Studies in polyelectrolytes. II. Gum arabate

STUDIES IN POLYELECTROLYTES. II. GUM ARABATE Sadhan Basu, Pares Ch. Dasgupta and Anil K. Sircar Indian Association for Cultivation of Science, Lady Willingdon Road, Calcutta 32, India Received May 17~ 1951

INTRODUCTION

Natural arabic gum is a mixture of potassium, magnesium, and calcium salts of a high-polymeric acid, which may be liberated by treatment with mineral acids. Gum arabic purified by repeated precipitation from water with alcohol has an equivalent weight of 1000-1200 and a molecular weight of about 240,000 (1). On hydrolysis it yields a mixture of galacrose, arabinose, rhamnose, and as high as 28% of an aldobionic acid which has been proved to be L-galacturonic acid 6-galactose (2,3). Very little is known as to the nature of the chemical union between these groups in the molecule of arabic gum. A ring formula suggested by Norman (4) has been rejected on the basis of various physicochemical considerations. Strain double refraction of a thread of arabic gum showed that the molecules are elongated and most probably linear; but the x-ray examination of the thread revealed no fiber diagram. A reconciliation between these two structures has been proposed and it has been suggested that the constituent units in a gum arabic molecule are arranged in a chain, but the chains are not very regular and therefore cannot fit into a well-ordered lattice owing to side-branching (5). An indirect way of deciding the chain-like character or otherwise of the gum arabic molecule, is to study the folding-unfolding phenomenon of such a molecule in solution. The folding-unfolding phenomenon in a high-molecular-weight compound may be strikingly demonstrated by simple physicochemical measurements if the compound can be converted into a soluble, ionizable compound; that is to say, into a polyelectrolyte, as suggested by Fuoss (6). THEORETICAL

By the term polyelectrolyte is meant a high-molecular-weight compound which contains a number of ionizable groups distributed along the polymer chain. Polyelectrolytes, unlike their analogs the simple electrolytes, on the one hand, and neutral polymers on the other, show some characteristic behavior in solution, e.g., viscosity, conductivity, osmotic pressure, etc. These peculiar behaviors of the polyelectrolytes arise, according to 539

540

SADHAN BASU, P A R E S Ctt. DASGUPTA AND ANIL K. SIRCAR

Fuoss, due to the presence of high charge densities on the polymer chain in dilute solution and flexibility of the polymer chain. If the degree of ionization of the polye!ectrolyte chain is decreased by any factor, say by an increase of concentration or by adding a suitable electrolyte, the average distance between the charge centers in the polymer chain increases; consequently the coulombic repulsion between the charge centers decreases and the polymer chain coils u p . Fuoss (6) studied a number of synthetic polyelectrolytes and interpreted their peculiar behaviors by his folding-chain theory. Katchalsky (7), by mathematical analysis, also interpreted the polyelectrolytie behavior in the same light. It has emerged from these investigations that the folding-chain theory of Fuoss (6) is almost the only explanation for the peculiar viscometric and other behavior of polyelectrolytes, and neither in neutral polymers nor in colloidal electrolytes could such peculiarities be detected. It has been shown that arabic gum disperses to molecular units in solution and the colloidal character of the solution is associated with the colloidal dimension of the molecule itseif (8). If in the gum arabic molecule the constituent units are arranged in a chain with the carboxyl group of the uronic acid distributed along the chain, the solium salt of the acid in dilute solution will behave as a polyelectrolyte. With this end in view, the present investigation was undertaken, the results of which are summarized in the present paper. EXPERIMENTAL

Preparation of the Gum Arabic Acid The arabic gum supplied by Messrs. Bengal Chemical & Pharmaceutical Works, Calcutta, was dissolved in water, filtered, and precipitated with alcohol. The process was repeated three times. The purified gum was then dissolved in water and brought to pH 2.5 by adding the requisite quantities of hydrochloric acid, kept at that pH for 24 hr., and then dialyzed till the dialyzate had an ash content of only about 0.05%, The acid was titrated electrometrically using a glass electrode in conjunction with a model G Beckman pH-meter. The equivalent weight was found to be 1208, a result, in good agreement with the recorded data in the literature, which is in the range of 1000-1200 (1). Preparation of Sodium Salt of Arabic Acid A solution of arabic acid was neutralized with sodium hydroxide solution, the sodium salt precipitated out of the solution with alcohol, redissolved in water, and dialyzed for 4 hr. against distilled water in order to remove any adhering alkali. The concentration of the solution was determined by evaporating and vacuum drying a portion of the solution. Ash estimation of the gum arabate corresponded to sodium calculated

