Studies on polyelectrolytes. III. Polyglucosamine hydrochloride

S T U D I E S O N POLYELECTROLYTES. III. P O L Y G L U C O S A M I N E HYDROCHLORIDE Sadhan Basu ~ and P a r e s Ch. DaN Gupta Indian Association for the Cultivation of Science, Calcutta 32, India Received December 3, 1952 INTRODUCTION

It has been shown by Basu and DaN Gupta (1) that the various physicochemical properties of solutions of sodium carboxymethylcellulose (SCMC) can be explained b y the folding-chain theory of Fuoss (2) taking into consideration the polyelectrolyte character of the sodium salt of carboxymethylcellulose. If these properties be attributed to the coilinguncoiling of the cellulose chain owing to variation in the charge density on the polymer chain, then it may be argued that the nature of the charge will have nothing to do with these properties, which will be governed mainly b y the charge distribution on the chain. In the case of SCMC, dissociation of the salt in water leaves the chain negatively charged and the mutual repulsion between the similarly charged centers in the same chain is responsible for the peculiar physicochemical properties of the compound in solution compared to neutral polymers. It may be expected that the polyglucosamine hydrochloride which on dissociation in water leaves the chain positively charged will also behave exactly similarly with respect to its physicochemical properties in solution. With a view to testing this point a series of investigations were undertaken, exactly similar to those reported previously. Since it is almost impossible to compare the compounds, S C M C and polyglucosamine hydrochloride (PGH) at exactly the same molecular weight, the comparison has been limited to the variation in the physicochemical properties rather than to the absolute values of these quaritities. The molecular nature of the P G H solution evidently removes any complications arising from the associated colloidal systems (3). EXPERIMENTAL

Polyglucosamine hydrochloride was prepared from chitin of carapace of S. Paeneus by the alkali hydrolysis method of Clark and Smith (4). The carapace was washed thoroughly with warm water and then treated with nitric acid (dilute) to remove calcium carbonate and coagulate some 1 Present address: Dept. of Chemistry, Indiana University, Bloomington, Indiana.

355

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SADttAN

BASU

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GUPTA

of the proteinous matter. T h e substance was freed of acid and h e a t e d with alkali (20%) for a b o u t 7 hr. to h y d r o l y z e the a c e t y l a m i n o group of chitin to acetic acid and free a m i n o group. T h e p r o d u c t was t h e n washed with w a t e r a n d dissolved in 0.1 N hydrochloric acid. The solution was dialyzed to free it from residual sodium salts a n d free chloride ions, filtered, precipitated with acetone, washed free of a n y hydrochloric acid with acetone, a n d dried at low temperature. The polyglucosamine hydrochloride thus o b t a i n e d was a h y d r a t e d salt with two molecules of w a t e r of crystallization per glucosamine ring (5). TABLE I

Viscosity of Polyglucosamine Hydrochloride Solutions Concentration of P G H in g./lO0 ml. of solution

(c)

~Sp/C

In water (~ = 78) 0.3120 0.2080 0.1387 0.0925 0.0619 0.0309 0.0155 0.0077

8.1 9.0 10.1 11.4 12.6 15.3 17.8 19.9

In dioxane-water 1 : 4 (~ = 62.8) 0.1248 9.1 0.0832 10.1 0.0416 12.2 0.0208 13.9 0.0104 15.3 In dioxane-water 1 : 2 (~ = 52.7) 0.1248 8.0 0.0832 8.9 0.0416 10.4 0.0208 11.7 0.0104 12.7 The equivalent weight of P G H b y t i t r a t i o n was 220.4 and the nitrogen was 5.76% (Kjeldahl), s o m e w h a t lower t h a n the theoretical a m o u n t (7.09) on the basis of one amino group per glucosidic ring. The viscosity m e a s u r e m e n t s were done with an Ostwald viscometer h a v i n g a flow time of 230 sec. with w a t e r at 35 ± 0.01°C. T h e relative a n d specific viscosities were calculated from the usual e q u a t i o n s (1). Conductances, m e a s u r e d at 35 ± 0.01°C., on a direct-reading, bridget y p e Philoscopic unit, were t a k e n at a cell voltage of 2 v. a n d at a freq u e n c y of 1000 cycles/sec. T h e c o n d u c t i v i t y cell was a K o h l r a u s c h - t y p e ,

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parallel-plate, fixed-electrode cell having a cell constant of 0.6723, checked against 0.01 N potassium chloride solution. The conductance of water used in preparing all solutions were 2.2 X 10-6 mho. RESULTS

The results of viscosity measurements on solutions of P G H at different concentrations in water and dioxane-water mixtures are summarized in Table I and the corresponding ~,p/c versus c curves are given in Fig. 1 (~ represents the dielectric constant of the solvent). 20 e-.-~

