6044
J . Phys. Chem. 1988, 92, 6044-6051
Forces between Two Poly(2-vinylpyrldlne)-Covered Surfaces as a Function of Ionic Strength and Polymer Charge Johan Marra and Michael L. Hair* Xerox Research Centre of Canada, Mississauga, Ontario LSK 2L1, Canada (Received: December 9, 1987; In Final Form: April 21, 1988)
A report is given of the direct measurement of surface forces between two poly(2-vinylpyridine)(P2VP)avered mica surfaces both in acid solutions (where P2VP is fully charged) and in alkaline solutions (where P2VP is uncharged). Results are obtained as a function of the chain length and the ionic strength. At low pH and low ionic strength, the adsorption causes a surface charge reversal, is independent of the chain length, and creates surface forces that can adequately be described with DLVO theory, suggesting an essentially flat chain conformation on the surfaces. Deviations from DLVO theory begin to occur in 0.1 M NaCl salt, thereby providing evidence for a more "loopy" chain conformation in high salt leading to an additional steric surface force. Following the neutralization of adsorbed P2VP chains, a large expansion of the adsorbed layer results which, given a nearly constant adsorbed amount, is essentially independent of the chain length. The results highlight the crucial role of the segments/surface binding affinity and the intersegmental repulsion with regard to the extension of the adsorbed layer. Some observations in the present study indicate the effects of a polymer depletion layer near a charged surface on the adsorption kinetics. These are caused either by electrostatic repulsion of charged polymers or through the presence of an electric double layer osmotic pressure gradient affecting the diffusion of uncharged polymers toward a charged surface. In both cases, the depletion effects decrease following the addition of electrolyte.
Introduction The manipulation of colloid stability by polyelectrolytes has become increasingly important once the effectiveness of polyelectrolytes at very low concentrations was recognized.ls2 Depending primarily on the concentration and the molecular weight, a given polyelectrolyte might be used both to stabilize or to destabilize a particular colloid. In either case, the result is a consequence of polyelectrolyte adsorption. As might be expected, adsorption is strongly favored when the polyelectrolyte and the particle surfaces carry opposite charges. In such systems, small amounts of adsorbed polyelectrolyte become effective coagulants due to a reduction of the surface charge and the resulting double-layer forces between the particles.' As a next step, this coagulation often leads to flocculation caused by a polyelectrolyte bridge formation between particles which enmeshes them in a three-dimensional n e t ~ o r k .Conversely, ~ at higher levels of adsorption, polyelectrolytes tend to enhance colloid stability as a result of surface charge reversal which again leads to the build up of strong double-layer forces. Moreover, dependent on the conformation of the adsorbed polyelectrolytes, a possible additional steric repulsion4 can contribute to stability. The latter effect, for example, explains the sometimes observed salt tolerance of commercial latices5 and the unexpected dependence of the zeta potential on the ionic strength6,' which arises when small amounts of polyelectrolyte are formed as a contaminant on the latex surfaces during their synthesis. Of particular interest is the influence of the ionic strength on the adsorbed amount and the conformation adopted by the adsorbed polyelectrolytes. Several studies have shown that the adsorbed amount increases with increasing salt concentration up to very high ionic strength.*-I0 At the same time, the influence (1) Black, A. P.; Birkner, F. B.; Morgan, J. J. J. Colloid Interface Sci. 1966, 21, 626, and references listed therein. (2) Pugh, T. L.; Heller, W. J . Polym. Sci. 1960, 47, 219. (3) LaMer, V. K. J . Colloid Sci. 1964, 19, 291. (4) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic: London, 1983. Scheutjens, J. M. H. M.; Fleer, G. J. J . Phys. Chem. 1979, 83, 1619; 1980,84, 178. (5) Goossens, J. W.; Zembrod, A. Colloid Polym. Sci. 1979, 257, 437. (6) Hidalgo Alvarez, R.; de las Nieves, F. J.; Van der Linde, A. J.; Bijsterbosch, B. H. Colloids Surf. 1986, 21, 259. (7) Bonekamp, B. C.; Hidalgo Alvarez, R.; de las Nieves, F. J.; Bijsterbosch, B. H. J . Colloid Interface Sci. 1987, 118, 366. (8) Greene, B. W. J . Colloid Interface Sci. 1971, 37, 144. (9) Marra, J.; Van der Schee, H. A.; Fleer, G. J.; Lyklema, J. In Adsorption from Solution; Ottewill, R. H., Rochester, C. H., Eds.; Academic: London, 1983: p 245.
0022-3654/88/2092-6044$01.50/0
of the molecular weight on the adsorbed amount becomes more pronounced. Techniques such as double-layer capacitance,' spin labeling,I2 photon correlation spectroscopy, and small-angle neutron scatteringlo have revealed that, especially at low salt concentrations, the polyelectrolyte configuration on the surface is essentially flat. The development of tails and loops was found to only occur at high ionic strength. A detailed polyelectrolyte adsorption theory has been developed by Van der Schee and Lyklema,I3 which was recently extended by Evers et al.I4 to include weakly dissociated polyelectrolytes. Based on a lattice model, this theory predicts a polyelectrolyte behavior at interfaces that is at least in semiquantitative agreement with experimental facts. In spite of these successes, there is a paucity of quantitative data on the modification of surface forces through polyelectrolyte adsorption in systems where all relevant parameters such as polymer molecular weight, surface charge density, charge density on the polyelectrolyte chain, ionic strength, etc. are well controlled. In the present study, we have examined the influence of poly(2vinylpyridine) (P2VP) adsorption on the surface forces existing between molecularly smooth mica surfaces. Use is made of the direct force measurement technique developed by Israela~hvili.~~ P2VP behaves as a basic polyelectrolyte in acid solutions and is insoluble at neutral or higher pH. It is commercially available in narrow molecular weight samples and has a reasonably well understood acid-base dissociation behavior in aqueous solutions. Adsorption is studied as a function of the pH, the ionic strength, and the molecular weight. The resulting surface forces are compared with those in the absence of P2VP. Finally, following the establishment of equilibrium P2VP adsorption from acid solutions, we studied the effect of the addition of a more than equivalent amount of base on the surface forces. This nullifies the polymer charge density without essentially altering the electrolyte concentration.
