Measurements of Interaction Forces between Polycations, between


Values for the adhesion (the pull back) force, Debye-length, and surface potential have also been evaluated. View: PDF | PDF w/ Links | Full Text HTML...
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J. Phys. Chem. B 2001, 105, 10579-10587

10579

Measurements of Interaction Forces between Polycations, between Clay Nanoplatelets, and between Polycations and Clay Nanoplatelets by Atomic Force Microscopy A. Szu1 cs,† T. Haraszti,† I. De´ ka´ ny,† and J. H. Fendler*,‡ Department of Colloid Chemistry and Nanostructured Materials Research Group of the Hungarian Academy of Sciences, Attila Jo´ zsef UniVersity, Aradi V.t.1., H-6720 Szeged, Hungary, and Center for AdVanced Materials Processing and Department of Chemistry, Clarkson UniVersity, Potsdam, New York 13699-5814 ReceiVed: February 21, 2001; In Final Form: June 8, 2001

Interaction forces have been measured between (i) apposing layers of 2.0 ( 0.05 nm thick polydiallyldimethylammonium chloride, PDDA, self-assembled onto a silicon wafer, silicon-substrate/PDDA, and onto a silicon microparticle, attached to a tipless AMF catilever, SMP-AFM-tip/PDDA; (ii) apposing layers of 2.0 ( 0.05 nm thick montmorillonite, M, platelets, self-assembled onto the silicon-substrate/PDDA (siliconsubstrate/PDDA/M) and onto the SMP-AFM-tip/PDDA (SMP-AFM-tip/PDDA/M); (iii) apposing layers of silicon-substrate/PDDA and SMP-AFM-tip/PDDA/M; and (iv) apposing layers of silicon-substrate/PDDA/M and SMP-AFM-tip/PDDA by scanning force microscopy. Interactions between the bare silicon substrate and the bare SMP-AFM-tip and those between the silicon-substrate/PDDA and the bare SMP-AFM-tip have also been measured. The experimentally obtained force/radius vs probe-sample separation distance plots have been fitted to a simple exponential, taking advantage of the Derjaguin approximation for assessing the doublelayer force between a charged sphere and a flat surface, and to the DLVO theory using the constant potential numerical approximation (with the exception of ii, where constant charge numerical approximation was used). Values for the adhesion (the pull back) force, Debye-length, and surface potential have also been evaluated.

Introduction Beneficial optical, electrooptical and electrical properties of nanostructured materials have prompted the increasing interest in the chemical assembly and self-assembly of nanoparticles and nanoplatelets into two- and three-dimensionally organized ultrathin films.1-3 The layer-by-layer self-assembly of oppositely charged polyelectrolytes, polyelectrolytes and nanoparticles, and polyelectrolytes and nanoplatelets has provided a particularly useful approach to the construction of ultrathin organized structures. Self-assembly is routinely employed for the fabrication of ultrathin films from charged nanoparticles (metallic, semiconducting, magnetic, ferroelectric, insulating, for example) nanoplatelets (clays or graphite, for example), proteins, pigments, and other supramolecular species.4-6 That any of these species in any order can be layer-by-layer adsorbed is the greatest advantage of self-assembly. The process is deceptively simple. It involves the immersion of a well-cleaned substrate (glass slide, mica, silicon wafer, HOPG, evaporated gold or silver film, and even Teflon) into a dilute aqueous solution of a cationic polyelectrolyte, for a time optimized for adsorption of a monolayer, rinsing, and drying. The next step is the immersion of the polyelectrolyte monolayer covered substrate into a dilute dispersion of surfactant-coated negatively charged nanoparticles (or nanoplatelets), also for a time optimized for adsorption of a monoparticulate layer, rinsing, and drying. These operations complete the self-assembly of a 2.0 ( 0.5 nm thick polyelectrolyte and a 2.0 ( 0.5 nm thick nanoparticle (or * Corresponding author. † Attila Jo ´ zsef University. ‡ Clarkson University.

nanoplatelet) sandwich structure on the substrate. Subsequent sandwich layers can be self-assembled analogously. We have previously reported the self-assembly of alternating layers of positively charged polydiallyldimethylammonium chloride, PDDA, and anionic montmorillonite platelets, M, to form ultrathin (PDDA/M)n films on a variety of substrates.7 Employing X-ray diffraction, X-ray reflectivity, atomic force microscopy, transmission electron microscopy and surface plasmon resonance spectroscopic measurements we have established the adsorption of M and PDDA and the changes in the surface roughness with increasing layers of PDDA/M deposited (i.e., increasing values of n in (PDDA/M)n).7 In particular, the M platelets were found to smooth pinholes (sometimes as wide as 700 nm and as deep as 30 nm) and cap smaller pits (sometimes as wide as 180 nm and as deep as 30 nm). Clearly, the ability of mineral platelets to smooth the surfaces of self-assembled films is of fundamental importance and is highly relevant for a variety of technical applications. To realize the full potential of this type of hybrid self-assembled films it is necessary to obtain an insight of the forces responsible for the self-assembly. We have measured, therefore, forces between 2.0 ( 0.5 nm thick PDDA films (self-assembled onto a silicon wafer, and selfassembled onto a silicon microparticle, attached in turn to the AFM tip) between 2.0 ( 0.5 nm thick M platelets (self-assembled onto a PDDA film coated silicon wafer and self-assembled onto a PDDA film coated silicon microparticle, in turn attached to the AFM tip), and between 2.0(0.5 nm thick PDDA films and 2.0(0.5 nm thick M platelets (the PDDA self-assembled onto a silicon wafer and the M platelets, self-assembled onto a PDDA film coated silicon microparticle, attached in turn to the AFM tip). We report here our initial findings.

