Surface and Interface Chemistry

Solid/liquid Interface

Valentim M. B. Nunes

Engineering Unit of IPT

2014

Wetting and dewetting phenomena are all around us. The formation of rain droplets sitting on a plant or hanging from a spiders web provide familiar examples for dewetting.

On the other hand, spreading of paint and adhesives on solid surfaces or the application of cosmetics onto the human skin rely on the wetting properties for this liquids.

In fact, you could not read these lines without the tear films which wet your eyes and which are stabilized by the closure of your eyelids.

The wetting of a surface usually involves the displacement of a gas (usually air) for a liquid at the solid surface. A surfactant has wetting power when it promotes this effect.

Wetting

Spreading Immersion Adhesion

Spreading

A liquid spreads in a solid surface increasing the SL and LG interfacial area and decreasing the SG interface.

LSLSS S 0

If S < 0 the liquid forms drops with a contact angle, defined by the equation of Young :

cosLSLS

Immersion

In the complete immersion of a solid into a liquid, the LG interfacial area remains unchanged.

If SG > SL, < 90º - the process is spontaneous.If SG < SL, > 90º - It is necessary to perform work in order to emerge the solid.

Adhesion

Dupré’s Equation:

SLLSaW Combining with the Young’s equation we obtain:

cos1 LaW

Water in Glass = 0º Wa = 2L = Wc

Water in Paraffin = 110º -Mercury in Glass = 140º -

The wetting of an hydrophobic solid surface by a aqueous solution is favored by the addition of surfactants: Wa increases and L decreases, then decreases.

Surfactants, particularly anionic, are used as wetting agents in many situations. For instance in the textile industry to obtain results in processes such as bleaching, inking, etc.

The fluctuation of a solid in a liquid surface depends on the contact angle θ which can be modified by factors such as adding oil to the surface, surfactants, etc.

Flotation

Treatment of ore by flotation

The ore is crushed, and mixed with water, to which is added an oil. This will adsorb to the surface of ore, which causes the increase in the contact angle of the SLG interface. Other particles remain wet by water.

The molecules of oil (oil collector) are amphyphylic, creating a hydrophobic surface. It is then added a sparkling agent and forced air into the bottom of the vessel. The particles adhering to the air bubbles comes to the surface, where they are collected rich in metal (see figure)

airair

air

Water

Particles of ore non wetted

Wet particles

For the method to be effective the contact angle should be between 50 to 75°. This method is also used for example in the treatment of effluents.

Detergents effectsDetergency consists in removing dirt from solid surfaces by means of surfactants, consisting in the main application of this kind of materials.

For centuries it was used the SOAP:

A good detergent should comply with the following characteristics :•Good wetting power to come into contact with the surface•Ability to remove dirt into the interior of the liquid•Ability to solubilize the dirtThe proper formulation is dependent on several factors:•Type of surface: glass, plastic, ceramic, natural fibers, synthetics, skin, hair, etc.•Dirt: polar/apolar characteristics, reactivity or chemical inertness•Wetting. In clothing, for example, it is necessary to reduce the surface tension to 40 mNm-1.

Removal of dirt: Designating by O the dirt (of Oil) we have :

WSO = OW + SW - SO

The action of the detergent consists to reduce OW and SW decreasing the work of adhesion between the Oil and solid surface.

If the dirt is liquid, the removal is a matter of contact angle. The addition of detergent lessens the angle of contact on the SOW interface. If 0 < < 90° removal is by mechanical means, and if 90° < < 180° the process involves solubilization.

The best surfactants are those who adsorb in the oil/water and solid/water interfaces, with better detergents effects. The adsorption on air/water interface with the consequent decrease of ϒ is not indication of good performance.

Adsorption from solutions

This is a topic with plenty of application in numerous processes, as in chromatography, clarification of solutions by the addition of activated carbon, etc.

The adsorption from solutions is more simple to study than the adsorption of gases on solids. Simply mix the solid with the solution until we observe the variation in the concentration of the solution. This way we obtain apparent adsorption isotherms, since there is a competition between adsorption and solute/solvent interactions.

ac

acmx

m

x max

1

/nkcm

x /1

The most commonly used equations are Langmuir and Freundlich equations, modified to:

Where (x/m) is the amount of adsorbed solute per mass of solid, c is the equilibrium concentration, a, k and n are constants; (x/m)max is the monolayer capacity.