Cellulose swelling by protic solvents: which properties of the biopolymer and the solvent matter?

Abstract

The question posed in the title has been addressed by studying the swelling of celluloses at 20°C by twenty protic solvents, including water; linear-and branched-chain aliphatic alcohols; unsaturated aliphatic alcohols, and alkoxyalcohols. The biopolymers investigated included microcrystalline cellulose, MC, native and never-dried mercerized cotton cellulose, cotton and M-cotton, and native and never-dried mercerized eucalyptus cellulose, eucalyptus and M-eucalyptus, respectively. In most cases, better correlations with the physico-chemical properties of the solvents were obtained when the swelling was expressed as number of moles of solvent/ anhydroglucose unit, nSw, rather than as % increase in sample weight. The descriptors employed in these correlations included, where available, Hildebrand's solubility parameters, Gutmann's acceptor and donor numbers, solvent molar volume, V S , as well as solvatochromic parameters. The latter, employed for the first time for correlating the swelling of biopolymers, included empirical solvent polarity, E T (30), solvent ''acidity'', a S , ''basicity'', b S , and dipolarity/ polarizability, p S * , respectively. Small regression coefficients and large sums of the squares of the residues were obtained when values of nSw were correlated with two solvent parameters. Much better correlations were obtained with three solvent parameters. The most statistically significant descriptor in the correlation equation depends on the cellulose, being p S * for MC, cotton, and eucalyptus, and V S for M-cotton and M-eucalyptus. The best correlations were obtained with the same set of four parameters for all celluloses, namely, solvent pKa (or a S ) b S , p S * , and V S , respectively. These results indicate that the supra-molecular structure of the biopolymer, in particular the average sizes of crystallites and micro-pores, and the presence of its chains in parallel (cellulose I) or anti-parallel (cellulose II) arrangements control its swelling. At least for the present biopolymer/solvent systems, use of solvatochromic parameters is a superior alternative to Hildebrand's solubility parameters and/or Gutmann's acceptor and donor numbers. The relevance of these results to the accessibility of the hydroxyl groups of cellulose, hence to its reactivity, is briefly discussed.

Understanding solvation

“…Data recalculated from ref. [7]. curs in these probes, from the phenolate oxygen to the heterocyclic nitrogen; the corresponding CT band is sensitive to the solvent, i.e., is solvatochromic [8].…”

“…Additionally, this treatment leads to the transformation of cellulose I to cellulose II, the latter has less inter-crystalline hydrogen bonding [6]. Thus the supramolecular structure of mercerized cellulose is more accessible to the solvent; this leads to less sensitivity to solvent properties, relative to its native counterpart [7].…”

“…Figure 3 shows the dependence of the swelling of cotton cellulose on the number of carbon atoms of n-chain aliphatic alcohols, from methanol to 1-octanol. The swelling is expressed as (n; solvent), the number of moles of solvent absorbed per anhydroglucose unit of cellulose; the Y-axis is given as a ratio between the swelling by water (most efficient) and that by alcohols [7].This example is interesting because of the large difference between the responses of native and mercerized fibers to a systematic variation of a solvent property, namely, its molar volume.…”

Solvation simplified

“…26 Our results have shown that the use of solvatochormic parameters in order to describe the dependence of cellulose swelling on solvent properties resulted in better correlations coefficients, as compared with those based on Gutmann´s, or Hildebrand´s solubility parameters. 27 In fact, we were able to correlate the swelling of twenty eight protic and aprotic solvents simultaneously with the solvent molar volume, b S , and p* S (a S was excluded because the dipolar aprotic solvents carry no acidic hydrogens). 27 This is remarkable because previous work on swelling has either used a very limited number of solvents, or employed solvents of the same nature, either protic or aprotic.…”

“…27 In fact, we were able to correlate the swelling of twenty eight protic and aprotic solvents simultaneously with the solvent molar volume, b S , and p* S (a S was excluded because the dipolar aprotic solvents carry no acidic hydrogens). 27 This is remarkable because previous work on swelling has either used a very limited number of solvents, or employed solvents of the same nature, either protic or aprotic. 28 This success indicates that the same solute-solvent interactions mechanisms are relevant to probe solvation and cellulose swelling.…”