Role of the Surface State and Structural Feature in the

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Chem. Mater. 2004, 16, 3995-4004

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Articles Role of the Surface State and Structural Feature in the Thermoreversible Sol-Gel Transition of a Zirconyl Aqueous Precursor Modified by Sulfuric Acid L. A. Chiavacci,†,‡ C. V. Santilli,*,† S. H. Pulcinelli,† C. Bourgaux,‡ and V. Briois‡ Institute of Chemistry, UNESP, P.O. Box 355, 14801-970 Araraquara, SP, Brazil, and LURE, UPS, Baˆ t. 209D, BP34, 91898 Orsay Cedex, France Received December 2, 2003. Revised Manuscript Received June 22, 2004

The sols produced by admixture of ZrOCl2 acidified solutions to hot H2SO4 aqueous solutions were studied to clarify the effects of Cl- and SO42- ions on the kinetic stability of nanoparticles and to obtain some new evidence concerning the mechanism of a thermoreversible sol-gel transition observed in this system. The study of suspensions prepared with different molar ratios RS ) [Zr]/[SO42-] and RCl ) [Zr]/[Cl-] revealed domains of composition of formation of thermoreversible gels, thermostable sols, and powder precipitation. The effects of RS and RCl on the structural features of nanoparticles and on the particlesolution interface were systematically analyzed for samples of thermoreversible and thermostable sol domains. Small-angle X-ray scattering measurements revealed the presence of small fractal aggregates in all samples of thermoreversible domains, while compact packing aggregates of primary particles are present in the thermostable sol. Extended X-ray absorption fine structure and elemental chemical analysis revealed that irrespective of the nominal value of RS and RCl all studied samples of the thermoreversible domain are constituted by a well-defined compound possessing an inner core made of hydroxyl and oxo groups bridging together zirconium atoms surrounded on the surface by complexing sulfate ligands. ζ potentials of powders extracted by freeze-drying from the thermoreversible gel revealed a point of surface charge inversion attributed to the specific adsorption of SO42ion. Thermoreversible gel formation is rationalized by considering the effect of the specific adsorption on the electrical double-layer repulsion together with the temperature dependency of the physical chemical properties of ions in solution.

1. Introduction The appeal of intelligent materials showing a reversible change between fluidlike and solidlike states by the application of external stimuli has grown during the past few years.1 Typical examples are the electro- and magnetorheological suspensions, in which the applied electric or magnetic fields are used to alternate the properties of materials between the viscous behavior of a fluid and the elasticity of a solid.2 These kinds of materials have various applications such as in clutches, valves, shock absorbers, and variable catheters.3,4 Similar applications could be expected for some polymer solutions exhibiting a reversible sol-gel transition induced by a controlled change of temperature. Familiar polymer-solvent systems presenting this thermorevers* To whom correspondence should be addressed. E-mail: santilli@ iq.unesp.br. † UNESP. ‡ UPS. (1) Ball, P. Nature 1998, 391, 232. (2) Martin, J. E. Phys. Rev. E 2001, 6301, 1406. (3) Kim, J. W.; Choi, H. J.; Lee, H. G.; Choi, S. B. J. Ind. Eng. Chem. 2001, 7, 218. (4) Rogers, C. A. Scientific American 1995, September, 154.

ible sol-gel transition are poly(vinyl alcohol) in water, gelatin in water, and poly(vinyl chloride) in dibutyl phthalate.5,6 In these systems, some properties such as refractive index, ionic conductivity, rheological properties, volume, and geometric shape present reversible changes during the sol-gel-sol transition. It is generally agreed that thermoreversible sol-gel transitions in polymer-solvent systems should proceed via a firstorder phase transition,6 arising from the creation of a minimum molecular order out of the initial disordered solution. The network formation and the subsequent gelation can be induced either by crystallization or by association of macromolecules, due to the occurrence of cooperative van der Waals forces or hydrogen bonds of energy on the order of kT.6 Despite the growing interest in the use of sol-gel processes for the syntheses of glasses, ceramics, and hybrid materials,7 only two papers report the thermo(5) Keller, A. Faraday Discuss. 1995, 101, 1. (6) Guenet, J. M. Thermoreversible Gelation of Polymers and Biopolymers; Academic Press: London, 1992. (7) Brinker, C. J.; Scherer, G. W. Sol-gel Science; Academic Press: San Diego, 1990.

