- Immobilized water. This is meant to be water that does not leak
out of a solidlike food. It is present in closed cells, in open pores in a solid
matrix (like a sponge), or between chains of coiled polymers. Binding sites
for water need not be present for the water to be held, and ‘‘bound water’’
clearly is a misnomer. Better names are held, trapped, or imbibed water. Its
amount can be large: in several gels, one g of polymer can readily hold 100 g
of water. Actually, it is generally an aqueous solution that is held, rather
than pure water.
What is the mechanism by which water is held? A simple explanation is
thatwater fills space. A coiled polymer molecule has a certain equilibrium
conformation (Section 6.2.1), and there are water molecules in the spaces
between polymer segments. Removal of water means shrinking of the coil,
which costs free energy, because it implies a decrease in conformational
entropy. Something similar is true for a polymer network or gel (Section
6.4.4) and for other gellike structures (Section 17.2): deformation of the
network needs a force and thereby energy. This implies that the water in the
network has a decreased activity, but the decrease is extremely small.
Equation (8.3) gives the relation betweenawand osmotic pressure. The same
relation would hold for the mechanical pressure needed to remove water.
From (8.3), the change inawequals the change in pressure multiplied by
FIGURE8.7 Negative adsorption (steric exclusion) of a solute S from the surface of
a protein molecule or particle. (a) Schematic explanation. (b) Relation between
nonsolvent waterwnsand molecular radiusRsof the solute (mostly sugars) for
micellar caseinate; a scale of molar mass (M, in Da) is also given.Rwis the radius of a
water molecule.