AK X-RAY DIFFRACTIOS STUDY OF T H E SWELLING ACTION OF SEVERAL QUATERKARY AMMOSIUM HYDROXIDES OK CELLULOSE FIBERS’ WAYSE A. SISSOS
AND
WILLIAlM R. SANER
Cellulose Department, Chemical Foundation, Boyce Thompson Institute f o r Plant Research, Inc., Yonkers, New York Received December 3, 1938
Swelling is of fundamental importance in cellulose chemistry, and, as pointed out by Katz (ll), one of the most satisfactory experimental methods for studying the intimate mechanism of swelling is by x-ray diffraction analysis. The present investigation was undertaken to obtain more information concerning the swelling action of quaternary ammonium hydroxides on cellulose fibers. I n the present work it was found that certain of the quaternary ammonium bases react to form a “swelling compound” or complex with the crystalline cellulose, as indicated by the appearance of a new x-ray diagram (see figures 2A and 2C). This change in lattice structure is accompanied by a shrinkage of the fiber length and a noticeable increase in fiber diameter, as shown by a comparison of the photomicrographs in figures 1A and 1B. Upon removal of the swelling agent the fiber shrinks in diameter (figure 1C) and the diffraction pattern reverts to that of mercerized or hydrate cellulose (figure 2F). The quaternary ammonium hydroxides investigated are listed in table 1. The action of these reagents may be divided into three more or less overlapping steps: (a) swelling of the fiber and formation of a complex which gives a new crystalline x-ray diagram, ( b ) further liquid imbibition during which the fiber continues to swell and the x-ray diagram of the complex is displaced by an amorphous or liquid pattern, and ( c ) fiber dispersion in the excess reagent. This paper is concerned primarily with the formation and behavior of the quaternary ammonium hydroxide-cellulose complex, as indicated by x-ray diffraction analysis. Review of literature The action of quaternary ammonium hydroxides on cellulose appears analogous, in some respects, to that of certain other bases whose swelling compounds with cellulose have been studied with x-rays. I Presented before the Division of Cellulose Chemistry at the Ninety-fifth Meeting of the American Chemical Society, held a t Milwaukee, Wisconsin, September 5-9, 1938.
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WAYNE A. SISSON AND WILLIAM R . SANER
The mercerizing action of sodium hydroxide and other alkali hydroxides on cellulose has been known for a long time, and the addition compounds of cellulose and sodium hydroxide (soda-celluloses) have been studied very carefully by many investigators (8, 16). Carbon disulfide has been reported to react with soda-cellulose I during xanthation to form a product that shows a characteristic x-ray diagram (14, 15). Cellulose has also been reported to form two cellulose-copper compounds when swollen with cuprammonium hydroxide (10, 18). In all of the above cases, upon removing the swelling reagent the x-ray diagram reverts to that of mercerized or hydrate cellulose. Trogus and Hess (19) made a study of the action of aqueous solutions of hydrazine, ethylenediamine, and tetramethylenediamine on cellulose and found that swelling compounds are formed which give definite x-ray diagrams, the nature of which depends on whether native cellulose or mercerized cellulose is used as the starting material. Upon removing the TABLE 1 Quaternary ammonium hydroxides NAME OF REAQENT
I -. ___ Tetramethylammonium hydroxide. . . , . . Trimethylethylammonium hydroxide. . , . Trimethylbenzylammonium hydroxide . . . Dimethyldibenzylammonium hydroxide. . I Choline. , , , , , , . . . . . , , . , . , . . . . . . . , . . . . . .
