Surface chemistry and colloids

In short

Surface chemistry studies what happens at interfaces — solid-gas, solid-liquid, liquid-liquid — where atoms have unsatisfied valencies and so adsorb, catalyse and stabilise. Colloid science applies this to dispersions of 1-1000 nm particles (sols, emulsions, gels, foams, aerosols) that are neither true solutions nor suspensions. The topic carries 8-12 marks in B.Sc physical chemistry papers and appears every year in GATE Chemistry (Section: physical chemistry, surface phenomena) and CSIR-UGC NET. Employers in paints, detergents, pharmaceuticals, water treatment, petroleum and food processing test it directly — zeta potential, CMC, emulsion stability and BET area are everyday working numbers at Asian Paints, HUL, Pidilite, Cipla and IOCL R&D.

Explained

Surfaces, surface tension and wetting

• Molecules at a surface have fewer neighbours, so they carry excess free energy. Surface tension γ is the work needed to create unit new area (N/m = J/m²); it falls with rising temperature and vanishes at the critical temperature.
• Water γ ≈ 72 mN/m at 25 °C; ethanol ≈ 22 mN/m; mercury ≈ 485 mN/m. Measured by capillary rise, stalagmometer (drop weight), Du Noüy ring or Wilhelmy plate.
• Surface excess and the Gibbs adsorption isotherm: a solute that lowers γ accumulates at the surface (positive adsorption — soaps, alcohols, called surface-active); one that raises γ is depleted (negative adsorption — inorganic salts, surface-inactive).
• Wetting: Young's equation relates contact angle θ to the three interfacial tensions. θ < 90° wets (water on clean glass); θ > 90° does not (water on wax, mercury on glass); θ → 0 is complete spreading.
• Curved surfaces: excess pressure inside a drop or bubble follows the Young-Laplace relation; the Kelvin equation explains why small droplets have a higher vapour pressure, giving capillary condensation and hysteresis in porous solids.

Adsorption: types, isotherms and measurement

• Adsorption is the concentration of a substance at a surface; absorption is uniform penetration into the bulk; sorption covers both. The adsorbed substance is the adsorbate, the surface is the adsorbent. Desorption is the reverse; adsorption is always exothermic (ΔH negative) because the adsorbate loses freedom (ΔS negative), and ΔG = ΔH − TΔS must be negative.
• Physisorption: van der Waals forces, ΔH ≈ 20-40 kJ/mol, non-specific, reversible, multilayer at high pressure, favoured at low temperature, no activation energy.
• Chemisorption: chemical bond formation, ΔH ≈ 80-400 kJ/mol, highly specific, usually irreversible, monolayer only, needs activation energy so it first rises with temperature then falls.
• Factors raising adsorption: larger surface area (powdered/porous/activated adsorbent), higher pressure of gas, lower temperature (physisorption), easily liquefiable gases (high critical temperature — SO₂, NH₃, Cl₂ adsorb more than H₂, N₂).
• Freundlich isotherm: x/m = k·p^1/n, an empirical fit; a log-log plot is linear with slope 1/n (0 < 1/n < 1). Fails at high pressure because it predicts no saturation.
• Langmuir isotherm: assumes a monolayer, energetically uniform sites, no lateral interaction, dynamic equilibrium between adsorption and desorption. θ = Kp/(1 + Kp). At low p, θ ∝ p (first order); at high p, θ → 1 (zero order) — this explains the order change in surface-catalysed reactions.
• BET isotherm: extends Langmuir to multilayers and is the standard route to specific surface area (m²/g) from N₂ adsorption at 77 K. Typical values: activated carbon 800-1500 m²/g, silica gel 300-800, zeolite 13X ≈ 600.
• Adsorption from solution follows Freundlich-type behaviour and is used in decolourising sugar syrup with animal charcoal and in chromatography (adsorption, TLC, column).
• Applications: gas masks (activated charcoal), creating high vacuum (charcoal at liquid-air temperature), desiccation with silica gel, ion-exchange softening, froth flotation of sulphide ores, heterogeneous catalysis, chromatographic separation, removal of colour and organics in water treatment.