'
(10) Cosgrove, T.; Obey, T. M.; Vincent, B. J . Colloid Interface Sci. 1986, 1 1 1 , 409. (1 1) Van der Schee, H. A.; Lyklema, J. In The Effect of Polymers on Dispersion Properties; Tadros, Th. F., Ed.; Academic: London, 1982; p 81. (12) Williams, P. A.; Harrop, R.; Phillips, G. 0.;Robb, I. D.; Pass, G. In The Effect of Polymers on Dispersion Properties; Tadros, Th. F., Ed.; Academic: London, 1982; p 361. Cafe, M. C.; Robb, I. D. J . Colloid Interface Sci. 1982, 876, 41 1. (13) Van der Schee, H. A.; Lyklema, J. J . Phys. Chem. 1984,88, 6661. (14) Evers, 0. A,; Fleer, G. J.; Scheutiens. J. M. H. M.; Lvklema, J. J . Colloid Interface Sci. 1986, I 1 I , 446. (15) Israelachvili, J. N.; A d a m , G. A. J . Chem. SOC.,Faraday Trans. 1 1978, 7 4 , 975.
0 1988 American Chemical Society
Forces between Two P2VP-Covered Surfaces
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 6045
Experimental Section r Poly(2-vinylpyridine) (P2VP) of molecular weights (MW) 30 000 and 240 000 ( M , / M , = 1.07 and 1.15, respectively) were obtained from Polysciences. They were further purified by a double precipitation from propanol into hexane and a final precipitation from propanol into distilled water. The resulting polymer paste was dried in vacuo in the presence of phosphorous pentoxide. Solutions of P2VP were prepared by dissolving a weighed amount of polymer in aqueous HCl solutions where the HCl concentration was at least 3 times the equivalent P2VP segment concentration. The P2VP solution was gently stirred overnight. All solvents used in the present study were purchased Reagent 0 8 16 24 32 40 48 56 Grade from Aldrich Chem. Co. and distilled just prior to use. D (nml Water was purified by using the following procedure: distillation, activated charcoal treatment, filtration through a 0.05 pm pore Figure 1. Force vs distance profiles between mica surfaces in aqueous sized nucleopore filter, and a final distillation from an all-glass solutions at pH 3.8, 2.7, and 2.0. Theoretical fits with DLVO theory still. (dashed curves) were carried out using a van der Waals Hamaker constant A = 2.2 X J and double-layer parameters: (i) pH 3.8, conForce measurements were accomplished using the apparatus stant +o = -55 mV, or constant uo = 1 charge/94 nm2; (ii) pH 2.7, developed by Israelachvili. This instrument permits an accurate constant +o = -28 mV, or constant uo = 1 charge/57 nm2; (iii) pH 2.0, determination of attractive, adhesive and repulsive surface forces constant +o = -45 mV, or constant uo = 1 charge/l5 nm2. At short F(D) as a function of the surface separation D. It is essential that distances, the force curves calculated at constant charge are always more the solid surfaces are rigorously smooth since in most instances repulsive than those at constant surface potential. Inward arrows indicate the surface force will strongly depend on the surface separation. inward jumps. Adhesive forces have a strength of about -40 mN/m. To satisfy this requirement, two molecularly smooth mica sheets Inward arrows indicate inward jumps to D = 0. The inset gives the (each with an identical thickness between 5 and 10 pm) were dependence of +o on the pH. cleaved from the crystal, silvered on one side with a 460 8,thick silver layer, and then glued (silvered sides down) with an epoxy a few representative force curves measured in the pH 2.0-4.0 resin (Shell Epon 1003) on two cylindrically curved glass disks. range. At large separations, the force is always repulsive due to The disks are mounted in the apparatus in a crossed cylinder the electrostatic double-layer forces. As a result of the screening configuration. Forces are determined by inferring the deflection effect, the distance range of this force decreases with increasing of a cantilever spring on which one of the glass disks is mounted HCI concentration. Closer in, below D = 4 nm, attractive van while the surface separations are measured with a resolution of der Waals forces become important causing the surface forces 0.2 nm by an optical interferometry technique. Details of these to go through a maximum. In our experimental setup this leads procedures have been described more extensively e l ~ e w h e r e . ’ ~ - ~ ~to the occurrence of an inward jump to D = 0 (adhesive contact) At the beginning of each experiment, the mica surfaces are from a distance which nearly coincides with the force maximum brought into contact in an atmosphere of dry nitrogen in a sealed and where the gradient of the surface force equals the spring apparatus. Observation of smooth adhesive contact ensures that constant of the cantilever spring. To separate the surfaces, an the surfaces are free from debris or other types of contamination. adhesive force has to be overcome whose magnitude was typically After separating the surfaces, the apparatus is filled up with around -40 mN/m. distilled water and left to equilibrate for at least 