Electrostatic Adhesion of Nanosized Particles: The Cohesive Role of

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J. Phys. Chem. C 2008, 112, 8534–8544

Electrostatic Adhesion of Nanosized Particles: The Cohesive Role of Water Leonardo F. Valadares, Elisaˆngela M. Linares, Fa´bio C. Bragança, and Fernando Galembeck* Institute of Chemistry, UniVersidade Estadual de Campinas, Caixa Postal 6154, 13083-970 Campinas SP, Brazil ReceiVed: NoVember 10, 2007; ReVised Manuscript ReceiVed: March 14, 2008

Adhesion between chemically dissimilar solids is not often observed for fundamental reasons, especially the large solid-solid interfacial tensions involved that, in turn, derive from the fundamental characteristics of van der Waals and other intermolecular interactions. However, this difficulty can be overcome in many cases by mixing particulate solids within aqueous media and drying the resulting dispersion. In this work, transmission electron microscopy (TEM) and scanning probe microscopy (SPM) were used to obtain evidence for strong adhesion between the following pairs of organic and inorganic nanoparticles: Sto¨ber silica and poly(styreneco-butyl acrylate-co-acrylic acid) (SA) latex, calcium montmorillonite and the same latex, and titanium dioxide and another SA latex. Adhesion was observed even though the particles in each pair are highly dissimilar and thus are expected to have a high interfacial tension. Bulk or aggregate particle nanohybrids were obtained by drying mixed aqueous dispersions at different particle concentrations and were examined using bright-field and energy-filtered imaging in TEM, as well as intermittent-contact and phase-contrast SPM. Association between silica, clay, or TiO2 and the latex particles was observed under several conditions, and partial particle segregation was also observed. A general mechanism for the formation of hybrid or composite monoliths is proposed, based on the action of capillary forces during the drying process followed by electrostatic interactions within the dry solid, between negative particles and cationic domains formed by dry serum solutes. Using this model, calculated electrostatic adhesion energy between dissimilar particles can be used to explain previous literature data. This mechanism is suitable for making hybrid monoliths from nanosized particles. Introduction Possibilities and constraints for mixing substances have been understood for many years,1 thanks to a great amount of experimental and theoretical work from a very large number of researchers. In the case of solutions, landmarks include the theoretical and experimental work that contributed to current understanding even in difficult cases such as electrolyte2 and polymer3 solutions and the respective phase equilibria. In biphasic systems, intimate mixing can be achieved only if interfacial tension is sufficiently low, and this is the basis of many successful efforts to achieve phase compatibility through surface modification or adsorption,4 as in the classical case of microemulsion formation.5 Many ingenious approaches have been developed to circumvent mixing difficulties especially in those cases involving hydrophobic organic polymers and polar inorganic particles.6 The morphology of silica-polymer nanohybrid structures is strongly dependent on particle-polymer compatibility,7 and functionalized surfaces are often used to obtain core-shell8 and inverse core-shell9,10 nanoparticles and nanocomposites.11 Current interest in the outstanding properties of polymer nanocomposites12–14 and the significant difficulties in dispersing clay lamellae within a polymer matrixes have led to many different approaches for making these materials. The pioneering procedure used by the Toyota research group is based on a previous treatment of montmorillonite clay with ω-amino acids15 followed by an intercalative ring-opening polymerization in which ε-caprolactam reacts chemically with the clay-bonded amino acids.16 Other approaches based on the reduction of * To whom correspondence should be addressed. Phone: +55-19-35213080. Fax: +55-19-3521-2906. E-mail: [email protected].

polymer-clay interfacial tension have been used for intercalation in solution and melt intercalation methods. This is usually achieved by clay surface modification using quaternary ammonium salts17,18 or by oxidation of the polymer.19 Recent results from this group20 show that polymer-clay nanocomposites can be prepared by mixing an aqueous polymer latex and an aqueous clay dispersion in which the lamellae are extensively exfoliated21 and then drying the resulting dispersion. By doing this, a stable dispersion of lamellae within the polymer matrix can be obtained without using any compatibilizer or surface-active agent beyond water, which is not a solvent for either the polymer or the clay. Moreover, clay-polymer nanocomposites prepared by drying clay-latex dispersions have mechanical properties that are strongly dependent from the dispersion counterions.22 This result can be interpreted according to the following hypothesis: The formation of solid monoliths by very dissimilar particles is the outcome of particle capillary adhesion during dispersion drying, followed by electrostatic forces in the dry solid in addition to van der Waals interactions and eventually hydrogen-bond formation. Following this interpretation, it should be possible to produce mechanically resistant multiphase solids with finely divided domains from widely different substances that combine many different properties just by dispersing them in water and drying. In this way, well-known powerful limitations to making of hybrid or composite materials should be easily overcome, making possible the fabrication of a number of hitherto-unattainable materials with attractive properties. Water is especially useful as a solvent or dispersant compared to other liquids because it is volatile, nontoxic, and nonflammable. Its high surface tension contributes to strong capillary adhesion of wetting surfaces,23 and its large dielectric constant

