A colloid is a substance microscopically dispersed evenly throughout another one.[1]
A colloidal system consists of two separate phases: a dispersed phase (or internal phase) and a continuous phase (or dispersion medium). A colloidal system may be solid Solid is one of the major states of matter. It is characterized by structural rigidity and resistance to changes of shape or volume. Unlike a liquid, a solid object does not flow to take on the shape of its container, nor does it expand to fill the entire volume available to it like a gas does. The atoms in a solid are tightly bound to each other,, liquid Liquid is one of the three classical states of matter. Like a gas, a liquid is able to flow and take the shape of a container, but, like a solid, it resists compression. Unlike a gas, a liquid does not disperse to fill every space of a container, and maintains a fairly constant density. A distinctive property of the liquid state is surface tension,, or gaseous Gas is one of four classical states of matter. Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons are so energized that they leave their parent atoms.
Many familiar substances are colloids, as shown in the chart A colloidal system consists of two separate phases: a dispersed phase and a continuous phase (or dispersion medium). A colloidal system may be solid, liquid, or gaseous below. As well as these naturally occurring colloids, modern chemical process industries utilize high shear mixing technology A high shear mixer disperses, or transports, one phase or ingredient into a main continuous phase (liquid), with which it would normally be immiscible. It is used in the adhesives, chemical, cosmetic, food pharmaceutical and plastics industries for emulsification, homogenization, particle size reduction and dispersion to create novel colloids.
The dispersed-phase particles have a diameter of between approximately 5 and 200 nanometers A nanometre (Ancient Greek: νάνος, nanos, "dwarf"; μέτρον, metrοn, "unit of measurement") is a unit of length in the metric system, equal to one billionth of a metre.[2] Such particles are normally invisible to an optical microscope A microscope is an instrument to see objects too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy. Microscopic means invisible to the eye unless aided by a microscope, though their presence can be confirmed with the use of an ultramicroscope or an electron microscope The electron microscope uses electrostatic and electromagnetic "lenses" to control the electron beam and focus it to form an image. These lens are analogous to, but different from the glass lenses of an optical microscope that form a magnified image by focusing light on or through the specimen. Homogeneous A substance that is uniform in composition is a definition of homogeneous in Chemistry. This is in contrast to a substance that is heterogeneous mixtures with a dispersed phase in this size range may be called colloidal aerosols, colloidal emulsions, colloidal foams, colloidal dispersions, or hydrosols. The dispersed-phase particles or droplets are affected largely by the surface chemistry Surface science is the study of physical and chemical phenomena that occur at the interface of two phases, including solid-liquid interfaces, solid-gas interfaces, solid-vacuum interfaces, and liquid-gas interfaces. It includes the fields of surface chemistry and surface physics. Some related practical applications are classed as surface present in the colloid.
Some colloids are translucent because of the Tyndall effect The Tyndall effect, also known as Tyndall scattering, is the scattering of light by colloidal particles or particles in suspension. It is named after the 19th century scientist John Tyndall. It is similar to Rayleigh scattering, in that the intensity of the scattered light depends on the fourth power of the frequency, so blue light is scattered, which is the scattering of light by particles in the colloid. Other colloids may be opaque or have a slight color.
Colloidal systems (also called colloidal solutions or colloidal suspensions) are the subject of interface and colloid science Interface and colloid science is a branch of chemistry dealing with colloids, heterogeneous systems consisting of a mechanical mixture of particles between 1 nm and 1000 nm dispersed in a continuous medium. This field of study was introduced in 1861 by Scottish Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the southwest. In addition to the mainland, Scotland scientist Thomas Graham Thomas Graham FRS was a nineteenth-century Scottish chemist who is best-remembered today for his pioneering work in dialysis and the diffusion of gases.
