Physical Chemistry of Foods

Note A certain amount of the fatty acid may adsorb on the oil–water

interface. If the oil droplets are small, this may be an appreciable amount.

If the adsorption isotherm is known, the effect can be calculated.

Note If the aqueous phase contains protein, it may well be that some fatty

acid becomes associated with the protein, whereby the activity coefficient is

lowered and the concentration in the water will thus be increased.

2.3 ELECTROLYTE SOLUTIONS

Ionizable substances, like salts, acids, bases, and polyelectrolytes, partly
dissociate into ions when dissolved in water. As a consequence, the
osmolarity will be higher than the molarity (see Section 2.2.4). More
important, ionic species generally are reactive because of their electric
charge, and the charge generally is shielded to a certain extent by the
presence of ions of opposite charge, called counterions. This implies that the
activity coefficient may be greatly diminished if the concentration of
counterions is high.
We will briefly recall some basic facts about dissociation and its
consequences, before discussing the magnitude and the importance of ion
activity coefficients.

2.3.1 Dissociation

A simple salt like NaCl will in water dissociate according to
NaClÐNaþþCl
. We now have for thedissociation constant

KD¼

aðNaþÞ?aðCl
Þ

aðNaClÞ

¼

½NaþŠ?½Cl
Š

½NaClŠ

6

gþðNaþÞ?g
ðCl
Þ

g 0 ðNaClÞ

ð 2 : 21 Þ

The first right-hand expression is written in activities, and this quotient gives
theintrinsicdissociation constant. The second right-hand expression is made
up of two factors, a quotient of (molar or molal) concentrations that may be
called thestoichiometricdissociation constant, and a quotient of activity
coefficients. All dissociation constants, association constantsðKA¼ 1 =KDÞ,
and solubility products in reference books are intrinsic constants. They
apply to concentrations only if the solution is extremely dilute for all ionic
species. In other cases, one has to know the activity coefficients.g 0 , i.e.,gfor
a nonionic species, will mostly be close to unity, butgþandg
will generally
be<1, the more so for a higher ion concentration. One may define thefree