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Studies on polyelectrolytes
STUDIES ON POLYELECTROLYTES. VI. GUM AGAR Sadhan Basu and Anil K. Sircar Indian A~sociation for the Cultivation of Science, Calcutta, India Received August 31, 1954
INTRODUCTION In a previous communication it has been shown by Basu et al. (1) that the sodium salt of arabic acid behaves as a polyelectrolyte; this has been taken to indicate a linear structure of the gum arabate molecule. The work has been extended to other gums, and the present paper reports the results of similar studies on gum agar. The difference between the acidic character of arabic acid and agar acid lies in the fact that whereas the acidity in arabic acid is due to the C 0 0 H group of the uronic acid segment of the molecule, in agar acid this is due to the S03H group, agar being a calcium salt of the sulfuric acid ester of a complex polycarbohydrate. The present study has been confined to the free acid and its sodium salt and is expected to throw some light on the nature of the molecule and the physical properties of its solution. ~]XPERIMENTAL Preparation of Free Agar Acid and Its Sodium Salt The usual method of liberating agar acid from the gum is to treat the aqueous dispersion with hydrochloric acid when the gum is decomposed into calcium chloride and free agar acid which can be precipitated out by alcohol or acetone. It has, however, been shown by Fairbrother and Mastin (2) that the reaction reaches an equilibrium; hence in order to effect a complete removal of calcium the gum solution must be made rather strongly acidic, which introduces the chance of extensive degradation. In order to avoid this complication the gum solution was treated with a solution of sodium oxalate when calcium precipitated out as oxalate. The solution was filtered and then dialyzed free of sodium oxalate. The dialyzed solution was made acidic with HC1 and the agar acid precipitated with alcohol. The acid was again dissolved in water and dialyzed free of chloride ions. The equivalent weight, obtained by electrometric titration with alkali, was 2213. The ash content of the free acid was 0.7 %. 574
STUDIES ON POLYELECTROLYTES. VI
575
T h e free acid was converted into its sodium salt by treating it with a solution of sodium hydroxide, somewhat in excess of the equivalent amount, precipitated with alcohol, again dissolved in water, and dialyzed until neutral. The molecular weight of the sodium salt, determined osmometrically in a NaC1 solution, was about 30,000. It is evident, therefore, that even in this operation the molecule has undergone degradation. On an average there are about 14 acidic groups distributed along the chain per molecule. The solutions of free acid and its sodium salt in water were stable for at least two weeks When preserved at 15°C.
Measurement of Viscosity The viscosity measurements were done with two OstwMd capillary viscometers of flow times 3 minutes 22 seconds and 4 minutes 37 seconds, respectively, with water at 35 ° :/: 0.1°C. The specific and relative viscosities were calculated with the use of the following relations: relative viscosity = ~/~0 - pl tl pt specific viscosity = v~p = - -- 1 T0
where v and ,10 are the viscosities, 01 and p the densities, and h and t the time of effiux (in seconds) for the solution and solvent, respectively. In the range of dilute solutions used, the difference in densities between solution and water was insignificant and no density measurements were made. The results of viscosity measurements with and without added electrolytes and also at various pH's are shown graphically in Figs. 1-3. DISCUSSION
It may be observed from Figs. 1 and 2 that the wp/c versus c plot for both free acid and its sodium salt in solution increases rapidly with dilution, which evidently indicates an increase in hydrodynamic volume unit. This observation m a y reasonably be explained from the consideration of folding-unfolding properties of high molecular chain compounds, i.e., repulsion between similarly charged centers on the agar chain causing the molecule to extend, which evidently increases with an increase in the extent of electrolytic dissociation, i.e., with increased dilution. Various ~,/c versus c curves for agar acid in the presence of different amounts of H2SO~ and for sodium salt in the presence of different amounts of NaC1 are also given in Figs. 1 and 2. I t m a y be observed that the sharp rise at lower concentration disappears and is replaced by a bell-shaped curve with a well-defined maximum which shifts towards higher concen-
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FzG. 1. Viscosity of ag~r acid in presence of H~S04.
