36
H. N. BHARGAVA AND D. C. SRIVASTAVA
Degradation of Graham’s Salt in Presence of Water-Miscible Organic Solvents by H. N. Bhargava’ and D. C. Srivastava Department of Chemistry, Gorakhpur University, Oorakhpur, India (Received May 19, 1969)
Graham’s salt, the polymeric sodium phosphate glass, undergoes depolymerizationin presence of certain watermiscible organic solvents, e.g., acetone, methyl, ethyl, n-propyl, and t-butyl alcohol, and dioxane. A 40% aqueous solution of these polar organic solvents has the same effect irrespective of the dielectric constant of the medium. The nucleophilic organic solvents catalyze the hydrolytic degradation of Graham’s salt by taking part in the reaction. Kinetic results for 40% ethanol show that the energy of activation is about 16 kcal/mol, which is equal to what one would expect if 0.1 M hydrochloricacid were to induce hydrolytic degradation.
Introduction Graham’s salt, the polymeric sodium phosphate 0 /I
glass, OH-(P-O),-H, I
exists as an open-chain poly-
Strauss, et aZ.,’ intrinsic viscosity [ q ] determined in this way is related to M , determined by light-scattering measurements. Thus [q] =
1.76 X 10-6M,
(1) End group molecular weights, Me, were obtained by employing the simple equations
ONa electrolyte. It hydrolyzes in the presence of acids.2 Recently, Martens and Rieman3 mentioned in passing, and without any detailed explanation, that certain polar solvents catalyze the hydrolytic degradation of Graham’s salt. Thilo4 found that lowering of the dielectric constant of the medium enhanced the catalytic effect of cations in causing hydrolytic degradation of Graham’s salt. We5 observed this phenomenon while attempting the fractional precipitation of Graham’s salt with the help of acetone and reached four important conclusions : (i) the degradation is negligible at low and moderate temperatures, (ii) the degradation in pure water is slight in comparison to that in presence of even a small amount of acetone, (iii) the degradation is probably not due to any increased acidity because the addition of acetone itself does not alter the pH of the solution of Graham’s salt (as measured with the glass electrode) significantly, and (iv) the degradation is favored by increasing amounts of acetone. I n the present paper, a more detailed study of the phenomenon is being presented. The degradation has been observed in several other water-soluble organic solvents and a kinetic study has been made in 40% solution of ethanol.
where a is the volume of 0.1 N NaOH solution required to titrate 1.02 g of Graham’s salt in the pH range of 5.5 to 8.5, the molecular weight of the monomer unit NaPOa being 102. Of the samples mentioned in Table I, the one with M, = 9140 was prepared by heating NaH2P04at 700” for 6 hr and the one with M, = 9950 was prepared by heating NaH2P04at 700” for 9 hr. The values of M e for these samples were found to be 4720 and 6370, respectively. The following experiment was performed to study degradation in 40% aqueous solutions of six different water-miscible organic solvents : acetone, methyl, ethyl, n-propyl, and t-butyl alcohol, and dioxane. All the organic solvents were purified by using standard purification methods for each of them. A weighed amount (about 0.2 g) of a sample of Graham’s salt of known molecular weight was taken in a thin glass test tube of about 10 ml capacity. Just enough solvent was added to it so that the concentration of Graham’s salt in the resultant solution was exactly 10%. The
Experimental Section The samples of Graham’s salt used in the present investigation were prepared by heating AR sodium dihydrogen phosphate (B.D.H.) in a platinum dish above 700” in a Gallenkamp muffle furnace (maintaining the temperature within *lOo) and quenching the melt by pouring it on a stainless steel plate and quickly pressing another on it. Their weight-average molecular weights (M,) were determined by measuring intrinsic viscosities in 0.035 N NaRr solution.6 As found by
(1) T o whom requests for reprints should be addressed. (2) J. R. Van Wazer, “Phosphorus and Its Compounds,” Vol. I, Interscience, New York, N . Y., 1958. (3) W. 8. Martens and W. Rieman, 111, J . Polyner Sci., 54, 603 (1961). (4) W.Wieker and E. Thilo, 2.Anorg. Allgem. Chem., 313,296 (1961). (6) H. N. Bhargava and D. C. Srivastava, I n d b n J . Chem., 5, 8 (1967). (6) A. C. Chatterji and H. N. Bhargava, J . Polyner Sci., 35, 236 (1959). (7) U. P.Strauss, E. H. Smith, and P. L. Wineman, J . Amer. Chem. Soc., 75,3935(1953).
