4480
Langmuir 1995,11, 4480-4485
The Order of Adding Polyelectrolyte and Salt Affects Surface Forces and Layer Structures Mats A. G. Dahlgren," Heleen C. M. Hollenberg,? and Per M. Claesson Laboratory for Chemical Surface Science, Department of Chemistry, Physical Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden, a n d Institute for Surface Chemistry, P.O. Box 5607, S-114 86 Stockholm, Sweden Received April 17, 1995. I n Final Form: July 24, 1995@ The order in which salt and polyelectrolyte are added to a solution influences the structure of adsorbed layers on surfaces in contact with the solution and the interactions between surfaces across such solutions. By performing surface force measurements, it is shown that the strong attraction observed between polyelectrolyte-coatedsurfaces in low ionic strength solutions is changed into a long-range repulsion with a solution of high ionic strength. This repulsion has a longer range and larger magnitude when the salt is added before the polyelectrolyte than when additions are done in the reverse order. When increasing the ionic strength in steps, the layer thickness and short-range repulsion increases. We suggest that this is due to an increased adsorption of the polyelectrolyte. Mean field calculations (Scheutjens-Fleer model) were used to rationalize experimental findings. The results show that the adsorbed layer is much more extended in solutions of high ionic strength. Furthermore, we show that the adsorption of polyelectrolyte is increased with ionic strength when the nonelectrostatic interaction between polymer segments and the solvent is unfavorable.
Introduction Polyelectrolytes are highly charged macromolecules. The long-range nature of charge-charge interactions makes the behavior of polyelectrolytes significantly different from that of uncharged polymers of the same size. This is reflected in both their solution properties and their adsorption onto solid surfaces. The adsorption of polyelectrolytes, which to a large extent is driven by electrostatic forces, is strongly affected by the ionic strength, polyelectrolyte charge density, and surface charge density. In the last few years, several experimental studies of polyelectrolytes have been presented, including surface force m e a s u r e m e n t ~ , l - ~ adsorption onto particles in s o l ~ t i o n , l and ~ - ~interaction ~ with surfactant system^.^^^^^ There are also several theoretical studies on polyelectro-
* Author to whom correspondence should be addressed. Fax:
[email protected]. Present address: Faculty of Mathematics and Computing Science,Room HG.6.57,Eindhoven University ofTechnology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. Abstract published inAduance ACSAbstracts, October 15,1995.
lytes in the literature, such a s mean-field calculations1s-21 and Monte Carlo (MC) simulation^.^^^^-^^ When comparing the findings from different studies, such as those in refs 3 and 8, there are striking differences, despite that the polyelectrolytes studied have similar molecular weight and charge density and that the ionic strength of the solution is about the same. It has been suggested3 that the conditions from which the initial adsorption takes place can explain the differences found. There are also other studies1J0J3J4indicating that, when polyelectrolytes are present, the order of addition of the components to the system is influencing the conformation of the adsorbed macromolecule. In this paper we address this question and investigate how the initial adsorption conditions affect the structure of adsorbed polyelectrolyte layers by studying forces acting between negatively charged mica surfaces in the presence of highly charged cationic polyelectrolytes and monovalent salt.
+46-8-790 8207. E-mail:
@
(1)Dahlgren, M. A. G. Langmuir 1994, 10, 1580-1583. (2)Dahlgren, M. A. G.; Claesson, P. M.; Audebert, R. J . Colloid Interface Sci. 1994, 166, 343-349. (3) Dahlgren, M. A. G.; Waltermo, A.; Blomberg, E.; Claesson, P. M.; Sjostrom, L.; h e s s o n , T.; Jonsson, B. J . Phys. Chem. 1993,97,1176911 775. (4) Hartley, P. G.; Bailey, A. I.; Luckham, P. F.; Batts, G. Colloids Surf. A: Physicochem. Eng. Apects 1993, 77, 191-198. (5) Claesson, P. M.; Ninham, B. W. Langmuir 1992,8,1406-1412. (6) Dhoot, S.; Goddard, E. D.; Murphy, D. S.; Tirrell, M. Colloids Surf. 1992, 66, 91-96. (7) Marra, J.; Hair, M. L. J . Phys. Chem. 1988, 92, 6044-6051. ( 8 ) Luckham, P. F.; Klein, J. J . Chem. SOC.,Faraday Trans. 1 1984, 80, 865-878. (9)Afshar-Rad, T.; Bailey, A. I.; Luckham, P. F.; Macnaughtan, W.; Chapman, D. Colloids Surf. 1987,25, 263-277. (10) Buchhammer, H.-M.; Petzold, G.; Lunkwitz, K. Colloids Surf. A: Physicochem. Eng. Apects 1993, 76, 81-85. (11) Taylor, P.; Liang, W.; Bognolo, G.; Tadros, T. F. Colloids Surf. 1991, 61, 147-165. (12) Denoyel, R.; Durand, G.; Lafuma, F.; Audebert, R. J . Colloid Interface Sci. 1990, 139, 281-290. (13) Meadows, J.;Williams, P. A,; Garvey, M. J.;Harrop, R. J . Colloid Interface Sci. 1990, 139, 260-267. (14) Meadows, J.; Williams, P. A,; Garvey, M. J.; Harrop, R. A,; Phillips, G. 0. Colloids Surf. 1988, 32, 275-288. (15) Wang, T. K.; Audebert, R. J . Colloid Znterface Sci. 1988, 121, 32-41.
