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##

SEDIMENTATION, CREAMING, CENTRIFUGATION AND DIFFUSION

*Sedimentation volume* (
sed) or *cream volume* (
cr) is the volume of sediment or cream formed in a suspension or emulsion. If the sediment is formed in a centrifugal field, the strength of this field should be explicitly indicated, otherwise normal gravity is understood.

*Sedimentation equilibrium* is the equilibrium between
sedimentation and diffusion.

*Rate of sedimentation* is the *velocity of
sedimentation* (
sed or ).

*Sedimentation coefficient* () is the rate of
sedimentation divided by acceleration, expressed in seconds () or in *Svedbergs* (Sv); Sv
.

*Limiting sedimentation coefficient*, , is the
sedimentation coefficient extrapolated to zero concentration of the sedimenting component,

*Reduced sedimentation coefficient*, (20)^{25} is the sedimentation coefficient reduced to a standard temperature, usually 20 and to a standard solvent, usually water.

where
coefficient of viscosity of the solution at temperature ,

coefficient of viscosity of standard solvent at 20,

partial specific volume of sedimenting substance at temperature ,

density of solution at temperature ,

density of the standard solvent at 20.
*Reduced limiting sedimentation coefficient*,
,
is the reduced sedimentation coefficient extrapolated to zero concentration of the sedimenting component:

*Differential diffusion coefficient*, , of species is defined by

grad

where is the amount of species flowing through unit area in unit time and
grad is the concentration gradient of species . Different diffusion coefficients may be defined depending on the choice of the frame of reference used for and
grad . For systems with more than two components, the flow of any component and hence its diffusion coefficient depends on the concentration distribution of all components.
*Limiting differential diffusion coefficient*, , is the
value of extrapolated to zero concentration of the diffusing species:

*Self-diffusion coefficient*, , of species is the
diffusion coefficient in the absence of a chemical potential gradient. It is related to the diffusion coefficient by

where is the activity of in the solution. If an isotopically labelled species () is used to study diffusion, the
*tracer diffusion coefficient*, is practically
identical to the self-diffusion coefficient provided that the isotope effect is sufficiently small.
*Rotational diffusion coefficient*,
, is defined by the
equation:

where
dd is the traction of particles whose axes make an angle between and
d with the direction
, and have an azimuth between and
d;
d is the fraction of particles having an azimuth between and
d, whose axis passes from values to values in unit time. The axis whose rotational diffusion is considered has to be clearly indicated.

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2002-09-05