10.1021/jp010673j CCC: $20.00 © 2001 American Chemical Society Published on Web 10/09/2001

10580 J. Phys. Chem. B, Vol. 105, No. 43, 2001 Experimental Section Sodium chloride and polydiallyldimethylammonium chloride, PDDACl, solutions were prepared from reagent-grade chemicals (Aldrich) without further purification in 18 MΩ deionized water (Millipore Co. Milli-Q system). The pH of water and the electrolyte (1.0 × 10-2 M NaCl) was maintained to be 5.7 and routinely monitored prior to and after each set of AFM measurements. Chloroform, acetone, and ethanol were reagent grade (Aldrich) and used as received. Sodium montmorillonite clay, M (MAD, Hungary) was purified by peptidization. Ten grams of it was suspended in 1.0 L of distilled water. The supernatant (containing < 2 µm diameter M particles) was removed, stirred with 1.O M NaCl, centrifuged, washed, and dialized to remove the excess electrolyte. Analysis: SiO2: 61.58%, Al2O3 ) 21.15%, Fe2O3 ) 4.5%, Ca ) 0.14%, Na2O ) 3.18%, alkaline earth metals ) 0.95%, water hydration ) 8.0%; specific surface area (determined by N2 adsorption by the BET equation) ) 58 m2/g, basal distance in air-dried M ) 1.25 nm (XRD). M suspensions were prepared by sonicating the purified, air-dried M in 100 mL distilled water for 40 min. The rougher fraction was sedimented and the supernatant was sonicated for 20 min. After centrifugation at 3000 rpm the top fraction was separated and used in the self-assembly. The charge density and cation exchange capacity, CEC, of M were determined by streaming potential measurements. 10 mL of aqueous M suspension (0.01 g of M in 100 mL water) was titrated by two different positively charged materials: aqueous polydiallyldimethylammonium chloride, PDDA, solution (0.01 g of PDDA/100 mL of water) and hexadcyl phosphate, HDP, solution (0.001 M). In every step, 0.1 mL PDDA or HDP solution was added to the M suspension until a positive potential was reached. From the zero surface charge point of titration curves (determined by interpolation) values for CEC were calculated to be 0.65 mmol/g (for HDP) and 0.27 mmol/g (for PDDA). Similarly, charge densities of 0.506 and 0.209 charge/ nm2 were obtained for HDP and PDDA, respectively. In both the CEC and charge density calculations we assumed the M surface area to be 774 m2/g. Adsorption isotherms for the binding of PDDA to M were determined by the standard method used previously for other systems.8 The amount of PDDA adsorbed was calculated from nS ) V(Co - Ce)/m, where nS is the adsorbed amount of PDDA (mg of PDDA/g of M), V is the total volume of the aqueous PDDA solution (in cm3), Co is initial concentration, Ce is equilibrium concentration of PDDA solution (mg/cm3), and m is the mass of M (in g). From the first linear range of isotherm 0.26 mmol PDDA/g M was calculated for CEC. Silicon (and silicon-oxide) wafers were planarized by chemical mechanical polishing (CMP); the excess particles were removed by megasonics in distilled water, 1% NH4OH, SC1 and distilled water. The established procedure was employed for the self-assembly of PDDACl (2% aqueous solutions, w/v) and M (supernatant of a 2 g of M/100 mL distilled water, centrifuged for 20 min at 3000 rpm) onto the substrates and the silica microparticle cantilever tips.5 Just prior to selfassembly, the silica microparticle cantilever tips were rinsed by ethanol and dried by a gentle stream of nitrogen. AFM topographies (scans) and force-distance curves were measured by means of the Explorer AFM stage of a Topometrix scanning force microscopy with a small liquid scanner (2.8 micrometer in the x and y range and 1.2 micrometer in the z range) or a small dry scanner (2.6 micrometer in the x and y range and 0.8 micrometer in the z range) and contact Si3N4 tips. In the absence of a special liquid cell, the measurements were