1 h. The presence The forces were analyzed by using an algorithm developed by of a double-layer force and a finite adhesion similar to those Chan et aLzl based on the DLVO theory. Given a certain elecpreviously reported” was used as a check for the continuing trolyte concentration, this algorithm permits distinction between absence of surface contamination. A small volume of a concenthe double-layer forces predicted at constant surface potential $o trated electrolyte solution or P2VP solution was then injected with or at constant surface charge density uo. At these two limits, either a syringe into the apparatus and mixed thoroughly to obtain the the surface potential or the surface charge is held independent desired final concentration. Whenever P2VP was added, the of the surface separation. surfaces were held widely separated to allow the polymer to freely In Figure 1, the dashed lines represent the corresponding diffuse into the gap between and adsorb onto the mica surfaces. theoretical fits. (The accuracy in the fitted values of the paAdsorption equilibrium was presumed to be established when the rameters I,Lo and uo is 5-IO%.) At close separation interaction surface forces remained essentially unchanged over a period of at constant charge is always more repulsive than interaction at several hours. All measurements were carried out at 22 f 1 OC. constant potential. The curves in Figure 1 show that the exThe measured forces F(D) are scaled with the mean radius of perimental points typically fall between these two limits. Pashley19 curvature R of the cylindrically curved mica plates. According has shown this to be the consequence of the fact that the surfaces to the Deryaguin approximation,18the ratio F ( D ) / 2 r R equals the are “regulating” surfaces with regard to ion binding, Le., neither surface interaction energy E(D) per unit area between two planar I,L0 nor uo is independent of D . In first-order approximation, surfaces at separation D. This equality is subject to the condition regulating surfaces can be accounted for through the introduction that the radius R is much larger than the distance range over which of an intrinsic ion-binding constant. Ideally, this constant should interactions are measured. Since the typical curvature radius was be independent of the ion concentration. However, although each usually around 2 cm and the interactions always had a distance individual force curve in Figure 1 can adequately be fitted with range less than about 100 nm, this condition is amply satisfied. a suitably chosen binding constant, the intrinsic binding affinity of the protons to the mica lattice surface sites is seen to decrease Results with decreasing pH. From the inferred surface potentials $o A . Forces in HCl Solutions. For an understanding of the forces (extrapolated to infinite surface separation, as shown in the inset measured between P2VP-covered mica surfaces in HCl solutions, of Figure l), we see the existence of a minimum in $o around pH it is essential to first investigate the surface forces in the absence 3.0. A weaker binding affinity of the protons at pH 2.0 leads to of P2VP. The basic charging mechanism of the aluminosilicate a (reproducible) increase in q0,whereas a concentration-indemica surface has been investigated b e f ~ r e . ” J ~ ~Figure * ~ 1 shows pendent binding affinity would continuously have lowered $o with (16) (17) (18) (19)
Israelachvili, J. N. J . Colloid Interface Sci. 1973, 44,259. Pashley, R. M. J . Colloid Interface Sci. 1981, 80, 153. Deryaguin, B. V . Kolloid-2. 1934, 69, 155. Pashley, R. M. J . Colloid Interface Sci. 1981, 83,531.
(20) Pashley, R. M.; Israelachvili, J. N. J . Colloid Inferface Sci. 1984, 97, 446. (21) Chan, D.
Y.C.; Pashley, R. M.; White, L. R. J . Colloid Interface
Sci. 1980, 77, 283.
6046
Marra and Hair
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988
increasing pH. The reason for this behavior is not immediately clear and a discussion about this matter is beyond the scope of this paper. For the present purpose, it is sufficient to note that the overall binding affinity of protons is substantially stronger than the binding affinity of previously investigated monovalent ionic specieslg and that the forces decay according to the theoretical prediction. Typical surface charges on the mica, as inferred from the theoretical fits, indicate that between 97% (pH 2.0) and 99.5% (pH 3.0) of all intrinsically negatively charged surface sites (each with a 0.48-nm2 surface area) are charge neutralized through adsorbed protons. Short-range hydration forces were never observed. B. Charging Behavior of P2VP. P2VP is only soluble in acidic aqueous solutions, where it acts as a weak base and acquires a net linear charge density due to protonation of the heteroatom in the pyridine rings. An apparent pK, is usually defined as pK, = pH
+ log ( a / l
loo[ pH = 3.6
.
z
,E
I.
