Physicochemical Studies of Ligninsulphonates: Iii. Properties of Fractions Prepared by Successive Sulphonation of Periodate Lignin

Abstract

Periodate lignin was sulphonated in stages by a series of short bisulphite coolcs. At each stagc the soluble ligninsulphonate was removed and purified: and the residue was resulphonatccl. 'The 14 fractions thus obtained had constant n~ethoxyl content and ultraviolet absorption coefficients. 'There was a gradual increase in the sulphur content as the integral reaction time increased, but 26% of the ligninsulphonate possessed a sulphur content of less than 3%. There was a marlced increase in both the intrinsic viscosity and rate of solution with increase in reaction time. 'The mechanism of the sulphonation reaction is discussed i n the light of these results. INTRODUCTIOXi\iIany uses of spent sulphite liquor depend on the colloidal and polyn~eric properties of ligninsulphonate (1, 2, 3, 4, 5). However, information on the size, shape, and configuration of the molecule is scarce.In an earlier paper in this series, Gardon and Mason ( 6 ) demonstrated that ligninsulphonates in spent liquor occurred in a wide range of molecular weights (from 4000 to 60,000). A similar co~lclusion was reached by NIcCarthy and co-worlters (7), who obtained fractions of ligninsulphonate having molecular weights from 400 to 130,000. In the sulphite process it is possible that the lignin becomes soluble as a uniform monodisperse material. This may be either degraded or condensed during later stages of sulphonation to give the polydispersity observed. I t is more likely, however, that lignins extracted a t various times during the reaction will differ considerably in molecular weight. The present worlt is an attempt to study the properties of soluble ligninsulphonates produced a t different stages of sulphonation.The problem was simplified by use of periodate lignin prepared by a method similar to that of Purves and co-workers (8, 9). Periodate lignin, like protolignin, is insoluble in most solvents, and undergoes many of the same reactions (10). The phenyl propane chain is probably not broke~l during periodate oxidation, although some methoxyl is lost. As a starting material for physical studies it is probably as suitable as any other isolated lignin.A kilogram of periodate lignin was prepared from white spruce. A large sample was coolted for successive short periods until most of the lignin was dissolved as sulphonate. After each stage of sulphonation the liquor and residue were separated. The residue was recoo1;ed and the soluble sulphonate was isolated from the liquor. The purified fractions were characterized by measurement of ultraviolet and visible absorption, methoxyl and sulphur content, and intrinsic viscosity.A parallel investigation based on the sulphonation of wood was published by McCarthy and co-worliers (11, 12) when this manuscript was in preparation. The results obtained are similar, although the present worlt is on an isolated lignin and is somewhat more detailed.'iManz~script received Marclz 31, 1058.

Physicochemical Studies of Alkali Lignins. I. Preparation and Properties of Fractions

“…Though there xvas a greater variation in visible light absorption from fraction to fraction, no clear maximu111 was found as in the case of lignin sulphonates (21). The average value of E~~~~ was 18.1 ( f 0.6) cm-I 1. g-I which is considerably greater than the corresponding value of 13.8 cm-I 1. g-I for the sulphonates (21) and nearly equal to the absorption noted by I-Iess for fractionated native black spruce lignin (43). These results suggest that the quick alltali treatment did not destroy the essential structure of the lignin.…”

“…(45) founcl the native and enzymatically liberated ligili~ls from oak, maple, and cork electrophoretically I~on~ogeneoiis while Scots pine lignin showed only s~nall secondary components. This is in contrast to the marked electrophoretic heterogeneity of lignin s u l p h o~~a t e s degraded by prolonged coolcing (21). Similarly, YIcCarthy and co-workers (46) have found several electrophoretically distinguishable components in the alkali degradation products of lignin sulphonates.…”

“…Digestion ii~as carried out in stdillless steel autoclave of 1.2-liter capacity fittecl with a motor stirrer (21). The digestion was of 15-1nin~1te duration a t 162" C with 10% NaOI-1 solution.…”

“…Optical densities were measured with a Beckman DU spectrophotometer as PI-eviously clescribecl (21). 3Ieasurenzents were made a t wavelengths of 3-1-60 and 2800 using concentrations of 0.2 and 0.002 g dl-' respectively.…”

“…As far as is ltnown, there have been no systematic studies of the solution properties of allcali lignins analogous to the worlc of Gardon and 1IIason (20), Goring et al (21,22), and JiIcCarthy et al (23,24) on lignin sulphonates. The present investigation deals with the fractionation and subsequent physicocl~emical characterization of the fractions.…”