Molar mass, lignins

The purpose of this work is to elucidate the role of hemilignins and glycolignin in the formation of color in pulp. The aim of the present part of the work was to develop a selective method for dissolution of low molar mass lignins without dissolving the glycolignin. 

The conclusion that all low molar mass lignins are hemilignins is, however, an oversimplification since it has been shown that, on heating an aqueous solution of glycolignin sulfonic acid acidified with hydrochloric acid or sulfur dioxide and bisulfite, a monomeric sulfonated hydrolysis product is formed (2). 

This observation leads to the view that the so-called residual lignin may not originate from undissolved glycolignin but from low molar mass lignins, i.e., hemilignins or breakdown products from glycolignin deposited on the fibers at an early stage of the cook. 

Fractionation on Sephadex G-25 using 0.5M sodium hydroxide as eluent causes the low molar mass lignin components in black liquor to elute in the relative retention volume range 0.3-1.3 with partial separation from each other, as shown in Figure 11. 

Figure 11. Fractionation of low molar mass lignin components in kraft black liquor. Column Sephadex G-25. Eluent 0.5M NaOH.

Fraction I, which consists mainly of low molar mass compounds, also contains a small amount of high molar mass lignin derivatives eluting with relative retention volumes of 0-0.1. These derivatives are polar and some may be bound to carbohydrates, or otherwise they would have been eluted by RPC along with the hydrophobic fractions II-IV. 

Many, but not all, macromolecules are created by the mutual chemical chain reactions of small molecules called monomers and the arising species contain repeated small units, mers. In that case they are designated oligomers or polymers depending on their molar mass. This means that all oligomers and polymers can be called macromolecular substances but not all macromolecular substances are of oligomeric or polymeric nature (lignin, humin substances, etc.). Properties of macromolecular systems depend on... 

In order to show the amount and molar masses of lignins that can be dissolved on heating wood with water, 5 g portions of wood meal extracted with cyclohexane-ethanol (4) were heated with 100 mL water at 150°C. 

Figure 11 reveals that the water-insoluble lignin has a broad molar mass distribution with an average molar mass close to Mw = 2000 g/mol. 

Forss, K. Kokkonen, R. Sagfors, P.-E. Determination of the Molar Mass Distribution of Lignins by Gel Permeation Chromatography ACS Symp. Ser. 1989, this volume. 

Determination of the Molar Mass Distribution of Lignins by Gel Permeation Chromatography... 

A method is needed for the determination of the molar mass and molar mass distributions of lignins. The method should give reproducible results when used in different laboratories. The procedure should not be complicated, and the calibration components must be readily available and reasonably priced. 

Figure 3. Molar mass as a function of retention volume for lignin sulfonates. Column Sephadex G-50. Eluent water. (Reprinted with permission from ref. 1. Copyright 1969 Wiley.)...

It should be noted that the relationships between molar mass and retention volume for lignin sulfonates shown in Figures 3 and 4 are strictly only valid for the samples studied in these experiments because lignin sulfonates are polyelectrolytes and thus interact with each other and with the gel matrix of the column. The shape of the calibration curve is thus affected by, among other things, the size and concentration of the sample (2). Interactions between molecular species can be eliminated by eluting with a suitable electrolyte. 

In the experiment described in Figure 6, four lignin sulfonate fractions with known molar masses were eluted. 

Because of the very high molar masses of enzymically polymerized lignin sulfonates, Sephacryl S-300 is used as the gel matrix. The fractionation range is 10 000 to 1 000 000 dalton. Even in this case proteins of known molar mass can be used as calibration standards (Figs. 7 and 8). 

Due to the dark color of alkali lignins, their molar masses cannot be determined by means of the light-scattering method. However, as shown by Figure 10, elution with sodium hydroxide also brings about a consistent elution pattern of lignin sulfonates and polypeptides. It is assumed that this also applies to the kraft lignins. 

It has been shown that the molar mass distributions of lignin sulfonates and kraft lignin can be determined by gel permeation chromatography. Calibration of the columns with lignin sulfonates of known molar mass or, alternatively, with commercially available proteins and polypeptides has been shown to give the same result. 

It should be noted that the broad molar mass distribution of fraction I in Figure 3 reflects the molar mass distribution of the lignin-carbohydrate compounds and not that of the lignin portion of the lignin-carbohydrate compounds. 

The figure shows that hydrophilic lignin derivatives (fraction I) elute in the retention time range 30-80 minutes, compared with 90-140 minutes for the hydrophobic compounds (fractions II-IV). Refractionation by analytical GPC of fractions I-IV (Fig. 9) shows that these fractions encompass wide molar mass ranges. 

Figure 9. Molar mass distribution of pine kraft lignin (fractions 1-IV in Figure 8).

Data concerning the elemental composition and methods for isolating lignin from the wood structure, as well as the mean molar masses and distributions of the obtained fragments, are given in Table II (1,5). 

Note The content of functional groups depends on the origin of the lignin in the cell wall. Great variations in molar masses and Mw/Mn-values are reported for technical lignins. 

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