ADSORPTION OF POLYELECTROLYTES ON MICA

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Orlando J. Rojas Universidad de Los Andes, Me´rida, Venezuela

INTRODUCTION ‘‘Adsorption of Polyelectrolytes on Mica’’ sounds like a very specific title and it may be difficult to envision, upon first reflection, why mica deserves a separate chapter in the study of adsorption of polymers on solid substrates. What makes mica such a special substrate? What are the driving forces involved in the adsorption of macromolecules on mica? What mechanisms participate in the adsorption phenomenon? What types of interactions and what factors govern systems containing polymers adsorbed on mica? These and other questions will be answered in this article, which is not meant to be exhaustive, but hopefully will help build an understanding of the intricacies involved in polyelectrolyte adsorption on such special substrates. MICA Let us start by briefly describing mica. Stoichiometrically, all members of the mica mineral group can be described as follows (1, 2): AB2
3 ðAl; SiÞSi3 O10 ðF; OHÞ2 where, A = K, Ca, Na, or Ba and sometimes other elements, B = Al, Li, Fe, or Mg. These atoms are bonded together into flat sheets, allowing a perfect cleavage of the mineral to produce clean, flexible, elastic, and tough sheets in a variety of colors (brown, green, black, violet, or colorless), often with a vitreous to pearly luster. What geologists call the mica group consists of more than 30 members, each with its specific crystal structure and composition. Mica materials commonly used in microscopy and other analytical applications consist of six forms found in nature; muscovite, biotite, phlogopite, lepidolite, fuchsite, and zinnwaldite. Muscovite, the most abundant type of mica, is found in igneous, metamorphic, and detrital sedimentary rocks and has been used extensively in studies of polymer, surfactant, or electrolyte adsorption. This mineral is con-

Encyclopedia of Surface and Colloid Science Copyright D 2002 by Marcel Dekker, Inc. All rights reserved.

sidered to be a classical silicate, and is geologically classified in the subclass of phyllosilicates. It is a layered mineral consisting of potassium aluminum silicate hydroxide fluorides with an ideal formula KAl2(AlSi3)O10(OH)2. ˚ thick, consists of two silicate Each sheet, about 10-A layers not strongly bonded together by aluminum atoms. Some silicon atoms in the lattice are replaced by aluminum and this isomorphous substitution results in a negative lattice charge that is neutralized by K+ ions (90 to 95%) (and to a much lesser extent Na+ ions, 5 to 10%) present between aluminosilicate sheets (3 –5). The electrostatic interaction between potassium and oxygen is weaker than the covalent bonds between the aluminosilicate layers, and muscovite mica can therefore be cleaved along the basal plane creating molecularly smooth, defect-free surfaces. Upon cleavage along the basal plane the potassium ions are evenly distributed between the two surfaces, and in air the mica surfaces are neutralized by these potassium ions. In aqueous solution the potassium and sodium ions will dissociate (4, 6), and the surface, which consists of Si, O, and Al joined together by Si O and Al O bonds, acquires a negative charge that is neutralized by protons. Ions situated in the mica crystal are not exchanged, as mica does not swell in water. The ion exchange capacity of mica plays an important role in many processes and has been recently studied (7). The total number of negative lattice sites on the mica ˚2 basal plane corresponds to one negative charge per 48 A 14 2 or 2.110 charges per cm (8, 9), i.e., the surface charge density is
0.33 C/m2. It is important to note that no OH groups are present on the mica basal plane. In aqueous solution, the surface charge density of mica is normally orders of magnitude lower than the lattice charge of
0.33 C/m2. The reason is that cations present in the solution adsorb on mica, and thus decrease the net charge (10). Other surfaces, such as glass, are not equally well defined as mica surfaces, and thus mica is the preferred substrate to be employed when studying, e.g., polymer adsorption. Mica is often used to model the adsorption

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behavior of other minerals, for reasons explained above (e.g., very thin, transparent and, atomically smooth sheets can be easily obtained) it is suitable for analysis with techniques such as the interferometric surface force apparatus (SFA) (one of the most useful techniques when investigating adsorption phenomena) (11, 12), atomic force microscopy (AFM) (13), x-ray photoelectron spectroscopy (XPS) (5), etc. The reader can find some AFM pictures of mica on-line (14, 15).

Adsorption of Polyelectrolytes on Mica

systems. Interaction forces in systems involving surfactant adsorption have been extensively studied (52 –58). Similar studies on interaction forces have been also conducted for neutral polymers and polyelectrolytes on mica (12, 31, 32, 40, 44, 54, 56, 59– 73), or in more complicated cases dealing with polymers, surfactants, and mica substrates in the same system (61, 74, 75). Additional reports on imaging (23, 24, 44, 76, 77), polymer conformation (78 –82), cryztallization (67), electrokinetics (83), electro-optic (84), and other studies on adsorption (65, 66, 68, 85 –92) have also been advanced.

