Water Potential - Decagon Devices AquaLab 4TE Operator's Manual

AquaLab
3. Water Activity Th eory
cause condensation. Th e AquaLab warns the user if a sample is more
than 4°C above the chamber temperature, but for high water activity
samples the operator needs to be aware that condensation can occur
if a sample that is warmer than the block is put in the AquaLab.

Water Potential

Some additional information may be useful for understanding what
water activity is and why it is such a useful measure of moisture
status in products. Water activity is closely related to a thermody-
namic property called the water potential, or chemical potential (μ)
of water, which is the change in Gibbs free energy (ΔG) when water
concentration changes. Equilibrium occurs in a system when (μ) is
the same everywhere in the system. Equilibrium between the liquid
and the vapor phases implies that (μ) is the same in both phases. It
is this fact that allows us to measure the water potential of the vapor
phase and use that to determine the water potential of the liquid
phase. Gradients in (μ) are driving forces for moisture movement.
Th us, in an isothermal system, water tends to move from regions of
high water potential (high a ) to regions of low water potential (low
w
a ). Water content is not a driving force for water movement, and
w
therefore can not be used to predict the direction of water move-
ment, except in homogeneous materials.
Factors In Determining Water Activity
Th e water activity of the water in a system is infl uenced by factors
that eff ect the binding of water. Th ey include osmotic, matric, and
pressure eff ects. Typically water activity is measured at atmospheric
pressure, so only the osmotic and matric eff ects are important.
Osmotic Eff ects
Osmotic eff ects are well known from biology and physical chemis-
try. Water is diluted when a solute is added. If this diluted water is
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