Mixed gels - Physical Gels from Biological and Synthetic Polymers - page 313

'Hairy' region

Cool

'Smooth' region

Ga lactomann an

Heat

Solution order

Random coil
(xanthan)

Mixed gel

Figure 10.13 Molecular origin of xanthan interaction with galactomannan: the mixed junctions may be formed
by cooperative association of mannan-backbone regions with the xanthan native structure, as
indicated schematically. In this figure the zigzag line represents a single helix of xanthan. Reprinted
with permission from Dea et al.( 1977 ) © 1977 Elsevier.

+GM

(a)

+GM

(b)

Disordered xanthan

+GM (c)

Five-fold helix

Figure 10.14 Proposed models for the interaction of xanthan with galactomannan or glucomannan (GM) chains:
(a) ' Unilever ' model; (b) ' Norwich ' model; (c) proposal from Goycoolea et al.( 1995 ). In each, the
upper zigzag line on the right-hand side represents the smooth region of the galactomannan or
glucomannan. Reprinted with permission fromGoycoolea et al.( 1995 ) ©1995 American Chemical
Society.

The controversy over the fine structure of these interactions continues, and

Figure 10.14 illustrates three models, the first being the original, and the other two

being versions adapted to different experimental results. That said, we need to make
two further comments. First, all of these

figures show the xanthan macromolecule as a
single ordered chain, although almost all current evidence suggests that the ordered
structure involves two chains. Second, the evidence for all of these figures is somewhat

indirect: there seems to be no data available to unequivocally establish any of these
macromolecular structures; instead, these are deduced from spectroscopy or other

'
probes. As we shall see, the evidence for some such binding is well established, for
example by DSC, but the exact form of this is not settled. In our view, the ideal techniques
to be used would seem to be either SAXS or, even better because of the potential to
deuterium-label parts of one or the other molecule, SANS, but so far no such experiments
appear to have been performed.

' local

10.6.2.2 'Synergistic' mixed polysaccharide gels

Succeeding sections describe current experimental observations, and cover a range of
other techniques. The sections gather the data collected on the most investigated mixtures
containing κ -carrageenans, gellan and xanthan.