Lyophilic(Hydrophilic)
Lyophobic(Hydrophobic)
Association
DispersedPhase
Single Macro-molecule
Large collectionof ions
Collection ofsurfactantmolecules(micelle)
Interactionwith DispersionMedium
Strong Weak Strong
Formation SpontaneousReversible
Specialtechnique
Spontaneousabove cmc.
Classification of Colloidal Systems
Classification Hydrophilic Hydrophobic Association
StabilizingMechanism
HydrationElectrical
repulsion
Electricalrepulsion
Hydration/Hydrophobic
interactionsViscosity Dramatic
increaseNo change Little change;
can increase athigh conc.
Examples GelatinMethyl-cellulose
Silver IodideGold Sols
Tween 80Bile Salts
DEFINITION OF A COLLOID
Mixtures of one material (the “dispersoid”)dispersed in another as particles in thesize range of:
1nm → 10µm.
Ostwald: “die Welt der vernachlässigten Dimensionen”
Chem E 455: Surface and Colloid Science Laboratory: Spring Quarter
ColloidsColloids• Colloids are suspensions in which the suspended particles are
larger than molecules but too small to drop out of the suspensiondue to gravity.
• Particle size: 10 to 2000 Å.• There are several types of colloid:
– aerosol (gas + liquid or solid, e.g. fog and smoke),– foam (liquid + gas, e.g. whipped cream),– emulsion (liquid + liquid, e.g. milk),– sol (liquid + solid, e.g. paint),– solid foam (solid + gas, e.g. marshmallow),– solid emulsion (solid + liquid, e.g. butter),– solid sol (solid + solid, e.g. ruby glass).
Dispersed
Phase
Dispersion
mediumCommon Name Examples
Liquid Gas Aerosol Fog, fine sprays
Solid Gas Aerosol Smoke, viruses, tear "gas"
Gas Liquid Micro-foam Shaving cream, beer foam
Liquid Liquid Emulsion Milk, mayonnaise
Solid Liquid Sol Paint, ink, clay
Gas Solid Solid micro-foam Foam plastics, marshmallow
Liquid Solid Solid emulsion Butter, cheese, opal, pearl
Solid Solid Micro(nano)
composite
Injection-molded objects
LYOPHOBIC COLLOIDS
ColloidsColloids• Tyndall effect: ability of a Colloid to scatter light.
The beam of light can be seen through the colloid.
ColloidsColloidsHydrophilic and Hydrophobic ColloidsHydrophilic and Hydrophobic Colloids• Focus on colloids in water.• “Water loving” colloids: hydrophilic.• “Water hating” colloids: hydrophobic.• Molecules arrange themselves so that hydrophobic
portions are oriented towards each other.• If a large hydrophobic macromolecule (giant molecule)
needs to exist in water (e.g. in a cell), hydrophobicmolecules embed themselves into the macromoleculeleaving the hydrophilic ends to interact with water.
ColloidsColloidsHydrophilic and Hydrophobic ColloidsHydrophilic and Hydrophobic Colloids• Typical hydrophilic groups are polar (containing C-O, O-
H, N-H bonds) or charged.• Hydrophobic colloids need to be stabilized in water.• Adsorption: when something sticks to a surface we say
that it is adsorbed.• If ions are adsorbed onto the surface of a colloid, the
colloids appears hydrophilic and is stabilized in water.• Consider a small drop of oil in water.• Add to the water sodium stearate.
ColloidsColloidsHydrophilic and Hydrophobic ColloidsHydrophilic and Hydrophobic Colloids• Sodium stearate has a long hydrophobic tail
(CH3(CH2)16-) and a small hydrophobic head (-CO2
-Na+).• The hydrophobic tail can be absorbed into the oil
drop, leaving the hydrophilic head on the surface.• The hydrophilic heads then interact with the water
and the oil drop is stabilized in water.
ColloidsColloidsHydrophilic and Hydrophobic ColloidsHydrophilic and Hydrophobic Colloids• Most dirt stains on people and clothing are oil-
based. Soaps are molecules with longhydrophobic tails and hydrophilic heads thatremove dirt by stabilizing the colloid in water.
• Bile excretes substances like sodium stereate thatforms an emulsion with fats in our small intestine.
• Emulsifying agents help form an emulsion.
ColloidsColloidsRemoval of Colloidal ParticlesRemoval of Colloidal Particles• Colloid particles are too small to be separated by physical
means (e.g. filtration).• Colloid particles are coagulated (enlarged) until they can
be removed by filtration.• Methods of coagulation:
– heating (colloid particles move and are attracted toeach other when they collide);
– adding an electrolyte (neutralize the surface chargeson the colloid particles).
• Dialysis: using a semipermeable membranes separateions from colloidal particles.
trattamento di flocculazione edecantazione. Il processo di coagulazioneviene agevolato da una lenta e continua
agitazione della massa.