Lynne Wong's PhD thesis

is much weaker and is reflected in the heats of adsorption measured particularly at EMC
values between 5 and 20% and the H m values calculated from the temperature dependence
of the GAB parameters.
The value of m h /m generally decreases with increase of water activity and with increase of
temperature. This observation can be easily rationalised from the foregoing. Since the
hydrated water is that fraction of water that binds first to the active sites on the surface of
the fibre, with increasing water activity the number of these sites which are unoccupied
will diminish and this is reflected in the m h /m ratio. The number of active sites decreases
with increase in temperature as observed before and hence this ratio also decreases with
increase in temperature. According to Norimoto and Yamada (1977), one hydrated water
molecule is bonded to about four hydroxyl groups by hydrogen bonds, whereas one
dissolved water molecule is bonded to not more than two hydroxyl groups. This further
corroborates the difference observed in the heats of binding for these two types of water.
Figures 6.13 – 6.15 show the experimental or calculated EMC values correspond closely to
the total water calculated as the sum of the hydrated and dissolved water (except in the
case of dry leaf fibre aged 36 weeks at 45 and 60 °C, green leaf fines aged 36 weeks and at
30 °C and reconstituted green leaf aged 36 weeks at 30, 45, 55 and 60 °C).
What is apparent from here is that the Hailwood-Horrobin dissolved water encompasses
both the multilayer water that has a heat of adsorption just somewhat larger than the heat of
vaporisation of pure water and free water with the same properties of bulk water and
probably situated in the voids in the fibre.
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