The Journal of Physical Chemistry
20,400
M, = a
DEGRADATION OF GRAHAM'S SALT
37
tube was then sealed off carefully and placed in an oven at 55 f 1". After 5 hr, it was taken out of the oven, broken from the top, and its contents transferred into a centrifuge tube. The polyelectrolyte was completely precipitated by adding an excess of the organic solvent whose solution was used. The precipitate was separated by centrifugation at 0" so that no further degradation could take place. The precipitate was dried in a vacuum desiccator. Experiments with all the solvents were done simultaneously and almost similar conditions were employed in all cases. This was done so that a comparison could be made on a sound basis. Dielectric constants of the solvents were determined by a method described by Bhargava and Srivastava.* Experiments done for the kinetic study in 40% aqueous solution of ethanol were similar to those described above. Six sealed tubes containing 10% solution of a particular sample of Graham's salt in 4oqib ethanol were placed in a preheated incubator maintained a t 45". They were taken out, one by one, after 4, 8, 16, 24, 28, and 32 hr. The contents were chilled by breaking the tubes in ice-cold ethanol and the precipitate (that is, the degraded polymer) was separated by centrifugation at 0'. The entire process was repeated at incubator temperatures of 65, 70, and 75". The weight-average molecular weights, M,, were determined in all cases by measuring intrinsic viscosity 1771 in 0.035 N NaBr and the number-average or endgroup molecular weight, M e , by titration against sodium hydroxide.s The sample used for the kinetic study had the value of M , = 12,040 and M e = 9260. It was prepared by heating NaH2P04at 800" for 12 hr and quenching the melt as mentioned earlier.
undegraded samples showed that it increased at the most by 1% after degradation. We can therefore draw two other conclusions: (1) that the composition of the water-organic solvent mixture is the main factor in inducing the degradation rather than the dielectric constant of the medium, and (2) that ring phosphates are not formed in any significant amounts during degradation. The dielectric constant of the medium may not be totally without significance. It is likely that there is a leveling effect and below a certain value the exact value of dielectric constant is not as important as the availability of organic molecules which themselves get involved in the process of degradation. All the organic solvents studied as catalysts favoring degradation of Graham's salt are nucleophilic agents. Their action can be visualized as an increased nucleophilic attack on P-0-P bonds. The same argumentslO which explain why solutions of Graham's salt are stable against the attack of a negatively charged nucleophile as OH- ion in alkaline solutions will explain why suddenly the P-0-P bonds become prone to a nucleophilic attack in media of low dielectric constant. I n alkaline medium, the polyelectrolyte is
,750
c
Results and Discussion A. The E$ect of the Dielectric Constant of the Medium. Table I shows the effect of the dielectric constant of the Table I : Degradation of Graham's Salt at 55" in Presence of Different Organic Solvents (Time of Degradation, 5 hr) Mw of
Organic solvent (40% in HnO) Methyl alcohol Ethyl alcohol Acetone n-Propyl alcohol t-Butyl alcohol Dioxane
Dielectric constant
59.6 55.0 54.6 50.3 43.9 43.0
-
Mw of
undegraded material
undegraded material =
9140
9960
Mw of degraded Mw of degraded
material
material
7570 7410 7400 7410 7380 7390
8450 8390 8430 8400 8410 8350
8
1 6
24
32
Time (hr.) Figure 1. Influence of temperature on the rat? of degradation of Graham's salt in 40% aqueous solution of ethanol (l/M,, 8s. time): 45', 0;65'' 0 ; 70"' @; 75', A. (8) H. N. Bhargava and D. C. Srivastava, KoUoirE 212,124 (1966).
medium in the case of two different samples of Graham's salt. A determination of the amount of phosphorus present as ring phosphatesg in the case of degraded and
Z. Z. Polyn.,
(9) H.N. Bhargava and D. C. Srivastava, Anal. Chim. Acta, 37,269 (1967). (10) F. G. R. Gimblett, "Inorgania Pofymer Chemistry," Butterworth and Co. Ltd., London, 1963, Chapter 7. Volume 74, Number 1 January 8,1070
H. N. BHARGAVA AND D. C. SRIVASTAVA
38 highly charged and strong negative charges on phosphorus atoms repel all nucleophilic attacks, but as soon as the dielectric constant of the medium is lowered the polyelectrolyte gets a more coiled up configuration and also loses a part of its charge density. A nucleophilic attack resulting in ultimate breakdown of P-0-P links becomes easier. Taking acetone as an example, the nucleophilic attack on the P-0-P can be visualized as follows.
