🫧 Surface Chemistry & Colloids

Surface chemistry examines phenomena occurring at phase boundaries. Essential competitive exam concepts include Freundlich & Langmuir adsorption isotherm algebra, micellization thermodynamics (CMC & Kraft temperature), Hardy-Schulze coagulation powers, and protective colloid Gold numbers.

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1. 🧲 Physisorption vs. Chemisorption

Characteristic	Physical Adsorption (Physisorption)	Chemical Adsorption (Chemisorption)
Forces Involved	Weak Van der Waals forces	Strong chemical (covalent / ionic) bonds
Specificity	Non-specific (all gases adsorb on all solids)	Highly specific (occurs only if bond can form)
Reversibility	Reversible	Irreversible
Enthalpy of Adsorption ($DeltaH$)	Low ($20-40\text{ kJ/mol}$)	High ($80-240\text{ kJ/mol}$)
Activation Energy	Almost zero	High (often requires activation energy)
Temperature Effect	Decreases continuously with increasing $T$	First increases, reaches maximum, then decreases
Layer Structure	Multi-molecular layers under high pressure	Strictly uni-molecular monolayer

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2. 📈 Adsorption Isotherms

2.1 Freundlich Adsorption Isotherm

$$\frac{x}{m}=k{P}^{1/n}(n>1,\text{ so }0\le 1/n\le 1)$$$$\log \left(\frac{x}{m}\right)=\log k+\frac{1}{n}\log P$$

• Slope of $\log (x/m)$ vs $\log P$: $\frac{1}{n}$ (lies strictly between $0$ and $1$).
• $y$-intercept: $\log k$.
• At Low Pressure: $\frac{1}{n}=1⟹\frac{x}{m}\propto P$.
• At High Pressure: $\frac{1}{n}=0⟹\frac{x}{m}=\text{const}$ (saturation plateau).

2.2 Langmuir Adsorption Isotherm

$$\frac{x}{m}=\frac{aP}{1+bP}⟹\frac{P}{x/m}=\frac{1}{a}+\frac{b}{a}P$$

• Plot of $\frac{P}{x/m}$ vs $P$ is linear with slope $b/a$ and intercept $1/a$.

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3. 🧪 Colloidal State & Micelle Formation

Colloidal systems have dispersed phase particle dimensions between $1\text{ nm}$ and $1000\text{ nm}$ ($10-10000\text{AA}$).

3.1 Classification of Colloids

1. Lyophilic (Solvent-attracting): Reversible, highly hydrated, stable, cannot be easily coagulated (e.g., Starch, Gelatin, Gum).
2. Lyophobic (Solvent-repelling): Irreversible, unstable, require stabilizers, easily coagulated by electrolytes (e.g., $Fe(OH{)}_3$ sol, $A{s}_2{S}_3$ sol, Gold sol).

3.2 Associated Colloids (Micelles)

Surfactants (e.g., Sodium stearate $C_{17}{H}_{35}CO{O}^{-}N{a}^{+}$) behave as normal electrolytes at low concentrations, but aggregate into colloidal clusters (Micelles) above:

1. Critical Micelle Concentration (CMC): Minimum concentration required for micellization (for soaps, $\text{CMC}\approx {10}^{-4}-{10}^{-3}\text{ M}$).
2. Kraft Temperature ($T_k$): Minimum temperature above which micelle formation takes place.

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4. ⚡ Colloidal Properties & Coagulation Rules

4.1 Charge & Zeta Potential

• Positively Charged Sols: Hydrated metal oxides ($F{e}_2{O}_3\cdot x{H}_{2}O,A{l}_2{O}_3\cdot x{H}_{2}O$), Basic dyes (Methylene blue), $Ti{O}_2$.
• Negatively Charged Sols: Metals ($Au,Ag,Pt$), Metallic sulfides ($A{s}_2{S}_3,CdS$), Acidic dyes (Congo red), Starch, Clay.

4.2 Hardy-Schulze Law of Coagulation

$$\text{Coagulating Power}\propto \mathbf{(}\text{Valency of Flocculating Ion}{\mathbf{)}}^{\mathbf{4}}$$

• For negatively charged sol ($A{s}_2{S}_3$): Coagulation power of cations:$$A{l}^{3+}>M{g}^{2+}>N{a}^{+}$$
• For positively charged sol ($Fe(OH{)}_3$): Coagulation power of anions:$$[Fe(CN{)}_6{]}^{4-}>P{O}_4^{3-}>S{O}_4^{2-}>C{l}^{-}$$

4.3 Protective Colloids & Gold Number (Zsigmondy)

$$\text{Protective Power}\propto \frac{\mathbf{1}}{\text{Gold Number}}$$

• Gold Number: The minimum mass (in milligrams) of a protective lyophilic colloid required to prevent the coagulation of $10\text{ mL}$ of a standard red gold sol upon adding $1\text{ mL}$ of $10\mathrm{\%}NaCl$ solution.
• Lower Gold Number $⟹$ Higher protective power (Gelatin: $0.005-0.01$, Casein: $0.01$, Gum Arabic: $0.15-0.25$, Starch: $20-25$).