Handbook of Solvents - George Wypych - ChemTech - Ventech!

12.2 Chain conformations of polysaccharides 725
Figure 12.2.12. Characteristic ratio, C n, as a function of degree of polymerization, n, for amylose (a) and cellulose
(b) in vacuo, water and DMSO.
By calculating of the conformational energy surface of a dimer as a starting point for
the prediction of mean chain properties, the effect of the solvation on the chain conformation
is taken into account by evaluation of the perturbations of such surface due to the presence
of the solvent. The characteristic ratio (eq. [12.2.6]) has been computed for (a)
[β-D-(1-4)-glc] n (cellulose) and (b) [α-D-(1-4)-glc] n (amylose) in different solvents,
namely, water and DMSO. 53 The results are reported in Figure 12.2.12 where, for comparison,
the in vacuo C n for both polymers is shown. Figure 12.2.12a shows that the chain extension
of cellulose decreases significantly in DMSO and much more so in water with a
reduction of about 50% when compared to that in vacuo data (C ∞=96). The solvation contribution
of water seems to favor those conformations, which have smaller cavity volumes and
thus force the chain to minor extensions. Tanner and Berry 66 reported, for cellulose derivatives
in solution, a value for the limiting characteristic ratio of between 30 and 60, which is
in good agreement with the theoretical data in Figure 12.2.12a.
Figure 12.2.12b shows the characteristic ratio of amylose chains computed in the same
solvents as in Figure 12.2.12a. The amylose chain extension is considerably lower than that
of cellulose and, as in the case of [β-D-(1-3)-glc] n shown above, presents at low n a remark-