an i.r. investigation of polyion-counterion interactions

11
SpectroohimicaActa,1967, Vol. 23A. pp. 1397to 1407. PergsmonPresa Ltd. PrintedinNorthem Ireland An i.r. investigation of polyion-counterion interactions J. C. LEYTE, L. H. ZUIDERWEQ and H. J. VLEDDER Laboratorium voor Fysische Chemie der Rijksuniversiteit te L&den, Netherlands (Received 12 Azlgust 1966) AbstractThe infra-red spectra of a number of polymethacrylic and polyaorylic acid salts in the solid state and in De0 solution have been obtained. It is concluded that no site-binding occura in solution for the alkali and earth alkali salts of these polyacids. The influenceof the conformational transition of polymethacrylic acid on the carboxylate group frequencies is discussed. 1. INTRODUCTION THE interaction of polyelectrolytes and their counterions has been the subject of much experimental and theoretical work. An attempt has been made to describe the behaviour of these systems by using simple models. No quantitative agreement with experimental results has been achieved however [l]. The following points seem to be generally accepted. (a) The electrostatic field around a charged polyion influences the behaviour of the counterions seriously by confining a portion of these ions in restricted regions of the solution, without seriously affecting the mobility of these ions within this region. (b) Apart from this, counterion binding to discrete reactive sites along the polymer chain may occur depending on the chemical nature of ions and sites. It may also be noted that a certain specificity in the interaction is observed which may not generally be interpreted as site-binding. The specificity in polyion- counterion interaction, in the sense that different species of counterions tend to create effects which at least differ in magnitude, may be illustrated with a few examples from recent literature. From transference experiments, NOLL and GILL [2] conclude that Csf-ions are much less bound by polyacrylic acid than Naf-ions. MANDEL and JENARD [3] observe differences in the influence of Mg 2+, Sr2+ and Ba2+ ions on the polarizability of partially neutralized polymethacrylic acid. IKE~AMI and IMAI [4] use the concept of site-binding to interpret the precipitation of polyacrylic acid by Mg2+, Ca2+ and Ba2+ ions at different polyion charges. From molar volume changes IKE~AMI [5] concludes that Ba2+ and La3+ ions disturb the water molecules close to the carboxylic ions of polyacrylic acid in contrast with sodium ions. The general [l]S.A. RICE and M. NAGASAWA, PoZyeZectroZyte Solutiona, Academic Press (1961). [2] L. A. NOLLand S. J. GILL, J. Phys. Chem. 67, 498 (1963). [3] M. MANDEL and A. JENARD, Truns.Fav-udayh'oc. 59,217O(1963). [4] A. IKEGAMI and N. IMAI, J. Polymer Sci. 56, 133 (1962). [5] A. IKEGAMI, Rep. Progr. Polymer Phys. (Japan) 6, 323 (1963). 1397

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Page 1: An i.r. investigation of polyion-counterion interactions

SpectroohimicaActa,1967, Vol. 23A. pp. 1397to 1407. PergsmonPresa Ltd. PrintedinNorthem Ireland

An i.r. investigation of polyion-counterion interactions

J. C. LEYTE, L. H. ZUIDERWEQ and H. J. VLEDDER Laboratorium voor Fysische Chemie der Rijksuniversiteit te L&den, Netherlands

(Received 12 Azlgust 1966)

AbstractThe infra-red spectra of a number of polymethacrylic and polyaorylic acid salts in the solid state and in De0 solution have been obtained. It is concluded that no site-binding occura in solution for the alkali and earth alkali salts of these polyacids. The influence of the conformational transition of polymethacrylic acid on the carboxylate group frequencies is discussed.

1. INTRODUCTION

THE interaction of polyelectrolytes and their counterions has been the subject of much experimental and theoretical work. An attempt has been made to describe the behaviour of these systems by using simple models. No quantitative agreement with experimental results has been achieved however [l]. The following points seem to be generally accepted.

(a) The electrostatic field around a charged polyion influences the behaviour of the counterions seriously by confining a portion of these ions in restricted regions of the solution, without seriously affecting the mobility of these ions within this region.

(b) Apart from this, counterion binding to discrete reactive sites along the polymer chain may occur depending on the chemical nature of ions and sites.

It may also be noted that a certain specificity in the interaction is observed which may not generally be interpreted as site-binding. The specificity in polyion- counterion interaction, in the sense that different species of counterions tend to create effects which at least differ in magnitude, may be illustrated with a few examples from recent literature.