Colloids: classification and preparation

• Dispersed phase (the colloid) in a dispersion medium; particle size 1 nm-1000 nm (1 µm). Below 1 nm is a true solution, above 1 µm a suspension.
• By affinity for the medium: lyophilic (solvent-loving — starch, gum, gelatin, protein; reversible, thermodynamically stable, highly solvated, need no stabiliser) and lyophobic (solvent-hating — metal sols, As₂S₃, AgI; irreversible, only kinetically stable, stabilised by charge, easily coagulated).
• By particle nature: multimolecular (many atoms aggregate — gold sol, sulphur sol), macromolecular (single large molecules — proteins, cellulose, polymers), associated/micellar (soaps and detergents above the CMC).
• By physical state — the eight standard systems:

• Preparation — dispersion methods: mechanical colloid mill (paints, inks), Bredig's arc (metal sols by striking an arc between metal electrodes under ice-cold water with a trace of alkali), peptisation (breaking a fresh precipitate into a sol with a small amount of electrolyte — Fe(OH)₃ + FeCl₃ → Fe(OH)₃ sol, the peptising ion is adsorbed), ultrasonic dispersion.
• Preparation — condensation (aggregation) methods: double decomposition (As₂O₃ + H₂S → As₂S₃ sol), oxidation (SO₂ + H₂S → S sol), reduction (AuCl₃ + HCHO or SnCl₂ → gold sol), hydrolysis (FeCl₃ + hot water → Fe(OH)₃ sol), exchange of solvent (sulphur or resin in alcohol poured into water).
• Purification: dialysis (parchment/cellophane membrane; electrolyte diffuses out), electrodialysis (an applied field speeds it up), ultrafiltration (collodion-impregnated filter paper), ultracentrifugation. Over-dialysis destabilises a lyophobic sol by removing the stabilising electrolyte.

Properties of colloidal systems

• Optical — Tyndall effect: scattering of light by particles comparable to the wavelength gives a visible cone; true solutions do not scatter. Basis of the ultramicroscope (particles seen as points of light) and of nephelometry/turbidimetry.
• Kinetic — Brownian motion: random zig-zag motion from uneven molecular bombardment; it keeps particles suspended against gravity and was used by Perrin to get Avogadro's number. Smaller particles and lower viscosity give faster motion.
• Colligative: very low osmotic pressure and negligible ΔTf/ΔTb because the number of particles per unit mass is small; osmometry therefore gives molar masses of macromolecules.
• Electrical: particles carry like charges from preferential ion adsorption or ionisation. Electrical double layer = tightly held Stern (fixed) layer + loose diffuse layer; the potential at the shear plane is the zeta potential (ζ, mV). |ζ| > about 30 mV usually means a stable sol; near zero is the isoelectric point where coagulation is fastest.
• Electrokinetic phenomena: electrophoresis (particles move in a field), electro-osmosis (medium moves when particles are held), sedimentation potential and streaming potential (the reverse effects).
• Donnan membrane equilibrium: unequal distribution of diffusible ions across a membrane impermeable to a macro-ion — important for protein solutions and dialysis.
• Coagulation (flocculation): destabilising a sol by adding an electrolyte, heating, prolonged dialysis, mutual precipitation of oppositely charged sols, or electrophoresis. Hardy-Schulze rule: the coagulating ion is the one of charge opposite to the particle, and its power rises sharply with its valency — for a negative sol Al³⁺ > Ba²⁺ > Na⁺ (roughly in the ratio 700 : 60 : 1 by the Schulze-Hardy sixth-power trend, z⁻⁶).
• Flocculation value: minimum millimoles of electrolyte per litre needed to coagulate a sol; coagulating power = 1/flocculation value.
• DLVO theory: stability is the sum of van der Waals attraction and electrical double-layer repulsion; the energy barrier at intermediate separation holds the sol apart, and adding electrolyte compresses the double layer and lowers the barrier.
• Protective colloids: a lyophilic colloid added to a lyophobic sol adsorbs on it and protects it (gelatin on gold sol). Gold number = minimum milligrams of protective colloid that just prevents coagulation of 10 mL standard gold sol by 1 mL of 10% NaCl — the smaller the gold number, the better the protection (gelatin 0.005-0.01, albumin 0.1, gum arabic 0.15-0.25, starch 20-25).