10.1021/cm035258d CCC: $27.50 © 2004 American Chemical Society Published on Web 09/23/2004

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reversible sol-gel transition in inorganic systems.8,9 They deal with aluminum polyphosphate8 in water and chelated titania in isopropyl alcohol.9 Recently, we have reported a thermoreversible sol-gel transition in zirconyl chloride aqueous solution modified by sulfuric acid.10-12 The structural analysis of this system showed that the sol and the gel are both formed by polynuclear species such as the known Zr18O4(OH)38.8(OSO3)12.6‚ 33H2O compound,13 evidencing that the thermoreversible gelation is not induced by crystallization. Thus, we have proposed that the particle-particle association induces the thermoreversible gelation. Furthermore, the characteristic temperature at which the thermoreversible gelation occurs strongly depends on the Zr/SO42molar ratio (RS) and/or the Zr/Cl- molar ratio (RCl),11,12 but the role of these anions in the thermoreversible solgel transition mechanism has not yet been elucidated. It is well-known that zirconium exhibits pronounced polyoxo ion chemistry in aqueous solution, and depending on the acidity and on the complexing ability of the ligands, several compounds can be formed.14,15 On one side, sulfate groups show quite high complexing ability and are thus able to replace coordinated water molecules. Sulfate molecules can behave either like capping ligands of discrete molecules as in Zr18O4(OH)38.8(OSO3)12.6‚33H2O or like three-dimensional network formers, bridging together infinite chains, e.g., in Zr(OH)2SO4.13-15 On the other side, the chloride ion has a low complexing ability and is able to replace the hydroxyl present in basic zirconium sulfate compounds, leading to the depolymerization of polyoxo species.14 However, besides these two main opposite effects of Cl- and SO42- (network former/depolymerization), other phenomena may occur, inducing the thermoreversible sol-gel transition which is dependent on the specific features of both the polymeric species and the ions in the aqueous solution. The aim of the present work is to obtain some new evidence concerning the mechanism of the thermoreversible sol-gel transition discussed above by studying the structural aspects and the surface features of samples prepared with different molar ratios, RS) [Zr]/[SO42-] and RCl ) [Zr]/[Cl-]. 2. Experimental Section 2.1. Sample Preparation. Suspensions were prepared by the procedure described previously,10,11 essentially consisting of ZrOCl2‚8H2O powder (Aldrich) dissolution in hydrochloric aqueous solution (2 mol‚L-1) followed by the addition to a hot (80°C) aqueous sulfuric acid solution (0.2 mol‚L-1), drop by drop, under magnetic stirring. The molar ratios RS and RCl for each sample were systematically varied by mixing appropriate amounts of zirconyl chloride and acid solutions. (8) Lima, E. C.; Galembeck, F. J. Colloid Interface Sci. 1994, 166, 309. (9) Haridas, M. M.; Datta, S.; Bellare, J. R. Ceram. Int. 1999, 25, 601. (10) Chiavacci, L. A.; Pulcinelli, S. H.; Santilli, C. V.; Briois, V. Chem. Mater. 1998, 10, 986. (11) Chiavacci, L. A.; Bourgaux, C.; Briois, V.; Pulcinelli, S. H.; Santilli, C. V. J. Appl. Crystallogr. 2000, 33, 592. (12) Chiavacci, L A.; Pulcinelli, S. H.; Santilli, C. V.; Briois, V. In Ceramics: Getting into the 2000’s; Vicenzini, Ed.; Techna: Faenza, 1999; Part C, pp 191-198. (13) Squattrito, P. J.; Rudolf, P. R.; Clearfield, A. Inorg. Chem. 1987, 26, 4240. (14) Clearfield, A. Rev. Pure Appl. Chem. 1964, 14, 91. (15) Jolivet, J. P. De la Solution a` l’Oxide, InterE Ä ditions: Paris, 1994; Chapter 5.

