Solution

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Mixed Adsorption Layers of Uncharged Polymers at Particle/Solution Interfaces Ferenc Csempesz* and Katalin F. Csa´ki Department of Colloid Chemistry, Eo¨ tvo¨ s Lorand University, Budapest, P.O. Box 32, H-1518 Budapest 112, Hungary Received January 14, 2000. In Final Form: April 8, 2000 Simultaneous competitive adsorption from binary mixtures of uncharged polymers on negatively charged colloidal dispersions and the structure of polymer layers formed at particle/solution interfaces were studied. From the adsorption isotherms determined in competitive adsorption at equal concentrations of two polymers in the bulk solution, preferential adsorption parameters for the polymers in pairs have been established, and used as a measure of the affinity of chemically different macromolecules for the solid surfaces. The diffusion coefficient of (monodisperse) particles with and without adsorbed polymer was measured by photon correlation spectroscopy, and the hydrodynamic thickness of adsorbed polymer layers has been calculated. The exchange of polymeric species adsorbed at the interfaces was detected by measuring the thicknesses of mixed layers after various contact times. The preferential adsorption parameters proved to be relevant markers for the various interfacial processes. Simultaneous adsorption onto particle surfaces of polymers with significantly different segment affinities resulted in considerable alterations in the structure of mixed adsorption layers. Close correlation was found between the affinity of the competing macromolecules and the spatial properties of mixed layers. From suitable polymer pairs, at partial surface coverages where enough surface sites were available for the various macromolecules, irregularly extended adsorption layers formed. At high surface coverages, the preferentially adsorbed species displaced the weakly adsorbed polymers from the interfaces.

1. Introduction Polymers find extensive use in controlling interfacial processes in a variety of disperse systems, as in water treatment, mineral processing, papermaking, biological and pharmaceutical applications, and other fields. The effects observed result from the change of interparticle interactions due to the macromolecules adsorbed at the particle/solution interfaces, but in certain cases, the presence of free polymer in the solution may also play a role. For most adsorption-controlled processes, the preferred polymer chains should not only be well anchored to the particle surface but should also give a thick adsorption layer extending into the dispersion medium.1-7 Considerable difficulties arise, however, in selecting polymers that fulfill these requirements in various practical applications. In many instances, the most effective way is to modify the chemical structure of homopolymers or to use copolymers. On the other hand, there are applications where a mixture of homopolymers, e.g., as flocculants or steric stabilizers, can achieve an enhanced effectiveness. The extensive studies relating to the interfacial behavior of macromolecules are mainly concerned with individual polymers. Much less is known about the adsorption from * Corresponding author. E-mail: [email protected]. (1) Takahashi, A.; Kawaguchi, M. Adv. Polym. Sci. 1982, 46, 1. (2) Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983. (3) Tadros, Th. F. In Polymers in Colloid Systems; Tadros, Th. F., Ed.; Elsevier: Amsterdam, 1986. (4) Cohen Stuart, M. A.; Cosgrove T.; Vincent, B. Adv. Colloid Interface Sci. 1986, 24, 143. (5) Lipatov, Y. S. Colloid Chemistry of Polymers; Elsevier: Amsterdam, 1988. (6) Robb I. D. Comprehensive Polym. Sci. 1989, 2, 733. (7) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman and Hall; London, 1993.