10.1021/jp710770v CCC: $40.75  2008 American Chemical Society Published on Web 05/15/2008

Electrostatic Adhesion of Nanosized Particles

Figure 1. Schematic drawing of the model used to calculate the electrostatic contribution to interfacial adhesion in a model formed by two negative surfaces bound by a positive layer. Surfaces A and C contain discrete 1- charges, and surface B contains discrete 2+ charges. The distance between adjacent charges on a surface is a, and the distance between adjacent surface planes is d.

allows electrostatic charge separation24 in the liquid dispersion that, in turn, gives rise to charged domains in the solid prepared by drying dispersions. To verify the possibility of dissimilar particle adhesion at the microscopic level, a detailed examination was made of dry submonolayers of mixed polymer latex and inorganic particles prepared by drying aqueous mixed dispersions. Morphology patterns providing evidence for dissimilar particle adhesion are reported in this work. A procedure for the calculation of electrostatic adhesion is also presented and successfully applied to interpret the Young moduli of a set of polymer-clay nanocomposites. Experimental Methods Latex. Poly(styrene-co-butyl acrylate-co-acrylic acid) latex (styrene-acrylic latex) was prepared by emulsion polymerization25 in a kettle glass reactor using two magnetic drive gear pumps (Cole Parmer model 7144-02) to add the redox initiator (4.3 g of sodium persulfate from Synth dissolved in 85 mL of water) and an emulsion containing monomers, water, and surfactant. The monomer emulsion was prepared by dissolving 19 g of the surfactant [an equimolar mixture of Renex 40 and Renex 1000 (Oxiteno), which are ethoxylated nonylphenols with 4 and 100 ethylene oxide units, respectively) in 280 mL of deionized water and then adding the monomers addition [266.0 g of styrene (CBE), 306.5 g of butyl acrylate (BASF), and 11.5 g of acrylic acid (BASF)] and stirring using a Cowles disperser at 2000 rpm for 10 min. The monomer emulsion and the redox initiator were added simultaneously for 4 and 4.5 h, respectively. The temperature was kept at 60-65 °C. After reactant addition had been completed, the temperature was held for 1 h to decrease the residual monomer content. One gram of t-butyl hydroperoxide and 1 g of sodium formaldehyde sulfoxylate were each dissolved in 10 mL water, and the solutions were added simultaneously to eliminate the remaining monomer residue. The final dispersion was cooled to room temperature and filtered through a 90-µm polyamide screen, where a small amount of coagulated latex (4.5 g) was collected. The pH was adjusted to 7.0 using 100 mL of an ammonium hydroxide aqueous solution (20% v/v). The effective latex particle diameter (measured by photon correlation spectroscopy, PCS) and ζ potential were determined in a Brookhaven Zeta Plus instrument and found to

