Trattamento delle sospensioni acquose Trattamento sospensioni colloidali.

• Proprietà e classificazione colloidi. • (Filtrazione ) • Aggregazione. – Coagulazione (Coalescenza) – Flocculazione LONG-RANGE VAN DER WAALS ATTRACTION

s0 a

Potential energy Attractive force of interaction of interaction Aa = d Aa 12s F = = 0 att ds 12s2 0 0 0 s0 Hamaker constant, A, p roportional to intermolecular attraction constant ELECTROSTATIC REPULSION RETARDS AGGREGATION

van der Waals attraction

electrostatic repulsion ELECTROSTATIC REPULSION DEPENDS ON….

1. The Stern potential, ψδ 2. The thickness of the diffuse double layer, κ-1 Electrical Repulsion

------+ + - Electrical Repulsion + + - - - + + - - + + + + + - + + + - - + + - + + + + - + + - + + - + + - - - + + + - - + + + -+ - -+ - - -+ - - -+ - strato dell'acqua legata" in quanto si muove con essa; il secondo strato, di spessore relativamente molto maggiore è detto "strato diffuso" o "strato dell'acqua libera" s

Forces between particles (surfaces) Attractive forces Repulsive forces • van der Waals forces • electrostatic forces DLVO Theory (aqueous or high ε media) ! kT Derjaguin ! Landau kT 0 s Verwey s Overbeek 0 ! kT

0 s When (Φ/kT) ≥ 10, → “stability” SOMMA DI POTENZIALI DI ATTRAZIONE E REPULSIONE Controlling DLVO interactions

! ! 0.064"aN C & ze$ ) A a kT = Av tanh2( % +e ,#s , 212 kT # 2 ' 4kT * 12kTs 0 s water, 25ºC 2e2z2n 2 2z2C ! = " = F $ 0.329z C Å-1 ## kT ## RT 0 0

Stabilizing colloids against van der Waals 1. Charge Stabilization

DLVO potential (1940’s) Derjaguin Landau Verwij Overbeek

picture1968 Stabilizing colloids against van der Waals 1. Charge Stabilization

2. Steric Stabilization

3. Nano-halo Stabilization? Classification of Colloidal Systems

Classification Lyophilic Lyophobic Association (Hydrophilic) (Hydrophobic) Dispersed Single Macro Large collection Collection of Phase -molecule of ions surfactant molecules (micelle) Interaction Strong Weak Strong with Dispersion Medium

Formation Spontaneous Special Spontaneous Reversible technique above cmc. Classification of Colloidal Systems

Classification Hydrophilic Hydrophobic Association

Stabilizing Hydration Electrical Hydration/ Mechanism Electrical repulsion Hydrophobic repulsion interactions Viscosity Dramatic No change Little change; increase can increase at high conc. Examples Gelatin Silver Iodide Tween 80 Methyl- Gold Sols Bile Salts cellulose Attractive Interactions

V = van der Waals Attractive Energy A Pair interactions are additive DEFINITION OF A COLLOID

Mixtures of one material (the “dispersoid”) dispersed in another as particles in the size range of: 1nm → 10µm.

Ostwald: “die Welt der vernachlässigten Dimensionen”

Chem E 455: Surface and Colloid Science Laboratory: Spring Quarter CCoollolloididss • Colloids are suspensions in which the suspended particles are larger than molecules but too small to drop out of the suspension due 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). LYOPHOBIC COLLOIDS

Dispersed Dispersion Common Name Examples Phase medium 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) Injection-molded objects composite CCoollolloididss • Tyndall effect: ability of a Colloid to scatter light. The beam of light can be seen through the colloid. CCoollolloididss HydrophilicHydrophilic andand HydrophobicHydrophobic ColloidsColloids • 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), hydrophobic molecules embed themselves into the macromolecule leaving the hydrophilic ends to interact with water. CCoollolloididss HydrophilicHydrophilic andand HydrophobicHydrophobic ColloidsColloids CCoollolloididss HydrophilicHydrophilic andand HydrophobicHydrophobic ColloidsColloids • 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. CCoollolloididss HydrophilicHydrophilic andand HydrophobicHydrophobic ColloidsColloids CCoollolloididss HydrophilicHydrophilic andand HydrophobicHydrophobic ColloidsColloids • 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. CCoollolloididss HydrophilicHydrophilic andand HydrophobicHydrophobic ColloidsColloids CCoollolloididss HydrophilicHydrophilic andand HydrophobicHydrophobic ColloidsColloids • Most dirt stains on people and clothing are oil- based. Soaps are molecules with long hydrophobic tails and hydrophilic heads that remove dirt by stabilizing the colloid in water. • Bile excretes substances like sodium stereate that forms an emulsion with fats in our small intestine. • Emulsifying agents help form an emulsion. CCoollolloididss RemovalRemoval ofof ColloidalColloidal ParticlesParticles • 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 to each other when they collide); – adding an electrolyte (neutralize the surface charges on the colloid particles). • Dialysis: using a semipermeable membranes separate ions from colloidal particles. INSTABILITY OF LYOPHOBIC COLLOIDS

Aggregation:

etc.

Coalescence: Coagulation of a Colloid trattamento di flocculazione e decantazione. Il processo di coagulazione viene agevolato da una lenta e continua agitazione della massa. ELETTROFORESI elettrodialisi