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FORMULA
K(CH8),0H li(CHs)3(CnHa)OH K (CH&(CH2CsHs)OH N(CHa)p(CH&sH&OH HO(CH&KCHpCHpOH
NORMALITY
2.0 1.9
1.6 11.15
reagent, x-ray diagrams of the original materials, either native or mercerized cellulose, were obtained. The interaction of cellulose with a series of non-aqueous alkyl amines has been reported by Barry, King, and Peterson (2). , Ammonium hydroxide does not affect the lattice structure of cellulose, but it has been shown by several investigators (3, 5 , 7, 9) that liquid ammonia reacts to form an ammonia-cellulose complex. Barry, Peterson, and King (3) found that this complex, upon heating, formed a new modification. Clark and Parker (5) showed the 101 planar extension to be a function of the ammonia content. Hess and coworkers (7) found two further modifications a t -20°C. and -3OOC. The action of quaternary ammonium hydroxides on cellulose has not heretofore been studied by x-ray diffraction methods. Their fiber-dispersing properties, however, have been discussed by Lieser and Leckzyck (12), and Bock (4) has described a new method of preparing water-soluble cellulose ethers based upon the alkylation of cellulose fibers dispersed in these reagents. Although all of the above basic reagents are similar in that they form
SWELLING OF CELLULOSE FIBERS
69 1
complexes with crystalline cellulose, as indicated by a new x-ray diagram, they d 8 e r greatly in their fiber-swelling and fiber-dispersing properties. In reagents such as the diamines or sodium hydroxide the fiber swelling is limited, while in others, such as cuprammonium hydroxide or the quaternary ammonium hydroxides, swelling continues until the fiber is disrupted and finally dispersed in the reagent. The fiber-dispersing properties of a reagent, therefore, appear to depend not only upon its ability to form a complex, but also upon the mechanism of its swelling action. MATERIALS AND METHODS
Commercially purified flax fibers in the form of linen thread were used as a source of cellulose in the present investigation, since they gave a well-oriented pattern which agreed well with the data for native cellulose. Kier boiled and bleached ramie and cotton fibers were also used to check the results. The mercerized fibers were prepared by treatment with 18 per cent sodium hydroxide and were then washed and dried. Prior to treatment, the fibers were placed parallel in bundles about 1.5 mm. in diameter, which were kept under slight tension by being clamped in a stretching frame. This simple device is a slight modification of that used by Astbury and Street (1) for stretching wool fibers. Fibers mounted in this fashion were treated dropwise a t room temperature with one of the quaternary ammonium hydroxides, care being taken to impregnate the fibers as uniformly as possible. Usually from four to six drops of base were found sufficient to effect a change in the x-ray diagram. Too much base, especially in the case of dimethyldibenzylammonium hydroxide, was likely to change the fiber material into a gummy mass which no longer gave a distinct fiber pattern. Patterns were taken of the fibers after treatment with the organic bases, after the impregnated fibers had been heated a t different temperatures for varying periods of time, and after both the heated and unheated samples were treated with dilute hydrochloric acid and water. Impregnated samples were also allowed to age for several months in order to determine the stability of the compound. As a starting point for the present work, the concentrations of the tetramethyl-, trimethylethyl-, trimethylbenzyl-, and dimethyldibenzyl-ammonium hydroxides were about 33, 36, 40, and 40 per cent, respectively. X-rays were obtained from a Philips Metalix tube, copper anode, operating a t 28 kilovolts and 25 milliamperes. The rays were defined through a pinhole system using 0.020-in. and 0.025-in. pinholes placed 10 cm. apart. I n studying the effect of various physical and chemical factors the specimen-to-plate distance was 4 cm. and the length of exposure was from l a to 23 hr. The measurements of interplanar spacings were made with a sample-to-film distance of 9 cm. and a 4-hr. exposure.
