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Langmuir 1994,10,1580-1583
Effect of Counterion Valency and Ionic Strength on Polyelectrolyte Adsorption Mats A. G. Dahlgren* Laboratory for Chemical Surface Science, Department of Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden, and Institute for Surface Chemistry, P.O.Box 5607, S-114 86 Stockholm, Sweden Received January 3, 1994@ The adsorption of cationic polyelectrolytes onto negatively charged mica from solutions of variable ionic strength was studied using the interferometric surface force technique. The results show that the structure of the adsorbed layer of polyelectrolyte is strongly dependent on the ionic strength of the solution from which the initial adsorption takes place, rather than the valency of the counterion. Within the time for one experiment (1week) it was not feasible to achieve the same layer structure by a stepwise increase of the ionic strength, as was obtained when the adsorption of the polyelectrolyte was carried out directly from the solution of the higher ionic strength. A mechanism for attraction due to counterion bridges when large, flexible counterions are present in the solution is suggested.
Introduction Polyelectrolytes have found many uses in a number of technical applications. These include paper-making,'g2 waste water treatment,3 and the food industry: just to mention a few. In all these applications, the adsorption behavior of polyelectrolytes is important. Several papers have been published focusing on the adsorption behavior of polyelectrolytes. These include experimental studies of adsorption to minerals,5s6 and adsorption to cellulose.2 Several theoretical works have also been published during the last few years, including extension of the Scheutjens-Fleer7fj model by van der Schee and L ~ k l e m aby , ~ Evers et al.,4J0and by Israels et a1.,l1as well as Monte Carlo simulations.12-16 Some surface force studies related to adsorption of cationic polyelectrolytes have also been presented,15J7-23 as well as a few on the adsorption of anionic polyelectrolytes.2P26 More-
* Address corresDondence to M.A.G.D. at the Royal Institute of Technology. * Abstract published in Advance ACS Abstracts, April 1, 1994.
(1)Jones, G. D. Uses for Cationic Polymers. InPolyelectrolytes;Frich, K. C., Klempner, D., Patsis, A. V., Eds.;Technomic: Westport, CT, 1976. (2) Tanaka. H.: Odberg, L.: Wigberg, - - L.; Lindstrom, T. J. Colloid Interface Sci. 1990,134, 219. (3)Eriksson. L.: Alm. B. Water Sci. Techn. 1993. 28. 203. (4)Evers, 0:A.f Fleer, G. J.; Scheutjens, J. M. H.'M.i Lyklema, J. J . Colloid Interface Sci. 1986,11 I, 446. (5)Durand-Piana, G.; Lafuma, F.; Audebert, R. J. Colloid Interface Sci. 1987,119, 474. (6)Denoyel, R.; Durand, G.; Lafuma, F.; Audebert, R. J . Colloid Interface Sci. 1990,139, 281. (7)Scheutjens,J. M. H. M.; Fleer, G. J. J. Phys. Chem. 1979,83,1619. (8)Scheutjens, J. M. H. M.; Fleer, G. J. J.Phys. Chem. 1980,84,178. (9)van der Schee, H. A.; Lyklema, J. J. Phys. Chem. 1984,88,6661. (10)Bohmer, M. R.; Evers, 0. A.; Scheutjens, J. M. H. M. Macromolecules 1990,23, 2288. (11)Israels, R.; Scheutjens, J. M. H. M.; Fleer, G. J. Macromolecules 1993,26,5405. (12) Akesson, T.; Woodward, C.; Jonsson, B. J . Chem. Phys. 1989,91, 2461. (13)Granfeldt, M. K.; Jonsson, B.; Woodward, C. E. J . Phys. Chem. 1991,95, 4819. (14)Beltrh, S.; Hooper, H. H.; Blanch, H. W.; Prasunitz, J. M. Macromolecules 1991,24,3178. (15)Dahlgren, M. A. G.; Waltermo, A.; Blomberg, E.; Claesson, P. M.; Sjostrom, L.; Akesson, T.; Jonsson, B. J . Phys. Chem. 1993,97,11769. (16)Sjostrom, L.; Akesson, T.; Jonsson, B. J . Chem. Phys. 1993,99, 4739. (17)Claesson, P. M.; Ninham, B. W. Langmuir 1992,8,1406. (18)Dahlgren, M. A. G.; Claesson, P. M.; Audebert, R. Nord. Pulp Pap. Res. J . 1993,8, 62. (19)Luckham, P. F.; Klein, J. J.Chem. SOC.,Faraday Trans. 1 1984, 80,865.
