if k is in nm^1 and the ionic strengthI in mol?L^1. It follows from
Eq. (6.10a) thatcis reduced toc 0 =eð& 0 :37 timesc 0 Þat a distancehthat
equals 1=k.1=kis called thethickness of the electric double layerorDebye
length. It is a measure for the distance over which electrostatic interactions
are significant. In Figure 6.8a, the dependence of the electric potential on
distance is illustrated for some values of the ionic strength. It is seen that at
highI,1=kis very small, comparable to the size of a very small molecule.
Figure 6.8b shows the dependence of 1=konI. To give an idea of the values
ofIto be encountered in foods, examples are given. Some fabricated foods,
especially when pickled or salted, have much higher ionic strengths, leading
to very small values of 1=k and very weak electrostatic interaction.
Figure 6.9 schematically depicts theconformationof a polyelectrolyte
(a polyacid) at various ionic strength. At highI(small 1=k) the negative
charges can only sense each other if they are very close. This implies that the
conformation is not greatly different from that of a similar but uncharged
polymer. As the ionic strength decreases, the molecule becomes more
expanded, because the charges sense each other over a longer distance. The
FIGURE6.9 Examples of the conformation of a polyacid molecule at various ionic
strengths, which would roughly be 400, 80, and 7 mmolar for (a), (b), and (c). The
dotted lines are at a distance of about 1=kof the charges. Only ions that are part of
the polymer are indicated. Highly schematic.