PHYSICAL CHEMISTRY IN BRIEF

CHAP. 10: TRANSPORT PROCESSES [CONTENTS] 340

10.4 Flow of matter—diffusion

Diffusionis a process during which concentrations are spontaneously equalized in a system

with different concentrations. For the sake of simplicity we will deal solely with diffusion in

binary mixtures.

10.4.1 Fick’s first law of diffusion

The flow of substance is caused by differences in its concentration. The driving force is a change

of a component concentration with location, or, to be more precise, a derivative of concentration

with respect to location,

dci

dz

.

In a binary mixture we have

1

S

dni

dτ

=−Dij

dci

dz

, i, j= 1,2 [T, p] (10.14)

whereDijis the diffusion coefficient of componentiin a mixture with componentj, while

D 12 =D 21. (10.15)

10.4.2 Diffusion coefficient.

The diffusion coefficient gives the number of moles of a given component in a mixture which
passes through unit area per unit time at unit concentration gradient of the component.
U Main unit:m^2 s−^1.

10.4.2.1 Dependence on state variables.

In gases, the diffusion coefficient increases with temperature and decreases with pressure; ac-

cording to kinetic theory the diffusion coefficient does not depend on the mixture composition,

see section10.5.3, equations (10.24) and (10.23).

The diffusion coefficients in liquids are smaller than in gases (diffusion is slower). In solids

they are yet smaller than in liquids.