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Studies onCarboxymethylcellulose:Osmotic PressureMeasurements. IIH. C. Trivedi a & R. D. Patel aa Department of Chemistry , Sardar PatelUniversity Vallabh , Vidyanagar, 388120,Gujarat, IndiaPublished online: 24 Feb 2007.

To cite this article: H. C. Trivedi & R. D. Patel (1982) Studies onCarboxymethylcellulose: Osmotic Pressure Measurements. II, Journal ofMacromolecular Science: Part A - Chemistry: Pure and Applied Chemistry, 18:4,555-563

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J. MACROMOL. SC1.-CHEM., A18(4), pp. 555-563 (1982)

Studies on Carboxymethylcellulose: Osmotic Pressure Measurements. I I

H. C. TRIVEDI and R. D. PATEL

Department of Chemistry Sardar Pate1 University Vallabh Vidyanagar 388120, Gujarat, India

A B S T R A C T

The osmotic coefficient g* is calculated for sodium carboxy- methylcellulose (NaCMC) samples of different degrees of sub- stitution. The dependence of g* on =, concentration of NaCMC, and added NaCl has been investigated for all samples. The true osmotic coefficient (p has also been calculated for all the samples

P by applying Donnan's equilibrium concept. (p is found to be inde-

P pendent of the amount of extraneous sal t accounting for Donnan's membrane equilibrium. The dependence of (p on the charge

P density of polyions can be explained quantitatively by Oosawa's and Katchalsky's theories.

I N T R O D U C T I O N

In a previous paper [ 11 the behavior of different samples of NaCMC in NaCl solutions of various ionic strengths has been studied by using a high-speed membrane osmometer. In the present study the osmotic pressure resul ts of all NaCMC samples [ 11 a r e used to calculate the osmotic coefficient (g*) and the osmotic coefficient of counterions in

555

Copyright 0 1982 by Marcel Dekker, Inc.

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556 TRIVEDI AND PATEL

TABLE 1. Characteristic Parameters of NaCMC Samples"

Monomer

0.80 0.36 7.20 1.10 2.75 226.0 1.08 0.89 2.90 1.49 3.72 248.2 1.35 0.42 6.15 1.84 4.60 271.6

1.70 0.38 6.80 2.33 5.82 298.0

?RT = 2.592 X lo4 L'cm/mol a t 33°C.

the NaCMC solution (4 ). The results obtained are also treated in

light of Oosawa's and Katchalsky's theories. P

E X P E R I M E N T A L

M a t e r i a 1 s

The methods of preparation, purification of all NaCMC samples, and their measurements of degree of substitution were the same a s in our earlier publication [ 11.

The NaCMC samples used in the present work a re characterized in Table 1. The average degree of substitution, number-average molecular weight, the charge density, structural and adjusted values, and the monomer weights for each sample a r e shown in the table.

Sodium chloride was of A.R. grade.

O s m o t i c P r e s s u r e M e a s u r e m e n t s

The osmotic pressure measurements of a l l samples of NaCMC in NaCl solutions of varying ionic strength were carried out in the manner described in our ear l ier publication [ 11.

R E S U L T S AND DISCUSSION

In analogy to the osmotic coefficient in salt-free polyelectrolyte solutions, one can define an osmotic coefficient in a polyelectrolyte- salt solution as

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STUDIES ON CARBOXYMETHYLCELLULOSE. JI 557

D g* = 71/71

where 71 is the observed osmotic pressure and 7~ is that calculated from the Donnan expression at the same concentrations of the poly- ions and the added salt. Experimentally, the reduced osmotic pres- sure and the calculated Donnan rr/C are used to calculate g*. The Donnan term is calculated a s

D

RTr'C (:) D=4X+ (:) The first term in Eq. (2) depends on the concentrations of the poly- electrolyte and the added salt. The second term is experimentally obtained a s an intercept on the ordinate in the plot of 71 /C versus c. It is observed that the contribution of the second term is negligible in the presence of low salt content, while it makes a significant con- tribution to g* a t high salt content. Thus g* is calculated for NaCMC samples of different degrees of substitution by using Eqs. (1) and (2) from the osmotic pressure data given in Figs. 1-4 of our previous publication [ 11.

Figure 1 shows the typical behavior of g* with concentrations of NaCMC and added NaCl in the case of NaCMC with DS = 1.35. It is seen that g* decreases with polymer concentration and increases with salt concentration (X). Moreover, g* tends to unity when c approaches zero. The osmotic coefficient g* may be considered to represent the state of association of simple ions with the polyions since is calculated on the assumption that NaCMC is completely D dissociated.