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The relative viscosities of P G H solutions were measured at constant solute concentration with increasing and diminishing pH's. The P G H solution (0.162%) had a pH of 4.1. In one experiment the pH of the solution was lowered to pH 1.6 by the stepwise addition of hydrochloric acid, the viscosity being measured at each step. The solution at this pH was divided into two parts: the pH of one was increased step by step by dialysis and of the other by the addition of sodium hydroxide solution, viscosities being measured similarly. In another experiment the pH of the original solution was increased by the addition of sodium hydroxide solution to pH 6.6 and again lowered to its original value with hydrochloric acid, measuring the viscosity in the course. The results are sum-

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SADHAN BASU AND P A R E S

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marized in T~/ble I I and the corresponding ~/~0 versus p H curves are given in Fig. 2. T h e viscosity measurements were e x t e n d e d t o solutions of P G H containing different a m o u n t of sodium chloride, sodium bromide, potassium chloride, sodium sulfate and calcium chloride for which the vsp/c versus c curves are given in Figs. 3 and 4. I t is e v i d e n t f r o m these figures t h a t the TABLE II Change in Relative Viscosity of PGH Solution with pH pH diminishing by the addition of HCI pH ~/~o

4.10 3.55 3.05 2.58 2.15

2.62 2.52 2.25 1.93 1.67

1.80

1.53

1.60

1.48

pH increasing by dialysis pH ~/~o

(In acid region) 1.60 1.90 2.55 3.20

pH increasing by the addition of NaOH pH ~/~o

4.10 4.80 5.70 6.60

pH increasing by the addition of NaOH pH ~/~a

1.48 1.58 1.92 2.37

1.60 2.50 4.00

1.48 1.47 1.46

pH diminishing by the addition of HCI pH ~/~o

(In the alkaline region) 2.62 6.60 2.55 6.30 2.00 5.72 1.38 5.23 4.83 4.32

1.38 1.39 1.81 1.89 1.84 1.92

m a x i m a in the curves occur at c o n c e n t r a t i o n s (all expressed in 10-4 g. e q u i v . / l . ) 10.89, 27.78, and 48.10 units of P G H with 5.1, 9.28, and 18.56 units of NaC1, respectively. With 7.51 units of KC1 the m a x i m u m is a t 19.05 units of P G H , with 2.91 units of Na2SO4 at 21.8 units of P G H , a n d with 2.5 units of CaCl2 at 13.6 units of P G H . T h e results of conductivity m e a s u r e m e n t s are plotted as graphs of A-~/~ in Fig. 5 for the cases of P G H in w a t e r and d i o x a n e - w a t e r mixtures. DISCUSSION

I t will be evident from Table I a n d Fig. 1 t h a t the reduced v i s c o s i t y c o n c e n t r a t i o n relationship in water a n d w a t e r - d i o x a n e mixtures is exactly similar t o those of other polyelectrolytes (1,6-9), which can be explained almost u n i q u e l y in all cases b y the folding-chain t h e o r y of Fuoss (2). T h e general n a t u r e of the p H - v i s c o s i t y relationship of P G H is also similar t o t h a t of S C M C (1). T h u s f r o m T a b l e I I and Fig. 2 it is e v i d e n t t h a t t h e Viscosity of a solution of P G H decreases rapidly as the p H of the solution is diminished b y the addition of HC1. On bringing the p H b a c k

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to its original value with NaOH solution the viscosity did not increase back to its initial value. When, however, the pH is retraced by dialysis, the original viscosity of P G H is regained. If Na ions from added NaOH be held responsible for the lowering of the viscosity of SCMC solution, it is the C1 ion from the added HC1 t h a t is responsible for lowering the viscosity of P G H solution by suppressing the dissociation of the hydrochloride. Increasing the pH by neutralizing the excess HC1 by I'¢,aOH has 3.0

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Fro. 2. Effect of pH on the viscosity of a polyglucosaminehydrochloridesolution. no effect on the viscosity of the solution, since the concentration of chloride ion stays the same (as NaC1 after partial neutralization). When the pH is increased by dialysis the viscosity increases again since the removal of C1 ion out of the solution causes a better dissociation of the P G H with the subsequent uncoiling of the polymer chain. When to a P G H solution (pH 4.1) NaOH is gradually added to raise the pH, the viscosity curve (Fig. 4) also shows a similar fall. This is due to

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SADHAN BASU AND P A R E S

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conversion of PGH to polyglucosamine, which remains undissociated and is precipitated if the pH is increased beyond 6.6. When the polyglucosamine thus formed is converted into P G H by lowering the pH again by the addition of HC1, the viscosity of the solution again increases, but it fails to regain its initial value completely. By the addition of HC1 the P G H formed increases the viscosity owing to better dissociation of P G H 13 NoCI"o

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FIG. 3. Viscosityof a polyglucosaminehydrochloridesolution in the presence of sodium chloride. and the subsequent uncoiling of the chain. At the same time, however, the conversion of NaOH to NaC1 introduces some amount of free chloride ions in the solution which tend to suppress the dissociation of PGH; as a result the viscosity fails to regain its original value. The reduced viscosity (~sp/c) of P G H is considerably lowered by the addition of neutral electrolytes, and the ~.,/c versus c curves (Figs. 3 and 4) pass through a maximum for small concentrations of added electro-

