Difficulty: Intermediate | Prerequisites: Basic understanding of intermolecular forces, thermodynamics (enthalpy, entropy, Gibbs free energy)
Source: Surface Chemistry 1, General Chemistry (University of Minnesota Twin Cities)
Surface chemistry is the branch of chemistry concerned with what happens at the boundary between two phases, such as a solid and a gas or a liquid and another liquid. Adsorption is one of its central topics: it explains how molecules collect on surfaces rather than passing into the bulk, and it underpins everything from industrial catalysis to water purification. You will need a working grasp of intermolecular forces (van der Waals, chemical bonding) and basic thermodynamics (enthalpy, entropy, Gibbs free energy) to follow this material comfortably. If the words "exothermic" and "spontaneous" feel unfamiliar, revisit your thermodynamics notes first.
Adsorption is the accumulation of molecules on a surface, not into the bulk. It comes in two flavours: physisorption (weak van der Waals forces, reversible) and chemisorption (chemical bonds, often irreversible). The Freundlich isotherm gives the empirical relationship between how much gas adsorbs and the pressure at constant temperature.
Adsorption
The accumulation of molecular species at the surface of a solid or liquid, rather than in its bulk.
Think of it as molecules sticking to the outside of a material, like dust clinging to a window.
Adsorbate
The molecular species or substance that concentrates or accumulates at the surface.
In simple terms, this is the thing that sticks.
Adsorbent
The material on whose surface adsorption takes place. Common examples: silica gel, charcoal, colloids.
Think of it as the surface that does the collecting.
Absorption
The process in which a substance is uniformly distributed throughout the bulk of a solid, rather than sitting only on the surface.
In simple terms, the substance soaks all the way through, like water into a sponge.
Desorption
The process of removing an adsorbed substance from a surface. It is the reverse of adsorption.
Sorption
A general term used when adsorption and absorption occur simultaneously.
Physisorption (physical adsorption)
Adsorption in which the adsorbate is held to the surface by weak van der Waals forces. It is reversible and non-specific.
Think of it as a loose grip: the molecule sits on the surface but can be shaken off easily.
Chemisorption (chemical adsorption)
Adsorption in which the adsorbate is held to the surface by chemical bonds. It is often irreversible and highly specific.
Think of it as a strong grip: the molecule forms a proper bond with the surface and does not let go easily.
Adsorption isotherm
A graph showing the relationship between the amount of gas adsorbed (x/m) and pressure at constant temperature.
Freundlich adsorption isotherm
An empirical equation relating the quantity of gas adsorbed to pressure at constant temperature: x/m = k P^(1/n), where 0 < n ≤ 1.
Surface chemistry deals with phenomena that occur at the surface or interface between two phases.
The interface is represented by separating the bulk phases with a hyphen or slash. For example:
Solid and gas: solid-gas or solid/gas
Liquid and liquid: liquid-liquid
There is no interface between two gases, because gases are completely miscible.
Molecules on a surface experience unbalanced attractive forces, resulting in a net inward pull. This produces surface tension.
Molecules in the bulk phase experience balanced forces from all directions and are in a state of equilibrium.
Adsorption: the substance concentrates only at the surface and does not penetrate through to the bulk of the adsorbent.
Absorption: the substance is uniformly distributed throughout the bulk of the solid.
When both occur at the same time, the process is called sorption.
Why is adsorption always exothermic?
During adsorption, there is a decrease in residual forces at the surface, which lowers the surface energy. That energy is released as heat. Adsorption is therefore exothermic: ΔH is negative.
After adsorption, the randomness of the system decreases (the adsorbate is more ordered on the surface), so ΔS is also negative.
For adsorption to be spontaneous, ΔG must be negative. From ΔG = ΔH – TΔS, this works because ΔH is negative and has a larger magnitude than TΔS (at least initially).
As adsorption proceeds, ΔH decreases in magnitude while TΔS grows, until eventually ΔG = 0 and equilibrium is reached.
Feature | Physisorption | Chemisorption |
|---|---|---|
Forces involved | Van der Waals forces | Chemical bond formation |
Specificity | Non-specific (any gas on any surface) | Highly specific (requires chemical compatibility) |
Reversibility | Reversible | Often irreversible |
Enthalpy of adsorption | Low (weak forces) | High (strong chemical bonds) |
Surface area effect | Increases with surface area | Increases with surface area |
Temperature effect | Decreases as temperature rises | Increases initially, then decreases at high temperature |
Example | SO₂ on charcoal | H₂ on platinum |
Physisorption: as temperature increases, physisorption decreases. Higher kinetic energy allows adsorbate molecules to escape the surface more easily.
Chemisorption: first increases with temperature (activation energy needed to form bonds), then decreases at very high temperatures as equilibrium shifts toward desorption.
An adsorption isotherm is a plot of the amount of gas adsorbed (x/m) against pressure (P) at constant temperature.
x = mass of gas adsorbed
m = mass of adsorbent
Freundlich adsorption isotherm:
x/m = k P^(1/n), where 0 < n ≤ 1
k and n are constants that depend on the nature of the adsorbent and the gas.
