diffusion (redirected from diffusionists)

diffusion, in chemistry, the spontaneous migration of substances from regions where their concentration is high to regions where their concentration is low. Diffusion is important in many life processes. It occurs, for example, across the alveolar membrane of the lung, which separates the carbon-dioxide-rich blood from the oxygen-rich air. Oxygen diffuses across the membrane and becomes dissolved in the blood; carbon dioxide diffuses across the membrane into the air.

The spontaneous redistribution of a substance is due to the random motion of the molecules (or atoms or ions) of the substance. Because of the random nature of the motion of molecules, the rate of diffusion of molecules out of any region in a substance is proportional to the concentration of molecules in that region, and the rate of diffusion into the region is proportional to the concentration of molecules in the surrounding regions. Thus, while molecules continuously flow both into and out of all regions, the net flow is from regions of higher concentration to regions of lower concentration. Generally, the greater the difference in concentration, the faster the diffusion.

Since an increase in temperature represents an increase in the average molecular speed, diffusion occurs faster at higher temperatures. At any given temperature, small, light molecules (such as H₂, hydrogen gas) diffuse faster than larger, more massive molecules (such as N₂, nitrogen gas) because they are traveling faster, on the average (see heat

Measures of Heat

..... ; kinetic-molecular theory of gases). According to Graham's law (for Thomas Graham), the rate at which a gas diffuses is inversely proportional to the square root of the density of the gas.

Diffusion often masks gravitational effects. For example, if a relatively dense gas (such as CO₂, carbon dioxide) is introduced at the bottom of a vessel containing a less dense gas (such as H₂, hydrogen gas), the dense gas will diffuse upward and the less dense gas will diffuse downward. It is true, however, that at equilibrium the two gases will not be uniformly mixed. There will be some variation in the density and composition of the gas mixture; at the top of the vessel the gas mixture will be slightly less concentrated, and there will be a slight preponderance of molecules of the less dense gas. These differences, which are due to gravity, are almost impossible to measure in the laboratory, although they interact with other factors in determining the distribution of gases in planetary atmosphere.

Diffusion is not confined to gases; it can take place with matter in any state. For example, salt diffuses (dissolves) into water; water diffuses (evaporates) into the air. It is even possible for a solid to diffuse into another solid; e.g., gold will diffuse into lead, although at room temperature this diffusion is very slow. Generally, gases diffuse much faster than liquids, and liquids much faster than solids. Diffusion may take place through a semipermeable membrane, which allows some, but not all, substances to pass. In solutions, when the liquid solvent passes through the membrane but the solute (dissolved solid) is retained, the process is called osmosis. Diffusion of a solute across a membrane is called dialysis, especially when some solutes pass and others are retained.

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diffusion

Process by which there is a net flow of matter from a region of high concentration to one of low concentration. It occurs fastest in liquids and slowest in solids. Diffusion can be observed by adding a few drops of food colouring to a glass of water. The scent from an open bottle of perfume quickly permeates a room because of random motion of the vapour molecules. A spoonful of salt placed in a bowl of water will eventually spread throughout the water.

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diffusion

A semiconductor manufacturing process that infuses tiny quantities of impurities into a base material, such as silicon, to change its electrical characteristics. See chip.

Diffusion Process

This diagram shows the masking, etching and diffusion stages that build the tiny sublayers in a transistor.

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diffusion

1. Physics

a. the random thermal motion of atoms, molecules, clusters of atoms, etc., in gases, liquids, and some solids

b. the transfer of atoms or molecules by their random motion from one part of a medium to another

2. Physics the transmission or reflection of electromagnetic radiation, esp light, in which the radiation is scattered in many directions and not directly reflected or refracted; scattering

3. Physics the degree to which the directions of propagation of reverberant sound waves differ from point to point in an enclosure

4. Anthropol the transmission of social institutions, skills, and myths from one culture to another

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diffusion [də′fyü·zhən]

(acoustics)

The degree of variation in the propagation directions of sound waves over the volume of a sound field.

(electronics)

A method of producing a junction by difusing an impurity metal into a semiconductor at a high temperature.

(mechanical engineering)

The conversion of air velocity into static pressure in the diffuser casing of a centrifugal fan, resulting from increases in the radius of the air spin and in area.

(meteorology)

The exchange of fluid parcels (and hence the transport of conservative properties) between regions in space, in the apparently random motions of the parcels on a scale too small to be treated by the equations of motion; the diffusion of momentum (viscosity), vorticity, water vapor, heat (conduction), and gaseous components of the atmospheric mixture have been studied extensively.

