Diffusion

Diffusion is a concept describing the process by which a substance moves from an area of high concentration to an area of lower concentration. It is most commonly discussed in physics and chemistry to explain the natural motion of liquids and gases, for example how molecules flow within cells or how the scent of flowers spreads throughout a room. However, it can also be applied in fields such as sociology and business theory to explain the spread of ideas or information. In all applications and occurrences, the basic principles of the diffusion process are the same.

Visualizing Diffusion

The process of diffusion can be easily modeled using an array of marbles of two different colors and a box. Place the marbles in the box in a single layer, with all the marbles of one color on one side and the marbles of the second color on the other. When the box of marbles is vibrated, marbles of one color will become dispersed throughout those of the other color. When the rate and amplitude of vibration are low, the interspersion of the marbles will be slow, but as either the rate or the amplitude is increased, the marbles will intersperse much more rapidly. At the atomic and molecular levels, diffusion functions in essentially the same manner.

Diffusion of Atoms and Molecules

Because atoms have energy, they are constantly in motion. Single atoms spin, and the force of electrostatic repulsion between different atoms’ respective electron clouds imparts directional motion through space. Atoms bonded together in molecules have several more modes of movement available to them due to the restrictions of their respective chemical bonds. Atoms in a molecular structure move in accordance with the various vibrational modes of their bonds. They may move in and out as though attached to a spring, or they may move laterally as bonds bend and recover and adjacent bonds open and close like a pair of scissors. Various parts of a molecule are able to rotate about the different bonds that make up their structure. These motions are strongly affected by temperature and become increasingly energetic as temperature increases and the atoms and molecules acquire thermal energy. Interaction with light can also impart additional energy to atoms and molecules, though only at the level of the electrons within atomic and molecular orbitals. However, this energy can be sufficient to cause the molecule to separate into smaller component pieces, further adding to the disorder (entropy) of the system and raising it to a more energetic state.

All of these various motions result in random collisions between particles, which drive them further into motion. Impacted atoms and molecules exchange momentum and velocity in much the same way that the marbles in the box bounce off each other and set each other in motion. This can be observed on the macroscopic scale as Brownian motion, which occurs when microscopic particles suspended in a fluid medium collide with the molecules that make up the fluid, causing continuous random movements of the particles that result in macroscopic effects. Brownian motion is the engine that drives the process of diffusion.

Applications of Diffusion

A basic example of diffusion occurs naturally in the cell membranes of living systems. The plasma membranes of all animal cells are composed of a lipid bilayer, primarily containing phospholipids. They are hydrophilic (attracted to water) on the phosphate end of the molecule, while the long hydrocarbon chains of the fatty acid are very hydrophobic (repelled by water) in nature. The hydrocarbon chains essentially dissolve in each other, forming a dense hydrocarbon layer sandwiched between two hydrophilic layers that can interact with proteins, ions, and other polar molecules. Sugars, proteins, and other essential compounds enter the cell by passing through the cell membrane, while by-products of metabolism and respiration must also pass through the membrane in order to be transported away and eliminated.

Molecules pass through cell membranes by either active or passive transport. In active transport, various proteins bind to ions and other molecules that otherwise would not be able to diffuse through the membrane due to their high polarity and the hydrophobic nature; these proteins, known as carrier proteins, form a channel in the membrane through which the polar materials can pass. The difference in composition between the fluid within the cell and the interstitial fluid outside of it forms a concentration gradient. Active transport processes move materials against a concentration gradient, from a region of lower concentration to one of higher concentration. This enables the transfer of signaling compounds between neurons in the nervous system, the passage of metabolic by-products and water through the nephrons of the kidneys, and similar actions in many other such systems.

In strictly chemical systems, diffusion is somewhat more straightforward. The process involves a semipermeable membrane, which is a porous membrane in which the pores are large enough to allow the passage of solvent molecules, typically water, but not large enough to allow the passage of dissolved materials, such as ions. Water molecules that pass through the pores are not actively carried through the membrane; rather, they are driven by the molecular motions of diffusion in an overall process called osmosis. Normally, osmosis works in the same direction as the concentration gradient, from higher to lower concentration, and will continue until equilibrium is attained, at which point the concentrations of the two solutions on either side of the semipermeable membrane are equal and the concentration gradient has disappeared.

In the process of reverse osmosis, pressure is applied to the solution of higher concentration to overcome the osmotic pressure of the membrane and drive solvent molecules through the membrane against the concentration gradient. Reverse osmosis has numerous applications, including obtaining fresh potable water from salt-laden seawater and concentrating solutions without the application of heat that would alter or destroy compounds of value.

PRINCIPAL TERMS

• Brownian motion: the continuous, random motion of particles in a fluid medium, caused by impacts with the molecules that make up the medium.
• concentration gradient: the gradual change in the concentration of solutes in a solution across a specific distance.
• equilibrium: the state that exists when the forward activity of a process is exactly equal to the reverse activity of that process.
• osmosis: the passage of solvent molecules through a semipermeable membrane from a region of low solute concentration to one of higher concentration; also the primary mechanism by which water moves through cell walls.
• semipermeable membrane: a membrane that allows the passage of a material, such as water or another solvent, from one side to the other while preventing the passage of other materials, such as dissolved salts or another solute.

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