It has been pointed out by Inagaki e t al. [ 21 that different curves of g* versus C a t several values of X are superposed on a single curve when g* is plotted against Y or Y- l , where the parameter Y is defined as

Y = (rC + X)/rC (3)

according to Howard and Jordan [ 31. This parameter Y was origi- nally introduced by them in their treatment for diffusion and sedi- mentation coefficients of a polyelectrolyte in simple sal t solutions.

The behavior of g* with 1/Y is shown in Fig. 2 for NaCMC of DS = 1.35 and in Fig. 3 for NaCMC of DS = 1.70. In each case a single curve is observed, hence g* may be considered as a unique function of 1/Y. Again, g* tends to unity as 1/Y approaches zero, This condition is achieved either with a very low polyelectrolyte concentration o r with a high salt content in the polyelectrolyte

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558 TRIVEDI AND PATEL

0.61

CQ/ I 1 FIG, 1. The dependence of the osmotic coefficient g* on the

concentration of NaCMC (m = 1.35) solutionzat several ionic strengths. NaCl concentrations: 0.91 X 10- ( e ) , 5.0 x l o m 2 N ( d ) , 10.0 x l o - ’ N (o), 28.57 x lo-’ 45.5 X - N ( A), and 75.0 x l o - ’ R - ( 0 ) .

N ( o), 2.5 x lo-’ - N (v ),

FIG. 2. The relation between the osmotic coefficient g* and parameter Y at all the concentrations of NaCl and NaCMC (m= 1.35) indicated in Fig. 1.

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STUDIES ON CARBOXYMETHYLCELLULOSE. II

r I I I I I 1

0.8 i-

1 IY

FIG. 3. The relation between the osmotic coefficient g* and parameter Y for NaCMC (m = 1.7) solution.

559

FIG. 4. The dependence of g* on W o f NaCMC samples. ( 0 ) Present data on NaCMC. ( m ) Data on NaCMC reported by Inagaki e t al. [ 21.

solution, The observed value of g* is around 0.6 a t high salt concen- tration (X) in a l l NaCMC samples.

I t is interesting to see the behavior of g* at low salt and high polymer concentrations. The asymptotic values of g* a r e obtained f rom plots of the type given in Fig. 2 for a l l the NaCMC samples

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560 TRIVEDI AND PATEL

studied. The dependence of g* on is shown in Fig. 4. The osmotic coefficient g* decreases with increasing DS of NaCMC sample. The osmotic coefficient g* tends to unity as the charge density approaches zero. The osmotic coefficient of NaCMC (m = 0.45) reported by Inagaki e t al. [ 21 has been included in the figure. The value of g* r e - ported for NaCMC (m = 0.45) seems to be higher than the general trend indicated by the present data.

It has been pointed out by Katchalsky e t al. [ 4) that to a f i rs t approximation the values of I$ (the true osmotic coefficient) derived from the straight lines of n/C versus C of polymer-salt systems correspond closely to that obtained by the direct study of salt-free polyelectrolyte solutions. Hence the estimation of 4 was done from the osmotic pressure measurements. The osmotic coefficient of counterions in the solution, 4

P

P

is defined by P'

where

A2' = r2/4X (5)

The osmotic coefficient @ for NaCMC samples i s evaluated from osmotic pressure measurements using Eq. (4). A; was obtained from Eq. (5), and the activity coefficient of salt, @s, was computed from the literature [ 51. Table 2 shows the osmometry data on NaCMC samples including the values of osmotic coefficients a t several ionic strengths. It is seen from Eq. (4) that I#J should be independent of the ionic strength; however, some experimental scatter in 4 values is observed in each of the NaCMC sample as shown in Table 2. In the last column of the table, therefore, an average value of @ is shown for each NaCMC sample. It is revealed that the higher the m o f NaCMC, the lower is the value of @ and vice versa.