Szu¨cs et al. performed on a liquid droplet (2-4 mL water, pH adjusted by HCl or NaOH to desired value) placed onto the surface of the silicon wafer. The entire AFM head (scanner) was immersed into the liquid. Since the liquid droplet covered all the gap between the scanner and the substrate, no additional force acted on the cantilever and the probe during the experiment. The system had a thermal drift of 7.04 nA/h initially (as measured by the electrical signal difference between the top and bottom segment of the photodiode detector of the AFM), but after some time (typically 2 h) it settled to 0.53 nA/h. The surface of the substrates (the self-assembled films on the silicon wafer) was routinely imaged by contact and noncontact scans prior to force measurements. The surface roughness was 0.15 nm root-meansquare (RMS) for the silicon-wafer, 0.57 nm RMS for the polymer films and 1.23 nm RMS for the clays (see Figure 1 for typical images of polyelectrolyte and clay nanoplatelet surfaces and for the areas used for roughness measurements). Force measurements were performed by using tipless AFM cantilevers (B type cantilever, Park Scientific Instruments, Sunnyvale, CA) onto which spherical silica microparticles (12 µm diameter, Supelco, Bellefore, PA) had been glued by epoxy resin (Epikote 1004, Shell) employing a slight modification of the reported procedure.9-14 Specifically, a few (2-5) µL of freshly prepared acetonic solution of the epoxy resin (1: vol %) was dropped onto the cantilever at the desired position by a copper hair wire (0.025 mm diameter, Puratronic 99.995%, ALFA AESAR), and a silica microparticle was placed on top of it. Both the hair wire and the silica microparticle were held and positioned by a Leitz xyz micromanipulator. The positioning was monitored by an optical microscope (Olympus SZH) coupled to a video camera (NC-8 CCD, NEC) and a monitor. The positioned silica microparticles were dried for at least 12 h, and their dimensions were determined by scanning electron microscopy, SEM (JEOL, JSM-6300). No resin could be seen on the exposed surface of the silica particle by SEM. Figure 2 shows a typical SEM image of a silica microparticle glued onto the tipless AFM cantilever. Force-distance measurements were performed by using the “Point Force Spectroscopy” menu of the controller program of the Explorer AFM. The speed of the force probe (the silica microparticle cantilever tip) was set to 0.02-0.01 µm/second during the measurement (sample point approaching speed, in the setup menu), and during the measurement of the detector current vs. sample displacement curves the cantilever was stopped for 500-700 µs before each point (using the time delays before sample, advanced setup menu). Taking these steps decreased the errors of the unexpected resonances of the cantilever during the force measurement.15 Conversion of the detector signal (diode current) vs. sample displacement data to a force vs distance curve requires the definition of zero force and zero separation (definition of surface contact) and the conversion of the diode signal (I) to cantilever deflection. Zero force means that the deflection of the cantilever is constant (dI/ dZ ) constant); in this region the force is not changing with the distance (where the silica sphere and the surface are far from each other and, therefore, they do not interact). As the sample is moving toward the silica sphere, the cantilever deflects and this is registered by the photodiode. At some point, when the probe reaches the surface, in many cases there is a “jump into contact”. If the attractive forces are not large enough, this is not observable. When the probe is “in contact” with the surface, its movement is coupled so that a piezo displacement (Z) causes an equal displacement in the cantilever deflection; this is the

AFM Measurements of Interaction Forces

J. Phys. Chem. B, Vol. 105, No. 43, 2001 10581

Figure 1. Typical AFM images of a nanolayer of PDDACl, self-assembled onto a silicon wafer (a) and of the M nanoplatelets, self-assembled onto the PDDACl coated silicon wafer (b).

regime of constant compliance. In this region it is assumed that the Z-piezo movement only leads to deflection of the cantilever, i.e., that the surfaces are incompressible. At this latter region, the position of the probe does not change (the distance between the probe and the surface is zero); only the other end of the cantilever (fixed to the Z-piezo translator) is moving, and the tilt of the cantilever varies with it. Fitting a straight line to the data, the slope of the line (dI/dZ ) Ω) gives the conversion factor between the changes in z-position and the probe-surface separation (D). The I0 value corresponding to the undeflected cantilever (X ) 0) was obtained at large probe-sample separations where dI/dZ ) 0:

X ) I0Ω

(1)

Probe-sample separation was determined by summing the sample displacement and cantilever deflection:

D)X+Z

(2)

The surfaces between the substrate and the SMP-AFM-tip remain in contact until the pull-back force, applied by the

deflected SMP-AFM-tip cantilever, becomes equal or slightly larger than the adhesion force. Therefore, the lowest point (highest pulling force) in the pull-back force curve provides a value for the adhesion force. Pull-back forces were measured by setting the speed of the SMP-AFM-tip to 0.1 mm/s (by the pull-back point speed in the setup menu), and during the measurement of the detector current vs sample displacement curves the cantilever was stopped for 200 msec before each point (using the time delays before sample points in the advanced setup menu). After the completion of an approach and retraction cycle, the detector was stopped for 100 ms before the next measurement. Hook’s law (F ) -kX) gives the spring restoring force, which, at mechanical equilibrium, should be equal in magnitude to surface force but opposite in sign. As a part of our calibration, we have measured the interaction forces between the silica microparticle and silicon oxide in aqueous sodium chloride solutions. The satisfactory agreement between our values (empty circles) and those published9 lends credence to our experimental protocol (see Supporting Information 1). Values of the spring constant (k) were determined to be 0.119 N/m (see Supporting Information 2).