IMPORTANCE OF POLYMER ADSORPTION ON MICA The polyelectrolyte behavior at the solid –liquid interface plays an important role in many industrial processes where particles or macroscopic solid surfaces are present. Water treatment, papermaking, mineral extraction, oil field exploitation, and biological and biotechnological processes are only a few examples. In several other everyday situations, e.g., household, pharmaceutical, and personal care product applications, polyelectrolytes are an important part of the formulation of a product that is ultimately brought into contact with several surfaces (textiles, skin, hair, minerals, etc.) where the polymer is adsorbed to obtain a desired property or effect. Polyelectrolyte adsorption on charged surfaces has been the subject of numerous theoretical studies. Several of those deal with the effect of variables such as the nature and electrical charge of the solid surface and the polymer, adsorption energy, and ionic strength (16 –21). Experimental studies, on the other hand, make use of advanced techniques to elucidate the polyelectrolyte behavior at solid –liquid interfaces. Besides conventional analytical procedures, currently many sophisticated techniques are being applied to study polymer adsorption, such as atomic force and scanning electron microscopy, ellipsometry, reflectometry, calorimetry, small angle neutron reflectivity, electron spin resonance, surface-sensitive spectroscopic techniques (XPS, etc.), NMR, quasi-elastic light scattering (QELS), interferometric, and quartz crystal microbalance techniques. Studies involving polymer adsorption on mica (or mica-modified) surfaces are quite numerous. In the last few years, research has concentrated on biological systems, i.e., adsorption of biopolymers on mica (3, 22 –37). The adsorption of DNA on mica (38) has also been the subject of several characterization and imaging studies (39 – 44). Understanding surface forces and related interactions in systems consisting of bare mica surfaces in aqueous and organic solutions (45 –51) has served to elucidate many of the mechanisms involved in more complex

POLYELECTROLYTE ADSORPTION ON MICA Adsorption is strongly favored when the polyelectrolytes and the solid surfaces carry opposite charges. A complete description of macromolecular configurations, particularly at interfaces, remains very difficult to resolve experimentally. Ellipsometry, reflectometry, quasi-elastic light scattering, and hydrodynamic and electrokinetic techniques are among the methods that allow determination of some mean thickness of adsorbed polymer layers. Disjoining pressure methods can provide valuable information, not only of the so-called steric thickness (ds) (93), but also and most importantly of the forces acting between the surfaces as a function of their separation, which can be measured by, for example, the surface force apparatus (SFA). This last issue is of major significance in polyelectrolyte systems where charge effects play a key role. In such systems, small amounts of adsorbed polyelectrolyte, below or close to that needed to obtain charge neutralization, become effective coagulants due to a reduction of the surface charge and the double-layer force between the surfaces, as well as to the development of attractive bridging forces (94). The addition of polyelectrolytes may also increase particle stability if the conditions are such that long-range repulsive steric forces are generated. This is favored by a highly adsorbed amount and by an extended polyelectrolyte layer (95). For simplicity, most of the discussion presented here will deal with a few polyelectrolytes, taken as representative of many others. However, it must be stressed that the nature of the polyelectrolyte can influence the adsorption mechanism and generalizations should be made only with caution. For instance, most of the examples given here deal with random cationic copolymers and less consideration is given to other structures such as alternating, block, or grafted copolymers. A range of positively charged copolyelectrolytes with varying charge densities will be discussed herein. The

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rather than the charge density, to account for the concentration of charged groups [such as 2-hydroxy-3(trimethylammonium chloride) propyl units] in the macromolecule structure. Finally, a cationic hydrophobically modified polyelectrolyte with a random distribution of hydrophobic dodecyl side chains, referred to as 40 DT, will be briefly discussed (see in Fig. 1 the chemical structures of the units mentioned above). AFM Imaging of Adsorbed Polyelectrolytes on Mica One of the major advantages of AFM techniques as compared to, e.g., electron microscopy (EM) is that imaging can be performed under near-native conditions, thus eliminating sample damage and related artifacts that are usually introduced when using EM (13, 96). Imaging of adsorbed polymer on mica can be accomplished by scanning the microscope probe on the surface. A TappingMode AFM technique can be used to study the sample’s topography by tapping the surface with an oscillating tip. In this operation a piezo stack excites a cantilever’s substrate vertically, causing the tip to oscillate near its resonant frequency (97). Close to the sample surface the tip tends to be deflected due to its interaction with the surface ma-

Fig. 1 Molecular structure of the monomers (A) AM or acrylamide; (B) MAPTAC; (C) CMA; and (D) the two types of segments of the 40% hydrophobically modified polyelectrolyte 40DT.

cationic segments of the considered polyelectrolytes are either (2-acryloxyethyl)-trimethylammonium chloride (CMA) or (3-methacrylamidopropyl)-trimethylammonium chloride (MAPTAC). The polyelectrolyte charge density (t, %) depends on the ratio of positively charged segments and uncharged segments (e.g., acrylamide, AM) present in the copolymer. Another example that will be used is a cationic guar gum, a derivative of guaran, one of the highest molecular weight, naturally occurring water-soluble macromolecules. Guar gum consists of a chain of (1 –4)linked b-d-mannopyranosyl units with single a-d-galactopyranosyl units connected by (1 – 6) linkages. In these cases, the use of the degree of substitution is preferred,

Fig. 2 Tapping mode AFM height image (0% RH) of low degree of substitution cationic guar gums adsorbed on mica. The image shows interchain association through conformationally odered ‘‘junction zones’’ terminated by ‘‘kinking’’ residues.

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Adsorption of Polyelectrolytes on Mica

Fig. 3 Tapping mode image (0% RH) of adsorbed polyelectrolytes [poly(AM-MAPTAC), t = 100%] on mica after equilibrium adsorption in 0.1-mg/mL bulk polymer concentration. Left: height image, Right: phase image.

terial, but a feedback loop maintains the tip oscillation amplitude constant by changing the vertical position of the sample (using a piezoelectric tube on which the sample is

mounted). The vertical position of the scanner (z) at each (x,y) location is thus used to reveal the topographic image of the sample surface.

Fig. 4 XPS survey spectrum of mica (after immersion in water), showing the major photoelectron and Auger electron lines.