multicomponent macromolecular solutions where two or more polymeric species are competing for surface sites.7-17 Competition for particle surfaces may lead to greatly different adsorption characteristics for the various components, and in general, preferential adsorption plays a role. Chemically different polymers usually have different affinities for the surface, which leads to preferential adsorption of the polymer with the highest segmental adsorption energy.7,14,18 Adsorption preference with respect to molecular weight occurs when macromolecules that differ only in chain length are adsorbed. At equilibrium, larger chains adsorb preferentially over small ones.10,11 The conformation of chains at an interface also has a marked effect on how the macromolecular substances act. Understanding the interfacial behavior of different polymers requires, therefore, information on both the extent of their adsorption and the structure of the adsorbed layer.1,7,15,17 To date, only a few attempts have been made toward investigating the state of chemically different chains in a composite adsorption layer. This study is mainly focused on the spatial properties of mixed adsorption layers formed at particle/solution interfaces from binary polymer mixtures in simultaneous competitive adsorption. (8) Shick, M. J.; Harvey, E. N. J. Polym. Sci., Part B 1966, 7, 495. (9) Botham, R.; Thies, C. J. Colloid Interface Sci. 1973, 45, 512. (10) Koopal, L. K. J. Colloid Interface Sci. 1981, 83, 116. (11) Hlady, V.; Lyklema, J.; Fleer, G. J. J. Colloid Interface Sci. 1982, 87, 395. (12) Csempesz, F.; Rohrsetzer, S. Colloids Surf. 1984, 11, 173. (13) Kawaguchi, M.; Sakai, A.; Takahasi, A. Macromolecules 1986, 19, 2952. (14) Van der Beek, G. P.; Cohen Stuart, M. A.; Fleer, G. J. Langmuir 1989, 5, 1180. (15) Kawaguchi, M. Adv. Colloid Interface Sci. 1990, 32, 1. (16) Schneider, M. H.; Granick, G. Macromolecules 1994, 27, 4714. (17) Lipatov, Yu. S.;Todosijchuk, T. T.; Chornaya, V. N. In Polymer Interfaces and Emulsions; Esumi, K., Ed.; Marcel Dekker Inc.: New York, 1999. (18) Silberberg, A. J. Chem. Phys. 1968, 48, 2835.

10.1021/la000040g CCC: $19.00 © 2000 American Chemical Society Published on Web 06/08/2000

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Csempesz and Csa´ ki

To that end, the hydrodynamic thickness of both the individual and mixed adsorbed layers at various polymer concentrations on negatively charged polystyrene (PS) latex and silver iodide (AgI) sol, respectively, have been determined. The adsorption characteristics of the single polymers and of their binary mixtures have also been investigated, and possible correlation between the layer properties and the preferential affinity of the macromolecules competing for particle surfaces has been shown. 2. Experimental Section 2.1. Materials. 2.1.1.Polystyrene Latex. Monodisperse PS latex was used in these experiments. The polystyrene latex was prepared according to the Vanderhoff method,19 using Aerosol MA as the emulsifier. The details of preparation are described elsewhere.20,21 The mean size of the latex particles is 67 ( 1 nm, and the polydispersity index is less than 0.05. The particles are negatively charged with a 3.2 µC‚cm-2 surface charge density. The particle size and polydispersity of each dispersion were determined by dynamic light scattering measurements. The concentrations of the PS in the size determinations and in the adsorption measurements were 1.0 × 10-2 and 1.0 g‚dm-3, respectively. 2.1.2. Silver Iodide Sol. The sol was prepared by adding a 90 cm3 aliquot of 10-3 M AgNO3 at room temperature to 22 cm3 of an aqueous solution of 5 × 10-3 M KI by mixing the solutions at a constant rate at 298 K. Freshly prepared AgI sol with negatively charged particles was used for each measurement. The particle size and the polydispersity of the fresh sol in every preparation were checked by dynamic light scattering measurements. The polydispersity index 60 min after the preparation of the sol used did not exceed 0.1. This showed that the samples were fairly monodisperse with a 34 ( 2 nm volume-average mean particle size. The sol concentration in the size measurements was 1.26 × 10-2 g‚dm-3. For the adsorption measurements, AgI sol of higher solid concentration (7.47 × 10-1 g‚dm-3) was used that exhibited a wider size distribution. 2.1.3. Polymers. Water-soluble uncharged polymers, such as methylcellulose (MC), hydrolyzed poly(vinyl alcohol) (PVA), and poly(vinylpyrrolidone) (PVP), and their 1:1 (w/w) binary mixtures were used. The polymers were fractionated samples, prepared from commercial products of Tylose MH 50 methylcellulose (Hoechst A.G., Germany), Powal 420 poly(vinyl alcohol) (Kuraray Ltd., Japan), and GAF K-90 poly(vinylpyrrolidone) (GAF GmbH, Austria). The degree of polymerization of the MC (the number of glucose rings) was 490, but those of the PVA and PVP were 2450 and 8120, respectively. 2.2. Methods. 2.2.1. Photon Correlation Spectroscopy (PCS). The mean size, size distribution, and polydispersity of the particles with and without adsorbed polymer were measured at 25 °C by an advanced technique of PCS using a Malvern Zetasizer 4 apparatus (Malvern Instruments, U.K.) with an autosizing mode and auto sample time. Analysis of the fluctuations in the intensity of light scattered from particles undergoing random Brownian motion enables the determination of an autocorrelation function G(τ) that, in effect, is a measure of the probability of a particle moving a given distance in τ time (τ is the correlation delay time):