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Szu¨cs et al.

Figure 3. Experimental data points (O) in the force/radius vs probesample separation distance plotted linearly and logarithmically (inset in the upper right) for the interaction between a bare silicon substrate and a bare SMP-AFM-tip in water at ambient temperature. The data points are the mean of six independently determined measurements. The bars indicate standard deviations from the mean. The experimental data were fitted to a simple exponential, taking advantage of the Derjagin approximation (- - - -, eq 6), and to the DLVO theory, using the constant potential numerical approximation and a modified Poisson-Boltzman equation (X, eq 8). Schematics of the silicon substrate and the SMP-AFM-tip are shown in the upper left inset.

Figure 2. SEM image of a silica microparticle glued onto the tipless AFM cantilever.

Results and Discussion In the present work, we have investigated the interactions (i) between a well-cleaned silicon substrate and a silicon microparticle attached to the AFM tip (SMP-AFM-tip), as illustrated in the upper left inset in Figure 3; (ii) between an ultrathin (2.0 ( 0.5 nm thick) film of stretched out PDDA polycations, self-assembled onto the silicon substrate (silicon-substrate/ PDDA) and the bare SMP-AFM-tip, as illustrated in the upper left inset in Figure 4; and (iii) between a silicon-substrate/ PDDA and PDDA, self-assembled onto the SMP-AFM-tip (SMP-AFM-tip/PDDA, as illustrated in the upper left inset in Figure 5) in water. We could not adsorb the M platelets directly onto the silicon wafer or onto the SMP-AFM-tip. They had to be self-assembled onto a layer of PDDA to provide the siliconsubstrate/PDDA/M and SMP-AFM-tip/PDDA/M ultrathin films. It should be remembered that the surface roughness of M (on the PDDA silicon substrate) was found to be substantially greater than that of PDDA (see Experimental Section and Figure 1). Inevitably, measurements of the interactions between the silicon-substrate/PDDA/M and SMP-AFM-tip/PDDA/M (see the upper left inset in Figure 6), or between silicon-substrate/ PDDA and SMP-AFM-tip/PDDA/M (see the upper left inset in Figure 7), or between silicon-substrate/PDDA/M and SMPAFM-tip/PDDA (see the upper left inset in Figure 8) in water were less straightforward and more difficult to interpret. Interactions in these different systems will now be discussed sequentially.

Figure 4. Experimental data points (O) in the force/radius vs probesample separation distance plotted linearly and logarithmically (inset in the upper right) for the interaction between the silicon-substrate/ PDDA and a bare SMP-AFM-tip in water at ambient temperature. The data points are the mean of six independently determined measurements. The bars indicate standard deviations from the mean. The experimental data were fitted to a simple exponential taking advantage of the Derjagin approximation (- - - -, eq 6), and to the DLVO theory, using the constant potential numerical approximation and a modified Poisson-Boltzman equation (X, eq 8). Schematics of the silicon substrate and the SMPAFM-tip are shown in the upper left inset.

Interaction between the Bare Silicon Substrates and the Bare Silcon Microparticle. Typical interaction force/radius vs probe-sample separation distance curves are illustrated in Figure 9 for the interaction of the charged bare silicon microparticle with the bare silicon substrate in water at pH 5.9. During the first approach of the SMP-AFM-tip to the silicon substrate, very little interaction is seen until a separation distance of about 17 nm is reached. At shorter separation distances, a strong repulsion force manifests itself which exponentially increases until a hard contact is reached. An analogous behavior was reported previously for the interaction of a colloidal silica particle and mica substrate in aqueous sodium nitrate solutions at different pH values.16 Specifically, at pH ) 5.7, the repulsion

AFM Measurements of Interaction Forces

Figure 5. Experimental data points (O) in the force/radius vs probesample separation distance plotted linearly and logarithmically (inset in the upper right) for the interaction between the silicon-substrate/ PDDA and the SMP-AFM-tip/PDDA in water at ambient temperature. The data points are the mean of six independently determined measurements. The bars indicate standard deviations from the mean. The experimental data were fitted to a simple exponential, taking advantage of the Derjagin approximation (- - - -, eq 6), and to the DLVO theory, using the constant potential numerical approximation and a modified Poisson-Boltzman equation (X, eq 8). Schematics of the silicon substrate and the SMP-AFM-tip are shown in the upper left inset.