Adsorption of Polyelectrolytes on Mica

Fig. 2 shows an AFM image of a low degree of substitution guar gum molecule adsorbed on mica. The probe used consisted of a 125-mm-long, single-beam cantilever and a tip (5 to 10 nm nominal radius of curvature) as an integrated assembly of single crystal silicon produced by etching techniques. Since some polymers tend to swell (98) in aqueous systems and since a large adhesive force exists between the probe tip and the mica surface in such conditions, AFM in air (or nitrogen) is a better choice than imaging underwater. Fig. 3 shows the tapping mode AFM images of adsorbed copolymers of AM and MAPTAC [poly(AMMAPTAC)] on mica after adsorption at a polymer concentration of 0.1 mg/mL. As can be seen, it is possible to resolve ‘‘individual’’ polymer chains adsorbed on the mica surface. Polyelectrolytes of high charge density tend to adsorb to the oppositely charged surface (mica) in a flat and strongly bound configuration, whereas those of low charge density tend to adsorb more loosely and in a less tightly bound configuration. Although molecular resolution is limited by probe broadening, section profiles (not shown) indicate an apparent chain thickness of ca. 0.2 to 0.7 nm in agreement with an estimated polymer segment length of ca. 0.45 nm unit size (99). The reader is referred to on-line sites that

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include several AFM images of polyelectrolytes adsorbed on mica (see, e.g., Ref. 64). Polyelectrolyte Adsorbed Amount on Mica and Ion Exchange Process Fig. 4 shows a wide range or survey XPS spectrum for the mica surface (after immersion in water). For comparison, a typical spectrum after adsorption of poly(AM-MAPTAC) of low charge density (t = 1%) on mica is shown in Fig. 5. The adsorption process in this case can be irreversible, since the adsorbed amount remains the same after rinsing with water. However, the presence of salt or surfactants in the rinsing solution can dramatically affect the adsorbed amount, as will be discussed in later sections. Comparison of these two spectra reveals that the adsorption of the polyelectrolyte produces a distinctive nitrogen (N 1s) peak (from the nitrogen atoms present in the cationic units of the polyelectrolyte) and a relative increase of the carbon (C 1s) signal due to the incorporation of the organic layer on the mica surface. Upon adsorption of the polyelectrolyte on the mica basal plane the appearance of the N 1s peak is accompanied by an attenuation of the potassium signal at the same time as the C 1s peak increases. The intensities (raw areas) of these

Fig. 5 XPS survey spectrum after adsorption of poly(AM-MAPTAC), t = 100%, on mica showing the major photoelectron and Auger electron lines. (From Ref. 5.)

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spectra allow the calculation of the polyelectrolyte adsorbed amount (5). Fig. 6 includes detailed nitrogen spectra for polyelectrolytes of different charge densities. Fig. 6A shows the case of poly(MAPTAC) (t = 100%) and Fig. 6B – D shows the respective spectra for decreasing polyelectrolyte charge densities (30, 10, and 1%, respectively). Deconvolution of these nitrogen peaks allows the estimation of the relative distribution of nitrogen-containing groups in the adsorbed layer. The proportion of nitrogen from ammonium quarternarium groups (i.e., proportion of charged nitrogen atoms relative to the total number of nitrogen atoms), indicates good agreement between XPS measurements and expected values (calculated from the charge density and corresponding polyelectrolyte structure). The results for the 10% and 1% charge polyelectrolytes indicate a higher charge density of the adsorbed polymer compared to that calculated from the structure. It can be argued that there may be a slight preferential adsorption

Adsorption of Polyelectrolytes on Mica

of the fraction of these polyelectrolytes with charge density higher than the average one (one has to remember that there is polydispersity in both molecular weight and charge density). The amount of polyelectrolyte adsorbed on mica can be calculated using XPS spectra (5, 98) and Fig. 7 illustrates a typical adsorption isotherm for low charge-density (t = 1%) poly(AM-MAPTAC). This isotherm, which can be taken as representative of other cases involving adsorption of cationic polyelectrolytes, is of the high affinity type and shows that saturation is reached at a rather low polyelectrolyte concentration. In the present case, the plateau or saturation value is ca. 2.5 mg/m2. The surface charge for mica (due to isomorphous substitution of silicon for aluminum) gives a lattice charge (sO) of
0.34 C/m2 (or 2.11018 negative charges per m2). This charge in aqueous solutions, as explained before, is mostly neutralized by adsorbed cations (mainly protons) in the Stern layer (6, 100), while ions in the diffuse layer only compensate a small fraction of the total lattice charge.

Fig. 6 Nitrogen 1s spectra for poly(AM-MAPTAC) of different charge densities adsorbed on mica: 100% charge density (A); 30% charge density (B); 10% charge density (C); and 1% charge density (D). (From Ref. 5.)

Adsorption of Polyelectrolytes on Mica

Fig. 7 Adsorption isotherm for poly(AM-MAPTAC), t = 1%, on mica in aqueous 0.1 mM KBr solution determined by XPS. (From Ref. 98.)