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WATNE A. SISSON AND WILLIAM R. SANER
Although both products give the same type of x-ray diagram, the term “mercerized” is used to designate fibrrs treated with sodium hydroxide, while “hydrate” refers to material obtairird after removal of the quaternary ammonium hydroxides. RESULTS
Of the five bases tried (table l ) ,the tetramethyl-, trimethylethyl-, trimethylbenzyl-, and dimethyldibenzyl-ammonium hydroxides were capable of changing the lattice of both native and mercerized cellulose. Choline, which contains a substituted hydrocarbon group, produced no change in the diffraction pattern of cellulose. Change i n x-ray diagram The change in x-ray diagram may be divided into three steps. When very little base is added, the pattern is that of either native or mercedaed cellulose, depending upon the starting material (figure 2A). At the other extreme, when sufficient base is added, the pattern is that of the modified cellulose complex (figure 2C). Between these two extremes the patterns may indicate the presence of the original cellulose and of the modified cellulose existing together in the same sample (figure 2B). As the new diagram of the swelling compound is formed, that of the original cellulose loses in intensity. The final x-ray diagram of the quaternary ammonium hydroxidecellulose complex consists of a crystalline pattern which in most cases is superimposed upon an amorphous pattern. This dual nature of the x-ray diagram apparently arises from the dual structure of the fiber, which consists of cellulose crystallites and a n intercryitalline or cementing material. The sharpness of the crystalline pattern of the swelling compound indicates a crystalline structure as well defined as that of the original cellulose. If the fibers are not permitted t o shrink perceptibly during impregnation and excess reagent is not added, the orientation of the crystalline pattern may be quite as perfect as that which exists in the original fibers. Since the intensity of the amorphous pattern is related to L e amount of reagent added, the pattern probably arises partly from excess reagent in the fiber and partly from the swollen intercrystalline material, which swells indefinitely with the reagent as will be discussed later. Lattice extension Table 2 contains the results of measurements from the x-ray diagrams obtained when both native and mercerized cellulose are treated with the various quaternary ammonium hydroxides. For comparison, the measurements of native and mercerized cellulose also are added. It is obvious that the most striking change in the lattice occurs in the distance between
693
SWELLING OF CELLULOSE FIBERS
the 101 planes. From a minimum of 6.1 A. in native cellulose, this equatorial interplanar distance increases to a maximum of approximately 16.7 A. when native cellulose is treated with dimethyldibenzylammonium hydroxide. This expansion apparently allows room for the introduction of the quaternary ammonium hydroxide molecules in the unit cell. As the methyl groups of tetramethylammonium hydroxide are replaced by groupings of larger dimensions, there is an increase in the 101 interplanar distance of the corresponding cellulose complexes. By comparing the innermost equatorial spots of figures 2C and 2D, it is apparent that dimethyldibenzylammoriium hydroxide produces a greater distention (smaller distance between spots) than tetramethylammonium hydroxide. TABLE 2 Equatorial interferences ~
INTERPLANAR DISTANCE0 I N
101 plane
0UBSTANCE
1 -~
Native cellulose Mercerized cellulose I Tetramethylammonium hydroxide-cellulose Trimethylethylammonium hydroxide-cellulose Trimethylbeneylammonium hydroxide-cellulose 1 D~methyldibensylammonium hydroxide-cellulose Heated modification of dimethyldibenzylammonium hydroxidecellulose Heated modification of trirnethylbensylammo- 1 nium hydroxide-cellulose Cellulose obtained after removal of swelling agent
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*This value is approximated, since the compound gives a rather indefinite pattern.