Behaviour at different pressures:
At low pressure: x/m ∝ P¹ (linear relationship, n = 1)
At high pressure: x/m ∝ P⁰ (adsorption approaches saturation, independent of pressure)
At moderate pressure: x/m ∝ P^(1/n), where 0 < n ≤ 1
Logarithmic form (for plotting):
Taking the log of both sides: log(x/m) = log k + (1/n) log P
This has the form y = c + mx, so a plot of log(x/m) vs log P gives:
Slope = 1/n
Intercept = log k
Effect of temperature on isotherms:
As temperature increases, the extent of adsorption decreases (for physisorption). The isotherm curve at a lower temperature (e.g. 195 K) sits above the one at a higher temperature (e.g. 273 K).
Solids can also adsorb solutes from solutions. The Freundlich equation applies here too, with a difference: instead of pressure, concentration of the solution is used.
x/m = k C^(1/n)
log(x/m) = (1/n) log C + log k
Plotting log(x/m) vs log C gives slope = 1/n and intercept = log k, the same way as for gas-phase adsorption.
Freundlich isotherm (gas phase):
x/m = k P^(1/n) where 0 < n ≤ 1
Log form:
log(x/m) = log k + (1/n) log P
Freundlich isotherm (solution phase):
x/m = k C^(1/n)
log(x/m) = (1/n) log C + log k
Gibbs free energy of adsorption:
ΔG = ΔH – TΔS
For adsorption: ΔH < 0, ΔS < 0, and ΔG < 0 (spontaneous, at least initially).
Activated charcoal in gas masks and water purifiers works by adsorbing toxic gases and impurities on its large surface area.
Silica gel packets in packaging adsorb moisture from the air, keeping products dry.
Heterogeneous catalysis relies on chemisorption: reactant molecules adsorb onto the catalyst surface, react, and the products desorb. This is how catalytic converters in cars reduce harmful emissions.
Students often confuse adsorption with absorption. Adsorption is a surface phenomenon; absorption involves penetration into the bulk. They are different processes.
Students sometimes think chemisorption always increases with temperature. It does initially (because activation energy is needed), but at high temperatures it decreases again as equilibrium favours desorption.
The Freundlich isotherm is sometimes treated as a universal law. It is an empirical relationship that works well at moderate pressures but fails at very high or very low pressures.
Students sometimes assume that a higher enthalpy of adsorption means a "better" adsorbent. It depends on the application: a reversible process (physisorption, lower enthalpy) may be preferred when you need to regenerate the adsorbent.
⚠️ The distinction between adsorption and absorption is a perennial exam question. Know the comparison table cold.
⚠️ "Why is adsorption always exothermic?" appears frequently. The answer involves both ΔH (decrease in surface energy) and ΔS (decrease in randomness), tied together through the Gibbs equation.
⚠️ The Freundlich isotherm's log form (log(x/m) = log k + (1/n) log P) is commonly tested. You need to identify slope (1/n) and intercept (log k) from a graph.
⚠️ The physisorption vs chemisorption comparison table is a high-yield exam topic. Focus on forces, specificity, reversibility, and enthalpy.
True or False: Adsorption is an endothermic process. (False. Adsorption is always exothermic.)
Fill in the blank: The substance that accumulates on a surface is called the ________. (Adsorbate)
True or False: Physisorption is highly specific to particular gas-surface combinations. (False. Physisorption is non-specific.)
Fill in the blank: In the Freundlich equation x/m = k P^(1/n), the slope of the log-log plot is ________. (1/n)
True or False: Chemisorption always decreases with increasing temperature. (False. It first increases, then decreases.)
Q: Define adsorption. How does it differ from absorption?
A: Adsorption is the accumulation of molecular species at the surface of a solid or liquid. In adsorption, the substance concentrates only at the surface. In absorption, the substance is uniformly distributed throughout the bulk of the solid.
Q: Why is adsorption always exothermic? Explain using thermodynamic reasoning.
A: During adsorption, residual surface forces decrease, lowering the surface energy and releasing heat (ΔH < 0). The adsorbate becomes more ordered on the surface, so entropy also decreases (ΔS < 0). For adsorption to be spontaneous, ΔG = ΔH – TΔS must be negative, which holds because the magnitude of ΔH exceeds TΔS.
Q: Give two differences between physisorption and chemisorption.
A: Physisorption is caused by van der Waals forces, is reversible, and is non-specific. Chemisorption is caused by chemical bond formation, is often irreversible, and is highly specific to certain adsorbent-adsorbate pairs.
Q: Write the Freundlich adsorption isotherm equation. What do the constants k and n represent?
A: x/m = k P^(1/n). k and n are empirical constants that depend on the nature of the adsorbent and the gas, as well as the temperature. In the log-log plot, log k is the y-intercept and 1/n is the slope.
Q: What is the effect of temperature on physisorption and chemisorption?
A: Physisorption decreases with increasing temperature because the adsorbate gains kinetic energy and escapes the surface. Chemisorption first increases (activation energy is overcome) and then decreases at very high temperatures as the desorption rate rises.
This material connects directly to colloids (the next part of the surface chemistry unit), since colloidal particles have large surface areas and many of their properties, such as charge acquisition and stability, depend on adsorption of ions.
The thermodynamics here (ΔG, ΔH, ΔS reasoning) draws on your earlier work in chemical thermodynamics. If you found the Gibbs equation straightforward there, the same logic applies here.
Adsorption is also the foundation of heterogeneous catalysis, which you will encounter in chemical kinetics. The catalyst works by adsorbing reactants on its surface, lowering the activation energy.
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