J. Phys. Chem. C, Vol. 112, No. 23, 2008 8535 be 64 nm and -28 mV, respectively. The styrene-acrylic glass transition temperature (Tg) was measured with a TA Instruments model Q10 differential scanning calorimeter as -17.8 °C. Another sample of styrene-acrylic latex that was used was Denvercril RA193, supplied by Denver (Saõ Paulo, Brazil). Its effective latex particle diameter (determined by PCS) and ζ potential were 118 nm and -48 mV, respectively, and its Tg was 19.6 °C (as determined by differential scanning calorimetry). Silica. The silica nanoparticles were prepared by the method of Sto¨ber et al.26 Reagent-grade tetraethoxysilane (TEOS; Merck), absolute ethanol (Merck) as the solvent, and ammonium hydroxide (Synth) were used. Glassware was cleaned with 10% hydrogen chloride and rinsed with distilled water and absolute ethanol. Four milliliters of TEOS was added to 50 mL of ethanol in a screw-cap vial, in the presence of 2 mL of NH4OH, and the vial was placed in a water bath at (36 ( 0.1) °C under ultrasonic vibration (25 kHz, 200 W) for 120 min. The effective silica particle diameter and ζ potential determined by PCS were 169 nm and -65 mV, respectively. Titanium Dioxide. Ti-Pure R-902+ rutile titanium dioxide (TiO2) was obtained from DuPont. Clay. Sodium montmorillonite was acquired from Southern Clay Products. Calcium montmorillonite22 (Ca-MMT) was prepared by exchanging Na for Ca. For the exchange procedure, 50 g of sodium montmorillonite was dispersed in 10 L of deionized water, under stirring at 70 °C, and 35 g of CaCl2 (Merck) was added to the suspension. The dispersion was kept stirring for 24 h at 70 °C. The excess salt was then removed by dialysis, using regenerated cellulose dialysis bags immersed in deionized water that was exchanged daily until the conductivity of the external water was less than 2 × 10-6 S/cm (20 days). A calcium concentration of 89.8 mequiv/100 g was determined using ICP-OES as described in ref 22. Solids contents were determined gravimetrically on samples dried at 80 °C to constant weight. Submonolayer Sample Preparation. A 1:1 Sto¨ber silica/ latex dispersion (in weight of solids) was prepared by diluting the latex in water and mixing it with the dispersion of silica nanoparticles. This mixture was diluted in water to ca. 0.003% solids content, and a droplet was deposited and dried over the microscope substrate. Sample substrates were carbon-coated parlodion films over the microscope holder grids for transmission electron microscopy (TEM) and crystalline silicon wafers for scanning probe microscopy (SPM). The same colloidal mixture was deposited on both substrates. Ca-MMT-latex or TiO2-commercial latex submonolayers for TEM were prepared in the same way but using aqueous Ca-MMT or TiO2 dispersions in water (1%). Silica-Polymer Bulk Nanocomposite Formation. Sto¨ber silica dispersion in ethanol was dialyzed against water for 20 days to exchange ethanol for water and to remove the excess reagents. It was then stirred for 10 min with styrene-acrylic latex and dried in an oven at 60 °C to yield a solid containing 4.7 wt % silica. Cryo-ultramicrotomy. Ultrathin (ca. 50-nm) sections for transmission electron microscopy (TEM) analysis were cut normal to the nanocomposite film plane, with a diamond knife at -130 °C using a Leica EM FC6 ultramicrotome cooled with liquid N2. A drop of supersaturated sucrose was used to collect the thin cuts from the cooled microtome and transfer them to the microscope grids. To wash out the sucrose, the grids were left floating in deionized water in a beaker for 10 min. They were then removed and dried at room temperature.

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Figure 2. TEM bright-field images of (a) silica nanoparticles and (b) styrene-acrylic latex particles and histograms for particles sizes of (c) silica and (d) styrene-acrylic latex.

Electron Microscopy. Samples prepared by drying a drop of dilute dispersion of silica nanoparticles and styrene-acrylic latex on a microscope grid were analyzed using transmission electron microscopy associated with electron spectroscopy imaging (ESI-TEM). A Carl Zeiss CEM 902 (80 kV) transmission electron microscope fitted with a Castaing-Henry-Ottensmeyer energy filter spectrometer was used. When the electron beam passes through the sample, interactions with different elements result in characteristic energy losses. The spectrometer separates electrons according to their energies, and the microscope uses inelastic scattered electrons to form element-specific images. Characteristic energy losses from the interactions of electrons with C (303 eV), Si (132 eV), Ca (352 eV), and Ti (464 eV) were selected with an energy slit of 20 eV. Energy-filtered transmission electron microscopy (EFTEM) can be used to obtain bright-field images with low chromatic aberration when the energy slit is selected to zero-loss and to obtain high-contrast images when the slit is set to a low loss energy (i.e., 25 eV).27 A detailed description of the phenomena involved is provided in ref 28. The images were recorded using a Proscan high-speed slow-scan CCD camera and digitized (1024-1024 pixels, 8 bits) using AnalySis software. Scanning Probe Microscopy. SPM analyses were performed using a Discoverer TMX 2010 (Topometrix) microscope operating in intermittent-contact mode and using a silicon tip. The