The increase, however, is not always directly proportional to the size of the substituted groups, as, for example, when one of the methyl groups is replaced by a? ethyl group. Likewise. within the range of experimental error (=k 0.3 A,), there is not an increase of the 101 planar distance corresponding to the dimensions of an additional benzyl group when the organic base contains two benzyl groups instead of one. These results differ from those obtained by Trogus and Hess (19) on the diamines and by Barry, King, and Peterson (2) on the amines, where the 101 interplanar spacing is directly related to the size of the alkyl group of the entering molecule. The present results may be explained by assuming that the organic base molecules have a definite orientation in the crystal lattice, and that the
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WAYNE A. SISSON AND WILLIAM R. SANER
second benzyl group in dimethyldibenzylammonium hydroxide does not appreciably increase that dimension of the molecule which arranges itself perpendicular to the 101 &me of the cellulose unit cell. With each reagent the extension of the 101 plane of native cellulose appears to be slightly larger (about 0.3 A.) than is the case for mercerized cellulose (see table 2). Here, too, the results differ from those on diamines (19), where the extension of the 101 plane of mercerized is greater than that of native cellulose. The fiber period as determined from three layer lines is the same for all the corn unds studied in the present investigation. The apparent value identical with that for native cellulose. It is possible howof 10.3 ever, that the actual fiber period is some multiple of the value 10.3 I n some of the patterns there are extra spacings (8.0 and 6.3 A.), which apparently are not associated with the swelling compound. This indicates the existence of another product, such as a modified hydrate cellulose, and suggests the possibility of the compound being in equilibrium with native cellulose and with hydrate cellulose formed by the hydrolysis of the compound.
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Effect of concentration The change in the cellulose lattice is associated with a minimum concentration for each reagent, below which the x-ray diagram of native cellulose is not affected. For tetramethylammonium hydroxide at room temperature this minimum concentration lies between 16 and 20 per cent. For trimethylethyl-, trimethylbenzyl-, and dimethyldibenzyl-ammonium hydroxides the limiting concentrations are 17 to 22, 25 to 29, and 27 to 31 per cent, respectively. These percentages correspond roughly to normalities of 2.0, 1.9, 1.6, and 1.15 (table 1). The normalities of the reagents necessary to change the cellulose lattice decrease linearly when plotted against increasing molecular weights. These results coincide with those of Lieser and Leckzyck (12), who showed that a straight-line curve is obtained if the normality of base necessary to “dissolve cellulose” is plotted against the molecular weight of the base. The amount of cellulose converted to the swelling compound by the action of a given amount of quaternary ammonium hydroxide appears to be subject to a mass action effect. For example, when a given weight of finely ground native cotton fibers is mixed with a definite amount of a 42 per cent solution of trimethylbenzylammonium hydroxide and allowed to come to equilibrium, approximately 1.5 to 2.0 molecules of base per glucose unit are necessary to effect a complete change in the x-ray diagram. With a 34 per cent solution, each glucose unit requires from 2.0 to 2.5 molecules of base to produce complete conversion. The average value iS about 2 molecules of organic base to one glucose unit.
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Owing to the presence in the fiber of e x c w reagent, part of which is held by the swollen intercrystalline material, it is difficult to determine analytically the exact amount of reagent that reacts with the crystalline cellulose to form the quaternary ammonium hydroxide-cellulose complex. An approximate idea of this amount, however, may be obtained from calculations based upon the size of the entering organic base molecule and the increase in volume of the cellulose unit cell. The diameter of the entering quaternary ammonium hydroxide molecule, estimated from interatomic distances, is of the same order as that approximated experimentally by subtracting 7.4 (101 interplanar spacing for mercerized cellulose) from the observed 101 interplanar spacing of the swelling compound. The increase in volume of the unit cell resulting from complex formation may be determined from the data given in table 2. The increase is approximately of the order necessary to accommodate one quaternary ammonium hydroxide molecule per glucose unit. These calculated results are similar to the experimental values obtained by Trogus and Hess (19) on the diamines, in that the ratio of diamine to glucose unit waa found to be approximately 1:l.