Chiavacci et al. Aliquots of 15 mL of the so-prepared solutions were put inside acetylcellulose membrane tubing (12-14000 MW), and then submitted to static dialysis against 200 mL of twice distilled water at 25 °C, for 24 h, to obtain transparent suspensions or precipitates at pH 1.5-1.7, containing 0.16 mol‚L-1 zirconium. Information concerning the composition of the systems was obtained from chemical analyses of samples freeze-dried at -5 °C, under a 10-3 mm Hg vacuum. The Zr content was determined by ICP/AES (inductive conduction plasma coupled to atomic emission spectroscopy), whereas the S and Cl contents were determined by infrared detection during pyrolysis. 2.2. Sample Characterization. 2.2.1. Light and X-ray Scattering Measurements. The gel formation temperatures were determined by measuring the onset of turbidity in the solutions during heating from 20 to 95 °C at 0.5 °C‚min-1. The turbidity was monitored measuring the light scattered at 90° out, using a Del Lab Nefelometer equipped with a thermostated cell and tungsten polychromatic radiation. The effective hydrodynamic size of the particles was estimated from quasi-elastic light scattering (QELS) measurements carried out in situ during gel formation and gel liquefaction upon cooling at 25 °C. The measurements were carried out with a PSC100 photocorrelator (Brookhaven Instruments Corp.) using a solid-state laser (25 mW) of 532 nm wavelength. The gelation temperature was chosen as a function of the onset of turbidity during heating of the solutions: 70 °C for suspensions with RS ) 3 and 50 °C for solutions with RS ) 2.5. The nanostructural features of sols at 25 °C were analyzed by small-angle X-ray scattering (SAXS) measurements performed at the D24 SAXS beamline of the synchrotron X-ray source DCI at LURE (Orsay, France). The station is equipped with a bent germanium (111) crystal monochromator, which yields a monochromatic (λ ) 1.4956 Å) and horizontally focused beam. A set of slits defines the beam vertically. An ionization chamber placed downstream from the sample was used to monitor the intensity of the incident beam and to determine the sample transmission. A vertical and linear positionsensitive X-ray detector was used to record the scattered intensity (I(q)). Because of the small size of the incident beam cross-section at the detection plane, no mathematical desmearing of the experimental function was needed. The scattered intensity was determined as the difference between the experimental curves recorded for the sample and the parasitic scattering curve obtained for the solvent (water) taking into account the transmission. Two independent experiments with different sample-detector distances (1750 and 730 mm) were performed to obtain information on a large q range (0.008 < q < 0.2 Å-1 and 0.02 < q < 0.5 Å-1, respectively); q ) (4π sin θ)/λ, where 2θ is the scattering angle and λ the wavelength. The whole curve is the result of the overlap of curves obtained at each distance. 2.2.2. Extended X-ray Absorption Fine Structure (EXAFS) Experiments and Data Analyses. The EXAFS data at the Zr K-edge (17998 eV) were collected at 25 °C on the EXAFS IV spectrometer at LURE, using the DCI storage ring. The monochromator was a Si(311) double crystal, and the experiment was performed in the transmission mode using two ionization chambers filled with argon. The EXAFS spectra were recorded within a 1000 eV energy range with 2 eV steps and 3 s accumulation times per energy. The sol was put in a liquid cell with optical path x optimized to obtain an edge jump such that µx ≈ 1. All the EXAFS spectra were treated within the classical single-scattering approximation16 using the chain of programs developed by Michalowicz.17 After atomic absorption removal and normalization, the k3χ(k) weighted EXAFS signal was Fourier transformed to d distance space, using the 3.3-14.5 Å-1 Kaiser apodization window with τ ) 2.5. The contributions (16) Teo, B. K. EXAFS: Basic Principles and Data Analysis; Springer-Verlag: Berlin, 1986. (17) Michalowicz, A. EXAFS pour le Mac, Logiciels pour la Chimie; Socie´te´ Franc¸ aise de Chimie: Paris, 1991; p 102.