0743- 746319512411-4480$09.00/0
Experimental Section Surface Force Measurements. Adsorption of cationic polyelectrolyte onto muscovite mica was studied by means ofthe interferometric surface force technique developed by Israelachvili . ~ and ad am^,^^ using a setup designed by Parker et ~ 1 The polyelectrolyte was adsorbed onto molecularly smooth muscovite mica sheets (order of 1 cm2in size)immersed in the polyelectrolyte (16) Macdonald, P. M.; Staring, D.; Yue, Y. Langmuir 1993,9,381384. (17) Lee, B.-H.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. Langmuir 1991, 7, 1332-1335. (18) Dahlgren, M. A. G.; Leermakers, F. A. M. Langmuir 1996,1I, 2996-3006. (19) Israels, R.; Scheutjens, J. M. H. M.; Fleer, G. J. Macromolecules 1993,26, 5405-5413. (20) van de Steeg, H. G. M.; Cohen Stuart, M. A,; de Keizer, A,; Bijsterbosch, B. H. Langmuir 1992, 8, 2538-2546. (21) Bohmer, M. R.; Evers, 0. A,; Scheutjens, J. M. H. M. Macromolecules 1990,23, 2288-2301. (22) Siostrom, L.; Akesson, T.: Jonsson, B. J . Chem. Phys. 1993,99, 4739-4747. (23) Granfeldt. M. K.; Jonsson, B.; Woodward, C. E. J . Phys. Chem. 1992,96, 10 080-10 086. (24) h e s s o n , T.; Woodward, C . ; Jonsson, B. J . Chem. Phys. 1989, 91, 2461-2469. (25) Israelachvili, J. N.; Adams, G. E. J . Chem. SOC.,Faraday Trans. 11978, 74, 975-1001. (26)Parker, J. L.; Christenson, H. K.; Ninham, B. W. Rev. Sci. Instrum. 1989,60, 3135-3138.
0 1995 American Chemical Society
~
Langmuir, Vol. 11, No. 11, 1995 4481
Order of Addition of' Polyelectrolyte and Salt
L
c=o
CH3
I
cl-
It -CH3
0 -CH*-CH2-N
CH3
Figure 1. Molecular structure of PCMA, poly( [2-(propiony1oxy)ethylltrimethylammonium chloride}. solution. The mica sheets were thin (1-3 pm) and silvered on the back side to enable interferometric determination of the surface s e p a r a t i ~ nD. , ~ In ~ the setup used, D can be determined to within 0.2 nm or better. The force acting between the surfaces at a separation D, Fc(D),is determined from the deflectionof the double cantilever spring supporting the lower surface. The detection limit of the force is about 100 nN, corresponding to a normalized force of 5 pN/m. The upper surface is mounted on a piezoelectric tube. By varying the voltage applied over the piezoelectrictube, the surface separation can be controlled.Also, the cantilever spring holding the lower surface can be moved by means of a synchronous motor. The two surfaces are oriented in a crossed-cylinderconfiguration. The force measured is normalized by the local mean radius ofthe cylinders, R, which is on the order of 2 cm. The free energy of interaction per unit surface area between two flat surfaces at separation D, GAD), can be determined from Fc(D) via the Derjaguin approximation:28 (1)
This equation is valid provided that D