Figure 6. Experimental data points (O) in the force/radius vs probesample separation distance plotted linearly and logarithmically (inset in the upper right) for the interaction between the silicon-substrate/ PDDA/M and the SMP-AFM-tip/PDDA/M in water at ambient temperature. The data points are the mean of six independently determined measurements. The bars indicate standard deviations from the mean. The experimental data were fitted to a simple exponential taking advantage of the Derjagin approximation (- - - -, eq 6), and to the DLVO theory, using the constant charge numerical approximation and a modified Poisson-Boltzman equation (X, eq 8). Schematics of the silicon substrate and the SMP-AFM-tip are shown in the upper left inset.

force began to manifest itself at around 25 nm separation and the jump distance was barely perceptible (it was found to decrease with increasing pH, from about 5 nm at pH ) 3.1 to no detectable jump in at pH ) 8.8).16 Considering the differences in the substrates and the compositions of the medium bathing them, the agreement between these measurements is quite satisfactory and indicates a consistency with the DLVO theory. Retraction of the SMP-AFM-tip from the silicon substrate resulted in appearance of a marked attraction attributable to the adhesion of the SMP-AMF-tip to the silicon substrate (i.e., to the negative value of the pull-back force). Following the

J. Phys. Chem. B, Vol. 105, No. 43, 2001 10583

Figure 7. Experimental data points (O) in the force/radius vs probesample separation distance plotted linearly and logarithmically (inset in the upper right) for the interaction between the silicon-substrate/ PDDA/M and the SMP-AFM-tip/PDDA in water at ambient temperature. The data points are the mean of six independently determined measurements. The bars indicate standard deviations from the mean. Schematics of the silicon substrate and the SMP-AFM-tip are shown in the upper left inset.

Figure 8. Experimental data points (O) in the force/radius vs probesample separation distance plotted linearly and logarithmically (inset in the upper right) for the interaction between the silicon-substrate/ PDDA and the SMP-AFM-tip/PDDA/M in water at ambient temperature. The data points are the mean of six independently determined measurements. The bars indicate standard deviations from the mean. Schematics of the silicon substrate and the SMP-AFM-tip are shown in the upper left inset.

established procedure,17 we assigned the minimum in the first retraction curve to be the value of the pull back force (see Figure 9).18 The second compression and retraction curve in the force/ radius vs probe-sample separation distance plot was quite similar to those observed in the first time. Values of the pullback force decreased progressively, however, in subsequent retraction curves. We have, therefore, repeated the determination of the first compression and retraction curves at five different positions in the sample and obtained a mean value of -0.9 ( 0.30 mN/m for the for the pull-back force of SMP-AFM-tip from the silicon substrate in water (see Table 1). This value should be compared to that determined for the pull-back force of a colloidal silica particle from a mica substrate in aqueous sodium nitrate solutions (-0.1 mN/m).16 The mean experimental data points (of six independent determinations) in the force/radius vs probe-sample separation plots were fitted to a simple exponential taking advantage of the Derjaguin approximation19-22 assessing the double layer force between a charged sphere of radius R and a flat surface.

10584 J. Phys. Chem. B, Vol. 105, No. 43, 2001

Szu¨cs et al. We have also fitted the data to the DLVO theory using the constant potential numerical approximation22 and a modified Poisson-Boltzman equation in the form

∆Ψ ) -

Then the normalized force vs. distance function (F/R) was calculated from

( )

(6)

where F is the force, R is the radius of the particle, r is the relative dielectric constant of the medium (assumed to be about 79) and 0 is the dielectric constant of the vacuum, k is the Boltzmann constant, T is the temperature (22 °C), e is the charge of the electron, Ψ is the surface potential, and 1/κ is the Debye length. The decay constant of the exponential, κ, is proportional to the square root of the number concentration of the ions in the bulk (i.e., infinite distance from the surfaces), ni:

x

∑i ni Z 2i

e2

κ)

)

r 0 kT

x

2e2n

(7)

r 0 kT

)-

∑i Zieni exp(ZieΨ/kT) (8)

0

0

where ‘i’ is index for the different ions present, Zi is the valence of the ion ‘i’, ni is the number concentration. The numerical fit of the experimental data is also shown in Figure 3, and the Debye length and surface potential obtained from this fit are also included in Table 1. Fitting of the experimental points to the DLVO theory is quite satisfactory. Interaction between the PDDA-Coated Silicon Substrate and the Bare Silicon Microparticle. There is a difference in the interactions between the polycation-coated substrate (siliconsubstrate/PDDA) and the bare negatively charged silicon microparticle and that between the bare silicon substrate and the bare silicon microparticle. In the first approach of the SMPAFM-tip to the silicon-substrate/PDDA a small attraction becomes observable at a separation distance of about 170 nm. This attraction then increases somewhat until a separation distance of about 50 nm is reached, after which it changes to a repulsion until contact between the silicon-substrate/PDDA substrate and the SMP-AFM-tip is reached (Figure 10). The first retraction of the SMP-AFM-tip from the silicon-substrate/ PDDA substrate manifested itself in a substantial attraction corresponding to the pull-back force. Once again, the second compression and retraction curves in the force/radius vs probesample separation distance plots were quite similar to those observed the first time, but values of the pull-back force progressively decreased in subsequent retraction curves. We have, as before, observed several first retraction curves at different positions of the silicon-substrate/PDDA substrate and assessed the mean value of the pull-back force from five measurements to be -1.5 ( 0.1 mN/m (Table 1). Not unexpectedly, there is a greater adhesion of the SMP-AFM-tip to the silicon-substrate/PDDA substrate than to the uncoated silicon substrate. The mean experimental data points (of six independent determinations) in the force/radius vs probe-sample separation plots are illustrated in Figure 4, and, as expected, fitting the data to theoretical points is somewhat poorer than that observed for the interaction of the bare silicon substrate with the bare SMP-AFM-tip. Values for the Debye length and surface potential obtained from the fits are included in Table 1.