0743-7463/94/2410-1580$04.50/0
over, Dhoot et have reported on surface force measurements in systems containing both cationic and anionic polyelectrolytes. Recent studies have shown that, upon increasing the concentration of a 1:l salt (KBr), additional adsorption of polyelectrolytes onto mica occurs.15J8 In an early surface force study Luckham and Klein19adsorbed poly(L-lysine) onto mica from a solution of high ionic strength (0.1 M KN03) and found a more extended conformation of the adsorbate than in refs 15 and 18. A question addressed in this paper is whether it is the ionic strength per se that causes the adsorbate to adopt a more extended conformation or if, in addition, specific interactions between the polyelectrolyte and the various counterions are of importance. Experimental Section Surface Force Measurements. The effect of anion valency on the adsorption of cationic polyelectrolyteswas studied using the surface force technique developed by Israelachvili and Adams.2* Some of the measurements were done with a setup designed by Parker et aLZ9 The polyelectrolyteswere adsorbed onto molecularly smooth muscovite mica sheets (order of 1cm2 in size) immersed in the polyelectrolyte solution. The mica sheets were thin (1-3 pm) and silvered on the back to enable interferometric determination of the surface separation,mD. D can be determined to within 0.2 nm. The force acting between the surfaces at a separation D, Fc(D), is determined from the deflection of the double cantilever spring supporting the lower surface, to within 100 nN. The upper surface is mounted on a piezoelectric tube. The two surfaces are oriented in a crosscylinder configuration. The force measured is normalized by the local mean radius of the cylinders, R , which is of the order of 2 cm. (20)Afshar-Rad, T.; Bailey, A. I.; Luckham, P. F.; Macnaughtan, W.; Chapman, D. Colloids Surf. 1987,25,263. (21)Marra, J.; Hair, M. L. J.Phys. Chem. 1988,92,6044. (22)Ananthapadmanabhan, K. P.; Mao, G.-2.;Goddard, E. D.; Tirrell, M. Colloids Surf. 1991,61, 167. (23)Dahlgren, M. A. G.; Claesson, P. M.; Audebert, R. J. Colloid Interface Sci., in press. (24)Marra, J.; Hair, M. L. J. Colloid Interface Sci. 1989,128, 511. (25)Kurihara, K.; Kuitake, T. Langmuir 1992,8,2087. (26)Berg, J. M.; Claesson, P. M.: Neuman, R. D. J. Colloid Interface s~i. 1993,161,182. (27)Dhoot, S.;Goddard, E. D.; Murphy, D. S.; Tirrell, M. Colloids Surf. 1992,66, 91. (28) Israelachvili, J. N.: Adams, G. E. J . Chem. SOC., Faraday Tram. I 1978,74,975. (29) Parker, J. L.; Christenson, H. K.; Ninham, B. W. Reu. Sci. Instrum. 1989,60,3135. (30)Israelachvili, J. N.J . Colloid Interface Sci. 1973,44, 259.
0 1994 American Chemical Society
Langmuir, Vol. 10, No. 5, 1994 1581
Polyelectrolyte Adsorption
ll
h
E
3
m 1500
0 1 0 2 0 3 0 4 0 5 0 D (nm)
503
0 I!
0
b
a
Figure 1. Molecular structures of PCMA (a) and MAPTAC (b).
The free energy of interaction per unit surface area between two flat surfaces at separation D , GAD),can be determined from Fc(R) via the Derjaguin appro~imation:~~ Fc(D)/R = 2sGf(D)
(1)
This equation is valid provided that D 10 nm. In the regime 6 nm < D < 10 nm there is a steep increase in the repulsion. The surfaces can be brought in to D = 6 nm. No attractive jumps are seen (see inset in Figure 2), but upon separation there is a weak attractive force between the surfaces (see Table 1). The absolute magnitude of the forces in 0.1 m M K3Ci cannot be trusted above Fc(D)IR i= 10 000 pN/m due to surface d e f ~ r m a t i o n .However, ~~ the distance determination is not affected by the surface deformation. The final surface separations for the different cases are given in Table 1. All the repulsions in Figure 2 have a decay length which is longer than expected from DLVO theory for the salt concentrations used at surface separations >15 nm. For 10 ppm MAPTAC the results are similar, Figure 3. I n 0.1 m M KBr ( I = 0.1 mM) there is a very weak repulsion (35)Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 63rd ed.; CRC Press, Inc.: Boca Raton, FL, 1982.