POLYELECTROLYTES. If. GUM ARABATE

541

from the equivalent weight of the acid. The solution had a p H of 6.8, and was used as such, or with dilution wherever necessary, in all subsequent measurements. The solution remained unchanged, i.e., without any hydrolysis, for more t h a n 15 days. M e a s u r e m e n t of Viscosity

The viscosity measurements were done with two Ostwald viscometers having flow times of 230.7 sec. and 324.7 sec., respectively, with water at a t e m p e r a t u r e of 35 ± 0.1°C. The specific and relative viscosities were calculated from the following equations: relative viscosity = v _ pit1 7o

pt

specific viscosity = ~ , = v/v0 - 1 where ~ and T0 are the viscosities, p~ and p the densities and t~, and t the time of efflux (in seconds) for the solution and solvent, respectively. In the range of dilute solutions used, the difference in densities of the solution and water was insignificant and no density measurements were taken. RESULTS The results of reduced viscosity (7~,/c, where c is the concentration in g./100 cc.) measurements at different concentrations of gum arabate are summarized in Table I and the corresponding w , / c versus c curves are given in Fig. 1. TABLE I Change in Reduced Viscosity with Concentration of Gum Arabate Concentration g./100 cc, water

y~p/C

1.3140 0.8760 0.5840 0.3893 0.2595 0.1730 0.1153 0.0577

1.1640 1.1760 1.2630 1.3640 1.4810 1.6160 1.7870 1.9670

I t is evident from Table I and Fig. 1 t h a t ~ sp/c rises as the concentration of gum arabate is reduced. T h e measurements of reduced viscosity of gum arabate solutions were done at various constant sodium chloride concentrations, the results of which are given in Table I I and the corresponding v ,p/c versus c curves are drawn in Fig. 1. I t becomes evident from Table I I t h a t the viscosity of gum arabate solution is lowered by the addition of NaC1, the lowering increasing with

542

SADHAN BASU, PARES C t t . DASGUPTA AND -4,NIL K. SIRCAR

increasing concentration of NaC1. T h e curves at three NaC1 concentrations, viz., 11.518 X 10 -~, 17.277 X 10 -5, and 28.795 X 10 -~ g.-equiv./1., show well-defined m a x i m a at nearly equivalent concentrations of g u m arabates, i.e., 12.62 X 10 -5, 18.32 X 10 -5, and 29.403 X 10 -5 e q u i v . / 1 , respectively. At higher NaC1 concentrations, e.g., 17.852 X 10 -4 a n d 46.647 X 10 -4, the ~,p/c versus c curves are similar to those of neutral polymers.

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FIe. 1. Reduced viscosity versus concentration. Viscosity m e a s u r e m e n t s were extended to different solutions of g u m a r a b a t e neutralized to different extents. T h e results are s u m m a r i z e d in T a b l e I I I , a n d the corresponding ~l~p/Cversus a (extent of neutralization) curves are given in Fig. 2. Calculation of the extents of neutralization were done in the following w a y : when to a 10 cc, solution of arabic acid (2.628%) 2.65 cc. of 0.0819 N N a O H was added, the acid was completely neutralized, as obtained f r o m a potentiometric titration curve. T h e resulting solution was t h e n t a k e n as 100% neutralized. Therefore, when to 10 co. each of the s a m e

POLYELECTROLYTES.

ii.

543

GUM ARABATE

TABLE II

Viscosity-Concentration Effect in Presence of NaCl Concentration of gum arabate

Concentra~on of NaC1

nsp~

g./lO0 cc.

g. equiv.fl.