Figure 9. Force/radius vs probe-sample separation distance plots for the initial approaches (compressions) and retractions (decompression) of the bare SMP-AFM-tip to the bare silicon substrate in water at ambient temperature.

2 F 64 πr0(kT) eΨ ) κ tanh exp(-κ D) R 4kT e2

F

where Zi is the valency of ions. A satisfactory fit has been obtained in the 0-20 nm range (see Figure 3), from which we calculated the Debye length and the surface potential to be 4.6 nm and 65 mV (Table 1).

TABLE 1: Pull-Back Forces, Debye Length, and Surface Potentials for the Different Interactions Measured

silicon-substrate SMP-AFM-tip silicon-substrate/PDDA SMP-AFM-tip silicon-substrate/PDDA SMP-AFM-tip/PDDA silicon-substrate/PDDA/M SMP-AFM-tip/PDDA/M silicon-substrate/PDDA/M SMP-AFM-tip/PDDA silicon-substrate/PDDA SMP-AFM-tip/PDDA/M

pull-back force (mN/m)

1/κa (nm)

1/κb (nm)

-0.9 ( 0.3

-1.5 ( 0.1

-2.7 ( 0.4

4.6

12.8

4.6 4.6

surface potentialb (mV)

surface potentialc (mV)

-0.9 ( 0.2

-0.5 ( 0.2

-3.5 ( 0.6

13.4

21.7

12.9

18.0

32.0

>21.7d

12.8

12.0

70

125

150

65

156

220

1/κc (nm)

9.8e

>107.0

130

a Obtained from fitting the first fifty experimental points to an exponential regression. b Obtained from fitting the data to the DLVO theory (see Figures 4-9). c Obtained from fitting the data to equation 6 (see Figures 4-9). d Employing the constant surface charge model, using a charge density of 0.209 charge/nm2 or 0.03 C/m2 for M, determined by titration with PDDA, see Experimental Section. e Obtained by using 1/κ g D.

AFM Measurements of Interaction Forces

Figure 10. Force/radius vs probe-sample separation distance plots for the initial approaches (compressions) and retractions (decompression) of the bare SMP-AFM-tip to the silicon-substrate/PDDA in water at ambient temperature.

Figure 11. Force/radius vs probe-sample separation distance plots for the initial approaches (compressions) and retractions (decompression) of the SMP-AFM-tip/PDDA to the silicon-substrate/PDDA in water at ambient temperature.

Interaction between PDDA Polycations. The first approach of the SMP-AFM-tip, covered by 2.0 ( 0.5 nm thick layer of cationic PDDA (SMP-AFM-tip/PDDA), to the PDDA-covered silicon wafer substrate (silicon-substrate/PDDA) in water at pH ≈ 5.7 is governed by a very long range (> 100 nm) attractive force that becomes repulsive at a separation distance of 40 nm, with the repulsion force increasing exponentially below 20 nm, until a hard contact is reached. Retracting the tip (SMP-AFMtip/PDDA) had to overcome a substantial adhesion force between the intertwined polyelectrolytes. This resulted in a long range (> 300 nm) attraction force (Figure 11). Subsequent approaches and retractions resulted in progressively diminished attraction forces. Determining the minima in the first retraction curves at five different positions on the silicon substrate/PDDA substrate led to a mean value of the pull-back force between the self-assembled polycations to be -2.7 ( 0.4 mN/m (Table 1). This long-range pull-back force indicates the interdigitation of the self-assembled PDDA layers. Although adhesion forces between polyelectrolytes have been determined previously (to be between 0.2 mN/m and 0.7 mN/ m), they cannot be compared directly with the present results since they were observed by measuring forces as a function of

J. Phys. Chem. B, Vol. 105, No. 43, 2001 10585

Figure 12. Force/radius vs probe-sample separation distance plots for the initial approaches (compressions) and retractions (decompression) of the SMP-AFM-tip/PDDA/M to the silicon-substrate/PDDA/M in water at ambient temperature.

