Wetting

Key Concepts

Wetting is the displacement from a surface¹ of one fluid² by another.

All particles are attracted to each other by intermolecular forces.

When these intermolecular forces are acting between identical paricles they are referred to as cohesive forces.
The resulting property is known as cohesion.

When these intermolecular forces are acting between different particles they are referred to as adhesive forces.
The resulting property is known as adhesion.

Wettability, how well one fluid displaces another from a surface, is determined by the relative strengths of cohesion and adhesion:

Relative Strenth:		Wettability
Adhesion	Cohesion	Wettability
strong	weak	good wetting (high wettability)
weak	strong	poor wetting (low wettability)

If liquid water is used to displace air from the surface of solid, then:
(i) a hydrophilic surface is one that is wet by water
(ii) a hydrophobic surface is one that is not wet by water.

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Defining Wetting

Wetting is the ability of one fluid to displace another from a surface.
The surface most commonly used will be a solid such as glass, wax, paper, plastic etc.
The fluid to be displaced will most commonly be air.
The fluid used to displace the air will probably be a liquid such as water, various hydrocarbons, oils and alcohols.

Consider a solid such as a perfectly flat, clean, dry, glass slide in contact with air as shown below:

air	A fluid phase: gaseous air
	Surface: boundary between a solid and a gas
	Surface: boundary between a solid and a gas
clean glass slide	A solid phase: solid glass

One phase is a mixture of gases (air), this is a fluid phase. This phase is in contact with a different phase, the solid glass slide.

When a droplet of liquid such as water, which is a different fluid phase, is placed on the glass slide, some of the gaseous phase (air) will be displaced.
If all the gas phase is displaced by the liquid phase, the liquid spreads out on top of the solid phase as shown below:

air	A fluid phase: gaseous air
	A surface: boundary between a gas and a liquid
	A surface: boundary between a gas and a liquid
liquid water	A different fluid phase: liquid water
	Another surface: boundary between a liquid and a solid
	Another surface: boundary between a liquid and a solid
clean glass	A solid phase: solid glass

This is referred to as complete wetting or perfect wetting. The liquid completely, or perfectly, wets the solid.
If the liquid used is water, the solid surface is referred to as being hydrophilic (water loving).

Now consider placing a droplet of water onto a perfectly flat, clean, dry, layer of paraffin wax.
The water droplet retains its spherical shape and does not spread out on the surface of the paraffin wax as shown below:

Very little, almost none, of the gas phase (air) has been displaced by the liquid (water) phase.
The liquid does not wet the solid, so this referred to as non-wetting.
If the liquid used is water, the solid is said to be hydrophobic (water fearing).

Now consider placing a droplet of water onto a perfectly flat, clean, dry, piece of polymethylmethacrylate.
The droplet of water spreads out a little bit, but if you view it side-on, you will see that it looks almost like half a sphere.

The liquid (water) has displaced some of the gas (air) from the surface of the polymethylmethacrylate.
Chemists refer to this as partial wetting. The liquid phase has partially wet the solid phase. The solid phase has been partially wet by the liquid phase.

The more wettable a surface is, the more the liquid spreads out and the less spherical the droplet of liquid will look.

The less wettable a surface is, the less the liquid spreads out and the more spherical the droplet of liquid looks.

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Cohesion

Every particle making up a liquid is attracted to the other particles that surround it in the liquid.
The forces of attraction between molecules making up a liquid are known as intermolecular forces.
On the right, molecules in the liquid are represented as O and the attractive intermolecular forces between the particles in the liquid are shown as red arrows.

Because all the molecules in the liquid are the same, these attractive intermolecular forces of attraction are referred to as cohesive forces.
The particles in the liquid are said to cohere (literally "stick together").
The property of the liquid is known as cohesion.

Every molecule in a liquid is attracted to other molecules in a liquid by the weakest intermolecular forces, known as dispersion forces or London forces.
If a molecule is non-polar, only these weak London forces or dispersion forces attract one molecule to another in the liquid.
If a molecule is polar, then each molecule in the liquid will also experience another, stronger force of attraction.

	↓		↓		↓
→	O	→ ←	O	→ ←	O	←
	↓ ↑		↓ ↑		↓ ↑

→	O	→ ←	O	→ ←	O	←
	↓ ↑		↓ ↑		↓ ↑

→	O	→ ←	O	→ ←	O	←
	↑		↑		↑

These stronger intermolecular forces of attraction will be either:

hydrogen bonds
(for polar molecules in which H is covalently bonded to F, O or N in the molecule)

dipole-dipole interactions
(any other polar molecule)

The stronger the intermolecular forces of attraction acting between the particles in the liquid, the stronger the cohesive force holding the particles together.

Intermolecular Force	Relative Strength	Examples
Dispersion (London) Forces	weakest	hexane, benzene, paraffin wax
	↓
Dipole-dipole Interactions	↓	hydrogen bromide, chloroform (trichloromethane)
	↓
Hydrogen Bonds	strongest	water, ethanol

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Adhesion

Adhesion refers to the intermolecular forces attracting a molecule in the liquid phase to a molecular in the solid phase.

In the representation on the right:

molecules in the liquid are represented as O

attractive intermolecular forces between the particles in the liquid are shown as red arrows

Cohesive forces are therefore shown as red arrows

molecules in the solid are represented as O

attractive intermolecular forces between the particles in the liquid and the particles in the solid are shown as green arrows

Adhesive forces are therefore shown as green arrows

Liquid Phase
	↓		↓		↓
→	O	→ ←	O	→ ←	O	←
	↓ ↑		↓ ↑		↓ ↑	cohesion

→	O	→ ←	O	→ ←	O	←
	↑ ↓		↑ ↓		↑ ↓	adhesion

Solid Phase

The strength of the adhesive force (adhesion) between molecules in the liquid and molecules in the solid will therefore depend on the nature of the molecules making up the liquid and the solid.