From transference experiments, NOLL and GILL [2] conclude that Csf-ions are much less bound by polyacrylic acid than Naf-ions. MANDEL and JENARD [3]

observe differences in the influence of Mg 2+, Sr2+ and Ba2+ ions on the polarizability of partially neutralized polymethacrylic acid. IKE~AMI and IMAI [4] use the concept of site-binding to interpret the precipitation of polyacrylic acid by Mg2+, Ca2+ and Ba2+ ions at different polyion charges. From molar volume changes IKE~AMI [5] concludes that Ba2+ and La3+ ions disturb the water molecules close to the carboxylic ions of polyacrylic acid in contrast with sodium ions. The general

[l] S. A. RICE and M. NAGASAWA, PoZyeZectroZyte Solutiona, Academic Press (1961). [2] L. A. NOLL and S. J. GILL, J. Phys. Chem. 67, 498 (1963). [3] M. MANDEL and A. JENARD, Truns.Fav-udayh'oc. 59,217O (1963). [4] A. IKEGAMI and N. IMAI, J. Polymer Sci. 56, 133 (1962). [5] A. IKEGAMI, Rep. Progr. Polymer Phys. (Japan) 6, 323 (1963).

1397

Page 2: An i.r. investigation of polyion-counterion interactions

1398 J. C. LEYTE, L. H. ZUIDERWEG and H. J. VLEDDER

occurrence of specific effects is discussed by VON HIPPEL and KWOK-YIN~ WONG [6] for biologically important systems.

On the other hand there are indications that the occurrence of site-binding or even direct site-ion interaction in polyelectrolyte systems is not general. From proton magnetic resonance [7] and Raman [8] investigations it is concluded that polystyrene sulfonic acid is completely dissociated in aqueous solution. The chloride ions in polyethyleneimine-HCI solutions do not seem to be site-bound either [9].

From the specificity of polyion-counterion interaction on the one hand and the evidence for the non-generality of site-binding in these systems on the other hand, it seems that every polyion-counterion system must be investigated as such before conclusions can be formulated regarding the nature of the interaction in the system. If it is found that in a specific system no site-binding occurs, it may be concluded that the polyion merely restricts a portion of the counterions within its domain. The distribution of the hydrated counterions in the hydrated polyelectrolyte must then be investigated to explain observed specificity.

Spectroscopic methods directed at the reactive sites, the counterions or both should give valuable information on the absence or existence of site-binding. 1.r. and Raman methods have been sparingly used in this field however. This paper reports an i.r. investigation of the behaviour of the carboxylic acid groups of two polymeric acids in the presence of alkali ions and earth alkali ions. Both the solid state and D,O solutions (or gels) were investigated. For the optical path-lengths used D,O transmits in the 1300 cm-l-2000 cm-l region. This permits the observa- tion of some absorptions associated with vibrations of the carboxylic acid group and the carboxylate ion at approx. 1700 cm-r (C=O stretch, ycIo), 1550 cm-r (asymmetric stretching mode of the -CO,- group, Y,), 1400 cm-l (symmetric stretching mode of the -CO,- group, ys). These modes have been shown to be sensitive to hydrogen bonding [IO] and interaction with positive ions [ 1 l] and may therefore be used to investigate polyion-counterion interactions.

2. EXPERIMENTAL

Polymethacrylic acid (PMA) and polyacrylic acid were synthesized and fractionated by procedures described elsewhere [12, 131. The molecular weights of the samples used were 7.5 x lo5 and 7.9 x lo5 for PMA and PAA respectively. All inorganic salts used were analytical grade or recrystallized in this laboratory. All bases used were kept carbon dioxide free. NaOD solutions were prepared by dissolving freshly cut sodium in D,O in a nitrogen atmosphere. D,O of 99.8 per cent was obtained from the Dutch Reactor Center (RCN, Petten). The alkali salts of PMA and PAA were prepared by freeze drying of polyacid solutions neutralized

[6] P. H. VON HIPPEL and KWOK-YING WONG, Scielzce 145, 577 (1964). [7] L. KOTIN and M. NAGASAWA, J. Am. Chem. Sot. 83, 1026 (1961). [S] S. LA~ANJE and S. A. RICE, J. Am. Ghem. Sot. 83, 497 (1961). [9] S. LAPANJE, J. HAEBIG, T. DAVIS and S. A. RICE, J. Am. Chem. Sot. 82, 1690 (1960).

