Adsorption onto solids: physisorption, chemisorption, thin films and self-assembled monolayers

A clean high-energy surface grabs molecules to settle its energy

Intuition

A fresh, clean metal or glass surface is like a magnet in a drawer of paper clips. It quickly picks up whatever floats by, because sticking things to it lowers its energy. Even a single layer of picked-up molecules can change how the surface behaves.

High-energy surfaces adsorb molecules from air or solution to reduce their surface energy $\gamma_{SV}$. This can be chemical or physical. A single adsorbed monolayer can dramatically change wettability, conductivity, friction and reactivity. Thin organic films a few nanometres thick are used in sensors, detectors, displays and electronic circuit parts. Organic molecules can be built with the structure and function you want, and then laid down on a solid as self-assembled monolayers (SAMs). The lecture does not cover Langmuir-Blodgett films, the other route.

Self-assembled monolayers pack themselves into an ordered coat

A SAM is an ordered layer made when surface-active molecules stick to a surface. Each molecule has a head group that binds the surface, a chain, and a functional end-group such as SH or COOH. Common pairs from the lecture are:

• alkyl sulphides on gold;
• alkylchlorosilanes on hydroxylated surfaces (silicon oxide, quartz, mica);
• long-chain n-alkanoic acids on metal surfaces (aluminium oxide, copper oxide, silver).

Thiols on gold. Alkanethiols, di-n-alkyl disulfides and dialkyl sulfides are organosulfur compounds that form ordered SAMs on gold. Chemisorption on clean gold gives monolayers, probably of the gold thiolate species $RS^-Au^+$. The mechanism is probably oxidative addition of the S–S bond to the surface.

$RS\text{-}SR + Au^0_n \rightarrow RS^- Au^+ \cdot Au^0_n$

$R\text{-}S\text{-}H + Au^0_n \rightarrow R\text{-}S^- Au^+ \cdot Au^0_n + \tfrac{1}{2}H_2$

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Silanisation. The most common silane is octadecyltrichlorosilane, OTS, $\mathrm{Cl_3\text{-}Si\text{-}(CH_2)_{17}\text{-}CH_3}$, on silica. The trichlorosilane group reacts with silanols on the surface and grafts the hydrophobic chain on. It also reacts with other silane groups and polymerises, so it is rarely a simple monolayer. AFM of an OTS SAM on smooth Si/SiOₓ gives a layer thickness of 2.1 nm.

Physisorption is weak and chemisorption makes a bond

Compare

Physisorption

Physical adsorption by van der Waals forces. Heat released is typically about 20 kJ mol⁻¹.

Chemisorption

A chemical bond forms with the surface. The enthalpy is much higher, about 200 kJ mol⁻¹. Breaking bonds on chemisorption is why surfaces act as catalysts.

As the gas pressure rises, surface coverage grows through three stages: a single-site monolayer, then multilayer, then pore condensation. The next topic puts equations on the first two.

Depletion adsorption: what the surface took, the solution lost

You often cannot see the molecules on a surface directly. Instead, you watch the bulk. When you add a solid to a solution, some solute sticks to the solid and the bulk concentration falls. If the solute increases in concentration near the surface, there is a surface excess of solute compared with the bulk. Adsorption can happen from solution (concentration $[X]$) or from gas (pressure $p$).

$n_{ads} = \left([X]_{initial} - [X]_{eq}\right)V$

$[X]_{initial}$ is the concentration before the solid is added. $[X]_{eq}$ is the bulk concentration after adsorption has settled. $V$ is the solution volume.

Try it: worked example

[X] drops from 1.0 mol/L to 0.8 mol/L in 0.50 L of solution. How much has adsorbed? (Practice numbers, not from the lecture.)

Show answer and steps

Answer0.10 mol.

1. Write $n_{ads} = ([X]_{initial} - [X]_{eq})V$.
2. The drop in concentration is $1.0 - 0.8 = 0.2$ mol/L.
3. Multiply by the volume: $0.2\ \text{mol/L} \times 0.50\ \text{L} = 0.10\ \text{mol}$.

Exam detail

Learn the two reaction equations for thiols on gold, and which one gives off $\tfrac12 H_2$. Know that OTS is $\mathrm{Cl_3}$-Si-$(\mathrm{CH_2})_{17}$-$\mathrm{CH_3}$ and that it is not a simple monolayer.

Recall

Why does the lecture say chemisorption helps explain catalysis?

Chemisorption breaks bonds in the molecule as it binds to the surface. A molecule with weakened or broken bonds reacts more easily, so the surface helps a reaction along.