Indeed, force measurements in monovalent electrolyte solutions at the same ionic strength (0.1 mM) (6, 100) show that the charge in the diffuse layer (sd) is typically only 0.009 C/ m2. Hence, the electroneutrality condition requires that the charge in the Stern layer (ss) be 0.331 C/m2. Adsorbing polyelectrolytes bring positive charges close to the surface, which affects the number of small ions adsorbed through an ion exchange mechanism. The amount of small ions present in the layer can be calculated through a simple charge balance, as suggested by Norde and Lyklema (101, 102). The adsorbed polyelectrolyte contributes a charge (sp) to the adsorbed layer, which can be calculated from the average charge per molecule (Zp) and the number of adsorbed polyelectrolyte molecules per unit area (G) as determined, for example, by XPS: sp ¼

GNA Zp e MW

where e is the elementary charge, NA is Avogadro’s number, and MW is the polyelectrolyte molecular weight. On the basis of XPS measurements, sp [at the plateau concentration for poly(AM-MAPTAC), with t = 1%] is calculated to be 0.033 C/m2 (equivalent to a number density of 2.07  1017 charges per m2). The charge contribution from small ions in the compact layer (ss) can thus be easily calculated from the requirement of charge neutralization: s0 þ ss þ sp þ sd ¼ 0 i.e., ss = 0.298 C/m2. Clearly, the adsorption of low charge density cationic polyelectrolyte [e.g., poly(AM-

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MAPTAC) t = 1%] on mica is accompanied by a desorption of small positively charged ions. It is also clear that the charges of the adsorbed polyelectrolyte alone cannot neutralize the mica lattice charge since it can be easily calculated that 25.3 mg/m2 of poly(AM-MAPTAC) with t = 1% is required to attain charge neutralization, whereas only ca. 2.5 mg/m2 is actually adsorbed, i.e., only 10% of the small cations initially adsorbed are exchanged (98). In contrast, Dahlgren et al. (103) found that upon the addition of a 100% charged polyelectrolyte, the mica lattice charge was fully compensated (complete ion exchange) by the charges on the polyelectrolyte. Adsorption isotherms for poly(AM-MAPTAC) polyelectrolytes of higher charge densities show a reduced plateau adsorbed amount, i.e., the adsorbed amount is reduced as the polyelectrolyte cationicity is increased, as expected from an electrostatically driven adsorption mechanism. As the polymer charge is increased, less polyelectrolyte is needed to neutralize the substrate surface charge. Furthermore, as the polyelectrolyte cationicity is increased, the polyelectrolyte adopts a flatter configuration on the surface. Although XPS does not provide information on the polymer configuration at the solid – liquid interface, complementary techniques such as the surface force technique is valuable in obtaining a better understanding of such information.

INTERACTION FORCES BETWEEN MICA COATED SURFACES AND POLYELECTROLYTE CONFORMATION The forces acting between two surfaces in air or immersed in aqueous solutions can be directly measured with the interferometric surface force apparatus (SFA) (104, 105). Using this instrument, the distance resolution is about 0.2 nm, and the force sensitivity is about 10
7 N. In a typical experiment two molecularly smooth surfaces of freshly cleaved mica (of 1 to 3 mm of uniform thickness) are glued with an epoxy resin onto optically polished half-cylindrical silica discs. The surface forces are measured with a double cantilever leaf spring. The separation between the two surfaces is measured by multiple beam interferometry (12). The distance between the two surfaces is controlled by using positioning rods of mm (coarse driver) or nm (fine driver) sensitivity, whereas positioning to 0.1 nm uses a piezoelectric crystal tube. The interaction force is obtained by expanding or contracting the piezoelectric crystal by a known amount and then optically measuring how much the two surfaces have actually moved; any difference in the two values is accounted for by the force difference between the initial and

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final positions. The force, F, between crossed cylinders normalized by the local geometric mean radius, R, is related to the free energy of interaction per unit area between flat surfaces (Gf) via the Derjaguin approximation (106). For a detailed account of surface forces and measuring techniques, the reader is referred to Ref. 107. At the beginning of each experiment, the freshly prepared mica surfaces are brought into contact in an atmosphere of dry air in the sealed apparatus. Observation of good mica adhesive contact ensures that the surfaces are free from debris or contamination. After separating the surfaces, the apparatus is filled with deaerated water and left to equilibrate for at least 1 h. The presence of a double-layer force and a finite adhesion is used to check for the continuing absence of surface contamination on the bare mica surfaces. For adsorption experiments, a small volume of concentrated polyelectrolyte (in pure water or electrolyte aqueous solution) is injected into the SFA apparatus through a 0.45mm PTFE filter with a syringe and mixed thoroughly to obtain the desired final concentration. Whenever the polyelectrolyte is added, the surfaces are held widely separated (ca. 1 to 2 mm) to allow the polymer to freely diffuse into the gap and adsorb onto the mica surfaces. Low Charge Density Polyelectrolytes Adsorption on mica of 1% charge density poly(AMMAPTAC) in aqueous 0.1-mM KBr solution, produces a nonelectrostatic force– distance profile (see Fig. 8). The force curves for 50 mg/mL polyelectrolyte bulk concentration show a somewhat higher repulsive force and a larger compressed layer thickness (separation distance at maximum load) compared with the force curve obtained in the 10 mg/mL solution, thereby indicating additional adsorption as the polyelectrolyte concentration is increased. This observation is consistent with a small increase in the equilibrium adsorbed amount from ca. 2.3 mg/m2 (at 10 mg/mL) to ca. 2.5 mg/m2 (at 50 mg/mL), as determined by XPS. Force – distance profiles for even higher concentrations (100 and 200 mg/mL) do not show appreciable differences with respect to that for the 50 mg/mL polyelectrolyte concentration, which implies that the adsorption plateau value has been reached around the latter concentration (98). It should be pointed out that perturbations induced by force measurements are fully relaxed between consecutive runs (approach– separation) since resulting force curves are, within the experimental uncertainty, the same. Hence, since it is unlikely that the adsorbed amount has time to change during the measurement, the situation is that of restricted equilibrium.