topographic and phase-contrast images were obtained from the same area at the same time. The intermittent-contact mode is based on a tip oscillating at its natural resonance frequency above the surface of the sample, touching the surface periodically, and building the topographic image by using the variations in the amplitude of the tip caused by the tip-sample interaction. The phase-contrast image reveals areas with different abilities to dissipate mechanical energy, determined by the viscoelastic and adhesive properties of the sample. The effect of topography is minimized when the phase difference between the wave applied to the driving piezo and the wave from the photodetector is (90°. When this angle is equal to -90°, brighter regions in the image are more dissipative than darker regions.29 Electrostatic Adhesion Calculations. Electrostatic interactions at the particle-particle interface were evaluated using the following model: Two surfaces (denoted A and C) containing fixed negative charges (i.e., the clay and latex surfaces) are joined by a layer (denoted B) of positive charges (i.e., Ca2+ ions), representative of the Ca-MMT-latex nanocomposite. Negative ions (with charges equal to -1.603 × 10-19 C) are uniformly distributed on each surface forming a square lattice distribution of 10000 (100 × 100) discrete fixed charges on each surface A and C. Surface B is sandwiched between surfaces A and C and contains the same total charge as the sum of surfaces A and C. The distance between the charges in B is


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Figure 3. Low-magnification bright-field image of a submonolayer formed by drying a mixed colloidal silica and latex dispersion over a carboncoated support. This image was acquired by taking micrographs from contiguous fields and mounting them as a mosaic.

also the same, but each individual charge is equal to +3.206 × 10-19 C, to simulate the Ca2+ ions. The overall system is thus electroneutral. The distance between the individual charges along the surfaces is represented by a, and the distance between adjacent surface planes is d. Both a and d were set as parameters. Figure 1 shows a schematic drawing of the model used. Calculations were also performed for nonelectroneutral systems, by replacing a fraction of the positive charges (surface B) by either zero-charge spots or 2- charges (-3.206 × 10-19 C). MatLab 6.5 software was used to calculate the electrostatic contribution to interfacial adhesion by summing the interactions of each charge in the model with all other charges, as in eq 1, according to the superposition principle

ET )

1 4πεε0

(∑

∑

n

n

n q[A,(i,j)]q[B,(k,l)] q[B,(i,j)]q[C,(k,l)] + + r[A,B,(i,j),(k,l)] r[B,C,(i,j),(k,l)] i,j,k,l)1 i,j,k,l)1 n

)

q[B,(i,j)]q[B,(k,l)] q[A,(i,j)]q[C,(k,l)] + (1) r r[B,B,(i,j),(k,l)] [A,C,(i,j),(k,l)] i,j,k,l)1 i,j,k,l)1

∑

∑

where is the relative permittivity (in this work, ) 5, a typical value for dry MMT), 0 is the vacuum permittivity, n is the order of the square matrix (in this work, n ) 100), and r

represents the distances between any two charges, given by eqs 2–4

r[A,B,(i,j),(k,l)] ) r[B,C,(i,j),(k,l)] )

√[(i - k)a]2 + [(j - l)a]2 + d2

(2)

r[A,C,(i,j),(k,l)] ) √[(i - k)a] + [(j - l)a] + (2d)

(3)

r[B,B,(i,j),(k,l)] ) √[(i - k)a] + [(j - l)a]

(4)

2

2

2

(i, j)*(k, l)

2

2

Note that the interactions of charges on surface A or C with the other charges on the same surface were neglected because these charges are fixed in the particles. The electrostatic energy per mole of ions on surface A was also calculated according to eq 5

E)

(6.02 × 1023)ET n2

(5)

The code used is available in the Supporting Information and calculates the overall electrostatic energy in the model system. With minor changes, it was also used to calculate the adhesion energy for sodium montmorillonite-clay particles.


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Figure 4. Micrographs of polymer-silica nanoparticle clusters: (a) bright-field image, (b) EFTEM image (25 eV), (c) carbon map, and (d) silicon map.

Results Bright-field TEM images of the silica and latex nanoparticles used in this work are shown in Figure 2. The silica particles are quasi-spherical with a diameter of (143 ( 14) nm, and the latex particles have a diameter of (54 ( 8) nm. These measurements can be compared to those obtained by PCS, which are 169 nm for silica and 64 nm for the latex. The larger particle sizes observed in the PCS measurements can be understood by considering that they were obtained in aqueous media, where the particles are swollen.30,31 Moreover, the low-Tg latex particles are probably also flattened over the TEM sample holder, so that their diameter in the x-y plane would be larger than their height. The individuality of the silica particle is largely preserved in the submonolayers, as shown in Figure 2a, but the formation of thin “necks” between the particles is often observed, indicating that they are coated with a thin, deformable layer that forms a stable interparticle junction upon drying. On the other hand, the boundaries between adjacent polymer particles cannot be observed in Figure 2b, as expected considering that this polymer has a low Tg, so that adjacent particles easily coalesce as a result of interparticle chain diffusion.32 When colloidal dispersions of Sto¨ber silica and styrene-acrylic latex are mixed and the mixed dispersion is allowed to dry on