0
6
16
24
32
40
48
56
D (nm)
- a)
where (Y is the degree of polymer protonation. To understand the charging behavior of P2VP, it is important to evaluate the dependence of a on the pH. However, it is well-known that, for polyelectrolytes, as a result of the buildup of an electrostatic potential, the pK, is not a constant but varies with both a and the electrolyte concentration.22 Only if the electrostatic free energy of the chain is explicitly taken into account is it possible to define an intrinsic association constant. Unfortunately, this free energy depends in a complicated way on a host of factors such as ion specificities, counterion binding, and chain c o n f o r m a t i ~ n .Therefore, ~ ~ ~ ~ ~ we will merely estimate the degree of PZVP protonation between pH 2.0 and 4.0. Experimentally, this has been investigated by a number of worker^^^-^^ using potentiometric titration, spectrophotometry, conductometry, and dialysis. It appears that, provided a more than equivalent amount of protons is present, P2VP is at least 70% protonated. In the present experiments we used P2VP concentrations between 8 and 10 pg/mL, (Le., about M in P2VP segments), whereas the pH was always held at or below 3.6. Thus, the excess of protons ensures that P2VP chains are almost fully protonated; the more so when NaCl is present. C. Forces in Acidic P2VP Solutions. Surface forces measured between two mica surfaces in a 10 Mg/mL P2VP solution (MW = 240000) at pH 3.6 were found to progressively increase in magnitude with time. After about 4-5 h of incubation they reached a maximum and showed no further change with time. Figure 2 represents the force curves measured after about 6 h of incubation. The curve measured on initial approach was somewhat more repulsive than on subsequent approaches, but in each case the decay of the forces with distance was the same. Below D = 4 nm, an attraction was experienced. This led to an inward jump to about D = 1.1-1.3 nm which then acted as a hard wall. We noted that this jump took several seconds to complete as compared to a fraction of a second in Figure 1. A theoretical analysis of both curves, similar to that employed in Figure 1, shows an excellent agreement with double-layer theory when the surface potentials (at large separations) are taken to be 75 and 5 5 mV, respectively. At shorter range, the forces are again those typical of regulating surfaces.Ig The absolute values of the inferred surface potentials are clearly higher than those found between bare mica surfaces at pH 3.6. Although no evidence merges about any steric polymeric force (except perhaps a slight one below D = 4 nm), the above observations suggest that each surface is coated with a 0.6 nm thick P2VP layer and that the surface charge has reversed sign. (The latter claim is corroborated in the following section (22) See for example: Morawetz, H. Macromolecules in Solution; Wiley-Interscience: New York, 1975; Chapter VIII. (23) Nagasawa, M. In International Symposium on Macromolecules; Voorn, M. J., Ed.; Butterworths: London, 1970; p 519. (24) Muller, G.; Ripoll, C.; SClEgny, E. Eur. Polym. J . 1971, 7 , 1373. (25) Muller, G. In Polyelectrolytes; SEltgny, E., Ed.; Reidel: Dordrecht, Holland. 1974: D 195. (26) Metaye;, M.; Chabot, F.; SElCgny, E. J . Chim. Phys. Phys.-Chim. Biol. 1979, 76, 404.
Figure 2. Force vs distance profiles between mica surfaces in a I O pg/mL P2VP (MW = 240000) solution at pH 3.6. After 5 h of incubation, the forces measured on the first approach are indicated with ( X ) symbols (outward jump from JI) and those on subsequent approaches with ( 0 ) symbols (outward jump from J 2 ) . The inset gives the corresponding adhesive forces. On prolonged incubation following compression, the forces measured initially during subsequent approaches would again increase toward a limiting force profile as indicated with the (X) symbols, thereby lowering the adhesive forces: (0)symbols and outward jump from J 3 . With P2VP ( M W = 30000) the forces after 5 h of incubation were hysteresis-free and identical with those indicated with ( X ) symbols. Adhesion force was about -7 mN/m. DLVO fits were made using a constant J.o = 75 mV or a constant uo = 1 charge/40 nm2 for the "initial" force curve, and a constant q0 = 55 mV or a constant uo = 1 charge/66 nm2 for the "subsequent" force curve. Hamaker constant A = 2.2 X J . Forces predicted at constant uo are at short separations more repulsive than those predicted at constant $o.
by observing that the double-layer forces pass through zero before becoming again strongly repulsive when a more than equivalent amount of NaOH is introduced.) We conclude that the P2VP chains are adsorbed in an essentially flat conformation. As measured, a thin film with a thickness between 1.1 and 1.3 nm is present between the surfaces in adhesive contact. This film is too thin to perform refractive index measurements. However, from an estimate of 0.20 nm2 for the segmental area together with a density of 1 g/cm3, the predicted thickness of two compact P2VP monolayers is about 1.7 nm. This suggests that the surfaces only carry a submonolayer of P2VP and implies that some lattice sites may still carry adsorbed protons. On the basis of a mass action lawI9 and the inferred surface potentials in Figures 1 and 2, it is easy to show that actually more than 50% of the lattice sites could still carry an adsorbed proton. Of course, such a calculation only gives an upper limit because of a proton exchange with charged segments. An indication that the adsorbed layer may undergo rearrangement during the first approach is obtained when the adhesion forces below D = 4 nm are considered (see inset Figure 2). Typically, an outward jump occurred from D = 2.0 to 2.5 nm but the adhesive minimum was clearly deeper on separating the surfaces after the first approach. An initially slightly less flat adsorption mode could facilitate a larger degree of polymer bridge formation between the surfaces and hence a larger force of adhesion. Based on this observation, one might consider whether an initially somewhat different adsorption mode also has a bearing on the stronger double-layer forces during the first surface approach (see Discussion). Polyelectrolyte bridging was always apparent from a noteworthy feature accompanying the separation of P2VP-covered mica surfaces. Once the adhesive force was overcome, the initial outward movement of the surfaces was sluggish, Le., clearly hindered, and it would take a few seconds to reach a separation of about 100 nm beyond which the outward movement was suddenly very fast. No such effect was seen if the surfaces were withdrawn before they had come into adhesive contact. We propose that bridging only occurs following surface adhesion, thereby again implying an essentially flat chain conformation and
Forces between Two P2VP-Covered Surfaces
1 lo
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 6047
pH = 3.0
h
1-
0
8
16
24
32
40
48
56
D (nm)
Figure 3. Force vs distance profiles in 10 pg/mL P2VP (MW = 240 000) at pH 3.0 after at least 1 h of incubation. Similar as in Figure 2, the initial approach is more repulsive. Adhesion forces and their dependence on “history”were also similar as in Figure 2. Force curves were fitted by using a Hamaker constant A = 2.2 X J and, for the initial approach, a constant $o = 85 mV or constant uo = 1 charge/l7 nm2; for the subsequent approaches a constant $o = 74 mV or constant uo = 1 charge/22 nm2was used. Forces at constant uoare always more repulsive than those at constant $o.