Adsorption of Polyelectrolytes on Mica

Fig. 8 Normalized force – distance profiles between two mica surfaces immersed in aqueous 0.1-mM KBr solution after equilibrium adsorption of poly(AM-MAPTAC), t = 1%. Two polyelectrolyte concentrations are shown: 10 mg/mL (4) and 50 mg/mL (* and 6, on approach and separation, respectively). The solid line represents a fit using the Alexander – de Gennes theory for polymer chains anchored to a surface with an average distance between attachment points and layer thickness of 24 nm and 52 nm, respectively. (From Ref. 98.)

From the force curves the steric layer thickness (ds) is about 50 nm per adsorbed layer and the compressed layer thickness is about 5 to 7.5 nm per adsorbed layer. No attraction or adhesion is observed. Adsorption of low charge density polyelectrolytes is limited by steric constraints rather than by the surface charge or potential of the solid itself. Among the factors that limit the amount of polyelectrolyte that is able to be adsorbed, one can include the unfavorable lateral interaction (excluded volume effect) between polyelectrolyte molecules within the layer, the entropy of mixing in the layer, and the fact that not all charges on the polyelectrolyte, due to the conformation of the macromolecule, are able to approach the surface as closely as small ions. The effect of the adsorption behavior discussed above is reflected in the force– distance profiles for mica surfaces coated with poly(AM-MAPTAC) of t = 1% (Fig. 8). The force curves reveal the steric nature of the measured interactions. The layer thickness and surface force range, as compared with investigations reported for similar polyelectrolytes with higher charge densities, indicate a more extended configuration. This is simply explained by the fact that only the cationic groups of the polymer can be adsorbed (108) and therefore trains are very unlikely to occur for low values of t. Hence, it is not surprising that the interaction forces in this case start at large separation

Adsorption of Polyelectrolytes on Mica

as a result of a configuration of the polyelectrolyte where loops and tails extend out from the surface. Most charged segments are probably close to the mica surface, and therefore the charged segments anchor the polyelectrolyte to the surface while the loops and tails are predominantly uncharged (for entropic reasons, they may contain some charges but they must be significantly less than 1% of the segments). The experimental force curves are in qualitative agreement with Alexander’s theory (109), which was later extended by de Gennes (110), for polymer chains attached by a single anchoring end group to a surface. In this theory, a simple relationship is proposed with two fitting parameters, namely, the average distance between the attachment points for terminally attached polymers and the layer thickness for the adsorbed polymer. In the case of an adsorbed poly(AM-MAPTAC), t = 1%, the fitted parameters are 24 nm and 52 nm, respectively. The adsorbed polymer thus can be pictured as a phase where trains (charged segments in the present case) anchor loops and dangling tails that extend a long way into the solution (111, 112) (see Fig. 9). For a molecular structure with regular distribution of charged segments in the polyelectrolyte chain, and taking into account the molecular weight and charge density, the studied low charge density polyelectrolyte can be represented as (AM101MAPTAC)122. Considering the adsorbed amount as determined by XPS and that each polymer contributes with two tails, it is calculated that the tail density is 3.341015 tails/m2 and the tail extension is ca. 101 AM units, i.e., 46 nm [assuming a segment length of 0.45 nm in unit size (99)]. A maximum loop density (assuming that all charged segments adsorb on the mica

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surface) can be calculated to be 2.021017 loops/m2 with an average size of 101 AM units extending out ca. 23 nm (about 50 molecules of AM). It is therefore hypothesized that the tail fraction dominates the long-range region of the force –distance profile since the steric layer thickness (ds) was measured to be ca. 50 nm per adsorbed layer (see Fig. 8), which is very close to the calculated 46 nm or the fitted value from the Alexander and de Gennes’s theory of 52 nm. The average distance between tails can be calculated to be approximately 17 nm, which is close to the fitted value (from Alexander and de Gennes’s theory) of 24 nm. Although the assumptions made here may be an oversimplification, the presented results validate the proposed picture of the polyelectrolyte conformation (see Fig. 9). Medium and High Charge Density Polyelectrolytes The forces acting between mica surfaces coated with cationic acrylamides of various charge densities have been investigated in a series of papers (61, 82, 103, 113-117). In low ionic strength solutions (around 0.1 mM), the adsorption of the polyelectrolyte (together with that of small ions) leads to an almost uncharged surface. Force curves obtained for poly(MAPTAC) (t = 100%) (115) and the structurally similar poly(CMA) (103) have proved to be insensitive to the molecular weight of the polyelectrolyte as a consequence of the high polyelectrolyte – surface affinity that results in a very thin adsorbed layer. At low poly(CMA) (t = 100%) concentration, below 20 ppm, the long-range repulsion is found to have a decay length consistent with that of a double-layer force in 0.1

Fig. 9 Schematic illustration of the interfacial configuration of adsorbed polyelectrolytes of low charge density adsorbed on mica.

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mM 1:1 electrolyte solution (see Fig. 10) (103). Further, the adsorption of the cationic polyelectrolyte on the negatively charged surface reduces the surface charge density, and thus the magnitude of the repulsive double-layer force decreases with polyelectrolyte concentration. At smaller separations, an attractive force component, which increases in range and magnitude with increasing polyelectrolyte concentration, is observed. Under the action of this attractive force, the surfaces reach a separation of about 1 nm. Hence, the adsorbed layer is extremely thin. The long-range repulsive force increases somewhat when the polyelectrolyte concentration is increased to above 20 ppm. This indicates that the charges in the adsorbed polyelectrolyte layer slightly overcompensate the charge of the substrate surface, and that the polymercoated surface now has a net positive charge. It is interesting to note that under these conditions the decay length of the force is larger than the expected decay length of a double-layer force in the given electrolyte. Hence, the observed force also has a steric origin and it seems plausible that it is due to a few polyelectrolyte tails extending far away from the surface. The long-range force is dominated by a strong bridging attraction at below 15 nm. The force curves for mica surfaces coated with a polyelectrolyte having t = 30%

Fig. 10 Normalized forces as a function of surface separation between mica surfaces immersed in an aqueous 0.1-mM KBr solution. The solution also contained poly(CMA) at concentrations of zero (4), 1 ppm (&), 5 ppm (5), 10 ppm(^), 20 ppm (), 50 ppm (.), and 100 ppm (6). The dashed line represents the slope of a double-layer force in 0.1-mM KBr, and the arrows represent inward jumps. (Adapted from Ref. 103.)