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of the various shells of neighbors were extracted by a back Fourier transform in d space and then fitted using experimental phase and amplitude functions. So the phase shifts for the Zr-O pair and for the Zr-S pair and the oxygen and sulfur backscattering amplitudes were extracted from the Zr(SO4)2‚ 4H2O reference,18 backtransforming the first and second peaks, respectively, of its Fourier transform (FT) and setting N ) 8, d ) 2.18 Å, and σ ) 0.065 Å (for the Zr-O pair) and N ) 4.0, d ) 3.56 Å, and σ ) 0.11 Å (for the Zr-S pair). N, d, and σ are the coordination number, interatomic distance, and DebyeWaller factor corresponding to the considered coordination shell. For the Zr-Zr pair, the phase shift and backscattering amplitude were extracted from a well-crystallized tetragonal zirconia (t-ZrO2) powdered sample, setting N ) 12, d ) 3.60 Å, and σ ) 0.07 Å. The Debye-Waller factors reported above for Zr(SO4)2‚4H2O and t-ZrO2 were determined by fitting first the filtered signal using the theoretical functions obtained by the FeFF7 code.19 This approach allows us to obtain the experimental amplitude functions not yet convoluted with the Debye-Waller term but still convoluted with the photoelectron mean free path. The structural parameters related to each shell were obtained using a least-squares fitting procedure by keeping fixed the electronic parameters, Γ, related to the mean free path (λk ) Γ) of the photoelectron and E0, the threshold energy related to k (k ) [2m/p2(E-E0)]1/2), at the values determined previously for the references for each contribution. The fitting procedure was performed in two steps: first, the Zr-O contribution was simulated, and then the Zr-O, Zr-Zr, and Zr-S contributions were fitted. To avoid correlation between the oxygen coordination number and the number of second nearest neighbors, the structural parameters related to the oxygen contribution were kept at the values obtained in the first step, only the structural parameters for the Zr-Zr and Zr-S contribution being simulated. The quality of the fit was determined by the agreement factor F:

∑(χ

i

exptl

F)

(ki) - χcalcdi(ki))2

i

∑(χ

(1) i exptl

(ki))

2

i

2.2.3. Electrophoretic Mobility and Anion Concentration. Information on the particle surface charge and ion adsorption process was obtained from electrophoretic mobility and free anion concentration measurements carried out with a ZetaPals Analyzer (Brookhaven Instruments) and an ionic exchange chromatograph (Metrohn), respectively. Electrophoretic values were automatically converted to ζ potential values by the analyzer using the Smoluchowski equation.20 The ζ potential determined by this experiment is the electrical potential at the rather ill-defined surface shear between the charged particle surface and the aqueous dispersion medium.20 In a first set of experiments the ionic strength was kept constant by dispersing the freeze-dried powder in a 0.01 mol‚L-1 NaCl aqueous solution and the pH of dispersion adjusted with NaOH and HCl aqueous solutions. In a second set of experiments the freeze-dried powder was dispersed in water acidified to pH 1.5 with HNO3 and the ionic strength adjusted with NaNO3. Prior to the measurements, all suspensions were kept for 18 h under magnetic stirring at room temperature (25 °C) or above the sol-gel transition temperature (80 °C). All reported results correspond to measurements carried out at 25 °C. 2.2.4. Raman Spectroscopy. The Raman spectra of a thermoreversible sol was measured in situ during sol-gel-sol transitions during a cyclic variation of temperature (25-7525 °C) using a commercial Raman spectrometer (model HL5R (18) Singer, J.; Cromer, D. T. Acta Crystallogr. 1959, 12, 719. (19) Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J. Phys. Rev. B 1995, 52, 2995. (20) Hunter, R. J. Zeta potential in colloid science: Principles and applications; Academic Press: London, 1981; p 465.

of Kaiser Optical Systems, Inc.) equipped with a near-IR laser diode working at 785 nm, operating with 50 mW power at the sample for excitation. The average resolution is equal to 2 cm-1.

