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. A colloid has a dispersed phase and a continuous phase.
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 began in 1845 by Francesco Selmi, who called them pseudosolutions, and expanded by Michael Faraday and Thomas Graham, who coined the term colloid in 1861.

Definition

Since the definition of a colloid is so ambiguous, the International Union of Pure and Applied Chemistry formalized a modern definition of colloids: This IUPAC definition is particularly important because it highlights the flexibility inherent in colloidal systems. However, much of the confusion surrounding colloids arises from oversimplifications. IUPAC makes it clear that exceptions exist, and the definition should not be viewed as a rigid rule. D.H. Everett—the scientist who wrote the IUPAC definition—emphasized that colloids are often better understood through examples rather than strict definitions.

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.

Hydrocolloids

Hydrocolloids describe certain chemicals 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 and personal-care products, to 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 hydrocolloids have additional functionality in a dry form if, after solubilization, they have the water removed – as in the formation breath-strip films, artificial sausage casings, and wound-dressing fibers. There are many different types of hydrocolloids, each with differences in structure, function, and utility; which is best suited to a particular application area may depend on the control of rheology and the physical modification of form and texture. Some hydrocolloids like corn starch and casein are useful foods, as well as rheology modifiers; others have some limited nutritional value, usually providing a source of dietary fiber.
The term hydrocolloid may also refer to a type of wound dressing, designed to lock moisture in the skin and help the natural healing process of skin, to reduce scarring, itching, and soreness.

Components

Hydrocolloids contain some type of gel-forming agent, such as sodium carboxymethylcellulose or gelatin. They are normally combined with some type of sealant, like polyurethane, to stick to skin.

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 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, is short-range, and is attractive.
  • Steric forces: A repulsive steric force typically occurring due to adsorbed polymers coating a colloid's surface.
  • Depletion forces: An attractive entropic force arising from an osmotic pressure imbalance when colloids are suspended in a medium of much smaller particles or polymers called depletants.

    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, or if they are less dense, they will cream. 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:
where
and is the sedimentation or creaming velocity.
The mass of the colloidal particle is found using:
where
and is the difference in mass density between the colloidal particle and the suspension medium.
By rearranging, the sedimentation or creaming velocity is:
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 if the rate of sedimentation is equal to the rate of movement from Brownian motion.

Preparation

There are two principal ways to prepare colloids:
The stability of a colloidal system is defined by particles remaining suspended in solution and depends on the interaction forces between the particles. These include electrostatic interactions and van der Waals forces, because they both contribute to the overall free energy of the system.
A colloid is stable if the interaction energy due to attractive forces between the colloidal particles is less than kT, where k is the Boltzmann constant and T is the absolute temperature. If this is the case, then the colloidal particles will repel or only weakly attract each other, and the substance will remain a suspension.
If the interaction energy is greater than kT, the attractive forces will prevail, and the colloidal particles will begin to clump together. This process is referred to generally as aggregation, but is also referred to as flocculation, coagulation or precipitation. While these terms are often used interchangeably, for some definitions they have slightly different meanings. For example, coagulation can be used to describe irreversible, permanent aggregation where the forces holding the particles together are stronger than any external forces caused by stirring or mixing. Flocculation can be used to describe reversible aggregation involving weaker attractive forces, and the aggregate is usually called a floc. The term precipitation is normally reserved for describing a phase change from a colloid dispersion to a solid when it is subjected to a perturbation. Aggregation causes sedimentation or creaming, therefore the colloid is unstable: if either of these processes occur the colloid will no longer be a suspension.
Electrostatic stabilization and steric stabilization are the two main mechanisms for stabilization against aggregation.
  • Electrostatic stabilization is based on the mutual repulsion of like electrical charges. The charge of colloidal particles is structured in an electrical double layer, where the particles are charged on the surface, but then attract counterions which surround the particle. The electrostatic repulsion between suspended colloidal particles is most readily quantified in terms of the zeta potential. The combined effect of van der Waals attraction and electrostatic repulsion on aggregation is described quantitatively by the DLVO theory. A common method of stabilising a colloid is peptization, a process where it is shaken with an electrolyte.
  • Steric stabilization consists absorbing a layer of a polymer or surfactant on the particles to prevent them from getting close in the range of attractive forces. The polymer consists of chains that are attached to the particle surface, and the part of the chain that extends out is soluble in the suspension medium. This technique is used to stabilize colloidal particles in all types of solvents, including organic solvents.
A combination of the two mechanisms is also possible.
A method called gel network stabilization represents the principal way to produce colloids stable to both aggregation and sedimentation. The method consists in adding to the colloidal suspension a polymer able to form a gel network. Particle settling is hindered by the stiffness of the polymeric matrix where particles are trapped, and the long polymeric chains can provide a steric or electrosteric stabilization to dispersed particles. Examples of such substances are xanthan and guar gum.