Adsorption of Polyelectrolytes on Mica

Fig. 11 Normalized forces as a function of surface separation. The measurements were carried out using two mica surfaces in a solution containing 20 ppm 40DT and 0.1 mM KBr (&), and after diluting this mixture by a factor of 6000 using a polyelectrolyte-free 0.1-mM KBr solution (6). (Adapted from Ref. 118.)

are in several respects similar to the ones observed for the 100% charged case, i.e., no long-range repulsion, bridging attraction at distances below 10 to 15 nm and no further compression of the adsorbed layer under high loads. As the cationicity of the adsorbed polyelectrolyte is reduced (61), the adsorbed layer thickness, the range of steric –bridging forces and the layer compressibility increase. In addition, the magnitude of the normalized pulloff force is reduced. As compared with the medium to high charge density cases, the data discussed for the 1% charge density polyelectrolyte show a larger layer thickness, and therefore no bridging attraction is present as a consequence of the low probability of having charged segments close to both surfaces. Furthermore, as pointed out previously, the long-range steric forces is the dominating feature in these force curves. The forces measured between mica surfaces across a solution containing 20 ppm 40DT, a hydrophobically modified, highly charged cationic polyelectrolyte, are rather complex. A strong repulsive steric force component dominates the long-range interaction (see Fig. 11). The force measured on approach is more repulsive than that measured on separation, and the range and magnitude of the force decrease for each consecutive approach. Once the

Adsorption of Polyelectrolytes on Mica

surfaces have been brought close together, a sudden in˚ to a ward jump occurs from a separation of 80 to 110 A ˚ separation of 15 to 20 A. At this position a strong attractive force is observed upon separation (118). It should be noted that the layer thickness is larger and the adhesion force smaller than those for the nonhydrophobically modified, highly charged polyelectrolytes poly(AM-MAPTAC) and poly(AM-CMA). This indicates that the hydrophobic attraction between layers of 40 DT (due to the grafted dodecyl chains) cannot compensate for the decreased bridging attraction due to the larger final layer thickness (as compared with nonhydrophobically modified, highly charged polyelectrolytes) (118). Hen egg white lysozyme is a small compact globular protein that carries a net positive charge of + 9 at pH 5.6 and posses a relatively large hydrophobic area. Lysozyme adsorbs strongly on mica (32) and balances the mica surface charge at about a 0.2-mg/ml concentration at pH 5.6 (119). The forces measured under these conditions are presented in Fig. 12 (35). A weak double-layer repulsion dominates the long-range interaction, whereas a steric force due to compression and dehydration of the adsorbed layer is the most dominating feature at distances below ˚ . The layer thickness obtained under a high com100 A pressive load is consistent with contact end-on adsorbed

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protein. This does not, however, mean that all molecules are adsorbed in such a configuration (35). As a summary, it can be pointed out that at the charge neutralization concentration in low ionic strength solutions, the layer thickness, layer compressibility, and range of steric– bridging force decreases with polyelectrolyte charge density, whereas the magnitude of the pull-off force (needed to separate surfaces) increases. Mica Surface Interaction Across Low-Polarity Medium The interactions between surfaces across low-polarity media are of great interest in several applications, e.g., food colloids and pharmaceutical systems. In these systems, phospholipids (e.g., lecithin) and other polymers are often present. A representative of food additives, polyglycerol polyricinoleate (PGPR), has been studied recently (120, 121). Results obtained by direct measurements of the forces acting between polar mica surfaces interacting across solutions of triolein containing phosphatidylethanolamine (PE), polyglycerol polyricinoleate (PGPR), and a PE – PGPR mixture, showed that PE adsorbs on mica (from anhydrous triolein) rendering the surface nonpolar. The change in the ordering of the liquid triolein molecules induced by bringing two such surfaces together gives rise to a structural force with two force barriers. In contrast, the adsorption of PGPR from anhydrous triolein resulted in a ˚ . It was also found steric force barrier with a range of 120 A that from the mixture of PE and PGPR in triolein, both additives adsorbed as a complex on mica surfaces. The presence of these aggregates on the surfaces gives rise to a very long-range, strong repulsive force (121). These results have great implications in colloid stability of particle dispersions in nonpolar media. Furthermore, the presence of water affects the adsorbed layer structure in different ways (121). Adsorption Dynamics

Fig. 12 Normalized forces as a function of surface separation between mica surfaces across a 0.2-mg/ml lysozyme solution in 1 mM NaCl (&). The forces measured after removing the protein from the solution (5) and after adding 0.8 mM SDS (.) are also shown. The solid lines are calculated DLVO forces. (Adapted from Ref. 103.)