O - -~" in p~esence O~wa{el~ (~-- > . . . . 14"8~X15~t~ ~tc[ ~-~" ,, 29"TXIS"N N~lcl • - ` + ,, . t9"8 X16SN gael
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FIG. 2. Viscosity of Na-agar~te in presence of NaC1. tration of solute as the added electrolyte concentration is increased. At sufficiently high concentrations of electrolyte the curves in b o t h cases are perfectly linear, resembling those of a neutral polymer. This type of viscosity behavior has been noticed in the case of a large number of high
STUDIES ON POLYELECTROLYTES. ¥ I
1.E
577
pH diminishi~9by HC1
1.'~
1.Z
pH 12 Fro. 3. Effect of
pH on t h e viscosity of N a - a g a r a t e solution.
molecular weight ionic compounds and fits the general behavior of polyelectrolytes. Although both free agar acid and its sodium salt behave as polyelectrolyres, signifying a ehainlike character of the agar molecule, there is a characteristic difference in the structural details of the free acid and its sodium salt in solution. Although the reduced viscosity rises with dilution in both eases, the absolute Value of reduced viscosity for free acid is much higher than that for its sodium Salt. We may explain this on the assumption of the formation of hydrogen bonds in free acid, but there is no way of establishing this point unequivocally. The variation of relative viscosity of a solution of the sodium salt of agar acid with pH also follows a course similar to that of the sodium salt of arabic acid (Fig. 3). Over a pH range of 6 to 8, v/T0 stays almost constant and drops rather sharply on either side of the range. The curves could not be retracted by neutralizing the acid or alkali, but could be by dialysis. Substantial paIi of this curve can therefore be explained by the folding-unfolding phenomenon of the chain molecule in the presence of electrolyte---an explanation that has also been suggested for arabic acid. Incidentally it may be noted that the variation of relative viscosity with pH is exactly similar to the variation of relative swelling with the pH of gum agar (2). If we are permitted to draw a conclusion by analogy, the swelling may be explained as the expansion-contraction of the crosslinked polyelectrolyte in a medium of different pH's. In fact it has been shown by Katchalsky (3) that a network of a cross-linked synthetic polyelectrolyte does expand and contract reversibly on changing the degree of ionization. Our present interpretation receives further support from the fact that free agar acid does not form gel, while its calcium salt, in which
578
SADHAN BASU AND ANIL 1(. SIRCAR
interchain cross-linkings may be established through the calcium atoms, does.
An important fact emerges from our studies on arabic and agar gums: the polyelectrolyte character is dependent on the charge on the chain molecule and not on the chemical nature of the group contributing the charge, because both arabic and agar gums show similar physical characteristics, although in one case the charge-producing group is the COOH group, whereas in the other it is the S03H group. AC1(NOWLEDGEMENTS Thanks are due to Dr. S. R. Palit for laboratory facilities and to the Council of Scientific & Industrial Research, India, for a fellowship to one of us (A. K. S.). SUMMARY
Measurements of viscosity with varying concentration of solute, witb and without added electrolyte, and at different pH's of solutions of agar acid and its sodium salt are reported. A connected explanation of the observations has been formulated on the assumption of the polyelectrolytic character of the molecule. A tentative explanation has been put forward for the phenomenon of the swelling of agar gum in water at different pH's. REFERENCES 1. BASU, S., DASOUPTn, P., ~tND SIRCA~, A. K., J. Colloid Sci. 6, 539 (1951). 2. FAI~BROTH~R, F., ~_ND M.tSTIN, H., J . Chem. Soc. 123, 1412 (1923). 3. KATC~ALSKY, A., J. Polymer. Sci. 7, 571 (1951).
