DIFFUSION Flashcards
The transfer movement of the individual molecules through a
fluid by means of the random individual movements of molecules
Fick’s Law of Molecular DIFFUSION/Transport
Diffusion in which temperature and
total pressure are constant
Molecular Diffusion in Gases
no bulk motion
Equimolar Counter – Diffusion
much more convenient to use. The equation
was obtained by correlating many recent data and uses
atomic volumes which are summed for each gas molecule.
Fuller et al
a gas mixture of dilute A in B is dimensionless. It is the
dimensionless ratio of the molecular momentum
diffusivity to the molecular mass diffusivity. Values for
gases range from about 0.5 to 2. For liquids range from
about 100 to over 10 000 for viscous liquids.
Schmidt Number of Gases
Molecules in a liquid are packed together much more closely
than in gases, the density and the resistance to diffusion in a liquid
are much greater. Also, because of this closer spacing of molecules,
the attractive forces between molecules play an important role in
diffusion. Since the kinetic theory of liquids is only partially
developed, equation for diffusion in liquids is written similarly to
those for gases.
Diffusion of Liquids
(B is non-diffusing) – solute A
is diffusing and solvent B is stagnant or nondiffusing.
Unicomponent Diffusion
one of the first theories, was
derived for a very large spherical molecule (A) diffusing in
a liquid solvent (B) of small molecules. The equation was modified by assuming that all molecules
are alike and arranged in cubic lattice, and by expressing
the molecular radius in terms of the molar volume.
Stokes – Einstein Equation –
– correlation can be used for most general
purposes where the solute (A) is dilute in the solvent (B).
“associated parameter” of the solvent where 2.6 (water),
1.9 (methanol), 1.5 (ethanol), 1.0 (benzene), 1.0 (ether),
1.0 (heptane), and 1.0 for other unassociated solvents.
Wilke – Chang
An
example is a dilution solution of propionic acid (A) in a
water (B) solution being contacted with toluene. Only the
propionic acid (A) diffuses through the water phase, to the
boundary, and then into the toluene phase. The toluene –
water interface is a barrier to diffusion if B and NB = 0
Unicomponent Diffusion
Used to describe the drag on the moving solute molecule.
Stokes – Einstein Equation
Diffusion through a stationary gas. Only the Component is moving toward the other.
Unicomponent diffusion