1582 Langmuir, Vol. 10, No. 5,1994
Dahlgren
Table 1. Surface Separation and Adhesion at Final Molecular Contact final surface adhesion separation force polyelectrolyte solution (mM) (nm) (mN/m) PCMA 0.1 mMKBr 0.1 1 170 33pMK3Ci 0.2 1 250 0.1 mM K2SO4 0.3 2 0 0.1 mM K&i 0.6 6 1 MAPTAC 0.1mM KBr 0.1 1 180 0.1 mM K2S04 0.3 3 35
Z
1
-100
Bc 0
10
a3
30
40
50
D (nm) Figure 5. Force-distance profiles for adsorbed polyelectrolytes under conditions close to the cnp: (m) 20 ppm PCMA in 0.1 mM KBr, (0) 20 ppm PCMA in 33 pM K&i, ( 0 )10 ppm MAPTAC in 0.1 mM KBr. Arrows indicate attractive jumps; the broken line indicates the surface separation after the jumps.
a,
10
0
30
50
40
61)
D (nm) Figure 3. Force-distance profiles for 10 ppm MAF'TAC (D) in 0.1 mM KBr and ( 0 )in 0.1 mM K2SO4. Arrows indicate attractive jumps; broken lines indicate the surface separations after the jumps.
1500
1" 0
1
0
2
0
3
0
4
0
5
0
6
0
D (nm) Figure 4. Force-distance profiles for adsorbed polyelectrolyte in 0.1 mM K2SO4: (m) 20 ppm PCMA inward measurement, (0) 20 ppm PCMA outward measurement, ( 0 )10 ppm MAPTAC. The broken line indicates the surface separation after the jumps.
at D < 50 nm, and the surfaces jump into an adhesive contact (at D = 1 nm) from -10-nm surface separation (indicated by an arrow). In 0.1 mM KzSO4 (I= 0.3 mM) there is a repulsion at 12 nm < D < 60 nm. A t D = 12 nm there is an attractive jump (arrow) to D = 3 nm. The final surface separations for the different cases are given in Table 1. The repulsion in 0.1 mM KzS04 in Figure 3 has a decay length consistent with DLVO theory ( K - ~ = 18 nm). In Figure 4 the forces acting in 0.1 mM K2SO4 between PCMA-coated and MAPTAC-coated surfaces are shown. The repulsion in 0.1 mM KzSO4 is somewhat more longrange in the MAPTAC case than in the PCMA experiments. In the former there is also an attractive jump from D = 12 nm to D = 3 nm. Measurements on separating PCMA-coated surfaces in 0.1 mM K&04 are also included in Figure 4. The extremely weak repulsions for polyeletrolyte systems close to the chage neutralization point,23 cnp, are shown on a larger scale in Figure 5. None of these
repulsions has a decay length consistent with DLVO theory. In all these situations there is an attraction causing the surfaces to jump from the regime 12 nm < D < 18 nm to D = 1 nm. The magnitudes of the attractions as measured when separating the surfaces are listed in Table 1.