Depending on the nature (molecular weight, charge density, etc.) of a polyelectrolyte, its adsorption on mica may be a very slow process. For polydispersed polyelectrolytes, the low-molecular-weight fractions are first adsorbed due to their faster diffusion rate. However, due to entropic effects, the adsorption of higher molecular weight fraction is more favorable. The decrease in entropy is less when one large molecule adsorbs compared to the entropy loss occurring when many small molecules adsorb. This leads to a displacement, with time, of the low-molecularweight molecules by the high-molecular-weight mole-

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Adsorption of Polyelectrolytes on Mica

cules. This exchange process, however, may last for weeks before equilibrium is reached. EFFECT OF IONIC STRENGTH ON POLYELECTROLYTE ADSORPTION The adsorption of polyelectrolytes onto oppositely charged mica (at low ionic strength) can be modified by varying polyelectrolyte ionicity, which modifies the polymer –surface and polymer – polymer interactions. However, another way to modulate these interactions in charged systems is to change the ionic strength of the medium by adding an inert electrolyte (122). The effect of ionic strength on polyelectrolyte adsorption studied by surface force techniques has been reported in a few studies. The first, by Luckham and Klein (60); dealt with the surface forces between mica surfaces in polylysine solutions at different salt concentrations. Later, Marra and Hair (123) reported on the effect of the ionic strength on the adsorption behavior of poly(2-vinylpyridine). Many other reports have appeared more recently (62, 82, 98, 114– 117, 124– 128). The adsorption of cationic polyelectrolytes on mica appears to be irreversible with respect to dilution. In fact, the adsorption is not truly irreversible, but the desorption process is extremely slow. Hence, mica surfaces precoated with polyelectrolytes can be easily obtained by first adsorbing the polyelectrolyte and then by removing the polymer from the bulk solution. For poly(CMA) (t = 100%), it has been noted that the small electrostatic double-layer force present before dilution is absent after dilution. This may indicate a slight desorption of the polyelectrolyte. The same trend is observed for lower charge density polyelectrolytes (around 30%). For polyelectrolytes with a charge density of 10 to 1%, the data are virtually identical before and after dilution. The effect of the addition of electrolytes on adsorbed polyelectrolytes on mica, after dilution with water, is presented in the next sections.

Desorption of Low Charge Density Polyelectrolytes XPS measurements show that desorption of preadsorbed poly(AM-MAPTAC) with t = 1% occurs upon increasing the ionic strength of the medium (see Fig. 13). However, in contrast to the findings of Dahlgren et al. (115), the force curves for this case (Fig. 14) show an increase in the range of the repulsive (steric) forces or a displacement further out of the force profiles for an increase in NaCl concentration.

Fig. 13 Desorption isotherm for poly(AM-MAPTAC), t = 1%, preadsorbed on mica upon immersion in NaCl aqueous solution of different ionic strengths studied by XPS. The polyelectrolyte (50 mg/mL solution) was first allowed to adsorb from aqueous 0.1-mM KBr solution, and then the mica was immersed in the respective electrolyte solution. (From Ref. 98.)

This behavior is explained by a reduction in the adsorbed amount and the formation of larger loops and tails upon increase of the ionic strength in cases where strong surface – polyelectrolyte segment (electrostatic) interactions are predominant and completely overrule the effects due to changes in conformational entropy. The distinct effect of ionic strength on the adsorbed amount, as presented before, is explained by the balance of four factors that accompany the screening of electrostatic forces by addition of salts: 1) a decrease in surface – polyelectrolyte attraction, 2) an increase in competition between the polyelectrolyte and the monovalent cations for adsorption at surface sites, 3) a decrease in free energy cost in creating a charged interface, and 4) a decrease in intra- and inter chain repulsion. Factor 4 facilitates an increased adsorption (screening-enhanced adsorption) as observed in some systems (124), whereas factors 1) and 2) promote a reduced adsorption. A simple rule is that the adsorbed amount is reduced by electrolyte screening whenever the distance between charges on the surface is sufficiently smaller than that along the polyelectrolyte chain or, in other words, the ratio of the surface charge to the polyelectrolyte cationicity is sufficiently large (129). The observed polyelectrolyte desorption with increased ionic strengths (in low charged polyelectrolyte systems) is therefore rationalized since the conditions that favor a (screening) reduced adsorption are satisfied. As the adsorbed layer thickness (d) usually increases with an increase in the adsorbed amount, a decrease in d due to polyelectrolyte desorption at higher ionic strengths

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surface, the macromolecules are attached by a decreasing number of segments when the ionic strength is increased, and therefore the behavior is akin to that of an adsorbed polyelectrolyte with low cationicity in pure water, that is, d increases. This phenomenon is accompanied by the development of more extended loops and tails and the incorporation of water (swelling) in the adsorbed layer, which explains why the range of interaction force increases with the ionic strength. Desorption of Medium to High Charge Density Polyelectrolytes

Fig. 14 Normalized force-distance profiles between polyelectrolyte-coated mica surfaces across NaCl solution of different ionic strengths. Poly(AM-MAPTAC) (t = 1%, 50 mg/mL solution) was first allowed to adsorb from aqueous 0.1-mM KBr solution. After equilibration, the bulk solution was replaced with pure water and subsequently NaCl solution was injected in the chamber up to the respective concentration: 0 M NaCl (&); 10 mM NaCl (6), and 500 mM NaCl (.). An additional force – distance profile after the replacement of the 500-mM NaCl electrolyte solution with pure water was measured at the end of the experiment (5). (From Ref. 98.)

