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DESIGN INFORMATION AND DATA
8.12.2. Liquids
The equation developed by Wilke and Chang (1955), given below, can be used to predict
liquid diffusivity. This equation is discussed in Volume 1, Chapter 10.
0.5
1.173 ð 10 13 M T
D L D 8.22
V 0.6
m
2
where D L D liquid diffusivity, m /s,
D an association factor for the solvent,
D 2.6 for water (some workers recommend 2.26),
D 1.9 for methanol,
D 1.5 for ethanol,
D 1.0 for unassociated solvents,
M D molecular mass of solvent,
2
D viscosity of solvent, mN s/m ,
T D temperature, K,
3
V m D molar volume of the solute at its boiling point, m /kmol. This can be
estimated from the group contributions given in Table 8.6.
The method is illustrated in Example 8.12.
The Wilke-Chang correlation is shown graphically in Figure 8.2. This figure can be
used to determine the association constant for a solvent from experimental values for D L
in the solvent.
The Wilke-Chang equation gives satisfactory predictions for the diffusivity of organic
compounds in water but not for water in organic solvents.

Example 8.12
Ž
Estimate the diffusivity of phenol in ethanol at 20 C (293 K).

Solution
2
Ž
Viscosity of ethanol at 20 C, 1.2 mNs/m .
Molecular mass, 46.
Molar volume of phenol OH from Table 8.6:

Atom Vol. No. of
C 0.0148 ð 6 D 0.0888
H 0.0037 ð 6 D 0.0222
O 0.0074 ð 1 D 0.0074
ring 0.015 ð 1 D 0.015
3
0.1034 m /k mol
0.5
1.173 ð 10 13 1.5 ð 46 293
2
D L D D 9.28 ð 10 10 m /s (8.22)
1.2 ð 0.1034 0.6
2
Experimental value, 8 ð 10 10 m /s