a submonolayer coverage. Although polyelectrolyte bridging should increase the adhesive force, the finite thickness of the adsorbed layer between the surfaces in “contact” will decrease the van der Waals contribution from the mica to the adhesion. Apparently the latter effect dominates since the adhesive minima were always less deep (see inset Figure 2) than the one between bare mica surfaces (-40 mN/m). In order to investigate possible effects of molecular weight, the adsorption experiments were repeated at pH 3.6 in a 10 kg/mL P2VP ( M W = 30000) solution. After about 5 h of incubation, the measured forces were identical with the force curve measured during the first approach in a P2VP (MW = 240 000) solution (Figure 2) but now they also remained unchanged during subsequent approaches. Furthermore, the sluggishness of the surface separation following adhesion as observed with P2VP (MW = 240000) was not observed with P2VP (MW = 30000). This is undoubtedly a consequence of the shorter chain length which must limit the length of the polyelectrolyte bridges. An adhesive minimum of about -7 mN/m was observed during both the first and the subsequent approaches. This value is clearly smaller than the adhesion measured with P2VP (MW = 240000) on the surfaces (inset Figure 2) and leads us to believe that a somewhat higher amount of P2VP (MW = 30000) became adsorbed during the first incubation. On further experimenting, we noted that, when adsorbed P2VP (MW = 240000) layers were first compressed and then allowed to reequilibrate with the solution, the forces eventually increased again to those measured during the initial first approach (and identical with those measured with P2VP (MW = 30000) on the surfaces). The adhesion also decreased. Apparently, the amount of P2VP adsorbed initially does depend somewhat on the chain length, probably involving the precise conformation of the adsorbed chains, but it is difficult to quantify. We conclude that the adsorbed amount depends mainly on the established surface potential. Since the latter can be changed by compressing the adsorbed layers of P2VP (MW = 240000), so therefore can the adsorbed amount. Eventually the measured surface forces become independent of the chain length, thereby indicating an equivalent amount of adsorption. Figure 3 gives the forces measured during the initial and subsequent approaches between mica surfaces after at least 3 h of incubation in a 10 pg/mL P2VP (MW = 240000) solution at pH 3.0. Again, the forces are in excellent agreement with those predicted from DLVO theory when surface potentials of 85 and 72 mV are used. The same features accompanying the surface forces in Figure 2 were also observed at this pH. Adhesive forces had a strength of about -6 mN/m.
0.1 :
0
8
16
24
D (nm)
Figure 4. Repulsive and adhesive forces in a 10 pg/mL P2VP (MW = 240000) solution at pH 2.0. Force curve A was measured immediately after lowering the pH from 3.0 (in which equilibrium adsorption was first established) to 2.0, or immediately after addition of 10 mM NaCI, thereby keeping the pH at 3.0. Typical adhesive forces under those conditions are given in the inset (see text). Force curve B was measured after at least half an hour of incubation in the above solutions. Arrows indicate inward or outward jumps. The two curves can be fitted with constant surface potentials of 85 and 95 mV, respectively, if the OHP is located at D = 0.6 and 0.8 nm, respectively.