(optics)

The distribution of incident light by reflection.

Transmission of light through a translucent material.

(physics)

The spontaneous movement and scattering of particles (atoms and molecules), of liquids, gases, and solids.

In particular, the macroscopic motion of the components of a system of fluids that is driven by differences in concentration.

(solid-state physics)

The actual transport of mass, in the form of discrete atoms, through the lattice of a crystalline solid.

The movement of carriers in a semiconductor.

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Diffusion

The transport of matter from one point to another by random molecular motions. It occurs in gases, liquids, and solids.

Diffusion plays a key role in processes as diverse as permeation through membranes, evaporation of liquids, dyeing textile fibers, drying timber, doping silicon wafers to make semiconductors, and transporting of thermal neutrons in nuclear power reactors. Rates of important chemical reactions are limited by how fast diffusion can bring reactants together or deliver them to reaction sites on enzymes or catalysts. The forces between molecules and molecular sizes and shapes can be studied by making diffusion measurements. See Semiconductor

Molecules in fluids (gases and liquids) are constantly moving. Even in still air, for example, nitrogen and oxygen molecules ricochet off each other at bullet speeds. Molecular diffusion is easily demonstrated by pouring a layer of water over a layer of ink in a narrow glass tube. The boundary between the ink and water is sharp at first, but it slowly blurs as the ink diffuses upward into the clear water. Eventually, the ink spreads evenly along the tube without any help from stirring.

Gases

A number of techniques are used to measure diffusion in gases. In a two-bulb experiment, two vessels of gas are connected by a narrow tube through which diffusion occurs. Diffusion is followed by measuring the subsequent changes in the composition of gas in each vessel. Excellent results are also obtained by placing a lighter gas mixture on top of a denser gas mixture in a vertical tube and then measuring the composition along the tube after a timed interval.

Rates of diffusion in gases increase with the temperature (T) approximately as T^3/2 and are inversely proportional to the pressure. The interdiffusion coefficients of gas mixtures are almost independent of the composition.

Kinetic theory shows that the self-diffusion coefficient of a pure gas is inversely proportional to both the square root of the molecular weight and the square of the molecular diameter. Interdiffusion coefficients for pairs of gases can be estimated by taking averages of the molecular weights and collision diameters. Kinetic-theory predictions are accurate to about 5% at pressures up to 10 atm (1 megapascal). Theories which take into account the forces between molecules are more accurate, especially for dense gases. See Kinetic theory of matter

Liquids

The most accurate diffusion measurements on liquids are made by layering a solution over a denser solution and then using optical methods to follow the changes in refractive index along the column of solution. Excellent results are also obtained with cells in which diffusion occurs between two solution compartments through a porous diaphragm. Many other reliable experimental techniques have been devised.

Room-temperature liquids usually have diffusion coefficients in the range 0.5–5 × 10^-5 cm² s^-1. Diffusion in liquids, unlike diffusion in gases, is sensitive to changes in composition but relatively insensitive to changes in pressure. Diffusion of high-viscosity, syrupy liquids and macromolecules is slower. The diffusion coefficient of aqueous serum albumin, a protein of molecular weight 60,000 atomic mass units, is only 0.06 × 10^-5 cm² s^-1 at 25°C (77°F).

When solute molecules diffuse through a solution, solvent molecules must be pushed out of the way. For this reason, liquid-phase interdiffusion coefficients are inversely proportional to both the viscosity of the solvent and the effective radius of the solute molecules. Accurate theories of diffusion in liquids are still under development. See Viscosity

Solids

Diffusion in solids is an important topic of physical metallurgy and materials science since diffusion processes are ubiquitous in solid matter at elevated temperatures. They play a key role in the kinetics of many microstructural changes that occur during the processing of metals, alloys, ceramics, semiconductors, glasses, and polymers. Typical examples of such changes include nucleation of new phases, diffusive phase transformations, precipitation and dissolution of a second phase, recrystallization, high-temperature creep, and thermal oxidation. Direct technological applications concern diffusion doping during the fabrication of microelectronic devices, solid electrolytes for battery and fuel cells, surface hardening of steels through carburization or nitridation, diffusion bonding, and sintering. See Phase transitions

The atomic mechanisms of diffusion are closely connected with defects in solids. Point defects such as vacancies and interstitials are the simplest defects and often mediate diffusion in an otherwise perfect crystal. Dislocations, grain boundaries, phase boundaries, and free surfaces are other types of defects in a crystalline solid. They can act as diffusion short circuits because the mobility of atoms along such defects is usually much higher than in the lattice. See Crystal defects