[lo] D. CHAPMAN, D. R. LLOYD and R. H. PRINCE, J. Chem. Sot. 550, (1964). [ll] K. NAKAMOTO, Y. MORIMOTO and A. E. MARTELL, J. Am. Chem. Sot. 83, 4528 (1961). [12] J. C. LEYTE and M. MANDEL, J. Polymer Sci. A2, 1879 (1964). [13] P. SELIER, Thesis, L&den (1966).

Page 3: An i.r. investigation of polyion-counterion interactions

An i.r. investigation of polyion-counterion interactions 1399

with the appropriate bases. The Ca and Ba salts of PAA were obtained in the same manner. The Mg and Sr salts of PMA and PAA and the Ca, and Ba salts of PMA have been obtained by freeze drying of solutions of the sodium salts and a 30-fold excess of the earth-alkali chlorides (partial precipitation occurred). After freeze drying chloride was washed out with ethanol (90 per cent and 70 per cent in the cases of Sr and Ba) until the alcohol was free of chloride. The polyacid solutions to which MgCl, had to be added were only 95 per cent neutralized to keep the pH of the solution below a value of 9.

Deuterium oxide solutions of PMA at different degrees of neutralization were prepared by adding calculated quantities of NaOD solution to weighed amounts of PMA and adjustment to 0.5 ml with D,O. The equivalent series for PAA was prepared by mixing NaOD solutions and PAA solutions in D,O and adjustment of the volume to O-5 ml with D,O in calibrated tubes.

All i.r. spectra were obtained with a SP 100 Unicam spectrophotometer using a sodium chloride prism; calibration was performed with indene or an indene- camphor mixture after each sample was run. The solid salts have been measured as Kel-F mulls or as films cast from H,O on CaF, or i.r.-tran plates.’ Where both methods were applied no significant difference in the results was found. The salts were dried in wacuo at temperatures up to 80” C either as powders before mulling (which was performed in a dry box) or as films in a specially designed cell so that spectra could be obtained without exposing the dried film to the atmosphere. The last procedure was necessary to obtain spectra of the most hygroscopic salts (K+ and Cs+-salts).

The solutions were measured between CaF, plates using teflon spacers. D,O was compensated for with a variable path length cell at 1250 cm-l. Cell lengths varied from 0.01 to 0.04 mm as determined by the interference method. The concentrations used were about 0.5 eq./l.

3. RESULTS AND DISCUSSION

The infra-red spectra of solid PAA, PMA and the sodium salts are presented in Figs. l-4. The wave numbers at which absorptions occur in the 2000 cm-l-1000 cm-l region may be seen in Tables 1 and 2 together with the results for D,O

Table 1. Polyacrylic acid

Solid D,O solution a=0 a = 1.0 a=0 a = 0.96

1730 (sh)

1709 (8) 1710 (8) 1707 (w) 1565 (8) 1570 (8)

1451 (m) 1452 (m) 1463 (m) 1453 (m) 1414 (w) 1406 (w)

1402 (s) 1412 (s) 1320-1345 (m) 1348 (w) 1356 (w)

1320 (w.sh.) 1327 (w) 1247 (m) 1170 (m) 1170 (V.W.) 1114 (V.W.)

I v(C=O)

~,WW WW 4CO)IWH) ~,(COO)

I ~toU%), W-W GWIWW (COOH)

Page 4: An i.r. investigation of polyion-counterion interactions

1400 J. C. LEYTE. L. H. ZUIDERWEU and H. J. VLEDDER

Table 2. Polymethacrylic acid

Solid D,O solution

a=0 a = 1.0 a = 0.1 a = 0.95

1709 (s.b.) 1556 (s)

1483 (m) 1475 (m) 1450 (w) 1445 (m)

1398 (8) 1390 (m)

1370-1350 (m)

1262 (s.b.) 1208 (m)

1173 (s.b.)

1699 (s) lJ(C=O)

1556 (m) 1551 (8) r,(CGG) 1474 (m) 1470-1485 (m) a(CH,) 1454 (m) 1450 (m) a(CH,)

;;;x) (w.b.) 1415 (m) r,(CGG)

r(CG)/a(GH) 1370 (w)

1330 (m) 1350 (w) I &CH,), I/,(CH,),

v(CO)/a(GH)

(COOH)

‘0 -

30 -

20 -

0 I I I I I I I I I 2000 ,900 moo I700 1600 1700 Cm.l 'wO 1300 I200 1100 IO00

Fig. 1. Polyacrylic acid. Film cast from H,O on ix.-tran plate.

