AquaLab 3 WATER ACTIVITY THEORY
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 va-
por phase and use that to determine the water potential of the liquid
phase. Gradients in (µ) are driving forces for moisture movement.
Thus, in an isothermal system, water tends to move from regions of
high water potential (high a
w
) to regions of low water potential (low
a
w
). Water content is not a driving force for water movement, and
therefore can not be used to predict the direction of water movement,
except in homogeneous materials.
Factors In Determining Water Activity
The water activity of the water in a system is influenced by factors
that effect the binding of water. They include osmotic, matric, and
pressure effects. Typically water activity is measured at atmospheric
pressure, so only the osmotic and matric effects are important.
Osmotic Effects: Osmotic effects are well known from biology and
physical chemistry. Water is diluted when a solute is added. If
this diluted water is separated from pure water by a semi-permeable
membrane, water tends to move from the pure water side through
the membrane to the side with the added solute. If sufficient pressure
is applied to the solute-water mixture to just stop the flow, this pres-
sure is a measure of the osmotic potential of the solution. Addition
of one mole of an ideal solute to a kilogram of water produces an
osmotic pressure of 22.4 atm. This lowers the water activity of the
solution from 1.0 to 0.98 a
w
. For a given amount of solute, increasing
the water content of the systems dilutes the solute, decreasing the
osmotic pressure, and increasing the water activity. Since microbial
cells are high concentrations of solute surrounded by semi-permeable
membranes, the osmotic effect on the free energy of the water is im-
portant for determining microbial water relations and therefore their
activity.
Matric Effects: The sample matrix affects water activity by phys-
ically binding water within its structure through adhesive and cohe-
sive forces that hold water in pores and capillaries, and to particle
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