Preferential action When a mixture containing equivalent amounts of tetramethylammonium hydroxide and trimethylbenzylammonium hydroxide (or dimethyldibeneylammonium hydroxide) is applied to cellulose fibers, the tetramethyl molecule exerts a preferential action, since the resulting x-ray diagram shows only the interference pattern characteristic of the tetramethylammonium hydroxide-cellulose complex. The explanation for such preferential action is not clear, but it is possible that the molecules of the tetramethyl compound, being smaller than those of either of the benzyl compounds, do not have to expand the cellulose chains so far apart and therefore may enter into the lattice with greater ease. There is the possibility, however, that this preferential action toward the crystalline cellulose may be related inversely to a preferential action of the reagents toward the intercrystalline material. All of the four reagents studied react essentially alike in that they form a swelling compound with the crystalline cellulose, but their action on the intercrystalline material is quite different: the gelatination of the intercrystalline material, as discussed later, is very small with tetramethylammonium hydroxide, but it increases with increasing molecular weight of the quaternary ammonium base, until it is quite pronounced with dimethyldibenzylammonium hydroxide. The failure of choline t o swell the fiber or to produce a change in the diffraction pattern is not explained by the present data, but there is the possibility that it may be related to the polarity of the molecule. Bock
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WAYNE A. SISSON AND WILLIAM R. SANER
(4)found, in general, “that those quaternary ammonium hydroxides which contain unsubstituted hydrocarbon groups will dissolve cellulose.’’ Eflect of heat When flax fibers, either native or mercerized, are pretreated with trimethylbenzyl- or dimethyldibenzyl-ammonium hydroxide and then heated at high enough temperatures for a sufficiently long period of time, they tend to shrink and to alter their external appearance. After heating there is obtained a different x-ray pattern, which consists of a new crystalline diagram superimposed upon an amorphous rtiagram (figure 2E). I n the crystalline pattern the 101 interplanar distance is reduced from 16.7A. to 13.4or 13.08. (see table 2). At 50°C.this chqnge takes place within about 15 min., while at lower temperatures more time is required. At room temperature no change in the x-ray diagram occurs even after aging for several months. When the heated samples are treated with trimethylbenzyl- or dimethyldibenzyl-ammonium hydroxide, the 101 interplanar distance increases again to its original dimension of 16.7b. Table 2 contains the values of interplanar distances as determined from interference patterns of the heated modifications. No change in the 101 planar distance could be noted when fibers treated with tetramethylammonium hydroxide and trimethylethylammonium hydroxide were heated for as long as 20 hr. at temperatures as high as 90°C. Removal of swelling agent Upon removal of the swelling agent the fibers shrink in diameter, and the x-ray diagram of hydrate cellulose is always obtained regardless of whether the starting material is native or mercerized cellulose. I n most cases the swelling agents were removed by treatment with dilute acids, followed by washing with water. The quaternary ammonium hydroxide cellulose complexes differ from those of the diamines in that the latter, after removal of the diamine, yield either native or hydrate cellulose, depending on the nature of the starting material (19). The modifications obtained by heating cellulose pretreated with trimethylbenzyl- and dimethyldibenayl-ammonium hydroxides also yield x-ray patterns of hydrate celiulose when treated with dilute acid and water (figure 2F). A comparison of figures 1B and 1C shows the shrinkage in fiber diameter upon removal of the swelling agent. FIBER SWELLING
Up to this point the discussion has been concerned primarily with the action of the quaternary ammonium bases on the cellulose crystal lattice. The action of these swelling reagents on the fiber as a whole will now be
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considered. The fiber-swelling reaction, which proceeds rather rapidly, is characterized by a relatively high rate of diffusion of the base throughout the fiber, as indicated by microscopic examination, and the x-ray pattern indicates an apparently equal modification of all the crystalline cellulose that reacts with the base. This type of reaction, according to Lorand and Georgi (13), is characteristic of the permutoid type. According to Katz (1I), the x-ray results are indicative of permutoid intramicellar swelling, accompanied by the formation of a compound of cellulose with the swelling agent, since the change in diagram is not a continuous function of the degree of fiber swelling, and since for each base