3. Results 3.1. Sol and Gel Formation Diagram. The gel formation diagram was obtained by visual observation of dialyzed solutions (pH 1.5-1.7) prepared with different RS and RCl values. The admixture of ZrOCl2HCl (25 °C) and H2SO4 (80 °C) aqueous solutions can produce four types of systems depending on the RS and RCl used according to the following patterns: (i) The mixture stays transparent and precipitation occurs after dialysis. (ii) The solution becomes opaque during mixing, and the particles settle during dialysis. (iii) The solution becomes opaque during mixing, and a redispersion of the precipitate occurs during dialysis, leading to a clear and transparent sol; however, it becomes turbid upon heating and is then transformed into a gel. When cooled back to room temperature, the gel spontaneously liquefies. (iv) As in the previous case the redispersion is observed but the system stays as a clear liquid upon heating. In this case irreversible gelation occurs at pH ≈ 2.7 upon addition of base. The zirconium (χZr ) nZr/(nZr + nCl + nSO4)), chloride (χCl ) nCl/(nZr + nCl + nSO4)), and sulfate (χS ) nSO4/(nZr + nCl + nSO4)) molar fraction regions in which the abovedescribed behaviors are observed are rather well defined and are depicted in Figure 1. The RS molar ratios in the initial mixture are almost preserved in the freezedried powder, while the chloride content decreases after dialysis. Thus, we have used to build this diagram the nominal amount of Zr salt and H2SO4(aq) initially added, while for the chloride we considered the content in the dialyzed solution. The RS ratio at which thermoreversible sol-gel transition (domain C) is observed is in the range =2.5-5. At higher and lower ratios a thermostable sol (domain D) and decanted precipitate (domains A and B) are formed, respectively. The diagram for gel formation when sulfuric acid is substituted by phosphoric acid, acetic acid, or nitric acid is similar to that given in Figure 1; however, the domain C corresponding to the thermoreversible sol-gel transition is not observed. These results indicate that the presence of sulfate ions in the reaction bath plays a key role in the formation of thermoreversible gels. To verify the effect of sulfate and chloride ions on the thermoreversible gel formation, selected samples of domains C and D were systematically studied. 3.2. Temperature of Gel Formation. The temperature of gel formation was determined as a function of RS and RCl, measuring the evolution of the turbidity during heating at 0.5 °C‚min-1. Figure 2a presents the effect of RS on the evolution of the turbidity for samples of domain C prepared with 2.5 e RS < 5 and RCl = 3.7 and for sols of domain D (RS > 5). The turbidities of samples with RS > 5 are almost invariant, showing that these suspensions are stable irrespective of temperature. In the case of samples with 2.5 e RS < 5, the turbidity increases as the temperature reaches a peculiar value, Tc. The onset of turbidity can be used to estimate the gelation temperature, Tc. Decreasing RS from 4 to 3 leads to a faint increase of Tc, while a

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Figure 1. Sol and gel formation diagram for Zr-SO42--Cl- molar fractions: domain A (O), precipitation occurs after dialysis of a clear sol; domain B (b), sedimentation from a turbid sol; domain C (+), thermoreversible gel formation; domain D (f), a thermostable clear sol. The compositions corresponding to the points assigned by numbers were selected to carry this study out. The domain borderlines are drawn as a guide to the eyes.

pronounced reduction of Tc occurs when RS changes from 3 to 2.5. Nevertheless, a more complex behavior, displayed in Figure 2b, is observed by maintaining RS at =3 and varying RCl. The lower value of Tc is verified for the sample with RCl = 0.75, while a faint variation of Tc is verified at higher and lower ratios. Furthermore, the curves of the sample with RCl * 0.75 present a pronounced minimum of turbidity near Tc, suggesting the occurrence of opposite phenomena such as dissolution S precipitation, depolymerization S polymerization, and redispersion S aggregation. This minimum near Tc is also verified for samples prepared with RS ) 4.0, despite the fact that it is not observed for that prepared with RS ) 2.5 (see Figure 2a). The distinct behavior observed as a function of RS and RCl indicates that sulfate and chloride ions play different roles in the stability of suspensions toward the thermoreversible gelation. The observed increase in the turbidity during heating can be due to the increase of both the number and size of the macromolecules or aggregates forming the gel. To verify the contribution of the latter effect, the effective hydrodynamic diameter of the particles was measured by QELS during isothermal gelation near Tc and subsequent liquefaction of the gel cooled back to 25 °C. The results related to the sample with RS ) 2.5 (Tc = 50 °C) and RS ) 3.0 (Tc = 70 °C) and RCl fixed at 0.75 are shown in Figure 3. Upon heating the sample for 100 min under these isothermal conditions the effective hydrodynamic size increases promptly from 15 ( 5 to 1300 ( 100 nm. The initial size of the particle is recovered during the subsequent cooling back to 25 °C.

