Food+Colloids_csm

= FOOD COLLOIDS =

Terminology

 * Colloidal Dispersion (SOL)---Dispersion of 2 or more immiscible materials, containing structural entities in the size range 1nm - 1um.
 * The simplest type of colloidal system consists of a single dispersed phase of particles in a second continuos phase called dispersion medium.
 * Suspension---a system containing particles > 1um, prone to settle under gravity.
 * Emulsion---a dispersion or suspension of liquid droplets in a coontinuos phase.
 * Foam---coarse dispersion of gas bubbles in a liquid/solid continuos phase.
 * Food colloids---dispersions containing large particles.

Interactions Between Dispersed Droplets

 * Effect of the intermolecular forces on the stability of dispersed systems---forces between the colloidal particles.
 * Two main interacting forces affecting colloidal system : Van der walls attractive forces & Electrostatic repulsive forces.
 * Attractive forces---destabilise colloids.
 * Repulsive forces---stabilise colloids.

__Van Der Waals Attractive Forces__

 * Originates from dipole-dipole interactions and always attractive for droplets.
 * Attractive forces increase as droplets approach.

__Electrical Double Layer__

 * Oppositely charged ions (counterions) are preferentially attracted to the surface, ions of the same charge (coions) tend to repelled away.
 * Electrical double layer---combination of charged surface & unequal distribution of coions & counterions near the surface.
 * Formation of double layer---important for stability of colloids.
 * Double layer---sensitive to electrolytes, temperature.

__Electrostatic Forces & DLVO Theory__

 * DLVO theory---named after 4 scientists (Derjaguin, Landau, Verway, Overbeek).
 * When two charged surface approach, electrical double layer overlap, repulsive forces is induced which tends to oppose further approach.
 * DLVO theory---repulsive forces will stabilise emulsion, when the electrolyte concentration phase is less than a certain value.
 * DLVO theory---relates stability of emulsified droplets to 2 independent potentials that come into action when 2 droplets approach each other.
 * For a small distances, sum of the 2 energies is always -ve. (net attractive causes aggregration).
 * When sum of 2 energies is +ve, (repulsive force).

Surface and Interfacial Tension

 * Surface tension is the property of liquid in contact with air that makes it behave like it was covered with a thin membrane under tension.
 * The molecule inside the liquid interact equally with other molecule from all sides.
 * The molecule at surface is affected only by molecule below it.
 * Surface tension (N/m)---force acting over the surface of liquid per unit length of the surface perpendicular to the force.


 * Surface tension ( γ s )---defined as the amount of energy ( ΔE) required to increase the surface area between a liquid and a gas (e.g. air and water) by an amount ΔA.   Δ E = γ  s ΔA
 * Interfacial tension ( γ i )---defined as the amount of energy required to increase the interfacial area between two immiscible liquids (e.g. oil & water).