solutions. Although no complete assignment will be attempted here some remarks may be made. The assignments for ycco and Y,(COO) present no difficulties [14, 15-j. For PAA ycco shows a shoulder at 1730 cm-r. This elect is discussed by SIMON et al. [16] in terms of hydrogen bonding; the Raman results of these authors are in agreement with our i.r. spectra. The absorption at 1451 cm-1 in the PAA spectrum may be assigned to the CH, bending mode [14, 171 as it is known that this mode is relatively stable as long as the CH, group does not partake in

[14] L. J. BELLAMY, The Infrared Spectra of Complex Molecules, Methuen (1962). [15] K. NAJUMOTO, Infrared Spectra of Inorganic and Coordination Compounds, John Wiley

(1963). [16] A. SIMON,M.M~~CELICH,D.KUNATH and G. HEINTZ,J. PoZyrrwrSci.30, 201 (1958). [17] R. ZBINDEN, Infrared Spectroscopy of High Polymers, Academic Press (1964).

Page 5: An i.r. investigation of polyion-counterion interactions

An ix. investigation of polyion-counterion interactions

0 I I I I '

I I I 2000 I900 ,800 1700 1600 1500 sm_, 140 1500 1200 II00 1000

Fig. 2. Sodium polyacrylate. Film cast from H,O on ix.-tran plate.

207

IO-

O- I I I I I I I I I 2000 I900 moo I700 1600 1500 1‘00 1300 1200 1100 NIOO

cm-'

Fig. 3. Polymethecrylic acid. Film cast from H,O on i.r.-tran plate.

multiple bonding and the neighbouring C-atoms are connected to carbon and hydrogen atoms only. For PMA the asymmetric CH, bending mode is expected in this region too [Id]; from comparison with PAA it seems that the absorption in the 1470-1485 region is probably representative of this mode. The symmetric stretching mode Y,(COO) may be assigned by observing its regular increase on ionization as shown in Fig. 5 for PAA. For the undissociated acids the bands in the 1400 cm-l and 1250 cm-l regions may be assigned to interacting CO stretching

Page 6: An i.r. investigation of polyion-counterion interactions

1402 J. C. LEYTE, L. H. ZUIDERWEU and H. J. VLEDDER

100

90 -T

BO-’

70--

60 -

JO -

40 -

30 -

*o-

IO-

a I I I I I 1 I I I 2000 I900 I800 I700 1600 1300

Cm-’ 140 1300 I200 1100 IO00

Fig. 4. Sodium polymethacqlrtte. Film cast from H,O on i.r.-tran plate.

‘\ \

n _ \ _---. I/:. ’

_‘\ \ 1

/ :,

/, i,’ ’

loo- 1) Ii ( I ’ II ’ 3 \ ’

/ ’ I,; ’

J ,

400 -

‘\ I ! . .

t,

! ,I _-

‘T$ , ‘,‘I /: -, t ,I ! : I!, ;I I 1 Lj z’ (:

500 -

600 - I I I I I

1800 1700 1600 I?00 1400 II Cm-’

Fig. 6. Polyacrylic acid solution in D,O at different degrees of neutralization a.

Page 7: An i.r. investigation of polyion-counterion interactions

An ix. investigation of polyion-counterion interactiona 1403

and OH deformation modes [ 181. The bands at 1171 (PAA) and 1173 (PMA) are clearly connected with the carboxylic acid group in some way, as may be seen from their disappearance on ionization. The bands in the 1350 cm-l region are expected to be due to symmetric CH, deformation, CH, wagging and CH bending [la, 171. From these results it seems that the intensities and frequencies of these bands are influenced by the ionization of the COOH groups. They are not found in the undis- sociated acids in the solid state for example.

The results for the PMA solutions in D,O are in agreement with the work of EHRLICH [19]. It should be noted that the films from which the spectra were taken were not completely dry as evidenced by some absorption in the region above 1600 cm-l for the salts.

The effect of increasing ionization of the polyacids was investigated by measuring the position and peak intensities of vcEo and v, as a function of the degree of neutralization of PMA and PAA in D,O solution. To reduce the inherent compensation uncertainties the difference in peak absorbances A(vc,o) - A(v,) has been determined. If the concentration of the polyacid is c eq./l we have at a degree of neutralization a:

A(bO) - -4va) c.1

= A(v~=~) - &v,) = J%,,_~ - (E,,_, + Eva) a

Here E is the extinction coefficient and I is the cell length in centimetres (cm). From Fig. 6 it may be seen that the increasing electrostatic field, the change of hydration characteristics and macromolecular conformation due to increasing ionization of the polyelectrolyte does not seem to influence the variation in electric moment during vibration appreciably for vcEo and v,. The same result was obtained for PMA. The ratio of EYo/E,,OrO is 2.4 and 3.2 for PAA and PMA respectively.