INTRODUCTION In a previous communication it has been shown by Basu et al. (1) that the sodium salt of arabic acid behaves as a polyelectrolyte; this has been taken to indicate a linear structure of the gum arabate molecule. The work has been extended to other gums, and the present paper reports the results of similar studies on gum agar. The difference between the acidic character of arabic acid and agar acid lies in the fact that whereas the acidity in arabic acid is due to the C 0 0 H group of the uronic acid segment of the molecule, in agar acid this is due to the S03H group, agar being a calcium salt of the sulfuric acid ester of a complex polycarbohydrate. The present study has been confined to the free acid and its sodium salt and is expected to throw some light on the nature of the molecule and the physical properties of its solution. ~]XPERIMENTAL Preparation of Free Agar Acid and Its Sodium Salt The usual method of liberating agar acid from the gum is to treat the aqueous dispersion with hydrochloric acid when the gum is decomposed into calcium chloride and free agar acid which can be precipitated out by alcohol or acetone. It has, however, been shown by Fairbrother and Mastin (2) that the reaction reaches an equilibrium; hence in order to effect a complete removal of calcium the gum solution must be made rather strongly acidic, which introduces the chance of extensive degradation. In order to avoid this complication the gum solution was treated with a solution of sodium oxalate when calcium precipitated out as oxalate. The solution was filtered and then dialyzed free of sodium oxalate. The dialyzed solution was made acidic with HC1 and the agar acid precipitated with alcohol. The acid was again dissolved in water and dialyzed free of chloride ions. The equivalent weight, obtained by electrometric titration with alkali, was 2213. The ash content of the free acid was 0.7 %. 574
STUDIES ON POLYELECTROLYTES. VI
575
T h e free acid was converted into its sodium salt by treating it with a solution of sodium hydroxide, somewhat in excess of the equivalent amount, precipitated with alcohol, again dissolved in water, and dialyzed until neutral. The molecular weight of the sodium salt, determined osmometrically in a NaC1 solution, was about 30,000. It is evident, therefore, that even in this operation the molecule has undergone degradation. On an average there are about 14 acidic groups distributed along the chain per molecule. The solutions of free acid and its sodium salt in water were stable for at least two weeks When preserved at 15°C.
Measurement of Viscosity The viscosity measurements were done with two OstwMd capillary viscometers of flow times 3 minutes 22 seconds and 4 minutes 37 seconds, respectively, with water at 35 ° :/: 0.1°C. The specific and relative viscosities were calculated with the use of the following relations: relative viscosity = ~/~0 - pl tl pt specific viscosity = v~p = - -- 1 T0
where v and ,10 are the viscosities, 01 and p the densities, and h and t the time of effiux (in seconds) for the solution and solvent, respectively. In the range of dilute solutions used, the difference in densities between solution and water was insignificant and no density measurements were made. The results of viscosity measurements with and without added electrolytes and also at various pH's are shown graphically in Figs. 1-3. DISCUSSION
It may be observed from Figs. 1 and 2 that the wp/c versus c plot for both free acid and its sodium salt in solution increases rapidly with dilution, which evidently indicates an increase in hydrodynamic volume unit. This observation m a y reasonably be explained from the consideration of folding-unfolding properties of high molecular chain compounds, i.e., repulsion between similarly charged centers on the agar chain causing the molecule to extend, which evidently increases with an increase in the extent of electrolytic dissociation, i.e., with increased dilution. Various ~,/c versus c curves for agar acid in the presence of different amounts of H2SO~ and for sodium salt in the presence of different amounts of NaC1 are also given in Figs. 1 and 2. I t m a y be observed that the sharp rise at lower concentration disappears and is replaced by a bell-shaped curve with a well-defined maximum which shifts towards higher concen-
576
SADHAN
BASU
AND
&NIL
9 'L.£"
K,
SIRCAR
Aga~a,old in w4er. •
L,
~
.,
e ~
"
I~'94XI~SNH.,80J 7"~:'g4"xI64hlHp"GO~41
6 b
0
0"06
e
(9 mgAoo~)
012
018
O'P.4-
FzG. 1. Viscosity of ag~r acid in presence of H~S04.
