On the anomalous temperature dependence of cellulose aqueous solubility

Abstract

The solubility of cellulose in water-based media is promoted by low temperature, which may appear counter-intuitive. An explanation to this phenomenon has been proposed that is based on a temperature-dependent orientation of the hydroxymethyl group. In this paper, this hypothesis is investigated using molecular dynamics computer simulations and NMR spectroscopy, and is discussed in conjunction with alternative explanations based on solvent-solute and solvent-solvent hydrogen bond formation respectively. It is shown that neither simulations nor experiments lend support to the proposed mechanism based on the hydroxymethyl orientation, whereas the two alternative explanations give rise to two distinct contributions to the hydration free energy of cellooligomers.

Influence of urea on methyl $$\upbeta$$-D-glucopyranoside in alkali at different temperatures

“…In addition to this, the 3 J HH couplings remained unchanged within our temperature window, which is in accord with the results found by others (Bergenstråhle-Wohlert et al 2016). This indicates that the conformational change of the hydroxymethyl group is not induced by a temperature change within the dissolution temperature window.…”

“…Concerning the influence of urea on the hydrophobic interactions in cellulose, Bergenstråhle-Wohlert et al (2012) and Wernersson et al (2015) reported on the accumulation of urea close to the hydrophobic surfaces on cellulose in water without alkali using MD simulations while Xiong et al (2014) and Isobe et al (2012) suggested from experimental results that an accumulation of urea in the cellulose/NaOH or LiOHsystem on the hydrophobic part of the cellulose occurred to prevent agglomeration of the chains. In the work by Cai et al (2008), experimental results indicated that NaOH hydrates were in favour to bind to cellulose chains through the formation of a new hydrogen-bonded network at low temperatures in contrast to urea hydrates.…”

“…The breakage of hydrophobic interactions could, however, result in conformational changes of the cellulose chains, which can be monitored through investigation of coupling constants between neighbouring 1 H's ( 3 J HH coupling), and 1 H's and 13 C's ( 1 J CH coupling). The J couplings as a function of temperature for cellotetraose in water were recently investigated by Angles dOrtoli et al (2015) and Bergenstråhle-Wohlert et al (2016) and found to be temperature independent. The conformational changes that occur in carbohydrates such as pyranosides are ring conformation, hydroxymethyl conformation and C-O rotation of the hydroxyl groups.…”

“…In terms of binding of urea to cellulose, it has also been suggested that urea accumulates on the hydrophobic surfaces of the cellulose chains and, thus, weakens the hydrophobic interactions, similar to protein denaturation, and hence facilitates dissolution in NaOH(aq) (Xiong et al 2014). The association of urea with cellulose has been studied by MD simulations, which were conducted in the absence of NaOH but indicated an accumulation of urea on hydrophobic surfaces of cellulose (Bergenstråhle-Wohlert et al 2012;Wernersson et al 2015). Experimental results on the cellulose/NaOH(aq) or LiOH(aq)-system with urea suggested the accumulation of urea to prevent agglomeration of the chains (Xiong et al 2014;Isobe et al 2012).…”

The CO2/CO32−chemistry of the NaOH(aq) model system applicable to cellulose solutions

“…Unfortunately, the equal change for H5 could not be observed due to overlap with H3. Interestingly, the same proton is affected upon dissolution of the b-MeO-Glcp in NaOH(aq) (the only 3 J HH changes observed when going from NaCl(aq) to NaOH(aq) solvent) due to the well-known conformation change of the primary hydroxyl from tg to gt in aqueous solutions (Horii et al 1983;Bergenstråhle-Wohlert et al 2016). Addition of CO 2 (g) prior to carbohydrate did not affect the 3 J HH couplings.…”