Total Hydroxyl Content

Since the hydroxyl groups are converted to O-acetyl groups by acetylation on a one-to-one molar basis, the total hydroxyl content of the lignin is equal to the total O-acetyl content of the acetylated lignin. Provided the methoxyl content of the acetylated lignin is known (see Chap. 7.6.3), the total hydroxyl content of the lignin expressed in terms of methoxyl content can be calculated  

The total hydroxyl content of the lignin may be also expressed in terms of a C9 unit, OH/C9 (mol/C9) as follows  

The total hydroxyl content of the lignin/C9 unit may also be calculated directly from the elemental composition of the acetylated lignin, i.e., % C, % H and the total O-acetyl and methoxyl contents (Freudenberg 1962). 

Note 11. The values for total and aliphatic hydroxyl are meaningful only if the lignin preparations under study are sufficiently pure and have a C9 

---

In Table 5.3, is compared with the total hydroxyl concentration (Ni, + N ) of the corresponding fully hydroxylated, sample. The results clearly demonstrate that the physical adsorption is determined by the total hydroxyl content of the surface, showing the adsorption to be localized. It is useful to note that the BET monolayer capacity n JH2O) (= N ) of the water calculated from the water isotherm by the BET procedure corresponds to approximately 1 molecule of water per hydroxyl group, and so provides a convenient means of estimating the hydroxyl concentration on the surface. Since the adsorption is localized, n.(H20) does not, of course, denote a close-packed layer of water molecules. Indeed, the area occupied per molecule of water is determined by the structure of the silica, and is uJH2O) 20A ... 

Functional Group Analysis. The total hydroxyl content of lignin is determined by acetylation with an acetic anhydride—pyridine reagent followed by saponification of the acetate, and followed by titration of the resulting acetic acid with a standard 0.05 W sodium hydroxide solution. Either the Kuhn-Roth (35) or the modified Bethge-Liadstrom (36) procedure may be used to determine the total hydroxyl content. The aUphatic hydroxyl content is determined by the difference between the total and phenoHc hydroxyl contents. 

Since lignin is not a uniform entity, chemical criteria for its characterization have centred around analytical detenninations of its functional groups, e.g. total hydroxyl content, phenolic hydroxyls (56), methoxyl and other ether groups, benzyl alcohol groups (7a), carbonyl groups (6), etc., and estimations of its content of special structural features, e.g. phenylcoumaran units (5), biphenylyl linkages (123), etc. 

Some results of the modification of lignin sulfonate Ultra B002 by reaction with terephthaloyl chloride are summarized in Table VI. The total hydroxyl content of the lignosulfonates as well as their derivatives are presented in Table VII. The hydrolytic resistance of selected products is evaluated in Table VIII. The results presented in Tables VI-VIII stress several advantages of the derivatives with terephthaloyl chloride. The modified lignin sulfonates were insoluble, or only very slightly soluble, in organic solvents. They were, however, soluble in dimethyl sulfoxide. Ordered structures were identified by X-ray studies (16,17). 

Table VII. Total Hydroxyl Content of Lignin Sulfonates and Their Derivatives with Terephthaloyl Chloride.

Symbol of Sample Tvne of Lignin Total Hydroxyl Content. X... 

Adapting these relative coverages to the total hydroxyl content as a function of temperature, yields the absolute numbers of figure 5.18. 

In general, the aliphatic hydroxyl content of a lignin preparation is determined by subtraction of the phenolic hydroxyl content (see Chap. 7.2) from the total hydroxyl content of the lignin. The total hydroxyl content of lignin (partial structure H) can be determined by acetylation with acetic anhydride-pyridine reagent (10) followed by saponification of the acetate (partial structure 12) and titration of the resulting acetic acid (14) with a standard 0.05 N sodium hydroxide solution (Roth 1953). 

The benzyl acetate (18) is quantitatively determined by capillary column gas chromatography (GC). Benzyl butanoate (20), formed from butanoic acid (19) concurrently with the formation of benzyl acetate (18), serves as the internal standard (Mansson 1983). Since 1 mol of acetic acid (14) is converted to 1 mol of benzyl acetate (18), the total hydroxyl content of the lignin preparation can be determined by quantitative determination of the benzyl acetate (18). The Mansson procedure is a modification of the Bethge-Lindstrom method for determinating O-acetyl groups in acetylated carbohydrates (Bethge and Lindstrom 1973). 

In a nutshell, the total hydroxyl content, including R-OH initiator and lactic acid impurities, determines the maximum attainable (number-average molecular weight) [4, 6]. The rate of polymerization is controlled by factors such as temperature and catalyst content, with the remark that a tin (II) octoate catalyst requires traces of the initiator to become active. 

Symbol of Samole Type of Lisnin Total Hydroxyl Content. %... 

---