Softwood (pinus silvestris) lignosulfonates from the acidic stage of a three-stage sulfite cook were fractionated and purified by ultrafiltration and ion exclusion chromatography. The fractions were characterized by 13C NMR spectroscopy together with conventional chemical methods.
In order to ascertain correct signal assignment of the 13C NMR spectra of lignosulfonate fractions, the chemical shifts of model compounds were studied. It was demonstrated that the principles of Karplus-Pople theory are valid for guaiacyl type lignin models, when guaiacol and various guaiacyl ethers are used as reference or parent compounds. The spectral data of various model compounds, including the substituent chemical shifts in the aromatic region of the spectra, is tabulated and discussed.
13C NMR spectroscopic analyses of lignosulfonate fractions suggested a considerably higher degree of condensation of C-6and/or C-5, then what has been reported for milled wood lignins (MWL) isolated from spruce and for kraft pine lignins. The degree of condensation in the lowest molecular weight fractions (MW < 4000) seemed to be affected by the post-cook treatment of spent liquor.
The degree of sulfonation decreased substantially with increasing molecular weight. The frequency of phenolic hydroxyl groups varied only slightly in the medium weight range (MW 5 000 – 20 000), but was considerably lower in the highest molecular weight fractions (MW > 20 000). The number of methoxyl groups per assumed C9-unit was almost constant (0.98-1.07) throughout the studied molecular weight region.
Gas chromatographic – mass spectrometric analyses of the KMnO4 – NaIO4 oxidation products obtained from methylated lignosulfonate fractions indicated substantial condensation at C-6 during acidic pulping. The degree of condensation at C-6 was remarkably higher than in spruce MWL or kraft pine lignins, and increased substantially with the molecular weight, whereas condensation at C-5 increased only slightly, and was comparable to that found in other lignin preparations.
A review of lignin reactions and the topochemical factors in sulfite pulping, together with our findings, suggest that condensation reactions apparently seem to occur in early phase of pulping, before substantial amounts of lignin fragments are removed from cell walls and middle lamellae. Topochemical factors lead to partial exclusion of active pulping chemicals from the cores of the lignin macromolecules, thus favouring condensation. This view is in good agreement with findings suggesting that only the outer surfaces of sorption of lignosulfonates from water solutions on both polar and nonpolar interfaces may be explained by conformational rearrangements exposing alternatively the polar sulfonate groups or the less polar and more condensed cores of the molecules to the interface.
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