← Chemistry — Std 11
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Adsorption and Colloids

Ch. 12Std 11

Easy Overview

Activated charcoal can remove toxins from water. Milk is an emulsion of fat in water. Fog is liquid droplets suspended in gas. These everyday phenomena involve surfaces — adsorption on solid surfaces and colloidal systems where one substance is finely dispersed in another. Adsorption is the accumulation of molecules on a surface. It's different from absorption (which penetrates the bulk). Adsorption depends on surface area, temperature, pressure, and the nature of adsorbate and adsorbent. Freundlich and Langmuir isotherms describe how much adsorbs at different concentrations. Physical adsorption (weak van der Waals forces) and chemical adsorption (strong covalent bonds) have different characteristics. Colloids (1-1000 nm particle size) are intermediate between true solutions and suspensions. They're everywhere — milk, blood, smoke, fog, paint, jelly, cheese. Colloids can be lyophilic (solvent-loving) or lyophobic (solvent-fearing). They exhibit the Tyndall effect, Brownian motion, and electrophoresis. Emulsions, gels, and foams are special types of colloids with immense practical importance.

Adsorption vs Absorption — Surface vs Bulk

Adsorption: molecules (adsorbate) accumulate on the surface of a solid or liquid (adsorbent). Absorption: molecules penetrate into the bulk of the absorbent (sponge absorbs water). Sorption: when both occur simultaneously. Adsorption is a surface phenomenon — depends on surface area (activated charcoal has huge surface area ~500-1500 m²/g). Highly porous materials are excellent adsorbents: activated carbon, silica gel, zeolites, alumina. Applications: gas masks (charcoal adsorbs toxic gases), water purification, sugar decolorization, chromatography separation.

Types of Adsorption — Physisorption and Chemisorption

Physisorption (physical adsorption): weak van der Waals forces (10-40 kJ/mol). Low specificity — any gas adsorbs on any surface. Multilayer adsorption possible. Reversible. Decreases with increasing temperature. Increases with pressure (Freundlich/Langmuir isotherms). Rate is fast (no activation energy). Examples: N₂ on charcoal, H₂O on silica gel. Chemisorption (chemical adsorption): strong covalent bonds (80-400 kJ/mol). Highly specific — requires bond formation. Monolayer only (chemically bonded). Often irreversible. Activation energy needed — rate increases with T (like chemical reactions). Examples: O₂ on platinum, H₂ on Ni (catalytic hydrogenation). Distinguishing: chemisorption has higher enthalpy, higher specificity, and is harder to reverse.

Factors Affecting Adsorption

Surface area of adsorbent: higher area = more adsorption (activated charcoal > graphite > diamond). Nature of adsorbate: easily liquefiable gases (higher critical temperature, more van der Waals) adsorb more — NH₃ > CH₄ > H₂ > He. Temperature: physisorption decreases with T (exothermic, ΔH < 0). Chemisorption initially increases (activation energy barrier), then decreases at very high T. Pressure: physisorption increases with P, reaches saturation (monolayer coverage for Langmuir). Concentration (from solution): follows Freundlich: x/m = kC^(1/n). Activation of adsorbent: increasing surface area by heating (removes adsorbed gases) or chemical treatment.

Adsorption Isotherms — Freundlich and Langmuir

Freundlich isotherm: x/m = kP^(1/n) (for gases) or x/m = kC^(1/n) (for solutions). log(x/m) = log k + (1/n)log P. n > 1 (usually 2-3), k depends on temperature and adsorbent. Empirical, works well at moderate pressures but fails at very high P and very low P. Langmuir isotherm: assumes monolayer adsorption, uniform surface, no interaction between adsorbed molecules. θ = KP/(1+KP), where θ = fraction of surface covered. At low P: θ ≈ KP (linear). At high P: θ → 1 (saturation). x/m = (x/m)_max × KP/(1+KP). Linear form: P/(x/m) = 1/K(x/m)_max + P/(x/m)_max. Langmuir is more theoretically sound but assumes ideal surface (real surfaces are heterogeneous).

Catalysis and Adsorption — The Surface Connection

Heterogeneous catalysis occurs on solid surfaces. Steps: (1) diffusion of reactants to surface (2) adsorption (3) surface reaction (4) desorption of products (5) diffusion away. Rate-determining step is usually surface reaction or adsorption. Examples: Haber process (Fe + promoters adsorbs N₂, weakens N≡N bond, H₂ adsorbs, NH₃ desorbs — 450°C, 200 atm). Contact process (V₂O₅: SO₂ + O₂ adsorb, SO₃ forms, desorbs). Catalytic converter (Pt/Pd/Rh adsorb CO, NO, unburned HC — surface reactions convert to CO₂, N₂, H₂O). Automobile catalyst: three-way converts CO, HC, NOx simultaneously. Poisoning: impurities (Pb, S) adsorb irreversibly, block active sites. Promoters: enhance activity (K₂O, Al₂O₃ in Fe catalyst for Haber).