Emulsions, micelles and gels

• Emulsion: a liquid dispersed in an immiscible liquid. O/W (milk, vanishing cream, paint) and W/O (butter, cod-liver oil, cold cream). Identified by the dilution test, dye test, conductivity test.
• Emulsifiers lower interfacial tension and form an interfacial film: soaps and detergents, proteins, gums, lecithin (egg yolk), and solid powders (lampblack, bentonite). Bancroft's rule: the phase in which the emulsifier is more soluble becomes the continuous phase. HLB scale: HLB 3-6 gives W/O, 8-18 gives O/W.
• Demulsification: heating, centrifuging, freezing, adding an electrolyte, or an opposing emulsifier — used in breaking crude-oil emulsions at refineries.
• Micelles: above the critical micelle concentration (CMC) and above the Krafft temperature, surfactant ions aggregate with hydrocarbon tails inward and ionic heads outward (aggregation number ~50-100). Sodium stearate CMC ≈ 10⁻³ mol/L; SDS CMC ≈ 8.2 mmol/L at 25 °C. CMC is detected by a sharp break in conductivity, surface tension or turbidity versus concentration.
• Cleansing action: the tail dissolves grease, the charged head keeps the droplet in water; solubilisation of oils into micelle cores also carries drugs and dyes.
• Surfactant classes: anionic (SDS, soaps), cationic (CTAB, fabric softeners), non-ionic (Tween, Triton X-100), zwitterionic (lecithin, betaines).
• Gels: elastic (gelatin, agar — reversible, show imbibition and thixotropy) and non-elastic (silica gel — rigid, irreversible). Syneresis is the slow exudation of liquid from a set gel; curd and paneer whey are the everyday examples.

Indian standards, practice and regulation

• Water and wastewater: IS 10500 : 2012 (with amendments) sets drinking-water limits — turbidity 1 NTU acceptable, 5 NTU permissible — and coagulation with alum or ferric chloride is the standard way to meet it. CPCB and the State Pollution Control Boards enforce effluent limits under the Water (Prevention and Control of Pollution) Act, 1974 and the Environment (Protection) Act, 1986.
• Alum dosing in municipal plants typically 10-50 mg/L depending on raw-water turbidity, with jar tests to fix the dose; the Jal Jeevan Mission and CPHEEO manual guide design practice.
• Detergents and soaps are covered by BIS specifications (soaps, synthetic detergent powders and bars) and by the Legal Metrology (Packaged Commodities) Rules, 2011 for declarations; cosmetics and creams fall under the Drugs and Cosmetics Act, 1940 and the Cosmetics Rules, 2020, administered by CDSCO.
• Food colloids — milk, ice cream, emulsifiers and stabilisers (E-numbered additives) — come under FSSAI regulations; permitted emulsifier lists and limits are notified under the Food Safety and Standards Act, 2006. Specific IS numbers and permitted-additive limits are revised periodically, so verify the current edition before quoting one.
• Catalyst and adsorbent specifications (activated carbon iodine number, zeolite surface area) are set by BIS and by customer specifications in Indian refineries and fertiliser plants; GST on most such industrial chemicals is 18%.

Key points

• Adsorption - accumulation at a surface; always exothermic, with negative ΔS and ΔH.
• Physisorption vs chemisorption - weak/multilayer/reversible/falls with T versus strong/monolayer/specific/rises then falls with T.
• Freundlich - x/m = kp^1/n, empirical, no saturation; linear on a log-log plot.
• Langmuir - monolayer, θ = Kp/(1+Kp); explains the shift from first- to zero-order kinetics on a surface.
• BET - multilayer model giving specific surface area from N₂ at 77 K.
• Colloid range - 1 to 1000 nm; smaller is a true solution, larger a suspension.
• Lyophilic vs lyophobic - solvated, reversible, stable versus charge-stabilised, irreversible, easily coagulated.
• Tyndall effect - light scattering that distinguishes a colloid from a true solution.
• Brownian motion - random motion that prevents settling and gave an early value of N_A.
• Zeta potential - potential at the shear plane; |ζ| above roughly 30 mV indicates a stable dispersion.
• Hardy-Schulze rule - the opposite-charged ion coagulates, and power rises steeply with valency.
• DLVO theory - stability is van der Waals attraction plus double-layer repulsion.
• Peptisation - converting a fresh precipitate back into a sol with a trace of electrolyte.
• Dialysis - purifying a sol through a semipermeable membrane; basis of the artificial kidney.
• Gold number - mg of protective colloid protecting 10 mL gold sol; lower means better protection.
• CMC and Krafft temperature - the concentration and temperature above which micelles form.
• Emulsion types - O/W and W/O, set by the emulsifier's solubility (Bancroft) and HLB value.
• Gels - elastic and reversible (gelatin) or rigid and irreversible (silica gel); syneresis and thixotropy.