Gi(τ) ∝

∑k exp[-τ/t i

c,i(ai)

(1)

The relaxation time (tc) of fluctuations is related to the diffusion coefficient (D) of particles

tc ) 1/DK2

(2)

from which the particle size can be calculated via the StokesEinstein equation (K is the wave vector). (19) Van Den Hul, H. J.; Vanderhoff, J. W. J. Electroanal Chem. 1972, 37, 161. (20) Csempesz, F.; Rohrsetzer, S.; Kova´cs, P. Colloids Surf. 1987, 24, 101. (21) Csempesz, F.; Csa´ki, K.; Kova´cs, P.; Nagy, M. Colloids Surf., A 1995, 101, 113.

Determination of the hydrodynamic layer thickness (δh) at particle/solution interfaces by dynamic light scattering is based on the fact that the adsorbed polymer layer limits the diffusion of coated particles. From the autocorrelation function determined for the dispersion in the presence and absence of adsorbed polymer, respectively, the diffusion coefficients and the mean hydrodynamic radii (ai) of particles have been evaluated. The hydrodynamic thickness (δh) of adsorbed polymer layers has been then calculated from the differences in the volume-average mean sizes between polymer-bearing and bare particles. Adequate determination of δh required, therefore, the use of monodisperse and kinetically stable dispersions.7,23 2.2.2. Adsorption Measurements. Two sets of adsorption measurements were carried out: (i) individual adsorption of the polymers and (ii) simultaneous competitive adsorption from binary polymer mixtures. In all cases, independent adsorption isotherms for each polymer were determined, by measuring the concentration difference (before and after adsorption) in the continuous phase. A 24 h time period was allowed for the adsorption in a thermostat at 298 K, and then the solid particles were separated from the polymer solution by ultracentrifugation. (After longer contact times no increase in the individual adsorption of the polymers could be detected.) The concentration of the polymers in the supernatant was measured spectrophotometrically as described elsewhere in detail.20,21 To characterize the affinity for solids of chemically different macromolecules in competitive adsorption, preferential adsorption parameters (f p) for the polymers in pairs have been established. In former papers12,21,22 we reported that these variables can be calculated from isotherm pairs for competing polymers determined under conditions in which, at constant total polymer concentration, the concentrations of both species in the equilibrium solution are the same. At a given initial concentration, for polymers of chemically different chains, the above requirements can be achieved at a well-defined ratio of the two polymers. On the basis of the isotherms determined at “equal equilibrium concentrations”, the preferential adsorption parameters for polymer pairs have been calculated as follows. For polymer i in competition with polymer j, f p is given by

f p ) Γi/(Γi + Γj)

(3)

where Γi and Γj are the adsorbed amounts per unit surface area, corresponding to “zero” concentration in the equilibrium solution of both i and j polymers.

3. Results and Discussion 3.1 Adsorbed layer thicknesses. The approaches to the interfacial behavior of polymer mixtures consider competition of macromolecules for surface sites as mainly affecting the adsorbability of chemically different chains. It is well documented that upon simultaneous adsorption from a multicomponent solution each polymeric species may reduce the surface excess of the competing partner(s), and at high surface coverages the less preferred molecules can be displaced from the interfaces by the preferentially adsorbed ones.8-17 Investigations of the adsorption characteristics of polymer mixtures together with the spatial properties of mixed adsorption layers formed at the interfaces may provide a way, however, for revealing the possible alterations also in the chain conformations that may be due to the competitive adsorption of different macromolecules onto particle surfaces. The effective thickness of an interfacial polymer layer may be defined and measured in different ways.23-26 (22) Csempesz, F.; Nagy, M.; Rohrsetzer, S. Colloids Surf., A 1998, 141, 419. (23) Pecora, R. Dynamic Light Scattering; Plenum Press: New York, 1985. (24) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163. (25) Shibayama, M.; Tanaka, T. Adv. Polym. Sci. 1993, 109, 1. (26) Zhu, P. W.; Napper, D. H. J. Colloid Interface Sci. 1999, 214, 389.