Figure 4 represents the forces measured in a 10 kg/mL P2VP solution ( M W = 240000) at pH 2.0. An equilibrium P2VP adsorption was previously established at pH 3.0. Curve A shows the forces measured immediately after lowering the pH from 3.0 to 2.0 and curve B the forces after at least half an hour of incubation at pH 2.0. In both cases, the decay of the forces with distance is well predicted from DLVO theory, thereby indicating that the conformation of the adsorbed chains is still essentially flat. N o indication of a hysteresis in the surface forces was apparent. At short distances, a much reduced adhesive minimum of about -3 mN/m was observed in curve A. During separation of the surfaces (inset Figure 4), the presence of polyelectrolyte bridges between the two surfaces could now clearly be demonstrated. After an initial outward jump J , from D = 2.5 nm the surfaces would come at rest between D = 8 and 9 nm. At this position, they are “suspended” from each other by a maze of P2VP bridges. These could only be disconnected by applying a stronger outward force which eventually caused a fast outward jump J2 from D = 27 nm. Such observations clearly indicate the contribution of polyelectrolyte bridges to the adhesive forces. After prolonged incubation (curve B), the repulsive forces were clearly increased and a far more shallow adhesion resulted in a overall sluggish outward motion of the surfaces without any occurrence of jumps. No specific proton effect from the increased proton concentration existed because essentially the same force curves A and B were measured when 10 mM NaCl was added to a P2VP solution, pH 3.0, to achieve the same ionic strength. Therefore, we attribute the stronger double-layer forces to an increase in P2VP adsorption, thereby noting that the increased ionic strength has screened the electrostatic repulsion between the charged segments and thus enhanced their adsorbability. It is recalled that at pH 3.0 the P2VP protonation should be nearly complete and will therefore remain unchanged both when the pH is lowered to 2.0 or when 10 mM NaCl is added instead. It is difficult to infer a surface potential from the force curves shown in Figure 4. If the outer Helmholtz plane (OHP), Le., the
6048 The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 100c
/i . -
pH = 3.0
+
E
O.lmM NaCl
2
E
. a U
t
I 0
I
8
16
24
D (nm)
Figure 5. Force vs distance profiles in a 10 pg/mL P2VP (MW = 240000) solution at pH = 3.0 and 0.1 M NaC1: ( 0 )indicate the forces measured immediately after addition of 0.1 M NaCl when prior to the addition an equilibrium adsorption was established at pH 3.0; (X) represent the forces after at least 2 h of incubation. The dashed line indicates the DLVO predicted decay length and is consistent with a constant Go = 75 mV when the OHP is assumed to be located at D = 0.5 nm. No
adhesion was apparent. plane where the diffuse double layer originate^,^' is taken to coincide with the mica surface, surface potentials of 140 and 200 mV (curves A and B, respectively) have to be used to obtain a fit with the experimental data. These are unrealistically high. A finite value of the O H P exists when the counterions experience an excluded volume close to the surface which can arise through their own excluded volume but also through the excluded volume of adsorbed segments. The latter factor is likely to be important in our system but the accurate location of the OHP presents a problem. Nevertheless, an estimate may be made from the film thickness of 1.2 nm between the two surfaces in adhesive contact (curve A). The positive surface charges are located in 0.6 nm thin films on each surface and this film must at least partially be impenetrable to C1- ions. Therefore, as a first estimate, we can locate the OHP at 0.6 nm from each surface. Taking this into account, a more realistic surface potential $, = 85 mV in curve A is inferred. Similarly in curve B, we infer a $o = 95 mV when, based on the observed incompressible film thickness of 1.6 nm, the OHP is located at 0.8 nm from each surface. Although evidence exists that OHP corrections have to be accounted for, it is stressed that in the present system the strong dependence of the surface potential on the OHP location does not allow them to be inferred accurately from the force curves in Figure 4. In Figures 2 and 3, where the O H P is assumed to coincide with the mica surface, this is not a problem because of the long decay length of the double layer. The estimated error in $o should then be less than 5 mV. Figure 5 shows the force vs distance profiles when, after adsorption equilibrium of P2VP (MW = 240000) at pH 3.0 has been established, 0.1 M NaCl is introduced. Forces were measured both immediately after NaCl addition, and several hours later when the forces no longer changed with the time. The dashed curve gives the predicted double-layer forces either at $o = 180 mV if the O H P is supposed to coincide with the mica surface, or at t+b0 = 75 mV if the O H P is located 0.5 nm from each surface. (27) See for example: Hunter, R. J. Zeta Potential in Colloid Science; Academic: London, 1981.
Marra and Hair Obviously, a reliable estimate of $o is impossible from the presented data. However, the important feature of the force vs distance profiles is that they clearly have a longer range than according to DLVO theory, especially after longer incubation times which increases the adsorption. We propose this to unambiguously indicate the change from a flat conformation at low ionic strength toward a more “loopy” conformation at high ionic strength, more akin to the typical conformation of uncharged polymer^.^ The forces will in this case consist of both a steric part and an electrostatic part. Since this implies that the polyelectrolyte charges become distributed within a finite volume near the surfaces, the notion of an O H P loses every physical meaning and the forces cannot be analyzed with DLVO theory. At this stage it would have been interesting to further increase the ionic strength. Unfortunately, due to the small surface/volume ratio in the experimental system, trace amounts of organic contaminants in molar concentrations of electrolyte can readily contaminate the surfaces and thereby drastically modify the forces. The problem is aggravated because of the long incubation times with the surfaces widely separated necessary to achieve equilibrium adsorption. These more delicate experiments, requiring rigorous purification procedures, will be the subject of a future investigation. D . Forces in Alkaline P2VP Solutions. The neutralization of P2VP chains adsorbed on mica from acid solutions allows the steric forces between surfaces covered with uncharged polymer to be investigated as a function of the chain length at constant adsorbed amount. The latter circumstance is achieved simply as a consequence of the insensitivity of polyelectrolyte adsorption to the chain length (provided the P2VP (MW = 240000) adsorbed layer is compressed at least once during the incubation). Figure 6 represents the forces when, after adsorption equilibrium of P2VP (MW = 30000 or 240000) was established at pH 3.6, the P2VP was neutralized through addition of 4 X lo4 M NaOH. On keeping the surfaces close together during the addition, a finite time is needed for the NaOH to neutralize the narrow gap between the surfaces. This allowed us to observe that the double-layer forces first completely disappeared before the surface forces at long range again became strongly repulsive. We conclude that a surface charge reversal has taken place. The bottom curve in M solution of NaOH in Figure 6 gives the forces in a 4 X the absence of P2VP. Clearly, in the presence of P2VP, the forces below D = 60 nm are more repulsive than in the absence of PZVP, whereas beyond D = 60 nm the forces merge. The steric forces were disentangled from the double-layer forces through the subsequent addition of M NaCl which, through electrostatic screening, eliminates any electrostatic forces beyond D = 20 nm. The results are shown in Figure 7. We note that, beyond D = 20 nm, the forces in the presence of P2VP are lowered by approximately the amount as measured in the lower curve of Figure 6. This observtion indicates that the double-layer forces are not appreciably affected by the P2VP presence on the surfaces. Therefore and because of the relatively weak double-layer forces between mica surfaces in M NaC1, the upper force vs distance profile in Figure 7 must essentially represent the steric forces. A measure of the spatial extension of the adsorbed layers is conveniently obtained from the distance range of the steric force^.^^^* From Figures 6 and 7 it is evident that a drastic expansion has accompanied the P2VP neutralization on the surfaces even though the solvent is now a worse than 0 solvent. Interestingly, given that the amounts of adsorbed P2VP (MW = 30000) and P2VP (MW = 240000) are nearly the same, the forces suggest that the extension is independent of the P2VP chain length. Weak attractive forces and a shallow adhesive minimum of about -0.03 mN/m between P2VP layers were observed around D = 60 nm (not shown). Although too weak to allow for an accurate measurement, their presence is expected because of the nominal insolubility of P2VP at alkaline pH. (28) Scheutjens, J. M. H. M.; Fleer, G. J. Macromolecules 1985, 18, 1882. Scheutjens, J. M. H. M . Ph.D. Thesis, Agricultural University, Wageningen, Holland, 1985.