a carbon film over a microscope holder grid, it is possible to observe patterns similar to that shown in Figure 3. Throughout the observation field, dark particles identified as silica appear dispersed within coalesced polymer matrix (gray continuous domains) forming hybrid films. An interesting feature is that, in many cases, the silica particles are at the tip of a lateral protrusion in the composite film. Moreover, the gray polymer film surrounding the silica particles is darker and thus thicker, as observed in Figure 4, thus indicating good silica-polymer adhesion. Figure 4 shows C and Si elemental maps of the area inside the circle in Figure 3, together with the respective bright-field and EFTEM images obtained using 25 eV loss electrons. The white areas in the carbon map (Figure 4c) indicate the location of the polymer around the silica particles, which are identified in the silicon map (Figure 4d). Polymer accumulation around the particles produces bright rings, thus confirming the strong adhesion between the polymer and the silica. In many areas, the polymer is actually inserted in between particles, showing that the silica particles adhere to the polymer as well as to each other. In the SPM topography image in Figure 5a, silica nanoparticles are easily seen with little contribution from the coalesced



Figure 5. SPM images of dilute colloidal silica and latex dried over a silicon crystal: (a) noncontact topography and (b) phase contrast.

Figure 6. (a) TEM bright-field image of polystyrene-acrylic/Sto¨ber silica nanohybrid thin cuts. (b) Diameter histogram of silica particle size in the nanohybrid slices (compare to Figure 1c).

flat latex particles. Measurements on such images show that the height of the polymer film is very low, ca. 30 nm. However, in the phase-contrast image shown in Figure 5b, the polymer surrounding the silica particles is clearly observed because of the difference in the viscoelastic properties of the polymer, the silica, and the background silicon wafer. The phase-contrast image also shows that the polymer does not make a thick coating on the silica particles. Consequently, its adhesion to the silica particles is sufficient to hold the latter on the substrate, as expected considering that the polymer is bonded by the action of capillary forces and the film persisting to the end of evaporation is adjacent to the substrate. Drying a mixed dispersion of silica and latex particles produces nanohybrid slabs. A thick film was sliced in a cryoultramicrotome, and the resulting thin cuts were observed by TEM. As an example, the bright-field image in Figure 6 shows silica particles (in dark tones) dispersed in the polymer matrix. Particles fully surrounded by polymer predominate, and silica-silica contacts are observed in only a few cases. Measurements on the silica particle diameters (in the x-y plane) indicate a broad bimodal distribution extending to smaller diameters than those shown in Figure 2c and thus indicating that the silica particles were also sliced. The white areas in Figure 6 are assigned to voids left by particles that were plowed out by the diamond knife, but these are as frequent as the particles that remain sticking to the polymer film. Altogether,

Figure 7. Bright-field image of the morphology obtained when thin nanocomposite cuts were collected from the cryo-ultramicrotome.

these observations indicate adhesion between the silica and the latex particles within the dry nanocomposite, even after water


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Figure 8. Micrographs of cluster made of TiO2 and commercial latex particles: (a) bright-field image, (b) EFTEM image (25 eV), (c) carbon map, and (d) titanium map.

evaporation is complete, that is sufficiently strong to allow for film cutting and handling. A few nanocomposite thin cuts exhibited unusual behavior: During the process of picking the cut with a sucrose solution, mounting it on the microscope grid, washing it, and drying it for examination, when the material was subjected to a large temperature change and to capillary forces, a new morphology developed, as shown in Figure 7. It shows stretched polymer wires forming an irregular web. Remarkably, silica particles are part of the web. In some cases, they were found along a wire, and in others, they were at the web nodes. Even though the formation of this morphology is not fully understood at this time, this is additional and unexpected evidence for strong polymer-particle adhesion. TiO2 particles were also mixed with latex and dried over a microscope grid. Figure 8 shows a set of images with a cluster formed by the two types of particles. The carbon map in Figure 8c shows latex particles adhering on the TiO2 surfaces. Latex is also trapped between the inorganic particles bridging them. It is possible to observe a thin organic layer covering all of the TiO2 that is probably due to a surfactant layer or previous organic treatment. Some titanium dioxide particles are in mutual contact, whereas some latex particles appear isolated. Dilute dispersions of styrene-acrylic latex and Ca-MMT were also allowed to dry on a microscopy holder and observed by TEM. The bright-field image (Figure 9a) obtained from the clay-latex sample show agglomerated structures of polymer and clay and also a few isolated spherical latex particles. Silicon