Mixed Adsorption Layers of Uncharged Polymers

Figure 1. Hydrodynamic thicknesses (δh) of individual and mixed adsorbed layers of methylcellulose and poly(vinyl alcohol) on polystyrene latex particles.

Figure 2. Hydrodynamic thicknesses (δh) of individual and mixed adsorbed layers of methylcellulose and poly(vinylpyrrolidone) on polystyrene latex particles.

Various static methods refer to noninvasive techniques and generally detect only the denser portions of the adsorbed layer near the interface, i.e., the train segments and loops, but are insensitive to the presence of farextended tails, which are at low segment concentrations. However, measurements of the hydrodynamic thickness by PCS of polymer-coated dispersions might be expected to reflect also the influence of tail segments protruding far from the particles, even when the latter are quite sparsely distributed over the surface.27-29 As characteristic results, the hydrodynamic thicknesses of adsorbed layers obtained for individual polymers and for their binary mixtures at various initial polymer concentrations (ci) on polystyrene latex particles are shown in pairs in Figures 1 and 2, respectively. The thicknesses of adsorbed layers have been calculated from the change in the size of (monodisperse) particles with and without polymer, respectively. In Figure 1, the thicknesses of individual methylcellulose and poly(vinyl alcohol) layers (dashed lines) and of (27) Killman, E.; Maier, H.; Kaniut, P.; Gutling, N. Colloids Surf. 1985, 15, 261. (28) Malmsten, M.; Tiberg, F. Langmuir 1993, 9, 1098. (29) Stenkamp, V. S.; Berg, J. C. Langmuir 1997, 13, 3827.

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Figure 3. Hydrodynamic thicknesses (δh) of individual and mixed adsorbed layers of methylcellulose and poly(vinyl alcohol) on silver iodide sol particles.

their mixed layers (solid lines) formed in simultaneous competitive adsorption from a 1:1 (w/w) MC-PVA binary mixture after 1 and 24 h contact times are shown together. The dependence of layer thicknesses on polymer concentration shows the expected trend. At low surface coverages very thin layers for either polymer form, but when the train density reaches a certain limit tails develop rapidly and a steep increase in δh is observed. At high polymer concentrations, the thickness of each layer approaches a plateau value. In comparison with the 1 h data, after 24 h of adsorption there are no detectable changes in the individual polymer layers, but a definite increase in the thickness of the mixed layer can be observed, especially at higher polymer concentrations. Similar interfacial behavior can be revealed for the methylcellulose-poly(vinylpyrrolidone) mixture, as well. The formation of a relatively thick layer from the MC and, despite its much higher molecular weight, a thinner layer from the PVP on the PS particles is illustrated by the curves in Figure 2. After a longer adsorption period, the average thickness of the MC-PVP 1:1 (w/w) layer, mainly at high polymer concentrations, increases and the thickness of the mixed layer approaches that of the individual MC layer. In all probability, the latter effect can be ascribed to a slow displacement of the PVP molecules by the methylcellulose molecules from the particle/solution interfaces, but some exchange between the MC molecules of different chain lengths may also take place.21 Somewhat different layer characteristics for the polymers adsorbed at silver iodide/solution interfaces are demonstrated by the results shown in Figures 3 and 4. In the presence of a small polymer amount, the kinetic stability of the electrically stabilized AgI sol decreases and slow flocculation may take place. After a 24 h contact time the sol has become polydisperse to such a degree that the thickness of the adsorbed layers can not be measured with adequate accuracy. Therefore, in the latter figures the layer thicknesses determined after 1 and 5 h of adsorption are shown. The formation of thin interfacial layers by the PVA and PVP on the sol particles but a relatively thick adsorbed layer with MC, at higher surface coverages, is illustrated also by these curves. The results for the MC-PVA and MC-PVP 1:1 (w/w) mixtures, however, reveal some unusual effects. δh values obtained at a 1 h contact time

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Csempesz and Csa´ ki Table 1. Preferential Adsorption Parameters for Polymer Pairs in Competitive Adsorption on Polystyrene Latexa fp

polymer MC PVA PVP a

0.81 0.19

0.60 0.07 0.93

0.40

Adsorbent concentration 1.00 g·dm-3. Table 2. Preferential Adsorption Parameters for Polymer Pairs in Competitive Adsorption on Silver Iodide Sola fp

polymer MC PVA PVP h

Figure 4. Hydrodynamic thicknesses (δ ) of individual and mixed adsorbed layers of methylcellulose and poly(vinylpyrrolidone) on silver iodide sol particles.

