Langmuir 1992,8, 338-340
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Revisit to the Intrinsic Viscosity-Molecular Weight Relationship of Ionic Polymers. 6. Influence of Counterions on the Viscosity Behavior of Dilute Aqueous Suspensions of Ionic Polymer Latices Junpei Yamanaka,? Sadako Hashimoto,? Hideki Matsuoka; Hiromi Kitano,* Norio Ise,*z*Takuji Yamaguchi,? Susumu Saeki,? and Masakazu Tsubokawat
Department of Materials Science and Engineering, Fukui University, Fukui 910,Japan, and Department of Polymer Chemistry, Kyoto University, Kyoto 606-01,Japan Received June 12, 1991.In Final Form: September 16,1991
Introduction As we have reported in previous papers, the electroviscous effects play an essentially important role both in suspensions of ionic polymer latices112and in solutions of ionic polymer^.^-^ However, the mechanisms of the effects have not been fully understood. The following mechanism has been widely accepted for the first-order electroviscous effect: Counterions of ionic polymer are distributed around the polyion by the balance of diffusional and electrostatic forces. Under a velocity gradient, counterions are subjected to a frictional force from solvent or suspension medium, so that an excess energy is dissipated. Thus, the solution or suspension viscosities of ionic polymers are larger than that for nonionic polymer^.^^^ A theoretical expression on the basis of this model was derived by Booth8 for suspensions of rigid sphere. Although satisfactory agreements of the Booth theory with the experiments have been reported by several researchers,+ll it seems to us that more detailed studies are necessary to confirm the theory. One of the predictions of the Booth theory is that the suspension viscosity increases with increasing frictional force between counterions and suspension medium. In other words, viscosity becomes larger with decreasing ionic mobility of the counterions. In the present paper, we examined the validity of this prediction by studying counterion effect on the suspension viscosity of ionic polymer latices. Experimental Section A. Materials. Polystyrene-basedlatex, N-100, was purchased from Sekisui Chemical Co. (Osaka, Japan). It was purified by ultrafiltration and the ion-exchange method as described in part 1.' Ultrafiltration was carried out by using a model UHP-76 cell
* To whom correspondenceshould be addressed. t
Fukui University.
* Kyoto University.
(1) Yamanaka, J.;Matsuoka, H.; Kitano, H.; Ise,N. J. Colloid Interface Sci. 1990, 134, 92. (2) Yamanaka, J.; Matauoka, H.; Kitano, H.; Ise, N.; Yamaguchi, T.; Saeki, S.; Tsubokawa, M. Langmuir 1991, 7, 1928. (3) Yamanaka, J.; Matsuoka, H.; Kitano, H.; Ise, N.; Hasegawa, M. J. Am. Chem. SOC. 1990,112, 587. (4) Yamanaka, J.; Araie, H.; Matauoka, H.; Kitano, H.; he, N.; Yamaguchi, T.; Saeki, S.; Tsubokawa, M. Macromolecules 1991,24,3206. (5) Yamanaka, J.; Matsuoka, H.; Kitano, H.; Ise, N.; Yamaguchi, T.; Saeki, S.; Tsubokawa, M. Macromolecules, in press. (6) Conway, B. E.; Dobry-Duclaux, A. In Rheorogy; Eirich, F., Ed.; Academic Press: New York, 1960; Vol. 3, Chapter 3. (7) Hunter, R. J. Zeta Potential in Colloid Science; Academic Press: New York, 1981; Chapter 5. (8) Booth, F. Proc. R. SOC. London 1950, A203,533. (9) Stone-Masui, J.; Watillon, A. J. Colloid Interface Sci. 1968, 28, 327.
(IO) Chan, F. S.; Goring, D.A.I. J. Colloid Interface Sci. 1966,22,371. (11) McDonough, R. W.; Hunter, R. J. J.Rheol. (N.Y) 1983,27, 189. (12) Einstein, A. Ann. Phys. (Leipzig) 1906, 19, 289.