Colloidal State — The In-Between World

True solution: <1 nm, particles invisible, pass through filter paper, do not scatter light. Colloidal dispersion: 1-1000 nm, visible with ultramicroscope, pass through filter paper but not semipermeable membrane, scatter light (Tyndall effect). Suspension: >1000 nm, visible to naked eye, settle on standing, scatter light. Colloidal particles are aggregates of many molecules (multimolecular: S₈ colloid) or single large molecules (macromolecular: starch, proteins, polymers). Associated colloids (micelles): molecules that aggregate above a critical concentration (CMC) — soaps and detergents. At low concentration, soap exists as individual ions; above CMC (~10⁻⁴ to 10⁻³ M), they form micelles.

Types of Colloids — Lyophilic and Lyophobic

Lyophilic (solvent-loving): strong affinity between dispersed phase and dispersion medium. Reversible — dried residue can be reconstituted. Stable — needs electrolyte to coagulate. Examples: starch in water, gelatin in water, rubber in benzene. High viscosity. Prepared by simple mixing. Lyophobic (solvent-fearing): no affinity. Irreversible — once coagulated, can't be reconstituted. Unstable — needs stabilizing agent. Examples: metal sols (Ag, Au), metal hydroxides (Fe(OH)₃), As₂S₃, sulfur. Low viscosity. Need special methods (chemical reduction, hydrolysis, condensation, peptization). Catalysis: lyophobic sols have large surface area, used as catalysts (colloidal Pt, Pd, Ni).

Preparation of Lyophobic Colloids

Condensation methods: build particles from smaller units. Chemical reduction: 2AgNO₃ + 2NH₂OH → 2Ag + N₂ + 2HNO₃ + 2H₂O (red colloidal silver). Hydrolysis: FeCl₃ + 3H₂O (hot) → Fe(OH)₃ sol + 3HCl (red-brown). Double decomposition: As₂O₃ + 3H₂S → As₂S₃ sol + 3H₂O (yellow). Oxidation: 2H₂S + SO₂ → 3S + 2H₂O (sulfur sol). Peptization: convert freshly precipitated particles into colloid by adding electrolyte (Fe(OH)₃ + FeCl₃ → Fe(OH)₃ sol). Bredig's arc method: electric arc between metal electrodes in water — metal vaporizes and condenses as colloidal particles. Dialysis removes electrolyte impurities through semipermeable membrane.

Purification of Colloids — Dialysis and Ultrafiltration

Dialysis: separate colloidal particles from dissolved ions/molecules using semipermeable membrane (cellophane, parchment, animal bladder). Ions pass through, colloids retained. Water changed continuously for efficient removal. Electrodialysis: electric field speeds ion removal. Applications: purification of blood (kidney dialysis — removes urea and toxins while retaining proteins and cells), desalting colloids. Ultrafiltration: filter paper with pores smaller than colloid particles (collodion or cellophane membrane under pressure). Retains colloids, passes true solutions. Ultrafilter paper: filter paper soaked in collodion (nitrocellulose in ether/alcohol), dried. Membranes for reverse osmosis: remove salts from water.

Properties of Colloids — How They Behave

Tyndall effect: scattering of light by colloidal particles — beam visible through colloid (true solutions don't scatter). Blue eyes: Tyndall effect from colloidal particles in iris. Brownian motion: zigzag path due to bombardment by solvent molecules — prevents settling. Confirms kinetic theory and random molecular motion. Electrophoresis: charged colloidal particles move toward oppositely charged electrode under electric field. Positive particles (Fe(OH)₃ sol) → cathode. Negative particles (As₂S₃ sol) → anode. Electro-osmosis: opposite — liquid moves through stationary colloid under electric field. Coagulation/flocculation: neutralization of charge by adding electrolyte — particles aggregate and settle. Hardy-Schulze rule: ion with opposite charge and higher valency is more effective (Al³⁺ > Ba²⁺ > Na⁺ for negative sols).

Emulsions — Colloids of Liquids

Emulsion: liquid dispersed in another immiscible liquid. Oil-in-water (O/W): oil dispersed in water — milk (butterfat in water), mayonnaise (oil in vinegar/egg), creams. Water-in-oil (W/O): water dispersed in oil — butter, margarine, cold cream. Emulsifiers stabilize emulsions by reducing interfacial tension: soaps (hydrophilic head + hydrophobic tail), detergents, proteins (casein in milk), lecithin in egg yolk, gums. HLB (hydrophilic-lipophilic balance) number: >8 forms O/W emulsion (Tween), <8 forms W/O emulsion (Span). De-emulsification: breaking emulsion by heating (cream separation), centrifugation, freezing, adding electrolyte (salting out), chemical demulsifiers. Applications: homogenized milk, cosmetics, paints, asphalt emulsions.

Gels — The Semi-Solid Colloids

Gel: solid dispersed in liquid (firm, jelly-like). Lyophilic sols form gels on cooling (gelatin: hot sol → cold gel). Gel formation: chain-like molecules cross-link to form 3D network, trapping liquid. Syneresis (weeping): liquid exudes from gel on standing (curd, jelly). Thixotropy: gel↔sol reversible with agitation — paint (stir → sol, stand → gel), drilling mud, ketchup (shake → flows). Applications: gelatin (food, photographic film), agar-agar (microbiology culture medium), silica gel (desiccant — adsorbs water), ion-exchange chromatography. Hydrogels: water-swollen polymer networks — contact lenses, wound dressings, superabsorbent polymers (diapers absorb 500× their weight).