1.0146 0.8117 0.6494 0.4329 0.2886 0.1924 0.1283 0.1015 0.0676

11.518X 10-~

0.9802 1.0040 1.0260 1.0890 1.1640 1.2310 1.2530 1.1760 1.1040

1.0146 0.8117 0.6494 0.4329 0.2886 0.1924 0.1283 0.0885

17.277 X 10-5

0.9270 0.9452 0.9614 0.9933 1.0390 1.0520 1.0360 0.9891

1.0146 0.8117 0.6494 0.2886 0.1924 0.1283 0.0855

28.795 X 10-5

0.8143 0.8268 0.8506 0.8722 0.8476 0.8130 0.8096

1.0146 0.4504 0.3006 0.2004 0.0891

17.852 X 10-4 •

0.5805 0.5012 0.4668 0.4321 0.4238

1.0146 0.6764 0.4509 0.3006

46.647 X 10-4

0.4058 0.3734 0.3474 0.3303

a r a b i c a c i d s o l u t i o n , 0.4, 0.8, a n d 2.2 cc., etc., of 0.0819 N N a O H w e r e a d d e d t h e s o l u t i o n s w e r e n e u t r a l i z e d t o t h e e x t e n t of 15.10, 30.19, a n d 8 3 . 0 3 % etc., r e s p e c t i v e l y . F i n a l l y t h e r e l a t i v e v i s c o s i t y of a s o l u t i o n w a s m e a s u r e d a t c o n s t a n t c o n c e n t r a t i o n of g u m a r a b a t e w i t h i n c r e a s i n g p H , t h e p t I i n c r e m e n t b e i n g o b t a i n e d b y t h e a d d i t i o n of a few d r o p s of a l k a l i t o a l a r g e v o l u m e of t h e g u m a r a b a t e s o l u t i o n s u c h t h a t p r a c t i c a l l y no v a r i a t i o n in t h e c o n c e n t r a t i o n a p p e a r e d . A t first t h e v i s c o s i t y fall w i t h p H w a s s m a l l w h i l e a b o v e a b o u t 9 p H t h e f a l l w a s m u c h m o r e r a p i d . W h e n a g a i n t h e p H of t h i s

544

SADHAN BASU, PARES CH. DASGUPTA AND ANIL K. SIRCAR

final solution was reversed and gradually b r o u g h t b a c k to the initial p H value b y the addition of acid (viz., 6.8), there was no rise in viscosity, which remained practically constant f r o m 11 to 6.8 p H . W h e n this final solution was dialyzed, the viscosity gradually increased with time, till a t last nearly the original value was regained. All these points are well illust r a t e d in Fig. 3. TABLE I I I

Viscosity Variation with the Extent of Neutralization Concentration of Per cent gum acid neutralized ~sp/C % 1.3140 100.00 1.164 93.09 1.223 83.03 1.141 72.97 1.094 50.31 0.908 30.19 0.748 15.10 0.615 0.00 0.518 0.6570

100.00 93.09 83.03 72.97 50.31 30.19 15.10 0.00

1.229 1.342 1.295 1.249 1.049 0.871 0.719 0.613

0.3285

100.00 93.09 83.03 72.97 50.31 30.19 15.10 0.00

1.406 1.483 1.588 1.444 1.246 1.053 0.864 0.735

DISCUSSION I t has been found t h a t the ~ p / c versus c plot (Fig. 1) of sodium a r a b a t e solution increased sharply with dilution, which evidently indicates some increase in the h y d r o d y n a m i c volume unit. This p h e n o m e n o n m a y reasonably be explained f r o m the consideration of the folding-unfolding properties of the high-polymeric chain compounds. At a finite concentration of the g u m a r a b a t e in solution some of the sodium ions dissociated f r o m one molecule, a n d are drawn b a c k b y a n o t h e r molecule, so t h a t on a time average, a polyion always has some a m o u n t of oppositely charged ions closely associated with it. As the solution is diluted, i.e., as the concentration of

POLYELECTROLYTEB. II. GUM ARABATE 16

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the gum arabate is reduced, the probability of a polyion finding an oppositely charged ion near its vicinity diminishes, and as a result the effective charge on the polyion increases with dilution. Repulsion between similarly charged centers on the gum arabate chain causes the molecule to extend which evidently increases with an increase in the dissociation of the gum arabate molecule, i.e., the extension of the chain increases with dilution. 2.7

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546

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This explains the rapid rise in viscosity of a gum arabate solution at lower concentrations. Various v~p/c versus c curves f o r g u m arabate solution in the presence of different amount of NaC1 are also given in Fig. 1. It has been found that by the addition of 11.518 X 10-5 g. equiv. NaCl/1. the sharp risein the ~,/c versus c curve vanishes, the curve showing a well-defined maximum which appears when the stoichiometric concentration of the Na ion from the polymer .becomes nearly equal to that from the added NaC1. With increasing NaC1 concentration, the maximum shifts toward higher values of sodium arabate concentration till at last it vanishes completely and the curves resemble those of the neutral polymer. This type of peculiar viscosity behavior has been noticed in the case of a large number of p01yelectrolytes, e.g., polyacrylic acid (11), pneumococcus polysaccharides (9), sodium pectinates (10), and a number of polyvinyl pyridonium derivatives (6), and fits with the general behavior of polyelectrotytes. There is another important characteristic of a solution of gum arabic acid which can also be explained by the folding-chain theory. As the acid is neutralized, the reduced viscosity of the solution increases. This is evident from the curves in Fig. 2. As the acid is neutralized with sodium" hydroxide, more and more sodium salt of the acid dissociates, the effective charge on the polymer goes up, and, hence, the reduced viscosity increases. Nearabout the poin t of complete neutralization, the ~ ~p/c value decreases although slowly. This fall in v ~,/c value has been explained by Katchalsky and Kuhn (7) as due to a high charge density on the polymer chain and an increased concentration of oppositely charged sodium ions. This viscosity behavior is exactly the opposite of what happens to, and is expected of, a weakly acidic Colloidal electrolyte. The viscosity of the gum arabate solution (even of a dilute solution) changes markedly with pit. It will be evident from Fig. 3 that at pit 9.5, obtained by adding few drops of alkali to the gum arabate solution, the viscosity of the 1.05% solution of sodium gum arabate falls sharply. When the pH is reversed from 11 to 6.8 by neutralizing the alkali with acid, the curve is not retraced and the viscosity remains nearly constant, showing a slight fall with diminishing pit. If, after bringing the pH back to the original value with acid, the solution be dialyzed, the viscosity gradually rises with time and the original viscosity is nearly recovered after about 8-10 hr. If this effect be due simply to pit, then there is no reason why the viscosity behavior should depend on the mode of pH variation. Further this pH effect cannot be due to breakdown of micellar units since in dilute solution gum arabate is dispersed to molecular units. It has been shown by Tendeloo (12) and also by us (Table II and Fig. 1) that a similar fall in viscosity takes place when, instead of sodium hydroxide, an increasing