Polyelectrolyte desorption upon the addition of salt starts at rather low salt concentration, depending on the polyelectrolyte. It is observed that as the polyelectrolyte charge density is increased, higher ionic strengths are needed in order to be effective in the removal of the polyelectrolyte from the surface. The observed trends are originated from the screening of the electrostatic interactions between the polyelectrolyte and the surface as the ionic strength of the medium is increased. The observations corroborate the indication that the adsorption of cationic polyelectrolytes on mica is driven by electrostatic attraction between the charged segments of the polyelectrolyte and the negatively charged surface.

would perhaps be expected if not all factors were considered. However, since small cations compete with charged segments of the polyelectrolyte for adsorption on the

Fig. 15 Desorption isotherm for poly(AM-MAPTAC), t = 1%, preadsorbed on mica upon immersion in SDS (.) aqueous solution of different concentrations studied by XPS. The polyelectrolyte (50-mg/mL solution) was first allowed to adsorb from aqueous 0.1-mM KBr solution, and then the mica was immersed in the respective surfactant solution. (Adapted from Ref. 5.)

Fig. 16 Normalized force – distance profiles for polyelectrolyte-coated mica surfaces across SDS solution of different concentrations. Poly(AM-MAPTAC), (t = 1%, 50 mg/mL solution) was first allowed to adsorb from aqueous 0.1-mM KBr solution. After equilibration, the bulk solution was replaced with pure water, and subsequently SDS solution was injected in the chamber up to the following SDS concentration: 0 cmc ( + ); 0.1 cmc (^); 0.3 cmc (6); 0.75 cmc (~); 1 cmc (5); and 2 cmc (.). (From Ref. 5.)

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EFFECT OF SURFACTANTS ON ADSORBED POLYELECTROLYTES Fig. 15 shows the adsorbed amount, as determined by XPS, after immersing mica sheets with preadsorbed low charge density poly(AM-MAPTAC) in SDS solutions at various concentrations. No desorption is observed at low SDS concentrations (lower than 0.01 units of cmc). But as the surfactant concentration is increased to a critical value of 0.01 units of cmc or higher, a monotonic reduction of the adsorbed amount with increasing surfactant concentration takes place. It is seen that about 0.3 mg/m2 of polyelectrolyte remains adsorbed upon immersion of mica (carrying the preadsorbed polyelectrolyte) in an SDS solution at a concentration of 2 units of cmc. A complete removal of the polyelectrolyte from the mica surface is expected if SDS solutions with higher concentrations are employed. Although the desorption isotherm seems to be similar to that observed for polyelectrolyte desorption upon increasing

Adsorption of Polyelectrolytes on Mica

ionic strength, the mechanism involved in the present case is totally different as will be explained later. Surface force measurements reveal important changes upon the addition of SDS to the solution surrounding a preadsorbed layer of polyelectrolyte on mica. Fig. 16 shows the force– distance profiles obtained after replacing the bulk solution with pure water and subsequent addition of SDS at various concentrations (0.1, 0.3, 0.75, 1, and 2 units of cmc). A reduction of the repulsive forces is observed as the surfactant concentration is increased; however, this reduction is very small for surfactant concentrations of 0.1 units of cmc or less, but it becomes appreciable at higher surfactant concentrations. In all cases, the forces measured on separation are very similar to those measured on the first approach and display features typical of steric forces. No attraction was observed when separating the surfaces. In pure water, the onset of repulsion forces occurs at a separation between the surfaces of ca. 80 to 90 nm, whereas in concentrated surfactant solution (2 units of cmc) this distance is shortened by about half its

Fig. 17 Desorption isotherms for poly(AM-MAPTAC) on mica in sodium dodecyl sulfate solutions of different concentrations. The corresponding systems are (A) poly(AM-MAPTAC), t = 1%; (B) poly(AM-MAPTAC), t = 10%; (D) poly(AM-MAPTAC), t = 30%; and (D) poly(AM-MAPTAC), t = 100%.

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original value. A similar trend is observed for the separation distance at high force load. These observations, together with XPS measurements, are rationalized in terms of the desorption of the polyelectrolyte from the solid surface accompanied by a reduction in the adsorbed layer thickness (130). The effectiveness of the surfactant in desorbing the low charge density polyelectrolyte is explained by the surface conformation of the polyelectrolyte, which is dominated by long loops and tails. The adsorbed polyelectrolyte is thus easily detached from the surface upon association (presumably driven by hydrophobic interactions) with surfactant molecules. The experimental results for the low charge density polyelectrolytes presented here are in line with observed trends for similar polyelectrolytes of higher charge densities. Fig. 17 shows the desorption isotherms after immersing mica substrates (with preadsorbed layers of the respective polyelectrolyte) in sodium dodecyl sulfate solutions of various concentrations. A more extensive account on polyelectrolyte-surfactant interactions at mica – liquid interfaces can be found in Refs. 75 and 127.

7.

FINAL REMARKS

8.

Studies on the adsorption of polyelectrolytes on mica have helped to elucidate the mechanisms by which different types of polymers adsorb on solid surfaces. The accumulated knowledge is an invaluable resource for the synthesis and formulation of new polymeric additives with properties that are targeted to facilitate different states of dispersion in solid –liquid systems. The charge density of the polyelectrolyte stands out among the factors that influence macromolecular behavior at the interface. It can markedly affect, in different ways, the conformation and the layer thickness of the adsorbed layer as well as the interaction forces involved. In this discussion, the molecular weight of the polyelectrolyte was not examined, but it is obviously another important variable that needs to be considered. The effect of the environment is also very important, especially the ionic strength and the presence of surface-active agents. The pH is also a factor, especially in the case of weak polyelectrolytes. Finally, the tools that are at our disposal have allowed for a better understanding of the adsorption phenomena—some cases were illustrated by employing AFM, XPS, and SFA techniques.

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