there is one pattern characteristic of the reacted cellulose. In this case the swelling is in no way explained by the change in x-ray diagram; it is accounted for by the gelatination of the intercrystalline or cementing material, and the permutoid change of the crystal structure of the cellulose is an aceessory phenomenon in the mechanism of swelling. In the present paper the term “swelling compound” has been used in the sense described by Katz ( l l ) , but the present data do not indicate whether the swelling reaction is identical with the mechanism described by Katz; this is considered by the authors still to be an open question. From the present data it is clear, however, that the expansion resulting from the formation of the swelling compound accounts for only a small portion of the increase in fiber diameter revealed by microscopic examination. The x-ray patterns show the swelling compound to be formed before appreciable increase iv fiber diameter takes place, and, after the swelling compound is formed, further addition of the swelling reagent produces no additional lattice expansion, while the fiber diameter continues to increase. This continued increase in fiber diameter, therefore, is apparently due to swelling of the intercrystalline material. This mechanism is substantiated by microscopic examinat ion, since the fibrils can be observed to be pushed apart and arranged at larger irregular angles to the fiber axis as the fiber diameter increases, while the fibrils themselves appear to remain intact (see figure 1B). The overall swelling reaction, therefore, may be thought of as a dual process; the crystalline cellulose apparently has a limited increase in volume, while the intercrystalline material has no limit. The intercrystalline material may be thought of as forming a true lyophilic colloid, which may swell indefinitely with the dispersion medium, while the crystalline cellulose may be thought of as being lyophobic to the extent that it is not changed beyond the formation of a swelling compound with the reagent. A similar suggestion has been made regarding the swelling of cellulose fibers in cuprammonium hydroxide (17). This dual mechanism of swelling is in agreement with the chemical and slit ultramicroscopic data of Compton (6), which indicate that the cellulose component is not molecularly dispersed in quaternary ammonium bases but rather is dispersed
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WAYNE A. S I S 8 0 N AND WILLIAM R. SANER
as cellulose particles. Fiber swelling and compound formation and its relation to fiber dispersion and reactivity are being studied further. SUMMARY
1. When cellulose fibers, either native or mercerized, are treated with a quaternary ammonium hydroxide, they increase in diameter and give a new x-ray diagram which differs from that of the original cellulose. These results are best explained by assuming the fiber to have a dual structure: the increase in fiber diameter is due to an intercrystalline material which swells indefinitely with the reagent, while the change in x-ray diagram is due to the formation of a swelling compound between the reagent and the crystalline cellulose. 2. The principal change in the x-ray diagram corresponds to a lattice distention of the 101 interplanar distance from 6.1 b. in native cellulose to approximately 13.0 b. for the swelling compound between native cellulose and tetramethylammonium hydroxide, and to 16.7 A. for dimethyldibenzylammonium hydroxide. The fiber period, 10.3 &, remains unchanged in all compounds. 3. The change in x-ray pattern is not a continuous function of the amount of reagent added, and for each reagent there is one pattern characteristic of the reacted cellulose. This change is associated with a minimum concentration below which the cellulose lattice structure is not changed. Concentration also affects the number of molecules of base per glucose unit necessary to change the x-ray pattern. 4. I n mixtures containing equivalent amounts of tetramethylammonium hydroxide and either trimethylbenzyl- or dimethyldibenzyl-ammonium hydroxide the former base exerts a preferential action upon cellulose, the x-ray diagrams being characteristic of the tetramethylammonium hydroxide-cellulose complex. 5. The effect of heat on the swelling compounds of cellulose with triniethylbenzyl- and dimethyldibenzyl-ammonium hydroxides is to decrease the 101 interplanar distance to 13.0 b. The tetramethylammonium hydroxidecellulose and the trimethylethylammonium hydroxide- cellulose complexes do not change upon heating. 6. The cellulose complexes from either native or mercerized cellulose or their heated modifications are completely reverted to hydrate cellulose by washing with dilute acid and water.
The quaternary ammonium hydroxides used in this research were obtained through the courtesy of Rohm and Haas Company, Philadelphia, Pennsylvania, whose cooperation is greatly appreciated.
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