separation between the silica bead AFM-tip and flat mica surface in the presence of different concentrations of aqueous polyelectrolyte solutions.17 Emphasis in the previous work was placed on the in situ adsorption of polyelectrolytes at the solid liquid interface during the force measurements.17 In contrast, well-controlled (2.0(0.5 nm thick) PDDA polyelectrolytes were self-assembled onto the SMP-AFM-tip and the silicon substrate prior to (see the upper left inset in Figure 3) the force measurements. The magnitude of the pull-back force is indicative of the strong interaction between the stretched out polyelectrolytes self-assembled onto the SMP-AFM-tip and the silica substrate. Values of the interaction forces in water at pH ≈ 5.7 (each value is a mean of five measurements taken at different positions after four initial compression and retraction curves) between the SMP-AFM-tip/PDDA and silicon-substrate/PDDA at different separation distances are presented in Table 1. The mean experimental data points (of six independent determinations) in the force/radius vs probe-sample separation plots and their fittings to eqs 6 and 8 are illustrated in Figure 5, and values for the Debye length and surface potential obtained from the fits are collected in Table 1. Interactions between Clay Nanoplatelets. The interaction between the large (≈1 × 105 nm2) negatively charged clay platelets is expected to be strongly repulsive (Figure 12). Indeed, the approach of the SMP-AFM-tip/PDDA/M to the siliconsubstrate/PDDA/M is felt further than 500 nm and characterized by a very substantial repulsion force (14 mN/m on contact with the hard wall). On the other hand, the pull-back force (-0.9 ( 0.2 mN/m) is comparable to that observed for the interaction of the bare SMP-AFM-tip with the bare silicon substrate (-0.9 ( 0.3 mN/m, Table 1). Apparently the large surface area of the clay platelets and the dangling negative charges on them preclude appreciable attraction between them. The mean experimental data points (of six independent determinations) in the force/radius vs probe-sample separation plots are illustrated in Figure 6. They could not be fitted with the DLVO theory, using the constant potential approximation. Employing the constant surface charge approximation, using charge densities determined experimentally (see Experimental Section) reproduced the magnitude of the repulsive force; however, the shape of the experimental curve could not be fitted. The lack of our ability to fit the data may originate in the

10586 J. Phys. Chem. B, Vol. 105, No. 43, 2001

Figure 13. Force/radius vs probe-sample separation distance plots for the initial approaches (compressions) and retractions (decompression) of the SMP-AFM-tip/PDDA/M to the silicon-substrate/PDDA in water at ambient temperature.

geometry of the large M platelets that produce pronounced edges on the SMP-AFM-tip which may have a slip movement due to repulsion. Values for the Debye length and surface potential obtained from the fits are collected in Table 1. Interactions between Polycations and Clay Nanoplatelets. Interactions between oppositely charged PDDA polycations and anionic M platelets have been examined in two alternative arrangements depending on whether M was on the tip, MPAFM-tip/PDDA/M (Figures 7 and 13), or on the substrate, silicon-substrate/PDDA/M (Figures 8). Interactions in the former (i.e., between MP-AFM-tip/PDDA/M and silicon-substrate/ PDDA) were found to be better behaved and more reproducible. One may speculate that the slightly tilted large M platelets, located on the MP-AFM-tip, cannot attract PDDA molecules as easily as can the alternative arrangement when M is flat on the substrate. The first approach of SMP-AFM-tip/PDDA/M is characterized by an attraction force that begins to manifest itself at a distance of approximately 120 nm from the silicon-substrate/ PDDA and changes to a repulsion force at around 15 nm (Figure 13). The pull-back force, determined in the first pull-back to be -3.5 ( 0.62 mN/m, is the strongest of all the systems investigated (Table 1). Not unexpectedly, the oppositely charged polyelectrolytes and clay platelets strongly adhere to each other. This behavior is, in fact, more pronounced in the interaction of MP-AFM-tip/PDDA with the silicon-substrate/PDDA/M. The refracting tip may well carry some M platelets adsorbed to PDDA on the SMP-AFM-tip/PDDA. The experimental observation that oppositely charged surfaces can give rise to a repulsion (Figure 13) is quite remarkable, although not unprecedented. A similar behavior has been recently reported for the interactions between cationic magnetite particles and negatively charged montmorillonite platelets.23 No aggregation could be observed between these oppositely charged colloids in aqueous solutions, which contained less than 5 mM NaCl, but diffusion-limited heterocoagulation was found to be induced by the addition of 10 mM sodium chloride.23 Repulsions between cationic polyelectrolytes and clay nanoplatelets, observed by us, and those between cationic magnetic particles and anion clay platelets, reported by Tomba´cz and co-workers,23 are counterintuitive. Indeed, the classical Hogg, Healy, and Furstenaus (HHF) treatment of heterocoagulation, using the linear Debye-Hu¨ckel approximation for low and constant

