Lynne Wong's PhD thesis

It has been shown in Chapter 5 that the best-fit model for the sorption isotherms of sugar
cane component parts is the Hailwood-Horrobin followed by the GAB model. The work
described in this chapter is aimed at investigating some properties of the sorbed water and
the sorbent, and the thermodynamic properties of sorption, that can be obtained from the
isotherm models and will assist in better defining the bound water in the above described
three regions.
6.1 THE MONOLAYER MOISTURE CONTENT
Three of the sorption isotherm models tested in Chapter 5 have the monolayer moisture
content, m o , as one of their parameters. These are the GAB, Caurie I and the modified
BET models. The m o value gives an indication of the total number of polar groups on the
sorbent binding water. The prediction of m o values is important since the deterioration of a
food material or fibre is very small below the m o value, where water is strongly bound to
the material, and is not involved in any deteriorative reaction either as a solvent or as one
of the substrates.
The GAB equation was derived independently by Guggenheim (1966), Anderson (1946)
and de Boer (1953). It is written as:
m =
( 1 − c a ) ( 1 − c a + b c a )
w
m
o
b c a
w
w
w
,
where m is the equilibrium moisture content (kg/kg dry solid, i.e. decimal), m o is the
moisture content corresponding to saturation of all primary adsorption sites by one water
molecule, i.e. monolayer moisture content (kg/kg dry solid, i.e. decimal), a w is the water
activity, and b and c are constants related to the energies of interaction between the
monolayer and multilayer molecules at the individual sorption sites.
Caurie I, BET and modified BET models (Table 5.1) also contain a constant term of m o in
their equation. However, since the BET equation did not fit the data well, not much
credence can be given to the m o values derived from that equation.
Caurie I
1
n
=
m
−
1
n
bm
o
+
2b
m
o
⎛ 1 − a
n
⎜
⎝ aw
w
⎟ ⎞
⎠
and modified BET
m =
m
o
ba
( − a ) [ 1 − bn
(1 − a )]
1
w
w
w
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