Colloids and Suspensions

Applied Chemistry

Colloid

A colloid dispersion is a system in which particles of colloidal size of any nature are dispersed in a continuous phase of a different composition.

Types of Interactions between colloidal particles

Van der Waals
Electrical Interaction
Interactions dues to polymers/surfactants

Types of van der Waals Forces

Permanent Dipole-Permanent Dipole- Keesom
Permanent Dipole-Induced Dipole - Debye
Fluctuating Dipole-Induced Dipole - London Dispersion

Charges can originate from 3 different sources

Dissolution of soluble ions, mainly metal ions, such as found in clays

Adsorption of ions to surfaces, generally anions, e.g. Iodide in silver iodide

Adsorption of surfactants or polyelectrolyte

The osmotic effect

The osmotic effect increases pressure in the overlap zone due to high polymer concentration, pushing particles apart.

The entropic effect

Arises from the compression of polymers, reducing their configurational freedom. This is a loss in entropy so it energetically favors keeping particles apart.

Aerosol (Fog, Mist, BOP)

Liquid in gas

Aerosol (Smoke Dust in air)

Solid in Gas

Foam (Whipped cream, shaving cream)

Gas in Liquid

Foam (Marshmallow, Polystyrene)

Gas in solid

Gel ( Cheese)

Liquid in Solid

Sol (Blood, glue)

Solid in Liquid

Emulsion (O/W , W/O, W/O/W, O/W/O)

Liquid in Liquid

Volume Ratio of Liquid

The phase present in greater volume tend to become the continuous phase.

Emulsifying Agent

The Hydrophilic-Lipophilic Balance (HLB) of the surfactant determines whether O/W or W/O forms.

Concentration

Affects droplet size and viscosity too much emulsifier can cause phase inversion.

Temperature

Alters interfacial tension and emulsifier behavior can trigger inversion.

Solubility

The liquid that dissolves the emulsifier better is more likely to be the continuous phase.

Solid Stabilization

Solid particles at the interface can dictate emulsion structure based on wetting behavior.

Wetting Behavior

The liquid that better wets the emulsifying surface tends to becomes the continuous phase.

Unstable System

An emulsion is an unstable system if there is a natural tendency for a system to separate and reduce its interfacial area and, hence, its interfacial energy.

Interfacial Energy

•The stability of emulsion depends on the  interfacial energy and the interfacial tension.

•There exist tension and stronger attractive at the interface between immiscible liquids.

•The formation of emulsions decrease the interfacial energy and tension.

Emulsion Breakdown

Creaming / Sedimentation – Phase separation by density.

Flocculation / Coalescence – Droplets cluster or merge.

Ostwald Ripening / Phase Inversion – Droplet size changes or role reversal