Surface chemistry is the branch of chemistry that studies phenomena occurring at surfaces and interfaces. It is a topic of NEET weightage of 1–2 questions per year and encompasses four major areas: adsorption, catalysis, colloids, and emulsions.

Adsorption and Its Types

Adsorption is the process by which molecules of a gas or dissolved substance (adsorbate) accumulate on the surface of a solid or liquid (adsorbent), unlike absorption where the substance distributes throughout the bulk. When both processes occur simultaneously, the combined process is termed sorption. Adsorption is always a surface phenomenon, driven by the tendency to minimize surface energy.

Adsorption is classified into two types based on the nature of forces involved. Physisorption (physical adsorption) involves weak van der Waals forces and has a low enthalpy of adsorption (20–40 kJ/mol). It is non-specific (any gas adsorbs on any solid to some extent), reversible, forms multilayers, requires negligible activation energy, and decreases monotonically with increasing temperature. Chemisorption involves the formation of chemical bonds between adsorbate and adsorbent surface with a high enthalpy (80–240 kJ/mol). It is highly specific, irreversible, forms only a monolayer (surface sites are finite and specific), requires appreciable activation energy, and shows a characteristic maximum with temperature — increasing at first (as temperature helps overcome Ea) and then decreasing at very high temperatures (as desorption dominates).

The Freundlich adsorption isotherm describes the relationship between amount adsorbed per gram of adsorbent (x/m) and the pressure (P) of the gas at constant temperature: x/m = kP^(1/n), where k and n are empirical constants and 1/n lies between 0 and 1. The logarithmic form, log(x/m) = log k + (1/n) log P, is a straight line with slope 1/n and y-intercept log k when plotted as log(x/m) versus log P. The Freundlich isotherm is valid only at intermediate pressures; it does not predict saturation. The Langmuir isotherm, derived theoretically, describes monolayer adsorption with saturation at high pressure: x/m = aP/(1 + bP).

Factors affecting adsorption include surface area (greater area = more adsorption), nature of adsorbent (activated charcoal with ~1000–3000 $m^{2}$/g is highly effective), nature of adsorbate (gases with higher critical temperature are more easily adsorbed due to stronger intermolecular forces), temperature, and pressure.

Catalysis

Catalysts increase the rate of a reaction by providing an alternative reaction pathway with lower activation energy without being consumed. Homogeneous catalysts are in the same phase as the reactants (e.g., $H^{+}$ in acid-catalyzed ester hydrolysis). Heterogeneous catalysts are in a different phase (typically solid catalyst with gaseous reactants). Important industrial examples: Haber process ($N_{2}$ + $3H_{2}$ → $2NH_{3}$, Fe catalyst with Mo promoter) and Contact process ($2SO_{2}$ + $O_{2}$ → $2SO_{3}$, $V_{2}O_{5}$ catalyst). Catalytic activity refers to the ability to increase rate; selectivity refers to directing the reaction toward a specific product. The same reactants CO + $H_{2}$ give $CH_{3}OH$ with ZnO-$Cr_{2}O_{3}$, HCHO with Cu, and $CH_{4}$ with Ni — illustrating selectivity.

Enzyme catalysis follows the lock-and-key model (substrate binds specifically at the active site). Michaelis-Menten kinetics describes the rate: v = Vmax[S]/(Km + [S]), where Km (Michaelis constant) is the substrate concentration at which v = Vmax/2. Lower Km indicates higher enzyme-substrate affinity.

Colloids

Colloidal dispersions have particle sizes of 1–1000 nm, intermediate between true solutions (<1 nm) and suspensions (>1000 nm). They exhibit four characteristic properties: (1) Tyndall effect — scattering of light making the beam path visible; (2) Brownian motion — continuous zig-zag movement due to unequal molecular bombardment, providing kinetic stability; (3) Electrophoresis — migration of charged particles under an electric field; and (4) Coagulation — settling upon charge neutralization.

Lyophilic colloids (starch, gelatin, proteins) are solvent-loving, stable, reversible, and self-stabilizing. Lyophobic colloids (gold sol, $As_{2}S_{3}$ sol, Fe(OH){3} sol) are solvent-hating, unstable, irreversible, and stabilized only by surface charge. The Hardy-Schulze rule governs coagulation: the greater the valency of the ion bearing a charge opposite to the colloid, the greater its coagulating power (empirically ∝ z^6). For negative $As_{2}S_{3}$ sol: $Al^{3+}$ > $Ba^{2+}$ > $Na^{+}$. For positive Fe(OH){3} sol: [Fe(CN)_{6}]^{4-} > $SO_{4}^{2-}$ > $Cl^{-}$.

The gold number quantifies protective colloid effectiveness: it is the mass in mg of a protective colloid that just prevents coagulation of 10 mL of standard gold sol by 1 mL of 10% NaCl. Lower gold number = better protective colloid. Gelatin (gold number 0.005) is the most effective; starch (25) is the least among common examples.

Colloid preparation methods include chemical reduction (gold sol), hydrolysis (Fe(OH)_{3} sol from $FeCl_{3}$), Bredig's arc (metal electrodes in water), and peptization (electrolyte addition to fresh precipitate). Purification methods include dialysis, electrodialysis, and ultrafiltration.

Emulsions

Emulsions are colloidal dispersions of two immiscible liquids. Oil-in-water (O/W) emulsions (milk, vanishing cream) have oil droplets dispersed in water and are dilutable with water. Water-in-oil (W/O) emulsions (butter, cold cream) have water dispersed in oil and are dilutable with oil. Emulsifiers (soap, proteins, lecithin) stabilize emulsions by forming an interfacial film at the oil-water boundary.