Classification of colloids
Because the size of the dispersed phase may be difficult to measure, and because colloids have the appearance of solutions In chemistry, a solution is a homogeneous mixture composed of two or more substances. In such a mixture, a solute is dissolved in another substance, known as a solvent, colloids are sometimes identified and characterized by their physico-chemical and transport properties. For example, if a colloid consists of a solid phase dispersed in a liquid, the solid particles will not diffuse Molecular diffusion, often called simply diffusion, is a net transport of molecules from a region of higher concentration to one of lower concentration by random molecular motion. The result of diffusion is a gradual mixing of material. In a phase with uniform temperature, absent external net forces acting on the particles, the diffusion process through a membrane, whereas with a true solution the dissolved ions or molecules will diffuse through a membrane. Because of the size exclusion, the colloidal particles are unable to pass through the pores of an ultrafiltration membrane with a size smaller than their own dimension. The smaller the size of the pore of the ultrafiltration membrane, the lower the concentration of the dispersed colloidal particules remaining in the ultrafiltred liquid. The exact value of the concentration of a truly dissolved species will thus depend on the experimental conditions applied to separate it from the colloidal particles also dispersed in the liquid. This is, a.o., particularly important for solubility Solubility is the property of a solid, liquid, or gaseous chemical substance called solute to dissolve in a liquid solvent to form a homogeneous solution of the solute in the solvent. The solubility of a substance fundamentally depends on the used solvent as well as on temperature and pressure. The extent of the solubility of a substance in a studies of readily hydrolysed Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations (H+) (conventionally referred to as protons) and hydroxide anions (OH−) in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by step-growth polymerization. Such polymer species such as Al, Eu, Am, Cm, ... or organic matter Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds. The definition of organic matter varies upon the subject it is being used for complexing In chemistry, a coordination complex or metal complex, is a structure consisting of a central atom or ion , bonded to a surrounding array of molecules or anions (ligands, complexing agents). The atom within a ligand that is directly bonded to the central atom or ion is called the donor atom. Polydentate (multiple bonded) ligands can form a chelate these species.
Colloids can be classified as follows:
In some cases, a colloid can be considered as a homogeneous mixture. This is because the distinction between "dissolved" and "particulate" matter can be sometimes a matter of approach, which affects whether or not it is homogeneous or heterogeneous.
Hydrocolloids
A hydrocolloid is defined as a colloid system wherein the colloid particles are dispersed in water Water is a chemical substance with the chemical formula H2O. Its molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state, water vapor or steam. A hydrocolloid has colloid particles spread throughout water, and depending on the quantity of water available that can take place in different states, e.g., gel A gel is a solid, jelly-like material that can have properties ranging from soft and weak to hard and tough. Gels are defined as a substantially dilute crosslinked system, which exhibits no flow when in the steady-state. By weight, gels are mostly liquid, yet they behave like solids due to a three-dimensional crosslinked network within the liquid or sol A sol is a colloidal suspension of solid particles in a liquid. Examples include blood, pigmented ink, and paint (liquid). Hydrocolloids can be either irreversible In science, a process that is not reversible is called irreversible. This concept arises most frequently in thermodynamics, as applied to processes. Irreversibility is also used in economics to refer to investment or expenditures that involve large sunk costs (single-state) or reversible. For example, agar, a reversible hydrocolloid of seaweed extract, can exist in a gel and sol state, and alternate between states with the addition or elimination of heat.
Many hydrocolloids are derived from natural sources. For example, agar-agar and carrageenan are extracted from seaweed, gelatin is produced by hydrolysis of proteins of bovine and fish origins, and pectin is extracted from citrus peel and apple pomace.
Gelatin desserts like jelly or Jell-O are made from gelatin powder, another effective hydrocolloid. Hydrocolloids are employed in food mainly to influence texture or viscosity (e.g., a sauce).Hydrocolloid-based medical dressings are used for skin and wound treatment.
Other main hydrocolloids are xanthan gum, gum arabic, guar gum, locust bean gum, cellulose derivatives as carboxymethyl cellulose, alginate and starch
Interaction between colloid 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.
- Entropic forces: According to the second law of thermodynamics, a system progresses to a state in which entropy is maximized. This can result in effective forces even between hard spheres.