Formulas and examples

Example 1 — Freundlich constants from two data points. A charcoal sample adsorbs 1.20 g of a gas per gram at 0.40 bar and 1.80 g/g at 1.20 bar.

• 1/n = log(1.80/1.20)/log(1.20/0.40) = log 1.5/log 3 = 0.17609/0.47712 = 0.369.
• log k = log 1.20 − 0.369 × log 0.40 = 0.07918 − 0.369 × (−0.39794) = 0.07918 + 0.14684 = 0.22602 → k = 1.682.
• At 2.00 bar: x/m = 1.682 × 2^0.369 = 1.682 × 10^{0.369×0.30103} = 1.682 × 10^0.11108 = 1.682 × 1.2917 = 2.17 g/g.

Example 2 — Langmuir coverage and its limits. For CO on a metal at 300 K, K = 0.75 kPa⁻¹.

• At p = 0.20 kPa: Kp = 0.15, θ = 0.15/1.15 = 0.130.
• At p = 20 kPa: Kp = 15, θ = 15/16 = 0.938 — nearly saturated.
• Ratio of pressures is 100 but coverage rises only about 7-fold, which is exactly why a surface reaction that is first order at low pressure becomes zero order at high pressure.

Example 3 — BET specific surface area. 0.500 g of a silica catalyst adsorbs a monolayer V_m = 32.0 cm³ of N₂ at STP (0 °C, 1 atm; molar volume 22,400 cm³/mol).

• Moles in the monolayer = 32.0/22,400 = 1.4286 × 10⁻³ mol.
• Molecules = 1.4286 × 10⁻³ × 6.022 × 10²³ = 8.603 × 10²⁰.
• Area = 8.603 × 10²⁰ × 0.162 × 10⁻¹⁸ m² = 139.4 m².
• Specific surface area = 139.4/0.500 = 279 m²/g.

Example 4 — Hardy-Schulze comparison by flocculation value. An As₂S₃ (negative) sol is coagulated by NaCl at 52 mmol/L, BaCl₂ at 0.69 mmol/L and AlCl₃ at 0.093 mmol/L.

• Coagulating power ratio Na⁺ : Ba²⁺ : Al³⁺ = 1/52 : 1/0.69 : 1/0.093 = 0.01923 : 1.4493 : 10.753.
• Normalising to Na⁺ = 1 gives 1 : 75 : 559 — power rises steeply with cation valency, as the rule predicts. The coagulating ion is the cation because the sol is negative.

Example 5 — Brownian displacement and settling. A 100 nm radius sol particle (density 2,000 kg/m³) in water at 298 K, η = 0.890 × 10⁻³ Pa·s.

• D = kT/(6πηr) = (1.381 × 10⁻²³ × 298)/(6π × 0.890 × 10⁻³ × 1.00 × 10⁻⁷) = 4.1154 × 10⁻²¹/(1.6776 × 10⁻⁹) = 2.45 × 10⁻¹² m²/s.
• Mean square displacement in 1 s: ⟨x²⟩ = 2Dt = 4.91 × 10⁻¹² m² → RMS displacement = 2.2 µm, about 11 particle diameters.
• Stokes settling: v = 2 × (1.0 × 10⁻⁷)² × (2000 − 997) × 9.81/(9 × 0.890 × 10⁻³) = 2 × 1 × 10⁻¹⁴ × 9839/(8.01 × 10⁻³) = 1.968 × 10⁻¹⁰/8.01 × 10⁻³ = 2.46 × 10⁻⁸ m/s ≈ 2.1 mm/day — far slower than Brownian mixing, so the sol does not settle.