O - -~" in p~esence O~wa{el~ (~-- > . . . . 14"8~X15~t~ ~tc[ ~-~" ,, 29"TXIS"N N~lcl • - ` + ,, . t9"8 X16SN gael
El
L8
tS'
1..--4 1 ~ 0.9
i
r
I
~
g "I~
I
I
c I -.,,..A 0"4
(grnS/~oo ec) f
I
[
~ "6 '
'
' ~'a'
FIG. 2. Viscosity of Na-agar~te in presence of NaC1. tration of solute as the added electrolyte concentration is increased. At sufficiently high concentrations of electrolyte the curves in b o t h cases are perfectly linear, resembling those of a neutral polymer. This type of viscosity behavior has been noticed in the case of a large number of high
STUDIES ON POLYELECTROLYTES. ¥ I
1.E
577
pH diminishi~9by HC1
1.'~
1.Z
pH 12 Fro. 3. Effect of
pH on t h e viscosity of N a - a g a r a t e solution.
molecular weight ionic compounds and fits the general behavior of polyelectrolytes. Although both free agar acid and its sodium salt behave as polyelectrolyres, signifying a ehainlike character of the agar molecule, there is a characteristic difference in the structural details of the free acid and its sodium salt in solution. Although the reduced viscosity rises with dilution in both eases, the absolute Value of reduced viscosity for free acid is much higher than that for its sodium Salt. We may explain this on the assumption of the formation of hydrogen bonds in free acid, but there is no way of establishing this point unequivocally. The variation of relative viscosity of a solution of the sodium salt of agar acid with pH also follows a course similar to that of the sodium salt of arabic acid (Fig. 3). Over a pH range of 6 to 8, v/T0 stays almost constant and drops rather sharply on either side of the range. The curves could not be retracted by neutralizing the acid or alkali, but could be by dialysis. Substantial paIi of this curve can therefore be explained by the folding-unfolding phenomenon of the chain molecule in the presence of electrolyte---an explanation that has also been suggested for arabic acid. Incidentally it may be noted that the variation of relative viscosity with pH is exactly similar to the variation of relative swelling with the pH of gum agar (2). If we are permitted to draw a conclusion by analogy, the swelling may be explained as the expansion-contraction of the crosslinked polyelectrolyte in a medium of different pH's. In fact it has been shown by Katchalsky (3) that a network of a cross-linked synthetic polyelectrolyte does expand and contract reversibly on changing the degree of ionization. Our present interpretation receives further support from the fact that free agar acid does not form gel, while its calcium salt, in which
578
SADHAN BASU AND ANIL 1(. SIRCAR
interchain cross-linkings may be established through the calcium atoms, does.
An important fact emerges from our studies on arabic and agar gums: the polyelectrolyte character is dependent on the charge on the chain molecule and not on the chemical nature of the group contributing the charge, because both arabic and agar gums show similar physical characteristics, although in one case the charge-producing group is the COOH group, whereas in the other it is the S03H group. AC1(NOWLEDGEMENTS Thanks are due to Dr. S. R. Palit for laboratory facilities and to the Council of Scientific & Industrial Research, India, for a fellowship to one of us (A. K. S.). SUMMARY
Measurements of viscosity with varying concentration of solute, witb and without added electrolyte, and at different pH's of solutions of agar acid and its sodium salt are reported. A connected explanation of the observations has been formulated on the assumption of the polyelectrolytic character of the molecule. A tentative explanation has been put forward for the phenomenon of the swelling of agar gum in water at different pH's. REFERENCES 1. BASU, S., DASOUPTn, P., ~tND SIRCA~, A. K., J. Colloid Sci. 6, 539 (1951). 2. FAI~BROTH~R, F., ~_ND M.tSTIN, H., J . Chem. Soc. 123, 1412 (1923). 3. KATC~ALSKY, A., J. Polymer. Sci. 7, 571 (1951).