and carbon elemental maps were acquired (Figure 9b,c), showing that clay lamellae and latex particles are well superimposed, forming a tight aggregate. Moreover, the calcium map (Figure 9d) shows that cations are located together with carbon and silicon in the aggregate, which leads us to conclude that the clay interacts strongly through ionic bridges formed by the calcium ions trapped between the latex particles and the clay platelets. The results of calculations on the contribution of electrostatic interactions to interfacial adhesion when cations are sandwiched between two negative surfaces are presented in Figure 10a, showing that a positive contribution from electrostatic interactions is obtained when the distances between neighboring ions and between surfaces are in the 1-nm range. The distance between charges in the Ca-MMT clay lamellar plane is ca. 1 nm (calculated from known data for ion-exchange ability or charge density),33 and the distance between the surfaces can be estimated as 1 nm. Using these values, the j increases with the surface size (n × energy per mole of ions E a), following eq 6 to within 1% for n > 10

E ) -9.72 × 105 + (8.26 × 105)n (J mol-1)

(6)

Significant negative electrostatic energy can be obtained by replacing 2+ charges in surface B by 0 or even 2- charges. Using interionic distances equal to 1 nm and keeping the surfaces 1 nm apart, partial replacement of the positive charges by neutral species leads to negative electrostatic energy between



Figure 9. Micrographs of polymer-Ca-MMT clusters: (a) bright-field image, (b) carbon map, (c) silicon map, and (d) calcium map.

43.0% and 57.6% replacement, as shown in Figure 10b. Negative values of energy are also found between 21.3% and 29.4% replacement of 2+ by 2- charges. The following equations hold for the electrostatic energy per mole of charges in plane A in function of the percentage, x, of charge replacement in surface B by 0 (eq 7) or 2- charges (eq 8)

E ) 8.20 × 106 - (3.33 × 105)x + (3.31 × 103)x2 (replacement by 0) (7) E ) 8.20 × 106 - (6.64 × 105)x + (1.31 × 104)x2 (replacement by 2-) (8) Discussion The formation of the particle distribution pattern observed in Figure 3 can be understood following the sequence of events represented schematically in Figure 11: When water evaporates, the liquid film breaks down as a result of a wetting transition,34 leaving the surface covered with droplets that contain clusters of the dispersed particles. These are concentrated during water evaporation, and their deposition pattern depends on particle height, wetting ability, and adhesion to the substrate. Most silica particles are dragged together with polymer and are thus enclosed within the polymer films. During water evaporation,

particles are pushed against their neighbors by capillary adhesion. The deformable polymer particles are coalesced with similar particles and are tightly bonded to the harder silica particles. When the particles are wet, the remaining serum contains counterions with charge opposite to that of the particles, and when water evaporation is complete, the counterions are sandwiched at the polymer-silica interfaces, uniting the two different phases. According to this mechanism, nanosized polymer and silica particles that are both negative in water at neutral pH form stable aggregates or even monoliths, depending on the amounts and particle concentrations in the specific sample under consideration. Thus, there is an electrostatic contribution to particle-polymer adhesion that adds up to hydrogen bonds and other van der Waals interactions to produce significant cohesion in the nanohybrid solid. The electrostatic adhesion in insulators remains an unsolved solid-state physics problem because of the difficulty of probing the charges on the surface.35 The simplified model of two uniformly and similarly charged planar surfaces interacting across a gap filled with counterions has been used to calculate forces acting on surfaces. Calculations using this model36,37 indicate the existence of attractive forces between two negative surfaces, mediated by intervening counterions, for some sets of the parameters. Figure 10 shows that the electrostatic


Valadares et al.

Figure 10. Electrostatic interaction energy calculated for three charged surfaces, where two negative surfaces with fixed negative charges enclose a positive surface with 2+ charges. (a) Curves plotted for three different values of distance between the charges on the surfaces. (b) Energy plot obtained by replacing 2+ charges by 0 and 2- charges, where the distance between surfaces and the distance between the charges in the plane with fixed charges were set to 1 nm. The ordinate axes on the left show the values of energy per mole of negative charges on surface A, whereas the axes on the right show the values of the total electrostatic energy for a set of two planes with 100 × 100 negative charges and with positive charges between them.