- 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. Such an effect is specifically searched for with tailor-made superplasticizers developed to increase the workability of concrete and to reduce its water content.
Stabilization of a colloidal dispersion (peptization)
Stabilization serves to prevent colloids from aggregating. Steric stabilization and electrostatic stabilization are the two main mechanisms for colloid stabilization. Electrostatic stabilization is based on the mutual repulsion of like electrical charges. In general, different phases have different charge affinities, so that a electrical double layer forms at any interface. Small particle sizes lead to enormous surface areas, and this effect is greatly amplified in colloids. In a stable colloid, mass of a dispersed phase is so low that its buoyancy or kinetic energy is too weak to overcome the electrostatic repulsion between charged layers of the dispersing phase. The charge on the dispersed particles can be observed by applying an electric field: All particles migrate to the same electrode and therefore must all have the same sign charge.
Destabilizing a colloidal dispersion (flocculation)
Unstable colloidal dispersions form flocs as the particles aggregate due to interparticle attractions. In this way photonic glasses can be grown. This can be accomplished by a number of different methods:
- Removal of the electrostatic barrier that prevents aggregation of the particles. This can be accomplished by the addition of salt to a suspension or changing the pH of a suspension to effectively neutralize or "screen" the surface charge of the particles in suspension. This removes the repulsive forces that keep colloidal particles separate and allows for coagulation due to van der Waals forces.
- Addition of a charged polymer flocculant. Polymer flocculants can bridge individual colloidal particles by attractive electrostatic interactions. For example, negatively-charged colloidal silica or clay particles can be flocculated by the addition of a positively-charged polymer.
- Addition of non-adsorbed polymers called depletants that cause aggregation due to entropic effects.
- Physical deformation of the particle (e.g., stretching) may increase the van der Waals forces more than stabilization forces (such as electrostatic), resulting coagulation of colloids at certain orientations.
Unstable colloidal suspensions of low-volume fraction form clustered liquid suspensions, wherein individual clusters of particles fall to the bottom of the suspension (or float to the top if the particles are less dense than the suspending medium) once the clusters are of sufficient size for the Brownian forces that work to keep the particles in suspension to be overcome by gravitational forces. However, colloidal suspensions of higher-volume fraction form colloidal gels with viscoelastic properties. Viscoelastic colloidal gels, such as bentonite and toothpaste, flow like liquids under shear, but maintain their shape when shear is removed. It is for this reason that toothpaste can be squeezed from a toothpaste tube, but stays on the toothbrush after it is applied.
Technique monitoring colloidal stability
Measurement principle of multiple light scattering coupled with vertical scanningMultiple light scattering coupled with vertical scanning is the most widely used technique to monitor the dispersion state of a product, hence identifying and quantifying destabilisation phenomena.[3][4][5][6] It works on concentrated dispersions without dilution. When light is send through the sample, it is backscattered by the particles / droplets. The backscattering intensity is directly proportional to the size and volume fraction of the dispersed phase. Therefore, local changes in concentration (e.g.Creaming and Sedimentation) and global changes in size (e.g.flocculation, coalescence) are detected and monitored.
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Fri, 13 Jul 2007 07:00:00 GM
Yin has put his . colloids. in glass capillaries of a few hundred micrometers in diameter and still observed the color changing effect. 'It seems that the miniaturized system has much higher sensitivity, which means that it can operate at ...
Q. I have some written down, but they kind of contradict each other...if anyone could clear it up, thanks so much! Colloids: - Let light through - Particles are large enough to reflect light - Are not affected by gravity - Cannot be filtered - Heterogeneous (but looks homogeneous) Suspension: - Pushed down to the bottom/does not dissolve - CAN be filtered - Light does not pass - Heterogeneous
Asked by Soo Young - Sun Mar 9 00:16:17 2008 - - 2 Answers - 0 Comments
A. Glen's right looks like you're correct. suspensions have a much smaller particle size than colloids too.
Answered by Hala - Sun Mar 9 01:07:26 2008
