may be expected. For very concentrated systems, as in partly dried foods,
very large deviations between concentration and activity often occur.
- Adsorption.Several kinds of molecules can adsorb onto various
surfaces or bind onto macromolecules, thereby lowering their activity
coefficients. Binding should not be interpreted here as forming a covalent or
an ionic bond, for in such a case the concentration of the substance is indeed
decreased. It is well known that for many flavor components the threshold
concentration for sensory perception is far higher in a particular food than
in water. This means that the activity coefficient is smaller in the food than
in water, and a decrease by a factor of 10^3 is no exception. It may be recalled
that many flavor components are fairly hydrophobic molecules, which
readily adsorb onto proteins. Because of this, the so-called head space
analysis for flavor components makes good sense, since the concentrations
of the various components in the gas phase (which is, in principle, in
equilibrium with the food) are indeed expected to be proportional to the
activities in the food.
A surface active component naturally adsorbs onto many surfaces, say
of oil droplets or solid particles or macromolecules, by which its activity is
decreased (see Section 10.2). Another example is the binding of cations,
especially of heavy metals, to proteins; when concentrating by ultrafiltration
a protein solution that also contains some Cu, almost all of the Cu is
concentrated with the protein, although Cu ions would be perfectly able to
pass the ultrafiltration membrane.
In the simplest case, the amount adsorbed is proportional to the
concentration (or rather activity) of the species in the solution, implying that
the activity is a constant fraction of the concentration. If there is a limited
number of binding or adsorption sites, as is often the case, we often have a
Langmuir type adsorption isotherm, given as
G¼
G?m
Cþmð 2 : 20 ÞwhereGis the surface concentration of adsorbed material (in mol?m^2 ),G?
its value if all adsorption sites are occupied, and the constantCdenotes the
concentration at whichG¼ 0 : 5 G?. Note thatm(mol?m^3 ) refers to the
concentration in solution; the total concentration equalsmþGA, whereAis
the specific surface area. The activity coefficient then would be given by
m=ðmþGAÞ, which increases with total concentration, as illustrated in
Figure 2.7b.
- Self association. Amphiphilic molecules, i.e., molecules that
consist of a hydrophilic (or polar) and a hydrophobic (or apolar) part,
often tend to associate in an aqueous environment. Good examples are