Figure 11. Schematic description of events involved in the adhesion of Sto¨ber silica to styrene-acrylic latex.

contribution to the free energy of the system reaches values in the range of -(100-200) kJ mol-1, equivalent to the contributions of interfacial covalent bonds. On the other hand, restrictions on ion motion introduce a decrease in entropy, but this makes a much lower contribution to the Gibbs free energy of the system than do the electrostatic forces. For instance, the increase in entropy for the formation of sodium chloride aqueous solution under standard conditions is ∆mS ) 43.4 J K-1 mol-1 and thus T∆mS ) 12.9 kJ mol-1, which is much less than the electrostatic contributions to the enthalpy.

Results of the electrostatic adhesion calculations were verified by comparison to force measurement data found in the literature.22 The work for 1% deformation of rubber-clay composites calculated from the force-deformation curves in ref 22 is reported in Table 1, for Li, Na, and K clays. The energy difference calculated for a 1% increase in the distance between the surfaces using the code given in the Supporting Information is 151 J for monovalent cations and 50.5% replacement of positive charges by voids, as shown in Figure 10. The calculated value is very close to that determined experimentally for the K clay composite, as was also the case for the Li and Na composites. This is excellent agreement between the deformation energy extracted from force measurement data and the elastic energy calculated using a rigorous method based on principles of physical chemistry. The role of capillary adhesion in particle association has been emphasized by many authors in regard to the problem of latex film formation.38,39 In the case of a mixed dispersion containing two or more types of different particles, arguments based on maximum particle-particle attractive interactions for particles with identical Hamaker constants40 lead to the conclusion that particle segregation is to be expected and the solid obtained by drying the dispersion should display separate domains formed by particles of one type or another. This has actually been observed in some cases for latex films.41 Moreover, slow evaporation can lead not only to particle segregation but also to the formation of unusual morphologies by particle selfassembly mechanisms.34 On the other hand, whenever the rate of water evaporation is high, particle segregation during drying is less important, and mixed particle domains are expected, as in the cases of flash evaporation, spray-drying, and freeze-drying of mixed dispersions. If water evaporation rates are high compared to particle motion rates, particles are trapped in random positions and



TABLE 1: Elastic Deformation Work for Rubber-Clay Nanocomposites Calculated from Experimental Curves in Figure 1 of Reference 22 work for 1% deformation (J) rubber rubber-Li+-MMT rubber-Na+-MMT rubber-K+-MMT

0.386 × 10-3 2.17 × 10-3 4.97 × 10-3 4.66 × 10-3

difference between nanocomposite and rubber (J) 1.78 × 10-3 4.58 × 10-3 4.27 × 10-3

pushed against their neighbors by capillary pressure, provided that the water-particle contact angles are less than 90°. The powerful effects of capillary adhesion in inducing elastic and plastic deformations of nanosized particles have already been described in detail.42 Routh and Russel showed that elastic deformation is powerful enough to bring particle surfaces sufficiently close to warrant film cohesion by van der Waals forces in the dry state. To this argument we now add the contribution of electrostatic forces attracting particles to the pool of dry serum counterions left behind by evaporated water. In another context, adsorbed water itself was shown to play an essential role in surface electrization, as recently described by Gouveia et al.43 The resulting solids are probably not in an equilibrium state, as they contain a large interfacial area uniting two different phases and the interfacial tensions are not expected to be as low as in the case of microemulsions. Nevertheless, the thermodynamic arguments applying to microemulsions5 are also applicable here, and one can expect disperse biphasic solids to behave as systems under thermodynamic equilibrium, at least at some concentrations, provided that the particles are sufficiently small to produce a favorable entropic contribution and the involved interfacial tensions are well below 1 mJ/m2. Water has been often cited as the “universal solvent”44 that can dissolve or disperse a large number of substances. In this work, we found that it is can also be used to unite solid phases that are immiscible and usually considered to be incompatible. The micrographs presented here show that two intrinsically apolar and polar solid phases are spontaneously joined upon drying from an aqueous dispersion, provided that their surfaces are sufficiently wetted by water. This phenomenon is largely dependent on two exceptional properties of water, namely, its surface tension and dielectric constant, which are both larger than those of most molecular liquids. The surface tension contributes to capillary adhesion according to the Young-Laplace equation,23 and the dielectric constant contributes to the formation of an electric double layer in the dispersion that becomes an electrostatically stabilized structure formed by domains with excess positive and negative charges. If necessary, wetting can be imparted by adsorption of a surfactant or polymer or even by surface chemical modification, so as to keep water intervening between different particles during water evaporation and thus producing powerful capillary adhesion. Conclusion Stable hybrid aggregates and monoliths are unexpectedly formed upon drying of aqueous dispersions of very different nanosized particles as a result of the formation of stable interfaces between the organic and inorganic phases. This is due to capillary adhesion during drying followed by electrostatic adhesion mediated by serum counterions in the dry solids. The elastic deformation energy of the monoliths formed by nanosized particles can be calculated using a new electrostatic adhesion