0743-746319212408-0338$03.00/0
and a membrane UK-10 (exclusion limit M, = 104, Advantec, Co., Tokyo). The diameter was 0.12 X 10+ m, according to the producer. The charge number and charge density determined by conductmetric titrations with an aqueous NaOH solution were 3.9 X 103 per particle and 1.4 X 10" C/cm2, respectively. The purified latex particle thus obtained had H+ (H30+) as counterions (hereafter designated H-type). By neutralizing a H-type latex suspension with sodium hydroxide and tetraethylammonium (Et4N) hydroxide, Na- and Et4N-type latices were prepared, respectively, and were used immediately for viscosity measurements. Sodium hydroxide of an analytical grade was purchased from Merck (Darmstadt, Germany). Et4NOH was prepared by ion exchange of a polarography analysis grade EtrNCl (Nacalai Tesque Co., Kyoto, Japan). Sodium chloride was of a suprapur grade (Merck, Darmstadt). Tetraethylammonium chloride used as a coexisting salt was prepared in an aqueous solution by neutralizing Et4NOH with HCl. Water used was purified by the same procedure as that described in part 3.2The specific conductivity of the water was (0.5-0.6) X 104 S cm-1. The preparation of the sample suspensions and concentration determination of latex were as described in part 3. B. Methods. Viscosity measurements were performed by an Ubbelohde viscometer at 25 i 0.02 "C. The details of the viscometer and viscosity measurement were described in part 1. In relation to the viscosity behavior for various latices observed in part 1, the presently used latex suspension was expected to show Newtonian behavior at salt concentrations higher than 5 X 10-6 M and at 4's less than 10+ (4, latex volume fraction). Consequently, the data obtained by the Ubbelohde viscometer at that condition were regarded as that a t zero shear. The electrical conductivity of latex suspensions was measured by a conductivity meter, type DS-14 (Horiba Co., Kyoto), and a glass cell with platinum electrodes having a cell constant of 1.17 cm-1. Temperature was controlled at 25 f 0.05 OC.
Results A. Determination of the Effective Valency of the Latices. First, we evaluated the fraction of free counterions, f, for H-type latices by electrical conductivity measurements. The details of the measurements and determination of the f values were described in part 1. The f values did not appreciably change with 4, for 4 = 1.5 X to 3.5 X The f value extrapolated to 4 = 0 by the first-order least-squares method was 0.47. Although the 4 range measured was relatively small, the present data were consistent with the expected value from the f-charge density relation obtained in part 1. B. Influence of Degree of Neutralization. Figure 1shows the reduced viscosity, qsp/4,for salt-free suspensions plotted against the degree of neutralization (DN), as neutralized by NaOH. With increasing DN, the vSp/4 increased, though slightly. C. Influence of Counterionson the Viscosity. The viscosities of H-, Na-, and EtdN-type latex suspensions were measured. Because of the sensitive nature of saltfree systems to ionic impurities, experiments were performed in the presence of simple salts. Chlorides of each counterion were chosen as added salts, taking an exchange of counterions with added cations into account. The latices were neutralized so as to have a DN of 0.9, because it was feared that additions of excess hydroxides might cause a rapid decrease of the viscosity. Figure 2 shows vsp/4for Na- and EtrN-type latices at r#I = 3 X and at various salt concentrations (C,). vSs/Q decreased with increasing C,, and approached the EinM. The viscosity stein theoretical value12at about C, = of Na-type latex suspensions was not larger than that for Et4N-type in C, > 5 X M. vaP/@-4 plot at C, = 5 X M is shown in Figure 3 for latex suspensions with three kinds of counterions. In the r#I range measured, vap/4for Na-type latex was almost the 0 1992 American Chemical Society
Langmuir, Vol. 8,No. 1, 1992 339
Notes
lines in the figure were obtained by applying the firstorder least-squares method at a 4 region between 3 x 10-3 and 1.2 X The slopes of the qEp/#-fj plot obtained were nearly the same for the three latices.
0.0
0.1
0.2
D N Figure 1. Influence of the degree of neutralization (DN) on vsp/4 (4 = 3.0 x 10-3). o : c o u n t e r i o n ; Na+
, salt
;
NaCl
0 10-6
10-5
10-4
cs
10-3
10-2
(")
Figure 2. Salt concentration (C,)dependence of qs,,/4 for Naand EtdN-type latex suspensions at 4 = 3 X and at DN = 0.9. The data in the bracket is the one for Na-type latex in water ([H+] = 1.4 X lo4 M). 0 :
10
counterion;EtqN+,salt;Et4NCl
A:
Ne+,
NaCl
0 :
H+
HC1
8
z 0
C
5
Einstein
Theory
0
0.0
0.5
1 .O
1.5
0 (10-2) Figure 3. vap/~-4plot for latex suspensions with various counterions (salt concentration 5 X 10+ M).
