Colloid

A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others extend the definition to include substances like aerosols and gels. The term colloidal suspension refers unambiguously to the overall mixture (although a narrower sense of the word '' suspension'' is distinguished from colloids by larger particle size). A colloid has a dispersed phase (the suspended particles) and a continuous phase (the medium of suspension). The dispersed phase particles have a diameter of approximately 1 nanometre to 1 micrometre.

Some colloids are translucent because of the Tyndall effect, which is the scattering of light by particles in the colloid. Other colloids may be opaque or have a slight color.

Colloidal suspensions are the subject of interface and colloid science. This field of study was introduced in 1845 by Italian chemist Francesco Selmi and further investigated since 1861 by Scottish scientist Thomas Graham.

Classification

Colloids can be classified as follows:

Homogeneous mixtures with a dispersed phase in this size range may be called ''colloidal aerosols'', ''colloidal emulsions'', ''colloidal suspensions'', ''colloidal foams'', ''colloidal dispersions'', or ''hydrosols''.

File:Aerogel hand.jpg, Aerogel
File:Jello Cubes.jpg, Jello cubes
File:Opaleszens Kolloid SiO2.jpg, Colloidal silica gel with light opalescence
File:Crème Chantilly.jpg, Whipped cream
File:Mist - Ensay region3.jpg, Mist
File:Why is the sky blue.jpg, Tyndall effect in an opalite: it scatters blue light making it appear blue from the side, but orange light shines through; opal is a gel in which water is dispersed in silica crystals
File:Milk and straw.jpg, Milk - emulsion of liquid butterfat globules dispersed in water

Hydrocolloids

Hydrocolloids describe certain chemicals (mostly polysaccharides and proteins) that are colloidally dispersible in water. Thus becoming effectively "soluble" they change the rheology of water by raising the viscosity and/or inducing gelation. They may provide other interactive effects with other chemicals, in some cases synergistic, in others antagonistic. Using these attributes hydrocolloids are very useful chemicals since in many areas of technology from foods through pharmaceuticals, personal care and industrial applications, they can provide stabilization, destabilization and separation, gelation, flow control, crystallization control and numerous other effects. Apart from uses of the soluble forms some of the hydrocolloids have additional useful functionality in a dry form if after solubilization they have the water removed - as in the formation of films for breath strips or sausage casings or indeed, wound dressing fibers, some being more compatible with skin than others. There are many different types of hydrocolloids each with differences in structure function and utility that generally are best suited to particular application areas in the control of rheology and the physical modification of form and texture. Some hydrocolloids like starch and casein are useful foods as well as rheology modifiers, others have limited nutritive value, usually providing a source of fiber.

The term hydrocolloids also refers to a type of dressing designed to lock moisture in the skin and help the natural healing process of skin, in order to reduce scarring, itching and soreness.

Components

Hydrocolloids contain some type of gel-forming agent, such as sodium carboxymethylcellulose (NaCMC) and gelatin. They are normally combined with some type of sealant, i.e. polyurethane in order to 'stick' to the skin.

Colloid compared with solution

A colloid has a dispersed phase and a continuous phase, whereas in a solution, the solute and solvent constitute only one phase. A solute in a solution are individual molecules or ions, whereas colloidal particles are bigger. For example, in a solution of salt in water, the sodium chloride (NaCl) crystal dissolves, and the Na⁺ and Cl⁻ ions are surrounded by water molecules.  However, in a colloid such as milk, the colloidal particles are globules of fat, rather than individual fat molecules. Because colloid is multiple phases, it has very different properties compared to fully mixed, continuous solution.

Interaction between particles

The following forces play an important role in the interaction of colloid particles:
* Excluded volume repulsion: This refers to the impossibility of any overlap between hard particles.
* Electrostatic interaction: Colloidal particles often carry an electrical charge and therefore attract or repel each other. The charge of both the continuous and the dispersed phase, as well as the mobility of the phases are factors affecting this interaction.
*van der Waals forces: This is due to interaction between two dipoles that are either permanent or induced. Even if the particles do not have a permanent dipole, fluctuations of the electron density gives rise to a temporary dipole in a particle. This temporary dipole induces a dipole in particles nearby. The temporary dipole and the induced dipoles are then attracted to each other. This is known as van der Waals force, and is always present (unless the refractive indexes of the dispersed and continuous phases are matched), is short-range, and is attractive.
* Steric forces between polymer-covered surfaces or in solutions containing non-adsorbing polymer can modulate interparticle forces, producing an additional steric repulsive force (which is predominantly entropic in origin) or an attractive depletion force between them.

Sedimentation velocity

The Earth’s gravitational field acts upon colloidal particles. Therefore, if the colloidal particles are denser than the medium of suspension, they will sediment (fall to the bottom), or if they are less dense, they will cream (float to the top). Larger particles also have a greater tendency to sediment because they have smaller Brownian motion to counteract this movement.

The sedimentation or creaming velocity is found by equating the Stokes drag force with the gravitational force:

:$m_Ag=6\pi \eta rv$

where

:$m_Ag$ is the Archimedean weight of the colloidal particles,

:$\eta$ is the viscosity of the suspension medium,

:$r$ is the radius of the colloidal particle,

and $v$ is the sedimentation or creaming velocity.

The mass of the colloidal particle is found using:

:$m_A =V(\rho_1 - \rho_2)$

where

:$V$ is the volume of the colloidal particle, calculated using the volume of a sphere $V = \frac\pi r^3$,

and $\rho_1-\rho_2$ is the difference in mass density between the colloidal particle and the suspension medium.

By rearranging, the sedimentation or creaming velocity is:

:$v = \frac$

There is an upper size-limit for the diameter of colloidal particles because particles larger than 1 μm tend to sediment, and thus the substance would no longer be considered a colloidal suspension.

The colloidal particles are said to be in sedimentation equilibrium