The Journal of Physical Chemistry, Vol. 92, No. 21, 1988 6049
Forces between Two P2VP-Covered Surfaces
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1
0.1
t 0
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24
32
40
64
56
48
D (nm)
Figure 6. Forces measured after addition of 4 X lo4 M NaOH to a 10 rg/mL P2VP solution at pH 3.6 in which, prior to the addition, equilibrium adsorption was established: ( X ) represent the forces when an equilibrium amount of P2VP (MW = 240000) was adsorbed, ( 0 )represent the forces when an equilibrium amount of PZVP (MW = 30000) was adsorbed. The forces between the bare mica surfaces in 4 X lo4 M NaOH are given in the lower curve (0). They can be fitted with a constant Go = -180 mV but are more repulsive than predicted below D = 2 nm through the occurrence
of hydration forces.I9
pH = 10.1
+ 1 0 m M NaCl
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.
I
I
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I
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Figure 7. Analogous to Figure 6 but the electrostatic double-layer forces have been screened through the extra addition of 10 mM NaCI. The forces between bare mica surfaces in a 10 mM NaCl solution are given in the lower curve and can be fitted with a constant Go = -100 mV. At short range, an additional hydration force is e~perienced.'~The difference between the top curve and the lower curve represents the steric forces between P2VP (MW = 30000) and (MW = 240000) covered surfaces ( 0 and X, respectively).
The adsorbed amounts in Figure 7 were independently investigated through refractive index measurements with the FECO fringe technique.16 At large separations, the refractive index was essentially that of water (n = 1.33) but on closer approach the refractive index increased. Specifically, the refractive index was measured at D = 5.0 nm for both molecular weights and found to be 1.42 f 0.01. If we choose n = 1.35 for the aqueous part of the medium between the surfacesz9 (thereby accounting for the high concentration of counterions) together with n = 1.60 for solid P2VP,30 it is easily inferred that the average P2VP volume concentration between the surfaces at D = 5.0 nm is about 0.28 f (29) Handbook Of Chemistry and Physics, 67th ed.; West, R. c., Ed.; CRC: Cleveland, OH, 1986. (30) Polymer Handbook 2nd ed.; Brandrup, J., Immergut, E. G., Eds.; Wiley-Interscience: New York, 1975.
0.04. This value is equivalent to a compact P2VP layer with a thickness of 0.6-0.8 nm, which approximately agrees with the 0.6 nm previously obtained in Figure 2 from the finite thickness of the film between the surfaces in adhesive contact. Consequently, our previous assumption that the P2VP adsorption was essentially unchanged after neutralization of the solution appears justified. A final observation concerns the kinetics of the increase in P2VP adsorption when the surfaces are held widely separated for a prolonged time in the turbid alkaline P2VP solution. Additional adsorption is expected from worse than 'b solvents because under such conditions, given an initially adsorbed amount, the surfaces act as nucleus for phase separation. For both molecular weights, we observed a rapih increaie in the range of the steric forces when incubation took place in the presence of 10 m~ NaC1, thereby confirming the adsorption increase. Although the Same increase M N a O H solution, with was eventually observed in a 4 X
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The Journal of Physical Chemistry, Vol. 92, No. 21 1988 ~
incubation times less than 1 h essentially no changes in the surface forces (as given in Figure 6 , upper curve) occurred. This suggests a kinetically inhibited P2VP adsorption process at low ionic strength.