same as that of H-type, and Et4N-type latex had obviously larger qE,14 values than H- and Na-type latices. With increasing 4, qEp/4increased almost linearly. The solid
Discussion We discuss the counterion effect on viscosity behavior of latex suspension. As was mentioned in the Introduction, we intended to clarify the influence of counterion mobility on the viscosity. It is necessary, therefore, to compare other properties for the presently used counterions. First, it should be examined whether Na- and Et4Ntype latices have the same f value as H-type latex. Although we could determine the f value for H-type latex by conductivity measurements, this method was not applicable for the other types of latices because of fairly small equivalent conductivities of Na and Et4N ions (50.10 and 32.66 S cm-' equiv-') relative to that of proton (349.81 S cm-l equiv-l). By performing transference measurements, It0et al.13showed that Na-type latex had pi dctically the same f value as H-typs latex. The dissociation behavior of tetraalkylammonium-type latex has not frequently been studied. However, for linear poly(styrenesulfonate), it was found by Ise et al.14 that the f value of tetramethylammonium salt was almost the same as that of Na salt in aqueous solutions. In light of these findings, we assumed that thefvalues of H-, Na-, and Et4N-type latices are not largely different from each other. There seems to be another support for this assumption. We noted earlier that the slope of the qEp/$+ plot for three latex suspensions were practically the same (Figure 3). One of the factors which determine the slope value is the second-order electroviscous effect, due to the electrostatic interparticle interactions. Since the second-order electroviscous effect would be affected significantly by the effective charge number of latex (and not by counterion mobility), the fact that there was no counterion effect on the slope seems to suggest that the difference in f value, if any, did not markedly affect the viscosity behavior. Second, it should be borne in mind that tetraalkylammonium ions have singular properties in aqueous solutions because of hydrophobicity of alkyl chains. In a systematic study of viscosity, Kay et al.I5 reported that tetrapropyland tetrabutylammonium ions were excellent structure makers for water and that tetramethylammonium ion was a structure breaker. However, for Et4N ion, they found that these two effects canceled out. In another paper,ls they also reported that dissociation of tetraethylammonium halides was complete in aqueous solutions. Thus, it seems that the change of the effective charge number with varying counterion and singularity of EtdN ion can be excluded from further discussion. Therefore, we consider the relationship between suspension viscosity and ionic mobility of counterions. The ionic mobilities of H, Na, and Et4N ions in water at 25 "C (proportional to the limiting equivalent conductivity of ions) are estimated cm*V-l s-l, respectively. to be 36.25,5.192, and 3.385 X The fairly large value for H+ is due to its unique conduction mechanism through a so-called proton bridge, which consists of hydrogen bonds between water molecules. The purely hydrodynamic mobility of H+ in water (H30+), which contributes to the first-order electroviscous effect, (13) Ito, K.; Ise, N.; Okubo, T.J. Chem. Phys. 1985, 82, 5732. (14) Mita, K.; Okubo, T.;Ise, N. J. Chem. SOC.Faraday Trans 1 1976, 7"
,h"n
[I,IWLI.
(15) Kay, R.L.;Vituccio, T.; Zawoyski, C.; Evans, D. F. J.Phys. Chem. 1966, 70,2336. (16) Evans,D.F.;Kay, R. L. J . Phys. Chem. 1966, 70,366.
Notes
340 Langmuir, Vol. 8, No. 1, 1992
could not be measured. However, its mobility is expected not to be greatly different from that of Na+ in view of its ionic radius relative to that of Na+.17 If this respect is taken into consideration, the suspension viscosity of Htype latex is to be almost the same as that of Na-type latex, and EtrN-type latex suspension is to have larger viscosity than H and Na type. This is what was observed. As was mentioned in the Introduction, this suggests that the first-order electroviscous effect is an important factor in viscosity behavior. The viscosity increased very slightly with increasing DN (Figure 1). If the above discussion can be applied, this DN dependence means that the ionic mobility of H+is slightly larger than that of Na+ in water. M were The data at a salt concentration of 5 X compared with Booth theory, which reads
qSp/4(4 = 0) = K' = 5/2[1 + q*(Qe2/eakT)2z(~a)I with N
N
q* = (ekT / q o e 2 ) ~ n i Z , 2 wx~n'i/Z , 2 1=1
i=l
where Q is the effective charge number of the particle, e (17) H30+has a relatively flat, trigonal-pyramidstructurewhose radius is 1.35 X 10-lom. Na+ is usually represented as a sphere, having a radius of 0.95 X 10-10 m. The primary hydration numbers of them are reported to be 4 and 5, respectively.
Table I. Experimental and Theoretical Values of the Intrinsic Viscosity for N-100 Latex SusDensions* counterion
H+ Na+
K'erptl
RthWt
3.7 4.1 5.2
6.0
EtdN+ 7.1 a KjeXpa, experimental value; K'thWr,theoretical value by Booth theory.
the elementary charge, a the radius of the particle, the dielectric constant, k the Boltzmann constant, T the tem) relaxation function, 1 / the ~ Debye perature, Z ( K ~the screening length, qo the viscosity of suspension medium, and N the number of ionic species, and ni, Zi, and W i are the concentration, valency, and mobility of the ions of species i , respectively. Q was estimated from the f value determined in section A of the Results. Table I shows the intrinsic viscosities obtained by applying the first-order least-squares method to the vsp/4-$ plot in Figure 3, together with Booth's theoretical values. The agreement between the experiment and theory is gratifying.
Acknowledgment. We express our gratitude to Professor N. Imai, Nagoya University, for his helpful discussions on the electroviscous effect. Registry No. N100,126040-50-4.