0
(CH&C
0 II
It will, therefore, become much easier
(-P-O-)%. I I
0for nucleophiles to attack P-0-P bonds. B. Kinetics of Degradation Catalyzed by 40% v / v Ethanol. Out of the several water-miscible organic solvents whose presence was found to catalyze hydrolytic degradation of Graham’s salt, ethanol was chosen
0
0
4
I
I I N II + (P)-O-P-O-P-O-(P)
O-Na+ O-Na+
0
--+
II (P)-O-P-OI
0 Q
C-(CH&
II + O-P-O-(P) e
I
0-Na+
0
0
I
I
O-Na+
O-Na+
0 e
II I
H + + 0-P-0-(P) O-Na+ Here (P) represents the rest of the chain besides the units shown. Since an unstable complex ion involving the combination of acetone and a part of the polyphosphate chain is formed as an intermediate, obviously the acetone takes part in the reaction. That increasing amounts of acetone help the process of degradation can therefore be explained on the basis of the mechanism suggested. As mentioned earlier, the rate of attack of a negative nucleophile (like OH- ion) on a highly charged polyanion is unfavorable. This is because of the electrostatic repulsion between similar charges which greatly decreases the entropy of activation.“ For a process involving the reaction of two negatively charged species, decreasing the dielectric constant would decrease the rate constant.ll A classical example is the reaction of hydroxide ion with bromophenol blue. l2 The rate constant decreases 200-fold by the addition of 30% ethanol to the water. However, the attack of a neutral molecule like water, acetone, or some other nucleophilic organic molecule would be expected to increase as the dielectric constant decreases.“ The decrease of dielectric constant would favor ion pairing by the sodium ions with the polyphosphate anion, The JOUTnal of Physical Chemistry
O-Na+
0
II
+HO-P-O-(P)
I
O-Na+ for a detailed kinetic study. The choice was arbitrary and it is believed that the other solvents would behave in exactly similar manner. The conclusions drawn on the basis of the present study can be easily taken to apply in the case of the other solvents. Graham’s salt is essentially formed by condensation polymerization and has a random size distribution of molecular weights or chain-lengths.’* A monodisperse or random heterodisperse polymer, whose bonds are broken at random, obeys the equation14 1 / 2 n = l/(zn)o
+ kt
(3)
where (zn),, and Zn are the number average chain lengths at t = 0 and t, respectively. This relationship holds quite generally for random scission reactions regardless of the chemical nature or the cause of deg(11) A. A.Frost and R. G. Pearson, “Kinetics and Mechanism,” John Wiley & Sons, Ino., New York, N . Y., 1961, Chapter 7. (12) E. S. Amis and V. K. La Mer, J . Amer. Chem. Soc., 61, 906 (1939). (13) F. G. R. Gimblett, “Inorganic Polymer Chemistry,” Butterworth and Co. Ltd., London, 1963, Chapter 7. (14) C. Tanford, “Physical Chemistry of Macromolecules,” John Wiley &Sons, Inc., New York, N. Y., 1961, Chapter 9.
DEGRADATION OF GRAHAM'S SALT
39 to Grassie and Grant,16 the slope of any line in such a plot should give the rate of the reaction for the particular temperature. Since the straight lines in Figure 1 have been obtained for the reaction at different temperatures, they also illustrate the influence of temperature on the rate of chain scission. Assuming the Arrhenius equation to hold good, they can be used for determining E , the energy of activation. I n Figure 2 is shown the plot of -In IC vs. 1/T obtained. A value of E equal to 16 kcal/mol could be deduced from it. Thilo and Wieker" gave a value of 25 kcal for hydrolysis at pH 8 and 15 kcal for pH 1. Obviously, in 40% aqueous solution, water-miscible organic solvents such as ethanol function as well as 0.1 M hydrochloric acid.
14.0)
2.9
30
I/T
x lo3
3.1
I
Figure 2. Evaluation of the energy of activation, E. Plot of -In k us. 1/T.
radation.16 Graham's salt should obey this equation. Figure 1 gives a plot of the reciprocal of the numberaverage molecular weight, M, (same as Me) against the time of degradation in hours. Within the limits of experimental error, (3) is obeyed quite well. According
Acknowledgment. We are grateful to the Council of Scientific and Industrial Research (India) for financial support. D. C. S. is grateful to C. S. I. R. for a fellowship during the tenure of which this work was carried out. Our thanks are also due to Professor R. E. Davis, Chemistry Department, Purdue University, for several discussions, and to Dr. J. R. Van Wazer of Monsanto Co., St. Louis, for his interest in this work. (16) H. H. G. Jellinek, Pure Appl. Chem., 4,419 (1962). (16) N. Grassie and E. M. Grant, European Polyner J., 2 , 266 (1966). (17) E. Thilo and W. Wieker, 2. Anorg. Allgem. Chem., 291, 184
(1967).
Volume 74,Number 1
January 8, lQ7O