POLYELECTROLYTES. II. GUM ARABATE

547

amount of sodium chloride solution is added step by step to the gum ara= bate solution. Thus if the fall in viscosity in the presence of N a 0 H be attributed to the presence of sodium ion, then the explanation of the above phenomenon will follow simply from the folding-chain theory of polyelectrolytes. When the pH is reversed from 11 to 6.8 by the addition of successive quantities of HC1, the N a 0 H present is simply neutralized to NaCI. There is no change in Na ion concentration and hence the vis' cosity of the solution remains unaltered because the coiling of the chain remains constant. But when this solution is dialyzed, the sodium salt present in the system passes out, and as a result the viscosity increases owing to an increased extension of the polymer chain. It is evident, therefore, that all the peculiar viscometric characteristics of the gum arabate solution may satisfactorily be explained by the folding= chain theory of polyeleetrolytes which presupposes the existence of flexible chain molecules in solution. As the chain is irregular due to side branching, the coiling up of the molecules cannot proceed to a great extent, and consequently the relative rise in viscosity on dilution is much less in the case of gum arabic as compared to other polyelectrolytes recorded in the literature (6,10). ACKNOWLEDGMENT Thanks are due to Prof. S. R. Palit, Indian Association for the Cultivation of Science, for his keen interest, helpful suggestions, and constant encouragement during the course of these investigations. SUMMARY

Measurements of viscosity with varying concentration of solute, added sodium chloride, and the extent of neutralization of the solutions of gum arabate and arabic acid have been reported. A connected explanation for all the different observations has been formulated b y the assumption of the existence of flexible chain molecules in solutions of gum arabate. From the same considerations the effect of pH variation with alkali on the relative viscosity of gum arabate solution has been attributed to the effect of sodium ion from alkali rather than the particular pH value. REFERENCES 1: 2. 3. 4.

O~KLEY, H. B., Trans. Faraday Soc. 31, 136 (1935). HEIDELBERGER, .¥[., AND KENDALL, F. E., J. Biol. Chem. 84, 639 (1929). CHALLINO~, S. W., HAWORTH, W. N., AND HlSST, E. A., J. Chem. Soc. 1931, 258. NORMAN,A. G., The Biochemistry of Cellulose, the Polyuronides, Lignin, etc., p. 125. Oxford Press, London, 1937. 5. IV[EYEa,K. H., Natural and Synthetic High Polymers, Vol. IV, p. 375. Interscienee Publishers Inc., New York. 6. Fuoss, R. M., Science 108, 545 (1948); J. Polymer. Sci. 3, 246, 602 (1948); ibid. 4, 97 (1949).

548 7. 8. 9. 1O. 11.

SADttAN BASU~ PARES OH. DASGUPTA AND ANIL K. SIRCAR

KATCHALSK¥,A., KUttN, W., AND KU~ZLE, O. Helv. Chem. Acta, 31, 1994 (1948). HARTLEV, G. S., Quarterly Rev. Chem. Soc. 2, 152 (1948). HEmELBERGER, M., ~ D KENDALL, F. E., J. Biol. Chem. 95, 127 (1932). PALS, D. T. F., AND HERMANS, J. J., J. Polymer Sci. 3, 897 (1948). STAVDINGER, It., Die hochmolekularen Organischen Verbindungen, Part I I D. Springer, Berlin, 1932. 12. TENDELOO, H. 5. C., Rec. tray. chim. 48, 23 (1929).