Szu¨cs et al. surface potential, allows only attractions between oppositely charged particles.24 Attempts to extend the HHH treatment to constant charge cases have not been very successful.25-27 Using the “compression” approach, Gregory demonstrated that charge repulsion may occur for unequal constant-charge double layers in the low κd region (where κ, the reciprocal Debye length, is proportional to the electrolyte concentration and d is the distance from the surface).28 Similar conclusion was reached by Shulepov and co-workers.29 Considering the value obtained for 1/κ for the interaction of PDDA and M (1/κ ) 12.9 nm, see Table 1), our finding of repulsive forces in the 15 nm region is not unreasonable. More important is Gregory’s conclusion that “in practice, neither constant potential nor constant charge assumption may be correct, mainly because of the presence of Stern layers and the uncertainty over which potential is relevant to the interaction of colloidal particles.”28 In light of these complications, we have not fitted the experimentally obtained force/radius vs probe-sample separation curves for the interaction of PDDA and M to any theoretical model. Acknowledgment. Support of this work by the U.S. National Science Foundation and by a NATO Science for Peace Program (SfP 972652) is gratefully acknowledged. We thank Dr. Tomba´cz for drawing our attention to her recent work on heterocoagulation. Supporting Information Available: A figure (S1) showing the date for the interaction forces between a silica microparticle and a silicon oxide wafer, determined in the present work and in ref 9, and the assessment of the cantilever spring constant. This material is available free of charge at the Internet at http:// pubs.acs.org. References and Notes (1) Fendler, J. H. Membrane Mimetic Approach to AdVanced Materials; Springer-Verlag: Berlin, 1992; Vol. 113. (2) Mann, S. Biomimetic Materials Chemistry; VCH: New York, 1996. (3) Fendler, J. H. Nanoparticles and Nanostructured Films - Preparation, Characterization and Applications, 1st ed.; Fendler, J. H., Ed.; WileyVCH: Weinheim, Germany, 1998; p 468. (4) Decher, G. Layered Nanoarchitectures Via Directed Assembly of Anionic and Cationic Molecules; Sauvage, J.-P., Ed.; Pergamon Press: Oxford, 1996; Vol. 9, pp 507-528. (5) Fendler, J. H. Chem. Mater. 1996, 8, 1616-1624. (6) Pileni, M. P. Supramol. Sci. 1998, 5, 321-329. (7) Kotov, N. A.; Haraszti, T.; Turi, L.; Zavala, G.; Geer, R. E.; Dekany, I.; Fendler, J. H. J. Am. Chem. Soc. 1997, 119, 6821-6832. (8) Dekany, I.; Turi, L.; Galbacs, G.; Fendler, J. H. J. Colloid Interface Sci. 1999, 213, 584-591. (9) Ducker, A. W.; Senden, J. T.; Pashley, R. M. Langmuir 1992, 8, 1831-1836. (10) Yalamanchili, M. R.; Veeramasuneni, S.; Azevedo, M. A. D.; Miller, J. D. Colloids Surf., A 1998, 133, 77-88. (11) Larson, I.; Drummond, C. J.; Chan, D. Y. C.; Grieser, F. J. Am. Chem. Soc. 1993, 115, 11885-11890. (12) Hillier, A. C.; Kim, S.; Bard, A. J. J. Phys. Chem. B 1996, 100, 18808-18817. (13) Rabinovich, Y. I.; Yoon, R. Langmuir 1994, 10, 1903-1909. (14) Toikka, G.; Hayes, R. A.; Ralston, J. Colloids Surf., A 1998, 141, 3-8. (15) Siedle, K.; Butt, H.-J. Langmuir 1995, 1065. (16) Hartley, P. G.; Larson, I.; Scales, P. J. Langmuir 1997, 13, 22072214. (17) Bremmell, K. E.; Jameson, G. J.; Biggs, S. Colloids Surf. A 1998, 139, 199-211. (18) It should be clarified, however, that with our instrument strictly speaking we do not have a true first compression or decompression. The very first approach and withdrawal is needed to set up the instrumental parameters of our Topometrix AFM. For the sake of clarity we shall, however, refer to the experimentally obtained force/radius vs sample separation plots (after setting up the instrument) in the order determined as first and second compression curves (and first and second retraction curves).

AFM Measurements of Interaction Forces (19) Bell, G. M.; Peterson, G. C. J. Colloid Interface Sci. 1972, 41, 542-566. (20) Israelachvili, J. N. Intermolecular and surface forces with applications to colloidal and biological systems; Academic Press: New York, 1985. (21) Butt, H.-J. Biophys. J. 1991, 60, 777-785. (22) Chan, D. Y.; Pashley, R. M.; White, L. R. J. Colloid Interface Sci. 1980, 77, 283-285. (23) Tomba´cz, E.; Csanaki, C.; Ille´s, E., Colloid Polym. Sci. 2001, 279, 484-492.

J. Phys. Chem. B, Vol. 105, No. 43, 2001 10587 (24) Hogg, R.; Healy, T. W.; Fuerstenau, D. W. Trans. Faraday Soc. 1966, 62, 1638-1651. (25) Bell, G. M.; Peterson, G. C. J. Colloid Interface Sci. 1972, 41, 542-566. (26) Gregory, J. J. Chem. Soc., Faraday Trans. 2 1973, 69, 1723-1728. (27) Kar, G., Chander, S., Mika, T. S. J. Colloid Interface Sci. 1973, 44, 347-355. (28) Gregory, J. J. Colloid Interface Sci. 1975, 51, 44-51. (29) Shulepov, Yu. V.; Koopal, L. K.; Lyklema, J.; Dukhin, S. S. Colloids Surf., A 1998, 131, 51-62.