A thermodynamic investigation of the cellulose allomorphs: Cellulose(am), cellulose Iβ(cr), cellulose II(cr), and cellulose III(cr)

Abstract

The thermochemistry of samples of amorphous cellulose, cellulose I, cellulose II, and cellulose III was studied by using oxygen bomb calorimetry, solution calorimetry in which the solvent was cadoxen (a cadmium ethylenediamine solvent), and with a Physical Property Measurement System (PPMS) in zero magnetic field to measure standard massic heat capacities Cp,w∘ over the temperature range T=(2 to 302)K. The samples used in this study were prepared so as to have different values of crystallinity indexes CI and were characterized by X-ray diffraction, by Karl Fischer moisture determination, and by using gel permeation chromatography to determine the weight average degree of polymerization DPw. NMR measurements on solutions containing the samples dissolved in cadoxen were also performed in an attempt to resolve the issue of the equivalency or non-equivalency of the nuclei in the different forms of cellulose that were dissolved in cadoxen. While large differences in the NMR spectra for the various cellulose samples in cadoxen were not observed, one cannot be absolutely certain that these cellulose samples are chemically equivalent in cadoxen. Equations were derived which allow one to adjust measured property values of cellulose samples having a mass fraction of water wH2O to a reference value of the mass fraction of water wref. The measured thermodynamic properties (standard massic enthalpy of combustion ΔcHw∘, standard massic enthalpy of solution ΔsolHw∘, and Cp,w∘) were used in conjunction with the measured CI values to calculate values of the changes in the standard massic enthalpies of reactionΔrHw∘∗, the standard massic entropies of reaction ΔrSw∘∗, the standard massic Gibbs free energies of reaction ΔrGw∘∗, and the standard massic heat capacity ΔrCp,w∘, for the interconversion reactions of the pure (CI=100) cellulose allomorphs, i.e., cellulose(am), cellulose I(cr), cellulose II(cr), and cellulose II(cr), at the temperature T=298.15K, the pressure p∘=0.1MPa, and wH2O=0.073. The “∗” denotes that the thermodynamic property pertains to pure cellulose allomorphs. Values of standard massic enthalpy differences Δ0THw∘, standard massic entropy differences Δ0TSw∘, and the standard massic thermal function Φw∘=Δ0TSw∘-Δ0THw∘/T were calculated from the measured heat capacities for the cellulose samples and for the pure cellulose allomorphs. The extensive literature pertinent to the thermodynamic properties of cellulose has been summarized and, in many cases, property values have been calculated or recalculated from previously reported data. The thermodynamic property data show that cellulose(am) is the least stable of the cellulose allomorphs considered in this study. However, due to the uncertainties in the measured property values, it is not possible to use these values to order the relative stabilities of the cellulose (I, II, and III) crystalline allomorphs with a reasonable degree of certainty. Nevertheless, based on chemical reactivity information, the qualitative order of stability for these three allomorphs is cellulose III(cr)>cellulose II(cr)>cellulose Iβ(cr) at T=298.15K. However, as evidenced by the fact that cellulose I(cr) can be reformed by the application of heat and water to a sample of cellulose III(cr), the differences in the stabilities of these three allomorphs appear to be small and may be temperature dependent.Standard thermodynamic formation properties as well as property values for the conversion reactions of the cellulose allomorphs to α-d-glucose(cr) have been calculated on the assumption that Sw∘→0 as T→0.The values for the standard massic Gibbs free energy of reactionΔrGw∘ for the conversion of the cellulose allomorphs to α-d-glucose(cr), with the exception of anhydrous cellulose(am), all have positive values and thus are thermodynamically not favored for mass fractions of water wH2O<0.073.