for the binary mixtures demonstrate the existence, in a concentration range, of irregularly extended mixed layers with nearly the same thickness as (Figure 3) or somewhat thicker than (Figure 4) that for either single polymer layer. This unusual behavior of both mixtures suggests that, as a result of competition between chemically different chains for particle surfaces, certain macromolecules in the mixed layers are in an “elongated state” compared to the conformation they adopt in the individual adsorbed layers. The order of the layer thicknesses is the same as that obtained from electrokinetic measurements.22 The displacement processes discussed above cannot explain these effects. Namely, the interfacial exchange of the chemically different macromolecules would account for a decrease in the extension of both mixed layers. The corresponding 1 and 5 h data well demonstrate that the adsorbed MC molecules are displaced in time from the silver iodide surfaces by both the PVA and the PVP molecules, and at high surface coverages, the mean layer thicknesses approach that of the displacer polymer. In accordance with theoretical considerations, these results strongly support the notion that the structure of composite interfacial layers depends indeed in a characteristic way on the affinity of the macromolecules competing for surface sites and also on the time scale of the exchange processes at the interfaces. 3.2 Preferential Adsorption. The adsorption preference of polymers competing simultaneously for particle surfaces with other polymer(s) can simply be established by comparing the adsorption isotherms for each species in individual adsorption and competitive adsorption, respectively. There are several factors, however, that have a great influence on the extent of preferential adsorption such as the concentration of all adsorbates in the bulk solution, the ratio of the competing species in the initial mixture, the extent of the interfacial area available for the macromolecules, etc. For an adequate characterization of the adsorption preference, the effect of the latter factors should be eliminated. It was shown21 that the preferential adsorption parameters (f p) meet these requirements and can be regarded as relevant characteristics of the affinity for the solid of the competing macromolecules. The preferential adsorption parameters determined in pairs for the polymers on two dispersions with different surface properties are collected in Tables 1 and 2. The f p parameters are sensitive markers for demonstrating the differences in the order and the extent of

a

0.41 0.59

0.22 0.27 0.73

0.78

Adsorbent concentration 7.47 × 10-1 g‚dm-3.

preferential affinity of the macromolecules for the different solids. The higher f p is for a given polymer (from 0 to 1.0), the more pronounced is its adsorption preference in competitive adsorption at the solid/solution interfaces. The data in Table 1 illustrate that in combination either with the poly(vinyl alcohol) or the poly(vinylpyrrolidone), the adsorption of methylcellulose molecules is preferred on polystyrene particles. On silver iodide sol, however, both the PVA and the PVP are adsorbed preferentially with respect to the MC molecules. When the PVA and the PVP molecules compete for the solid surfaces, the latter polymer is adsorbed preferentially. These results confirm the idea of displacement from the particle/solution interfaces of the weakly adsorbed polymers by the preferentially adsorbed ones and are in line with those of the hydrodynamic measurements, as well. It is worth recalling that extended mixed layers occur only at partial surface coverages where the surface sites are still available for each polymer to anchor, i.e., where the weakly adsorbed polymer has not been completely displaced from the interfaces. This fact reveals that, besides the exchange processes, the interfacial polymer chains of different segment affinities for particle surfaces change their conformation and at low surface coverages may take on an “elongated” form. Considering the preferential affinity of the polymers, the weakly adsorbed macromolecules are presumably constrained to adopt nonequilibrium conformations in the extended mixed layers, at least over some time. When polymers with different segment affinities are competing for surface sites, some alterations in the chain conformations at the interfaces for the weakly adsorbed macromolecules may certainly result. Nevertheless, the so-called “irregularly extended polymer layers” can only be detected by hydrodynamic measurements where the weakly adsorbed polymer is the main “thickness-determining” component in the mixed adsorption layer, i.e., where its contribution to the total layer thickness exceeds that of the preferentially adsorbed polymer. Similar alterations in the state of interfacial polymer chains with some polymer pairs were obtained on As2S3 sol, as well.20 Acknowledgment. This work was supported by the Hungarian Science Foundation under Grant OTKA T 022923. LA000040G