The more similar the molecules in the liquid and the molecules in the solid are, the stronger the adhesive force is likely to be.
For example, water molecules are polar and the surface of silica glass is polar so the force of adhesion between water and silica is likely to be quite strong.
Water is polar but paraffin wax is non-polar, so the force of adhesion between water and wax is likely to be quite weak.
Hexane is non-polar and paraffin wax is non-polar, so the force of adhesion between hexane and wax is likely to be quite strong.

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Wettability

How wettable a surface is depends on the strength of the intermolecular forces of attraction between:

particles in the liquid (called cohesion)

particles in the liquid and particles in the solid (called adhesion)

If the adhesive forces of attraction are greater than the cohesive forces of attraction, wettability is said to be high.
A droplet of the liquid will spread out on the surface of the solid and will not look very spherical in shape.

If the cohesive forces of attraction are greater than the adhesive forces of attraction, wettability is said to be low.
A droplet of the liquid will not spread out very much, if at all, on the surface of the solid and it will look almost spherical in shape.

High Wettability

High wettability means that most of the gas phase is displaced by a liquid phase at the surface of a solid.
High wettability is observed when:

a liquid spreads out over the surface of a solid

a liquid droplet does not retain its spherical shape on the surface of the solid

High wettability results when the forces of adhesion are greater than the forces of cohesion.

The surface of silica glass (SiO₂) is polar because oxygen atoms at the surface of the glass have made only one covalent bond to a silicon atom.
These oxygen atoms will covalently bond to any available hydrogen atoms resulting in a polar OH group at the surface of the glass.
Oxygen atoms are more electronegative than hydrogen atoms, so the electrons in the covalent bond are more attracted to the oxygen atom than to the hydrogen atom.
The oxygen atom takes on a partial negative charge, δ-, and the hydrogen atom takes on a partial positive charge, δ+.
The surface of silica glass is shown in the representation on the right.

H^δ+

O^δ-
|

-O

silica glass

Molecules of a liquid than can form hydrogen bonds with the polar OH groups at the surface of the silica glass will result in the liquid wetting the glass.

Water molecules, H₂O, are polar.
Oxygen is a more electronegative atom than hydrogen.
The electrons in the covalent bond between hydrogen and oxygen are more drawn towards the oxygen atom than they are towards the hydrogen atom.
Oxygen atoms in the water molecule take on a partial negative charge, δ-, and hydrogen atoms take on a partial positive charge, δ+.
The covalent bond between the oxygen atom and the hydrogen atom is therefore polar, ^δ+H-O^δ-.
The partial negative charge on the oxygen atom of a water molecule is attracted to the partial positive charge of a hydrogen atom on the surface of the silica glass so that a hydrogen bond can form between the water molecules in a droplet of water and the silica making up the surface of the glass.
These hydrogen bonds are the adhesive forces of attraction between liquid water and solid silica glass.
This adhesion is shown in the representation on the right.

liquid water

^δ+H
\

H^δ+
/

^δ+H
\

H^δ+
/

^δ+H
\

H^δ+
/

O^δ-

.
.
.
.

adhesion

H^δ+

O^δ-
|

-O

silica glass

The droplet of water spreads out over the surface of the silica glass because the adhesive forces of attraction between water molecules and silica glass is greater than the cohesive forces of attraction between one water molecule and its neighbouring water molecules.

Low Wettability

Low wettability means that very little of the gas phase is displaced by a liquid phase at the surface of a solid.
Low wettability is observed when:

a liquid does not spread out over the surface of a solid

a liquid droplet retains a spherical shape on the surface of the solid

Low wettability results when the forces of cohesion are greater than the forces of adhesion.

Paraffin wax is made up of non-polar hydrocarbon molecules.
The only intermolecular forces of attraction that can act between the hydrocarbon molecules in the paraffin wax solid and the molecules making up a liquid will be the weakest of all the intermolecular forces, dispersion forces (or London forces).
If the molecules in the liquid can form dipole-dipole interactions or hydrogen bonds with each other, then the cohesive forces of attraction between the molecules in the liquid will be stronger than the adhesive forces of attraction between the molecules in the liquid and the molecules in the solid, which results in poor wetting.

Water, for example is a polar molecule.
The cohesive forces of attraction between water molecules are the stronger intermolecular forces known as hydrogen bonds.
When a water droplet comes into contact with the non-polar paraffin wax surface, the adhesive forces between water molecules and hydrocarbon molecules are only the weak dispersion (London) forces.
Since the hydrogen bonds making up the cohesive forces between water molecules are stronger then the dispersion forces making up the adhesive forces between water and hydrocarbon, the water will not wet the paraffin wax surface.
The water will not spread out over the surface of the paraffin wax.
The water droplet will retain its spherical shape.

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1. The boundary between the two different phases is known as an interface, or as a surface.
The term surface is usually applied to the boundary between a solid phase and a gas phase, or, between a liquid phase and a gas phase.
The term interface is usually applied to all other boundaries between phases, for example, boundary between one liquid and different liquid is an interface, or the boundary between a liquid and a solid is an interface.

2. A fluid is a substance made up of particles than can move freely. Gases and liquids are both fluids. A solid is not fluid.