Applications of Colloids — From Medicine to Industry

Medicine: colloidal gold (diagnostic, drug delivery), silver sol (antimicrobial), milk of magnesia (Mg(OH)₂ sol — antacid), colloidal bismuth (stomach ulcer treatment), iron dextran (IV iron supplement — colloidal iron for anemia). Water purification: alum (Al₂(SO₄)₃) + lime → Al(OH)₃ coagulating sol — adsorbs and settles impurities. Sewage treatment: FeCl₃ or alum coagulates colloidal waste. Photography: AgBr in gelatin (colloidal suspension in photographic film). Rubber industry: latex (colloidal rubber in water), coagulation with acetic acid/formic acid. Paints and inks: pigments dispersed in vehicle (oil or water) with stabilizers. Food: mayonnaise (O/W emulsion), ice cream (foam — air in cream), butter (W/O), cheese, jelly, whipped cream. Purification of drinking water: coagulation with alum removes colloidal clay and bacteria.

Micelles and Critical Micelle Concentration

Surfactants (surface active agents): molecules with hydrophilic head and hydrophobic tail. Soap (sodium stearate C₁₇H₃₅COONa): ionic head, long hydrocarbon tail. Below CMC: surfactant molecules exist as individual ions or molecules at surface (reduce surface tension). Above CMC: spontaneously aggregate into micelles (spherical clusters, 50-100 molecules, tails inward, heads outward in water). CMC: 10⁻⁴ to 10⁻³ M for ionic surfactants, lower for nonionic. Factors: longer hydrocarbon chain = lower CMC; added electrolyte lowers CMC (reduces repulsion between ionic heads). Micelle shape: sphere, rod, or bilayer depending on concentration and structure. Reverse micelles in nonpolar solvents: heads inward, tails outward. Applications: detergency (dissolve grease in micelle core), drug delivery, enhanced oil recovery.

Colloids Around Us — Everyday Examples

Biological: blood (colloidal solution of proteins, RBCs), cytoplasm (gel of proteins/nucleic acids), milk (emulsion + colloid). Atmospheric: fog (liquid in gas), smoke (solid in gas), clouds, aerosol sprays (insecticides, deodorants). Industrial: paints (pigment + binder + solvent), printing inks, adhesives, lubricating greases (soap-thickened oil), drilling muds (bentonite clay in water). Food: mayonnaise, butter, cheese, ice cream, jelly, jam, whipped cream, mousse, chocolate. Household: cleaning products (detergents, soaps, shampoos), toothpaste (CaCO₃ + binder in water), cosmetics (creams, lotions). Geological: river deltas form when colloidal clay coagulates in salt water; soil colloids retain nutrients. Environmental: colloids transport pollutants in groundwater — need to understand their behavior for remediation.

Key Points

  • •Adsorption (surface) vs absorption (bulk); physisorption (weak) vs chemisorption (strong)
  • •Factors: ↑ surface area, ↑ pressure, ↑ T decreases physisorption
  • •Freundlich: x/m = kP^(1/n); Langmuir: θ = KP/(1+Kp) — monolayer, uniform surface
  • •Heterogeneous catalysis: adsorption → surface reaction → desorption (Haber, Contact)
  • •Colloids: 1-1000 nm, show Tyndall effect and Brownian motion
  • •Lyophilic: reversible, stable, needs electrolyte for coagulation
  • •Lyophobic: irreversible, unstable, needs special preparation methods
  • •Peptization: convert precipitate to colloid with electrolyte
  • •Dialysis: semipermeable membrane removes ions; electrodialysis uses electric field
  • •Electrophoresis: charged particles move to oppositely charged electrode
  • •Hardy-Schulze: higher valency ion → more effective coagulant
  • •Emulsions: O/W (milk) and W/O (butter); stabilized by emulsifiers
  • •Gels: 3D network traps liquid; thixotropy (stir → sol, rest → gel)
  • •CMC: surfactant molecules aggregate into micelles above critical concentration
  • •Applications: water purification (alum), medicine (colloidal drugs), food (mayonnaise)

Practice Questions

  • Distinguish between physisorption and chemisorption with examples.
  • Explain Freundlich adsorption isotherm. How is it different from Langmuir?
  • What is the role of adsorption in heterogeneous catalysis? Explain with Haber process.
  • What are colloids? Differentiate between lyophilic and lyophobic colloids.
  • Explain: (i) Tyndall effect (ii) Brownian motion (iii) Electrophoresis
  • How are lyophobic colloids prepared? Describe reduction and hydrolysis methods.
  • What are emulsions? Types, examples, and applications.
  • Explain Hardy-Schulze rule with examples.
  • What are micelles? Explain CMC and factors affecting it.