Colloids

=Colloids=

//Colloids// can be defined as two distinct phases stably dispersed within a continuous phases (dispersion medium).

Insight to colloidal systems
Consider mineral water containing ions dissolved in water. Ions dispersed in liquid is called a solution. This is not a colloid yet because ions alone do not constitute a single phase. However, image that the ions nucleated forming fine particles of nanometer diameters, or in some manner polymerized into chains. If the particles sink and phase segregate, they exhibit //lyophobic// behavior whereby the particle-particle surface interaction is more favorable than the particle-liquid surface interaction. If the fine particles remain suspended however, the particles exhibit a //lyophillic// (liquid-loving) behavior and remain colloidally stabilized. Colloidal suspensions (typically solid particulates dispersed in liquids) are called //sols//.

Some examples are as follows: Phase ||~ Dispersion Medium ||~ Name ||~ Examples || Table 1 - Examples of colloidal systems. Adapted from reference
 * ~ Disperse
 * = (s) ||= (g) ||= Aerosol ||= Smoke ||
 * = (l) ||= (g) ||= Aerosol ||= Mist, fog ||
 * = (s) ||= (l) ||= Suspension/Sol ||= Paint, printing ink ||
 * = (l) ||= (l) ||= Emulsion ||= milk, mayonnaise ||
 * = (g) ||= (l) ||= Foam ||= Fire extinguisher foam ||
 * = (s) ||= (s) ||= Solid dispersion ||= Ruby glass, alloys ||
 * = (l) ||= (s) ||= Solid Emulsion ||= ice cream ||
 * = (g) ||= (s) ||= Solid foam ||= insulating foam ||

Colloidal Stabilization
Attractive surface potentials are dominant for particles below 1 micron. The competing thermal energy associated with kT is approximately 1 order of magnitude less than VDW potential energy. Therefore, attractive particles will tend to stick together and thus energy must be invested or a repulsive system must be used in order to keep particles separated. Particle separation may include raising temperature, but is typically controlled by modifying surfaces through the following mechanisms :
 * Electrostatic stabilization: charging particle surfaces by (1) preferential adsorption of ions, (2) dissociation of surface groups, (3) isomorphic substitution, (4) adsorption of polyelectrolytes.
 * Steric stabilization: steric repulsion due absorption of uncharged polymers on particle surfaces.
 * Electrosteric stabilization: adsorption of charge polymers to the surface

Electrical Double Layer
Electric double layers apply to colloidal suspensions where the particle surfaces are charged. A layer of equal and opposite charge is produced by the solution via counterions. The most immediate layer of counterions adsorbed on particle surfaces is called the //Stern layer//. In absence of thermal motion, the absorbed charges would theoretically neutralize the charged particles created dense counterion layers. However, due to thermal motion a relatively thick layer of counterions known as the //diffuse layer// exists above the Stern layer. There counterions spread out a distance away from the surface as function of the electric field fall off with distance. The charge of the diffuse layer is sometimes counterbalanced with a defined concentration of co-ions. Repulsion occurs due to disruption in effective particle neutrality when double layers overlap. Therefore the double layer thickness, at times called the Debye screening length (1/K), dominates the repulsive potential of electrostatically stabilized colloids. 1/K is effected by factors such as particle zeta potential and salt concentration.

Polymer Adsorption
Polymers ideally collapse into solid balls due to VDW attractive forces of the segments. However, surface attraction competes against a high thermodynamic configurational entropy inherent in long polymer chains. Therefore, free floating chains in solution result open, random coiled chains, whereby the diameter is approximately by the root mean square end-to-end distance

math \scriptstyle \sqrt{\langle r^2\rangle} = l\sqrt{N} math

where l is the monomer length and N is the number of mers. Polymer coils expand better with increased //goodness// of solvent. However, solvents must be selected such that weakness in sufficient to facilitate adsorption, and balance in solvent quality is required. If the total segmental attractive potential is larger than the local environmental energy (kT), more polymer segments will absorb to the particle surface. This results in a flattened configuration and ultimately reduces the amount of polymer adsorbed. Brush-like structures can be produced using co-polymers wherein a small segment of the chain adheres to the particles surface while there remaining block repels segment-segment interaction. Examples include PVP/PS or PEO/PS in organic solvents. Similar adsorption behavior is observed in polyelectrolytes which preferably absorb to charged surfaces.

Repulsion between sterically stabilized particles can be described by:
 * 1) Mixing effect: The build up of osmostic pressure between two emerging particles due to solvent exclusion.
 * 2) Elastic effect: An elastic effect through a volume restriction effect and subsequent exclusion of configurational entropy caused by interpenetrating chains.