The osmotic coefficient was examined in light of the theories of rodlike polyions. In Oosawa's theory [ 61 the degree of free counter- ions f is replaced by the osmotic coefficient tb

charge density) was calculated by assuming the volume concentration of the NaCMC solution to be 0.01 for all NaCMC samples. The experimental values of @ could be fitted satisfactorily with the

P

P

P

I'

P

The value of Q (the P'

P theoretical 4 values with Q . = 2.5Qstruct. P adJ

Figure 5 shows a plot of 4 Q versus Q due to Oosawa. The P

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STUDIES ON CARBOXYMETHYLCELLULOSE. TI 561

TABLE 2. The Osmotic Coefficient Data of NaCMC Samples in Salt- Added Solutions

Concen- tration of NaCl ( A ~ / A ~ ' ) Average x lo2

@; @P @P x l o 2 -

DS

1.70 0.50 0.930 2.25 0.14 0.16 1.00 0.902 2.13 0.14 4.55 0.835 3.72 0.18 9.09 0.785 3.46 0.17

1.35 0.91 0.905 4.23 0.20 0.20 2.50 0.865 3.72 0.18 5.00 0.830 4.13 0.19

10.00 0.777 5.59 0.21

1.08 1.00 0.902 5.93 0.23 0.24 2.50 0.865 7.27 0.25 5.00 0.830 7.22 0.25 9.09 0.785 7.11 0.24

0.80 2.50 0.865 8.52 0.27 0.30 4.55 0.835 10.56 0.30 9.09 0.785 14.50 0.34

"See Ref. 5.

O , * ~ 0.4

1 2 3 4 5 6

Q

FIG. 5. The Oosawa plot of numbers of free counterions @Q versus the charge density Q for NaCMC of varying hs. The curve is due to Oosawa theory for Qadj = 2.5Qstruct. ( 0 ) Present data on NaCMC.

( 0 ) Data on NaCMC reported by Inagaki et al. [ 21. ( o ) Data on NaCMC reported by Pals and Hermans [ 71. ( A ) Data on NaCMC re- ported by Doty and Schneider [ 81.

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562 TRIVEDI AND PATEL

I I I I I I I

0

h

0 w

I I 1 I

005 0.10 0.15 0.20 0.25 0.30 0.35

Dxlb

FIG. 6. The Katchalsky plot of number of f ree counterions per unit monomer length

4 = 1 line is the ideal case and the curve is due to Katchalsky's theory for Qadj = 2.5 Qstruct' The points have the identities shown in Fig. 5.

*E/b versus the charge per unit length m / b . P

P

experimental points of all four NaCMC samples a re quite close to the theoretical line with Q on NaCMC are also included in the figure. The data reported by fimgaki et al. 121 are quite consistent with the present data on NaCMC. The experimental 4 value of NaCMC from the data of Pals and Hermans [ 71 l ies below the present data and that reported by Doty and Schneider 181 lies above the present data. This may be due to the inherent difficulties in osmometry and light scattering as applied to polyelectrolyte solutions.

where a, B / b versus DS/b is plotted. The present experimental points are best explained by the theoreticalline corresponding to Qadj = 2'5 Qstruct' a r e again in good agreement with the present data. Other literature data deviate from the present data a s before.

First , the osmotic pressure measurements of all the NaCMC samples a r e internally consistent so that all 4 can be explained by a single theoretical line. Second, the true osmotic coefficient a, and the degree of free counterions f give information about the association of counter- ions in polyelectrolyte solutions.

. = 2.5 Qstruct' Some of the l i terature data ad1

P

The analysis of 4 by Katchalsky's theory is also shown in Fig. 6 p -

P

The data on CMC of DS = 0.45 (Inagaki et al. [ 21)

Two important points are made clear from the above analysis.

P P

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STUDIES ON CARBOXYMETHYLCELLULOSE. II 563

A C K N O W L E D G M E N T S

The authors' sincere thanks are due to Prof Dr K. C. Patel and Prof Dr C. K. Patel for valuable suggestions and constant inspiration throughout the work.

R E F E R E N C E S

[ 11 H. C. Trivedi and R. D. Patel, J. Macromol. Sci.-Chem., Al?, 893 (1982).

[ 21 H. IAgaki, S. Hotta, and M. Hirami, Makromol. Chem., - 23, 1 (1957 ).

[ 31 G. H. Howard and D. 0. Jordan, J. Polymer Sci., 12, 209 (1954). [ 41 A. Katchalsky, Z. Alexandrowics, and 0. Kedem, =Chemical

Physics of Ionic Solutions (B. E. Conway and R. G. Barradas, eds.), Wiley, New York, 1966, p. 295.

[ 51 R. Parsons, Handbook of Electrochemical Constants, Butter- worths, London, 1959.

61 F. Oosawa, Polyelectrolytes, Dekker, New York, 1971. 71 D. T. F. Pals and J. J. Hermans, Rec. Trav. Chim., - 71, 433, 469

(1952). [ 81 P. Doty and N. Schneider, ONR Report, December 15, 1952.

t

Accepted by editor August 27, 1981 Received for publication September 6, 1981

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