Where it is used

• Water and effluent treatment: alum/ferric chloride coagulation and polyelectrolyte flocculation in municipal plants and ETPs; activated carbon for colour, taste, pesticide and COD removal; jar tests and zeta-potential control are routine process-chemist work.
• Paints, inks and adhesives: Asian Paints, Berger, Pidilite formulate pigment dispersions — stability, zeta potential, rheology and dispersant selection are core R&D roles.
• FMCG, detergents and cosmetics: HUL, Godrej, Marico, Dabur — emulsions, creams, shampoos and toothpaste are colloidal; CMC, HLB, emulsifier choice and accelerated stability testing are daily tasks.
• Petroleum and petrochemicals: IOCL, BPCL, Reliance — demulsifiers for crude-oil water separation, zeolite and alumina catalysts specified by BET area, drilling-mud thixotropy.
• Pharmaceuticals and food: Cipla, Sun Pharma, Amul, Nestlé India — nanosuspensions, liposomes, controlled-release coatings, milk and ice-cream stabilisers under FSSAI rules.
• Mineral processing and metallurgy: froth flotation of sulphide ores at Hindustan Zinc and HCL; electrostatic precipitators (Cottrell) for fly-ash in thermal power stations; delta formation and artificial rain are the classic natural examples.

Interview questions

• Q: Why is adsorption always exothermic? A: The adsorbate loses translational freedom, so ΔS is negative; for ΔG = ΔH − TΔS to be negative, ΔH must be negative.
• Q: How do you distinguish physisorption from chemisorption experimentally? A: By the enthalpy (20-40 versus 80-400 kJ/mol), reversibility, specificity, multilayer versus monolayer coverage, and the temperature dependence — chemisorption first increases with temperature because it needs activation energy.
• Q: State the assumptions of the Langmuir isotherm and where it fails. A: Monolayer coverage, identical non-interacting sites, dynamic equilibrium; it fails for multilayer physisorption, heterogeneous surfaces and strongly interacting adsorbates — BET is used instead.
• Q: Why do SO₂ and NH₃ adsorb on charcoal more than H₂ and N₂? A: They have higher critical temperatures and are more easily liquefiable, so their van der Waals interaction with the surface is stronger.
• Q: What is the Tyndall effect and what does it prove? A: Visible scattering of a light beam by colloidal particles comparable to the wavelength; its presence proves a heterogeneous colloidal dispersion rather than a true solution.
• Q: What is zeta potential and what value indicates a stable dispersion? A: The electrostatic potential at the shear plane of the electrical double layer; roughly |ζ| > 30 mV gives enough repulsion to keep a dispersion stable, while near zero it flocculates.
• Q: Why does alum clarify muddy water? A: Al³⁺ (and Al(OH)₃ flocs) neutralise the negative charge on clay colloids, following the Hardy-Schulze rule, so the particles aggregate and settle.
• Q: What is the CMC and how is it measured? A: The surfactant concentration above which micelles form; it shows as a sharp break in the plot of surface tension, conductivity or turbidity against concentration.
• Q: Why do soaps clean grease? A: Micelles trap oil in their hydrocarbon core while the ionic heads remain hydrated, so the grease is emulsified and washed away.
• Q: What does the gold number measure, and does a low value mean weak protection? A: It is the mg of protective colloid that just prevents coagulation of 10 mL standard gold sol by 1 mL of 10% NaCl — a low gold number means strong protection.
• Q: Why does prolonged dialysis destabilise a sol? A: It removes the electrolyte whose adsorbed ions provide the stabilising surface charge, so the double-layer repulsion collapses.
• Q: Explain emulsion type selection in a formulation. A: Bancroft's rule and the HLB value decide it — an emulsifier more soluble in water (HLB 8-18) gives O/W; one more soluble in oil (HLB 3-6) gives W/O.

Common mistakes

• Confusing adsorption with absorption, and treating adsorption as endothermic because it increases with "surface activity".
• Saying chemisorption decreases with temperature throughout — it rises first because of its activation energy, then falls.
• Applying the Freundlich isotherm at high pressure, where it wrongly predicts unlimited adsorption instead of saturation.
• Mixing up the sign convention in the Gibbs isotherm, or forgetting that salts raise γ and are negatively adsorbed.
• Reversing the gold number logic, or quoting the standard gold sol volume wrongly (10 mL sol, 1 mL of 10% NaCl).
• Choosing the wrong coagulating ion — for a negative sol it is the cation; and ignoring that valency, not concentration alone, drives the power.
• Unit slips: γ in dyn/cm versus mN/m (they are numerically equal), surface area per gram versus per sample, and using cm³ instead of m³ in Stokes and Einstein relations.

Related topics

Chemical kinetics and catalysis · Chemical thermodynamics · Electrochemistry · Solutions and colligative properties · Chemical and ionic equilibrium · Atomic structure and chemical bonding