moles of ions in the test sample 2.43 × 10-5 4.91 × 10-5 2.99 × 10-5

elastic energy per mole of ions (J/mol) 73.7 93.4 143

model, and the calculated energies are in good agreement with previously published experimental data. Acknowledgment. L.F.V., E.M.L., and F.C.B. acknowledge fellowships from CAPES, Fapesp, and CNPq. The MatLab code was written by Renato Galembeck, and the styrene-acrylic latex was synthesized by Mauro M. Murakami. This is a contribution from the Millenium Institute for Complex Materials, PADCT/ CNPq. Supporting Information Available: Micrographs of polymer-Ca-MMT cluster and modeling programs, including (a) code for writing the matrices representing point charges and their geometrical distribution and (b) code for calculating electrostatic energy for the set of point-charge matrices written in code a. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) McQuarrie, D. A.; Simon, J. D. Molecular Thermodynamics; University Science Books: Sausalito, CA, 1999; Chapters 10 and 11. (2) Corti, H. R.; De Pablo, J. J.; Prausnitz, J. M. J. Phys. Chem. 1990, 94, 7876. (3) Flory, P. J. J. Chem. Phys. 1945, 13, 453. (4) Vichi, F. M.; Galembeck, F.; Halstead, T. K.; Williams, M. A. K. J. Appl. Polym. Sci. 1999, 74, 2660. (5) Gradzielski, M.; Hoffmann, H.; Langevin, D. J. Phys. Chem. 1995, 99, 12612. (6) Landry, C. J. T.; Coltrain, B. K.; Teegarden, D. M.; Long, T. E.; Long, V. K. Macromolecules 1996, 29, 4712. (7) Amalvy, J. I.; Percy, M. J.; Armes, S. P.; Leite, C. A. P.; Galembeck, F. Langmuir 2005, 21, 1175. (8) Ni, K. F.; Shan, G. R.; Weng, Z. X.; Sheibat-Othman, N.; Fevotte, G.; Lefebvre, F.; Bourgeat-Lami, E. Macromolecules 2005, 38, 7321. (9) Tissot, I.; Reymond, J. P.; Lefebvre, F.; Bourgeat-Lami, E. Chem. Mater. 2002, 14, 1325. (10) Graf, C.; Vossen, D. L. J.; Imhof, A.; van Blaaderen, A. Langmuir 2003, 19, 6693. (11) Peng, C. C.; Gopfert, A.; Drechsler, M.; Abetz, V. Polym. AdV. Technol. 2005, 16, 770. (12) Hussain, F.; Hojjati, M.; Okamoto, M.; Gorga, R. E. J. Comp. Mater. 2006, 40, 1511. (13) Okamoto, M. Mater. Sci. Technol. 2006, 22, 756. (14) Pomogailo, A. D. Inorg. Mater. 2005, 41, S47. (15) Usuki, A.; Kawasumi, M.; Kojima, Y.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Mater. Res. 1993, 8, 1174. (16) Usuki, A.; Kojima, Y.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.; Kamigaito, O. J. Mater. Res. 1993, 8, 1179. (17) Giannelis, E. P. AdV. Mater. 1996, 8, 29. (18) Vaia, R. A.; Giannelis, E. P. Macromolecules 1997, 30, 8000. (19) Vu, Y. T.; Mark, J. E.; Pham, L. H.; Engelhardt, M. J. Appl. Polym. Sci. 2001, 82, 1391. (20) Valadares, L. F.; Leite, C. A. P.; Galembeck, F. Polymer 2006, 47, 672. (21) Norrish, K. Discuss. Faraday Soc. 1954, 18, 120. (22) Bragança, F. D.; Valadares, L. F.; Leite, C. A. D.; Galembeck, F. Chem. Mater. 2007, 19, 3334. (23) Rabinovich, Y. I.; Esayanur, M. S.; Moudgil, B. M. Langmuir 2005, 21, 10992. (24) Hunter, R. W. Foundations of Colloid Science; Oxford University Press: New York, 1986; Vol. 1, Chapter 6. (25) Keslarek, A. J.; Costa, C. A. R.; Galembeck, F. Langmuir 2001, 17, 7886. (26) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62.

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