Discussion The layered aluminosilicate mineral mica is negatively charged due to isomorphous substitution of silicon for a l u m i n ~ m 3(2.1 ~ X 10'8 such sites/m2 exist on the mica surface). This charge is compensated by ions (mainly potassium) located between the crystal layers. When a mica surface is brought into contact with water, the potassium ions are dissolved, giving rise to a negative surface charge of a t most 0.34C/m2, corresponding to an area per unit charge of 0.48 nm2. In dilute aqueous solutions, the observed surface charge is less by orders of magnitude, due to adsorption of proton^.^^^^^ When a cationic polyelectrolyte is added to a dilute aqueous solution, the electrostatic attraction between the polyelectrolyte and the mica surface will strongly favor adsorption. In solutions of low ionic strength (I I 0.1 mM), the adsorbed polyelectrolyte molecules adopt flat conformations and compensate the mica lattice charge (Figure 5). Under these conditions, the polyelectrolytes have a rather rodlike average conformation in solution,39 due to intramolecular repulsion between the charged monomers. However, a few tails and/or loops are present in the adsorbate layer; these give rise to the attraction, which can be attributed to bridging,ls where some polyelectrolyte molecules have monomers on both sides of the midplane between the surfaces.12 When the ionic strength of the solution is raised, the polyelectrolytes have less restricted conformations in sol~tionM*4~ (shorter bond parameteP). Hence, the polyelectrolytes adsorbing can adopt more extended conformations on the surface a t higher ionic strength, but the results presented in this paper show that (36) Griiven, N.2.Krystallogr. 1971, 134, 196. (37) Claesson, P. M.; Herder, P.; Stenius,P.; Eriksson, J. C.; Paehley, R.M.J. Colloid Interface Sci. 1986,109, 31. (38) Pashley, R. M. J . Colloid Interface Sci. 1981, 83, 531. (39) Liu, G.;Guillet,J.E.;Al-Takrity,E.T. B.; Jenkins,A. D.; Walton, D.R.M.Macromolecules 1991, 24, 68. (40)Granfeldt, M. K.;Jdnsson, B.; Woodward, C. E. J. Phys. Chem. 1992,96, 10080.
(41) Hodgson, D.F.;Amis, E. J. J. Chem. Phys. 1991,95, 7653.
Langmuir, Vol. 10, No. 5, 1994 1583
Polyelectrolyte Adsorption
I!
I
the surface force apparatus is the structure and interaction of short surface separations representing the equilibrium situation (or metastable state) when the surface separation is large (D> 10 pm). This is not necessarily the same as the thermodynamic equilibrium situation a t short separations. When the ionic strength of the solution is low ( I 5 0.2 mM), the adsorbed polyelectrolytes can neutralize the surface charge. Doing so, the polyelectrolytes adsorb in extemely flat conformations. The thickness of the adsorbed layer is a function of ionic strength rather than of the polyelectrolyte molecular weight. This is evident from the results in Figure 5, which show that the final molecular contact is only 1 nm from mica-mica contact, despite that there is a factor of 15 in the difference in molecular weight between PCMA and MAPTAC. The difference in the distance from which the surfaces jump into contact (which occurs at a surface separation of 10-18 nm) is also very small. This indicates that the molecular weight, as long as it is sufficiently large, has a very minor influence on the structure of the adsorbed layer under these conditions. This finding is also supported by theoretical calculations? It is also in line with results from Monte Carlo simulationsm which indicate that the intrachain interactions between monomers in the polyelectrolyte chain can be neglected for monomers some distance away, which is considerably shorter than the chain lengths of the polyelectrolytes used in this study (the MAPTAC used consists of approximately 450monomers). Also, the striking similarities in the force profiles for PCMA in 0.1 mM KBr and 33 pM K3Ci show that the ionic strength of the solution is more important to the structure of the adsorbed layer than specific interactions between PCMA and the counterions. If the opposite were true, larger differences in layer thickness would be expected, and possibly also the interaction a t moderate distarices (5 nm < D < 25 nm), which is not the case here.
Conclusion The structure of an adsorbed polyelectrolyte layer is strongly dependent on the ionic strength of the solution from which the adsorption takes place. For adsorption from solution of moderate ionic strength (II 0.6 mM) the layer structure is more significantly affected by the conditions from which the initial adsorption took place, than by the final conditions. Entropically induced bridging, as defined by Akesson e t aZ.12as due to polyelectrolyte molecules stretching across the midplane, vanishes as the ionic strength is increased. Instead, a mechanism including counterion bridges becomes important when large, flexible counterions are present in the solution. In solutions of low ionic strength (II 0.2 mM) the ionic strength is more important for the structure of the adsorbed layer than the valency of the counterions. (42) Largest oxygen-oxygen _ _ distance found in calculation by MOPAC 6.0 on a V.&X 6C60-310. (43) Lvov, Y.;Decher, G.; Mbhwald, H. Langmuir 1993, 9, 481. (44) Barford. W.;Ball, R. C.; Nex, C. M. M. J. Chem. SOC.,Faraday Trans. 1 1986,82,3233.
Acknowledgment. I want to thank Dr. Per M. Claesson and Prof. Jan Christer Eriksson for valuable discussions while preparing this paper. Asa Waltermo is acknowledged for leading me into this particular study. In addition, I thank NUTEK for financial support.
