OIA1008 W6 SURFACE INTERFACIAL_II

Solid-gas interface: physisorption & chemisorption

Adsorption isotherm: pressure of gas - amount gas adsorb at constant T

Freundlich Adsorption Isotherm - empirical (1909)

Limitations:

Applicable w/in certain pressure limits (high P show deviation)

Constant k & n change w T

No theoretical basis (empirical isotherm)

Langmuir Theory (1916)

Molecules/ atom of gas adsorbed on active sites of solid -> form one molecule thick layer (monolayer)

Assumptions:

Constant number of adsorption sites

Adsorption sites: all equal in size & shape

Each site occupy one gas molecule only w constant heat energy released

Dynamic equilibrium exist btwn adsorbed & free gas molecules

Monolayer adsorption

*Ka (adsorption - fw rxn), Kb (desorption - bw rxn)

Langmuir Adsorption Equation

Limitations:

Only monolayer

Only low P & high T

Not suitable for high P & low T: thermal energy gas decrease & multilayer adsorption occurs

BET (1938)

Type I Adsorption Isotherm

Monolayer adsorption - Langmuir adsorption isotherm

No interaction between adsorbed molecules on adsorbent surface

BET eqn: P/Po <<1 & C >>1

E.g., N2 on charcoal at -195°C

Type II Adsorption Isotherm

Large deviation of Langmuir model -> BET adsroption - describe multilayer adsorption

Intermediate flat region: monolayer

BET eqn: value C >>1

E.g., N2 gas on silica gel at -195°C

Type III Adsorption Isotherm

Large deviation of Langmuir model

Weak adsorbent-adsorbate interaction

BET eqn: C <<1

Explain multilayer formation

No flat portion - no monolayer

E.g., Br2/ I2 at 79°C on silica gel

Type IV Adsorption Isotherm

Lower P region similar to Type II - form monolayer follow by multilayer (+ capillary)

Saturation level reach P below saturation vapour pressure (P0) - possibility of gas condensed in tiny capillary pores of adsorbent below P0

E.g., benzene on Fe2O3 at 50°C & on silica gel at 50°C

Type V Adsorption Isotherm

Similar to Type IV

Weak adsorption

Both IV & V show capillary condensation of gas (due to saturation)

E.g., water (vpr) at 100°C on charcoal

Limitations:

Experiment agreement w BET only exist on narrow range of P/P0 (0.05-0.3)

Type II not often agree in range of 0.3-0.5 P/P0

**Freundlich, Langmuir and BET theories -> neglect interaction between adsorbed molecules (horizontal or lateral interaction - form surface associated molecules)

Applications of adsorption:

Purification of solution using 1% active charcoal

Adsorption of water using alumina, silica gel, CaCl2, P2O5

Medicinal: kaolin & charcoal adsorp gas, toxins & bacteria