In Fig. 7 the positions of v+o and v, are presented as a function of a. It may be seen that while v, and vcEo remain stable for PAA, slight changes in position take place in the case of PMA. It is interesting to compare these results with some studies on dicarboxylic acids [lo, 201. For D,O solutions of the monosalts of these acids low values for v+o (1650 cm-l-1680 cm-l) and high values for Y, (1580 cm-l- 1590 cm-l) are found for those compounds in which intramolecular hydrogen- bonding is present. On complete ionization of the hydrogen-bonded acids a large shift in v, to lower wavenumbers is observed (33 cm-l in the case of di-n-propyl- malonic acid). In PMA and PAA the value for vc=o and the absence of a large shift in v, seem to exclude intramolecular hydrogen-bonding for these polyacids in aqueous solution. The change in v,+o and v, for PMA on ionization may however be correlated to the general behaviour of this acid.

The ionization of PMA will be accompanied by an expansion of the densely coiled molecule and the environment of the carboxylic acid groups will change during this process. From the direction of the shift it seems that the expansion

[18] D. HADZI and N. SHEPPARD, Proc. Roy. Sot. A216, 247 (1953). [19] G. EHRLICH, J. Am. Chem. Sot. 76, 5263 (1954). [20] L. EBERSON, Acta Chem. Scan& 13, 224 (1959).

Page 8: An i.r. investigation of polyion-counterion interactions

1404 J. C. LEYTE, L. H. ZUIDERWEG and H. J. VLEDDER

n(v,,,)-n(v,) F__

d

-I -

-2-

-3-

-L-

-5-

-6-

Fig. 6. Polyacrylic acid. Absorption d.Berence for Y,,=~ and P, as a function of the

degree of ionisation a.

1 : i cm-~ cm-’

I700 I I- 1570

1690 +

1560

1

Fig. 7. vczO and v, as a function of the degree of ionisation a for polymethacrylic

acid (p) and polyacrylic acid ($) in D,O solution.

offers more favourable hydrogen bonding conditions for the carboxylic acid groups. This seems reasonable as, in view of the hydrophobic character of the PMA chain, the hydration of these groups will be unfavourably influenced in the densely coiled conformations. Apart from this more or less specific inference the replacement of the environment of the carboxylic acid groups by a more polar solvent may contribute to this effect as is known from studies of solvent shifts [21].

In PAA, where the methyl groups of PMA are replaced by hydrogen atoms, these effects may be expected to be less important. The influence of the relatively

[21] A. J. COLLINGS and K. J. MORGAN, J. Chem. Sot. 3437 (1903).

Page 9: An i.r. investigation of polyion-counterion interactions

An ix. investigation of polyion-count&on interactions 1405

small structural difference for PMA and PAA is also reflected in macroscopic properties of these polyelectrolytes. It has been shown [12] that the potentiometric and viscosimetric behaviour of PMA may be interpreted in terms of a conforma- tiona] transition from a group of conformations PMAa to another group PMAb. While at low values of the degree of ionization cc the densely coiled PMA” is stable, a transition to the more extended conformations PMAb may be observed in a region of a-values which depends on ionic strength and polymer concentration. For the concentrations used here the transition ends at a - O-5 [22]. The viscosimetric and potentiometric behaviour of PAA shows no evidence for a conformational transition.

It was observed that the position of the carbonyl absorption in PMA shifts gradually without splitting during the conformational changes. Thus, as far as the vibrational behaviour of the carboxylic acid groups is concerned, the conforma- tional transition in PMA is a continuous process. Two distinct molecular states in the sense that different vibrational states of the carboxylic acid groups exist are not observed simultaneously in PMA. This means that if the interpretation of the macroscopic behaviour of PMA in terms of two groups of conformations differing in overall properties is correct, these two groups in equilibrium at a certain degree of ionization do not differ in internal hydrogen bonding circumstances for the carboxylic acid groups for example.

Therefore, in contrast with the shifts of the amide I band in polypeptides [23] distinct values of yc=o cannot be ascribed to the hypothetical groups of conformations PMA” and PMAb.