Discussion Forces between Surfaces Covered with Charged P2VP Chains. Generally, our results with regard to the adsorbed chain conformation at low ionic strength (CO.1 M) agree with previous results obtained by investigators using totally different techn i q u e ~ . ~ - ’ ~ In , ~ ’acid solutions, the P2VP conformation is essentially flat because the measured surface forces can adequately be explained on the basis of DLVO theory. Not only will electrostatic segmental repulsions in low salt stretch the chains on the surface, but a strong segment-surface binding affinity should also flatten the conformation. Given that the mica surface has a high intrinsic negative surface charge density (1 charge per 0.48 nm2), electrostatic segment-surface interactions should create a high binding affinity for P2VP segments even though the segments have to compete with single protons for adsorption sites. (On the basis of purely entropic considerations one expects an adsorption equilibrium between segments and protons to be strongly in favor of P2VP segments.) The important influence of the segmentsurface binding affinity on the chain conformation was confirmed by Cosgrove et a1.I0 who measured an increase in the hydrodynamic thickness of polyelectrolyte layers adsorbed on low charge density surfaces, and especially on surfaces carrying the same charge as the polyelectrolyte. Only at salt concentrations of 0.1 M (Figure 5) does the force vs distance profile suggest that the P2VP conformation changes. Surface forces can no longer be explained on the basis of DLVO theory, thereby indicating the presence of an extra steric force originating from a more “loose” conformation with small tails and loops. Such behavior is expected for polyelectrolytes because at high ionic strength, where both intersegmental electrostatic repulsions and surface charge effects become screened, the polyelectrolyte chains should eventually approach the behavior of neutral p01ymers.I~ Similar conclusions were drawn by Cosgrove et a1.I0 in their investigation of the dependence of the hydrodynamic thickness of adsorbed polyelectrolyte layers on an oppositely charged surface as a function of the ionic strength. But, these authors also pointed out that much higher salt concentrations than 0.1 M need to be introduced to completely screen the charge effects on the polyelectrolyte chains and fully develop the train-loop-tail conformation characteristic of uncharged polymers. Considering the well-established fact that hydrophobic colloids are usually coagulated by not more than 0.1 M (1 :1) electrolyte, the very high salt concentrations might seem unexpected, but this feature has also shown up in viscosity measurements of polyelectrolyte solut i o n ~and ~ is also theoretically predicted.13 Using an advanced lattice theory for polyelectrolyte adsorption, Marra et al.9 and later Van der Schee and Lyklema,I3 showed that, even with monovalent ion concentrations of 3 M, the polyelectrolyte conformation was still less extended than that of an otherwise identical but uncharged polymer. In complete accordance with the present results at low ionic strength, this lattice theory predicts that adsorbed charged segments will mainly be confined to first lattice layer near the surface with the segment density in the second lattice layer usually at least an order of magnitude lower. Furthermore, the theory shows that depending on the (opposite) surface charge density, the segment density near the surface can reach an almost complete surface coverage. Our refractive index results on neutralized P2VP layers indicate within experimental error that the previously charged P2VP chains form a somewhat less than a fully compact monolayer. The adhesion results together with the adsorbed layer thicknesses in Figures 2, 3, and 4 corroborate this result. Because of the occurrence of P2VP bridging following adhesive contact, a finite bare surface area should still be available for the polyelectrolytes to adsorb (3 1) Bonekamp, B. C.; Van der Schee, H. A.; Lyklema, J. Croat. Chem. Acta 1983, 56, 695.
Marra and Hair upon and form bridges. Finally, a finite separation of the OHP from the mica surfaces had to be accounted for in Figure 4 in order to infer realistic values for the surface potentials from the measured surface forces. A separation of slightly less than the thickness of a full P2VP monolayer was found to give reasonable results. A nearly complete surface coverage with P2VP segments would necessarily mean that each negatively charged lattice site on the mica (area 0.48 nm2) is covered with about two P2VP segments (area 0.20 nm2), each positively charged. However, the inferred surface charge densities are rather low: 1 positive charge/5.7 nm2 in Figure 4 (at q0 = 85 mV) down to only 1 positive charge/40 nm2 in Figure 2 (at q0 = 74 mV). It is noted that this 10-fold change in surface charge occurs together with a much smaller change in the amount of adsorbed P2VP. Hence, we can only conclude that, apart from the segments necessary to neutralize the mica lattice, only a very small fraction of the remaining segments carry a net charge, even though the P2VP chains are nearly fully protonated in solution. The explanation must be that either part of the adsorbed segments are deprotonated following adsorption or that chloride counterion adsorption on the adsorbed segments has taken place. The latter situation would be the surface analogy of the well-known counterion condensation phenomenon on polyelectrolyte chains in solution.32 It is not possible at this stage to discriminate between these two possibilities but from Figures 2 and 3, where we inferred that both the surface charge and the surface potential vary with surface separation (i.e., regulating surfaces), we conclude that an ion binding regulating mechanism must exist. In spite of large surface charge density differences in Figures 2, 3, and 4, the concurrent changes of the surface potential are much smaller (subject to the validity of our O H P corrections) and typically lie between q0 = 70 mV and q0 = 90 mV. The latter observation indicates that, at low salt, the adsorbed amount is primarily limited through the buildup of a repulsive (positive) surface potential whose critical upper value is largely independent of the ionic strength (as long as the P2VP conformation remains flat). It should be possible to confirm this aspect theoretically using the theory of Van der Schee and L ~ k 1 e m a . l ~ The theory has also predicted the creation of a depletion zone near the surfaces following surface charge reversal, which is caused through the buildup of an electrostatic repulsive field (the ionic double layer). Since the spatial extension of this field is screened by electrolyte, the depletion zone becomes smaller on increasing the electrolyte concentration. As such, we infer that the kinetics of polymer adsorption will become a function of the ionic strength. It is likely that it is this depletion zone effect that is responsible for the much longer incubation times necessary to establish equilibrium adsorption at low ionic strength in Figure 2, (-4 h) as compared to Figures 3 and 4 at higher ionic strength (
