10.3.2 Polymers
In this section, we will consider only water-soluble polymers adsorbing at
homogeneous surfaces, A–W, O–W, or S–W. Theconformationin which
polymers adsorb will be considered first.
Adsorption ofhomopolymersis possible but not very common: most of
them are either insufficiently surface active or hardly soluble. The
conformation of an adsorbed homopolymer molecule will be roughly as
depicted in Figure 10.12a. Of course, the chains sticking out into the
aqueous phase show considerable random variation in conformation due to
Brownian motion.Copolymersthat contain both hydrophobic segments and
(usually a greater number of) hydrophilic segments are very suitable
surfactants. They would adsorb roughly as depicted in Figure 10.12b,
although the conformation will greatly depend on the distribution of the
hydrophobic segments over the chain.
Mostpolysaccharidesused in foods are predominantly hydrophilic and
not surface active. Some polysaccharides, however, notably gum arabic,
contain minor protein moieties, and do adsorb onto O–W (and presumably
A–W) interfaces. By chemical modification, hydrophobic groups can be
introduced. The best known examples are cellulose ethers, such as methyl
cellulose and hydroxypropyl cellulose, which substances are well soluble in
water (at least below 40 8 C) and strongly surface active.
The polymeric surfactants of choice in foods are proteins. The
polypeptide backbone is fairly polar, but several side groups are
hydrophobic (see Section 7.1). Protein adsorption is briefly discussed in
Section 7.2.2, subheading ‘‘Adsorption’’. All proteins are surface active and
adsorb at O–W and A–W surfaces. Globular proteins often retain a fairly
compact form, although conformational changes do occur: see Figure
10.12c. Nonglobular proteins, such as gelatins and caseins, tend to adsorb in
a way comparable to Figure 10.12b. Forb-casein the (average) conforma-
tion on adsorption is fairly well known: see Figure 10.12d. The picture
agrees well with the primary structure ofb-casein (see Figure 7.1): a very
hydrophilic N-terminal part, and a long tail containing several hydrophobic
side groups.
Thesurface activityof a protein and an amphiphile are compared in
Figure 10.13. It is seen that the protein is much more surface active. The
molar bulk concentration needed to reach G? differs by 4 orders of
magnitude. If the mass concentration is plotted, the curves are closer, but
the difference is still by more than two orders of magnitude. The main cause
is the larger molar mass of the protein. It implies that the free energy of
adsorption per molecule (roughly equal toP/GNAv) is very much larger
than that of the amphiphile; for the protein it would be about 60 timeskBT,