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lytes. On increasing the c o n c e n t r a t i o n of added electrolyte, the m a x i m u m shifts t o w a r d the higher concentration of P G H , and w h e n the concentration of the a d d e d e l e c t r o l y t e becomes sufficiently high t o keep almost all the polymer molecules in the solution more or less undissociated, the P G H molecule b e h a v e s as a neutral polymer and the ~.p/c versus c curves are linear. This b e h a v i o r is exactly similar to t h a t of o t h e r polyelectrolyres, especially S C M C , a n d the explanation put f o r w a r d in our previous 0 --~CoCl z _ .... . . . . ~ ~,L,I

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Fro. 4. Viscosity of a polyglucosamine hydrochloride solution in tile presence of NaBr, KCI, Na~SO4, and CaCl~. c o m m u n i c a t i o n holds in this case as well, with the exception t h a t in the case of S C M C it is the cation t h a t is active where as in the present case it is the anion. F u r t h e r , in the case of S C M C it was observed t h a t the m a x i m u m in the ~,,/c versus c curve occurred at a b o u t equal ionic strength, whereas in t h e present case the m a x i m u m occurs at a m u c h lower c o n c e n t r a t i o n of a d d e d electrolyte. This is due, in all probability, to partial hydrolysis of P G H producing free hydrochloric acid which acts together with the a d d e d electrolytes in suppressing t h e dissociation of

362

SADHAN BASU AND PARES CH. DAS GUPTA

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FIG. 5. Conductivity of a polyg]ucosamine hydrochloride solution. TABLE III Conductivity of P G H and NaCl Mixtures in Water ~ Concentration of PGH in g./100 ml. of soln.

0.2880 0.1728 0.1037 0.0622 0.0373

pl ~ # X 104 pl ~- p X 104 pl X 104 (observed) (calculated) Concentration of NaC1 soln. : 0.011% Specific conductance (p): 2.651 X 10 -4 12.030 14.160 14.681 7.649 9.906 10.300 5.034 7.281 7.685 3.301 5.633 5.952 2.254 4.592 4.905

=pl and p indicate the specific conductivities of P G H respectively.

Per cent lowering

3.55 3.86 5.26 5.36 6.38 and NaCl

solutions

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363

PGH; as a result, the maximum occurs at a lower concentration than it would have in the case of no hydrolysis. The conductivity of PGH, unlike strong electrolytes or colloidal electrolytes but similar to polyelectrolytes (e.g. SCMC), increases with dilution at first slowly and then rapidly, and the A-~/~ curves (Fig. 5) are concave upward at low concentration with no obvious tendency to approach a limiting value. The explanation put forward in our previous work also holds in this case. The fact that the dissociation of polyelectrolyte in the presence of a neutral electrolyte is suppressed is evident from the results of conductivity measurements in the presence of added electrolytes. (See Table III.) It is evident that the sum of the conductances of polyelectrolyte and the neutral electrolyte is greater than that of their mixtures, and their difference increases with a decrease in the polyelectrolyte to neutral electrolyte ratio. ACKNOWLEDGMENTS Thanks are due to the Council of Scientific and Industrial Research, Government of India, for financial assistance to one of the authors (P.D.G.) and to Dr. S. R. Palit, Indian Association for the Cultivation of Science, for his keen interest. SUMMARY

The viscosity, conductivity, and the effect of pH and other neutral electrolytes on the physicochemical properties of polyglucosamine hydrochloride solutions have been measured. The similarity in behavior of polyglucosamine hydrochloride to those of other similarly constituted substances in which the polymer ion is oppositely charged, has been shown. All these properties can be explained by the folding-chain theory of Fuoss. REFERENCES BASU, S., AND DAs GUPTA, P. C., J. Colloid Sci. 7, 53 (1952). Fuoss, R. M., Science 108, 545 (1948). TACHIBANA, T., AND NOKAGAWA, T., J. Japan, Chemistry 1, 31 (1947). CLARK, G. L., AND SMITH, A. E., J. Phys. Chem. 40, 863 (1936). MEYER, K. IX., AND WEHRLI, H., Helv. Chim. Acta 20, 353 (1937). FUOSS, R. M., AND CATHERS, G. T., J. Polymer Sc/. 2, 12 (1947) ; ibid. 4, 96, 121, 457 (1949). 7. PALS, O. T. F., AND HERMANS, J: J., J. Polymer Sc/. 3, 898 (1948). 8. HEIDELBERGER, M., AND KENDALL, F. E., J. Biol. Chem. 95, 127 (1932). 9. GOLDACRE, R. J., AND LOCI-I, I. J., Nature 166, 736 (1951).

1. 2. 3. 4. 5. 6.