Molecular diffusion: Difference between revisions
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'''Molecular diffusion''' is the motion of [[atom]]s, [[molecule]]s, or other [[particle]]s of a [[gas]] or [[liquid]] at [[temperature]]s above [[absolute zero]]. The rate of this movement is a function of temperature, [[viscosity]] of the fluid, size and density (or their product, mass) of the particles. This type of [[diffusion]] explains the net [[flux]] of molecules from a region of higher concentration to one of lower concentration. | '''Molecular diffusion''' is the motion of [[atom]]s, [[molecule]]s, or other [[particle]]s of a [[gas]] or [[liquid]] at [[temperature]]s above [[absolute zero]]. The rate of this movement is a function of temperature, [[viscosity]] of the fluid, size and density (or their product, mass) of the particles. This type of [[diffusion]] explains the net [[flux]] of molecules from a region of higher concentration to one of lower concentration. | ||
Once the concentrations are equal the molecules continue to move, but since there is no concentration gradient the process of molecular diffusion has ceased and is instead governed by the process of [[self-diffusion]], originating from the random motion of the molecules. The result of diffusion is a gradual mixing of material such that the distribution of molecules is uniform. Since the molecules are still in motion, but an equilibrium has been established, the result of molecular diffusion is called a "dynamic equilibrium". In a [[Phase (matter)|phase]] with uniform temperature, absent external net forces acting on the particles, the diffusion process will eventually result in complete mixing. | Once the concentrations are equal the molecules continue to move, but since there is no concentration gradient, the process of molecular diffusion has ceased and is instead governed by the process of [[self-diffusion]], originating from the random motion of the molecules. The result of diffusion is a gradual mixing of material such that the distribution of molecules is uniform. Since the molecules are still in motion, but an equilibrium has been established, the result of molecular diffusion is called a "dynamic equilibrium". In a [[Phase (matter)|phase]] with uniform temperature, absent external net forces acting on the particles, the diffusion process will eventually result in complete mixing. | ||
Consider two systems; S<sub>1</sub> and S<sub>2</sub> at the same [[temperature]] and capable of exchanging [[Molecule|particles]]. If there is a change in the [[potential energy]] of a system; for example μ<sub>1</sub>>μ<sub>2</sub> (μ is [[Chemical potential]]) an [[energy]] flow will occur from S<sub>1</sub> to S<sub>2</sub>, because nature always prefers low energy and maximum [[entropy]]. | Consider two systems; S<sub>1</sub> and S<sub>2</sub> at the same [[temperature]] and capable of exchanging [[Molecule|particles]]. If there is a change in the [[potential energy]] of a system; for example μ<sub>1</sub>>μ<sub>2</sub> (μ is [[Chemical potential]]) an [[energy]] flow will occur from S<sub>1</sub> to S<sub>2</sub>, because nature always prefers low energy and maximum [[entropy]]. | ||
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== Significance == | == Significance == | ||
[[File:Diffusion (1). | [[File:Diffusion (1).svg|thumb|280px|Schematic representation of mixing of two substances by diffusion]] | ||
Diffusion is part of the [[transport phenomena]]. Of mass transport mechanisms, molecular diffusion is known as a slower one. | Diffusion is part of the [[transport phenomena]]. Of mass transport mechanisms, molecular diffusion is known as a slower one. | ||
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Because chemical diffusion is a net transport process, the system in which it takes place is not an [[chemical equilibrium|equilibrium]] system (i.e. it is not at rest yet). Many results in classical thermodynamics are not easily applied to non-equilibrium systems. However, there sometimes occur so-called quasi-steady states, where the diffusion process does not change in time, where classical results may locally apply. As the name suggests, this process is a not a true equilibrium since the system is still evolving. | Because chemical diffusion is a net transport process, the system in which it takes place is not an [[chemical equilibrium|equilibrium]] system (i.e. it is not at rest yet). Many results in classical thermodynamics are not easily applied to non-equilibrium systems. However, there sometimes occur so-called quasi-steady states, where the diffusion process does not change in time, where classical results may locally apply. As the name suggests, this process is a not a true equilibrium since the system is still evolving. | ||
Non-equilibrium fluid systems can be successfully modeled with Landau-Lifshitz fluctuating hydrodynamics. In this theoretical framework, diffusion is due to fluctuations whose dimensions range from the molecular scale to the macroscopic scale.<ref>{{cite journal | last1=Brogioli | first1=Doriano | last2=Vailati | first2=Alberto | title=Diffusive mass transfer by nonequilibrium fluctuations: Fick's law revisited | journal=Physical Review E | publisher=American Physical Society (APS) | volume=63 | issue=1 | date=2000-12-22 | issn=1063-651X | doi=10.1103/physreve.63.012105 | | Non-equilibrium fluid systems can be successfully modeled with Landau-Lifshitz fluctuating hydrodynamics. In this theoretical framework, diffusion is due to fluctuations whose dimensions range from the molecular scale to the macroscopic scale.<ref>{{cite journal | last1=Brogioli | first1=Doriano | last2=Vailati | first2=Alberto | title=Diffusive mass transfer by nonequilibrium fluctuations: Fick's law revisited | journal=Physical Review E | publisher=American Physical Society (APS) | volume=63 | issue=1 | date=2000-12-22 | issn=1063-651X | doi=10.1103/physreve.63.012105 | article-number=012105| pmid=11304296 |arxiv=cond-mat/0006163| bibcode=2000PhRvE..63a2105B }}</ref> | ||
Chemical diffusion increases the [[entropy]] of a system, i.e. diffusion is a spontaneous and irreversible process. Particles can spread out by diffusion, but will not spontaneously re-order themselves (absent changes to the system, assuming no creation of new chemical bonds, and absent external forces acting on the particle). | Chemical diffusion increases the [[entropy]] of a system, i.e. diffusion is a spontaneous and irreversible process. Particles can spread out by diffusion, but will not spontaneously re-order themselves (absent changes to the system, assuming no creation of new chemical bonds, and absent external forces acting on the particle). | ||
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where D is the diffusivity of A through B, proportional to the average molecular velocity and, therefore dependent on the temperature and pressure of gases. The rate of diffusion N<sub>A</sub> is usually expressed as the number of moles diffusing across unit area in unit time. As with the basic equation of heat transfer, this indicates that the rate of force is directly proportional to the driving force, which is the concentration gradient. | where D is the diffusivity of A through B, proportional to the average molecular velocity and, therefore dependent on the temperature and pressure of gases. The rate of diffusion N<sub>A</sub> is usually expressed as the number of moles diffusing across unit area in unit time. As with the basic equation of heat transfer, this indicates that the rate of force is directly proportional to the driving force, which is the concentration gradient. | ||
This basic equation applies to a number of situations. Restricting discussion exclusively to steady state conditions, in which neither dC<sub>A</sub>/dx | This basic equation applies to a number of situations. Restricting discussion exclusively to steady state conditions, in which neither dC<sub>A</sub>/dx nor dC<sub>B</sub>/dx change with time, equimolecular counterdiffusion is considered first. | ||
==Equimolecular counterdiffusion== | ==Equimolecular counterdiffusion== | ||
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[[Category:Transport phenomena]] | [[Category:Transport phenomena]] | ||
[[Category:Diffusion]] | [[Category:Diffusion]] | ||