Physical Chemistry of Foods

6.3.2 Conformation and Viscosity

Electric charges on a polymer greatly affect its conformation in aqueous
solution, because like charges repel each other. This depends, of course, on
the number of charges per moleculeðzÞ, hence on pH, and also onionic
strength. Electric shielding of ions by other ions is discussed in Section 2.3.2,
and charged groups are shielded in the same way. The repulsive force due to
electric charges is proportional to the square of the electric potential that
they generate. If the potential equals c 0 at the surface of the charge,
shielding causes it to decrease with distanceðhÞfrom the charge according to

c¼c 0 ekh ð 6 :10aÞ

which applies for fairly lowjc 0 j(say,<50 mV) and a flat geometry. Around
a charged sphere of radiusrthe relation is

c¼c 0

r

rþh

ekh ð 6 :10bÞ

In water at room temperature,kis given by

k& 3 : 2 HI ð 6 :10cÞ

FIGURE6.8 The electric double layer. (a) The electrostatic potentialcrelative to
its value ath¼0, as a function of distancehfrom a flat charged surface in solutions
of various ionic strengthI(molar); see text. (b) The thickness of the electric double
layer 1=kas a function of ionic strength in aqueous solutions at room temperature;
approximate values ofIin (the aqueous part of) some materials are indicated.