SURFACE SITES

An oxide in water grows hydroxyls

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Hydroxyl groups

Water reacts with the oxide surface and leaves –OH groups on it

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Amphoteric

Each site can give up a proton or take one

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Three states

Negative SiO⁻, neutral SiOH, or positive SiOH₂⁺

The surface charge is set by how many sites are in each state.

SITE BINDING

Two reactions set the charge

SiOH ⇌ SiO⁻ + H⁺

The site-binding model describes each site with two equilibria. SiOH can lose a proton to become SiO⁻, or gain one to become SiOH₂⁺. Which way it leans depends on the proton concentration next to the surface.

More protons push sites positive; fewer push them negative.

POINT OF ZERO CHARGE

Where the surface is neutral

Below the pzc Acidic solution: net positive surface

At the pzc Positive and negative sites balance: net zero charge

Above the pzc Basic solution: net negative surface

Typical values SiO₂ near pH 2–3, Al₂O₃ near pH 8–9

In neutral water, silica is negative and alumina is positive.

SURFACE VS BULK

The surface has its own pH

[H⁺]surface = [H⁺]bulk · e^−qψ₀/kT

A charged surface sets up a potential ψ₀, and protons feel it through a Boltzmann factor. A negative ψ₀ crowds protons near the surface, and a positive one pushes them away. The pH at the surface can differ from the bulk by a unit or more.

The sites respond to the surface pH, which in turn feeds back on ψ₀.

SITE DENSITY

How many sites?

≈ 5 sites per nm²

A fully hydroxylated silica surface holds roughly 5 sites per square nanometer, or 5×10¹⁴ per cm². Only a fraction are charged at any pH. Change the pH and that fraction changes, and with it the surface charge.

A pH change becomes a charge change, and a charge change moves Vth.

UNIT 9 STUDY COMPLETE

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Ready for the Fab Challenge?

You've covered how an oxide in water picks up charge, and why that charge depends on pH.