The results obtained for the metal polymethacrylates and polyacrylates may now be considered. The wavenumbers at which v, and v, occur have been deter- mined for several PMA and PAA salts. For the PMA salts, which were examined as Kel-F mulls, the influence of bound water on the position of v, and v, was estimated by comparing results obtained from mulls containing different amounts of water. The absorption at 3400 cm-l was used as a measure for the amount of water in these mulls and difFerent samples were converted to the same “concen- tration and path-length” by using the C-H stretching region as an internal standard. The method is inaccurate however and the frequencies obtained in this way should be given rather large ranges (&2-3 cm-l). The salts which were measured as films with variable water content could be investigated more accurately as is shown in Fig. 8 where v, and v, are given for NaPAA as a function of the absorption due to water. For salts which could not be dried to zero absorp- tion in the 3400 cm-l range linear extrapolations were made to zero absorption. These extrapolations never involved shifts of more than 2 cm-l. The resulting values for v, and v, are displayed in Tables 3 and 4. For the dry compounds appreciable differences are observed, the trend being a decrease in frequency for v, and v, with increasing counterion radius at constant counterion charge. Only two possible exceptions are seen: LiPMA and SrPAA or BaPAA. For the alkali

[22] Unpublished results of this lctboretory. [23] T. MIYAZAWA, Polyanzino Acids, Polypeptides & Proteins, p. 201, Univ. Winsc. Press

(1952).

Page 10: An i.r. investigation of polyion-counterion interactions

1406 J. C. LEYTE, L. H. ZUDIERWEU and H. J. VLEDDER

1560 1565 1570 1100 UO5 cl-n-

Fig. 8. Sodium polyacrylate. Film from H,O on CaF,. Y, and Y, as a function of Hz0 absorption.

Table 3

Polymethacrylates

Dry DsO solution

VlI VS VlZ VS

Li 1553 1401 1549 1416 Na 1561 1396 1549 1413 K 1554 1392 1550 1415 CS 1546 1389 1550 1417 Mg 1557 1401 1546 1416 Ca 1540 1400 1546 1417 Sr 1538 1397 1546 1415 Ba 1529 1391 1547 1416

Li N&X K cs Mg Ca Sr Ba

Table 4

Polyacrylates

Dry D,O solution

V, VS V, r s

1571 1422 1570 1413 1570 1406 1571 1412 1570 1399 1569 1412 1565 1400 1571 1412 1582 1431 1565 1413 1562 1417 1563 1415 1550 1414 1563 1413 1553 1405 1564 1414

polyacrylates v, changes appreciably more than v, while the reverse may be noted for the earth-alkali polymethacrylates. The polyacrylates absorb at higher wavenumbers if compared with the equivalent polymethacrylates.

The solutions yield some interesting results. No significant diEerences are found within each of the following groups: alkalipolymethacrylates (va = 1549 cm-r; v, = 1415 cm-l) earth-alkali polymethacrylates (va = 1546 cm-l; v, = 1416 cm-l) alkali polyacrylates (va = 1579 cm-l; v, = 1412 cm-l) earth-alkali polyacrylates (va = 1564 cm-l; v, = 1414 cm-l).

Page 11: An i.r. investigation of polyion-counterion interactions

An i.r. investigation of polyion-count&on interactions 1407

As the earth-alkali salts could only be examined as gels the small difference with the alkali salts is not felt to be related to the difference in charge for these counterions especially in view of the observed constancy of v,.

The following conclusions may be drawn from the results of this investigation.

(a) In the dry solid state a specificity in polyion-counterion interaction may be observed which means that the carboxylate group vibrations are disturbed in a different way by different counterions.

(b) In aqueous solution the carboxylate groups do not differentiate between ions of the same charge and widely different radius, as shown by the behaviour of vibrations characteristic of these groups. Univalent and bivalent ions have no significantly diEerent influence on the vibrations mentioned.

From (a) and (b) it may be concluded that in the solutions and the gels there is no direct contact between the carboxylate groups and the counterions of the polyelectrolyte-counterion systems investigated. The many specific effects observed for these systems, of which some were mentioned in the introduction must therefore be interpreted in terms of the hydrated poly-ion on the one hand and the hydrated counterions on the other. It would be interesting to know quantitatively how much water per carboxylic acid group is necessary to obscure the differences in the perturbations the cations introduce in the vibrations of this group. Against the background formed by the results presented here systems in which site-binding occurs in solution may be investigated to obtain detailed information concerning the factors influencing chemical equilibria within the polyelectrolyte regions of solutions. Work in these directions is in progress at this laboratory.

Acknowledgenm+--We wish to thank Prof. M. MANDEL for his stimulating interest in thie investigation.