This behavior indicates that the thermoreversible solgel transition in this system is due to the reversibility of the aggregation process. 3.3. Structural Aspects of the Sol. The local arrangement around Zr was analyzed in detail by EXAFS. Figure 4 shows the Fourier transform (FT) of EXAFS spectra for selected as sols of domain C with RS ) 2.5 and 3 (RCl ) 3.75) and of domain D with RS ) 15 (RCl ) 0.83). The FTs are characterized by two peaks; the first one (labeled A) is related to the oxygen coordination shell as nearest neighbors, and the second one (labeled B) to zirconium and sulfur contributions at larger distances. The invariance of the shape of the FT observed for the thermoreversible sols indicates that the local order around Zr is not strongly affected by the sulfate contents. The comparison between the thermoreversible and the nonthermoreversible samples, however, shows a change of the peak intensity, in particular for peak B. As reported in ref 10 the contribution of peak A is satisfactorily reproduced by using a two-shell fitting procedure with Zr-O distances centered around 2.14 and 2.27 Å. The simulation of peak B was also achieved by a two-shell fitting procedure, one related to zirconium atoms and the second to sulfur atoms due to the complexation of zirconium by sulfate ligands. This modelization including S atoms was not used in the previous work,10 but the comparison of peak B of the FTs reported for a sample free of sulfate and that for samples of domains C and D clearly probes the necessity to take into account this contribution to perform a more accurate fit.

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Figure 4. Amplitude of the Fourier transforms of EXAFS signals for thermoreversible samples (RS ) 2.5 and 3.0, both with RCl ) 3.75) and for the thermostable sample with RS ) 15 and RCl ) 0.83. Table 1. Structural Parameters Derived from EXAFS for Thermoreversible (RS ) 2.5 and 3.0) and Thermostable (RS ) 15) Samples RS

RCl NZr-O

2.5 3.75 2.5 0.75 3.0 3.75 3.0 0.75 15

Figure 2. Effect of the molar ratios (a) RS (for RCl = 3.75) and (b) RCl (for RS ) 3) on the evolution of the turbidity (ftu ) formazide turbidity unit) during heating of the sol at 0.5 °C‚min-1.

0.83

Structural parameters derived from peaks A and B analyses are gathered in Table 1. The local order of samples belonging to the same region of the sol formation diagram (Figure 1) is similar, whatever the sulfate and chloride contents. The thermostable sols present a more ordered first coordination shell than the thermoreversible ones, as evidenced by the decrease of the Debye-Waller factors. Besides this reduction of struc-

2.12 2.26 2.12 2.26 2.13 2.25 2.13 2.25 2.12 2.25

0.06 0.07 0.05 0.07 0.05 0.07 0.05 0.07 0.04 0.05

7.3

3.58

0.11

3.5

3.59 0.19

6.1

3.57

0.10

2.4

3.60 0.19

7.2

3.58

0.11

3.9

3.59 0.19

6.6

3.59

0.10

4.1

3.60 0.19

6.7

3.62

0.11

5.9

3.59 0.19

Table 2. RS and RCl Determined from the Nominal Composition of Solutions and from Elemental Analysis of Freeze-Dried Solids domain

RS(solution)

RCl(solution)

RS(solid)

RCl(solid)

C

2.5 3.0 3.0 3.0 8.0 15

3.75 0.14 0.46 0.75 11.4 0.83

2.6 3.0 3.0 3.0 6.0 14

5.25 2.83 2.65 2.88 1.51 1.16

D

Figure 3. Evolution of the effective hydrodynamic size of particles during an isothermal sol-gel-sol transition at 50 and 70 °C for samples with RS ) 2.5 and 3.0, respectively, followed by liquefaction at 25 °C.

3.6 3.5 3.2 3.4 3.2 3.7 3.3 3.7 3.4 3.6

dZr-O σZr-O dZr-Zr σZr-Zr dZr-S σZr-S (Å) (Å) NZr-Zr (Å) (Å) NZr-S (Å) (Å)

tural disorder, we also observe a lengthening of the average Zr-Zr distance from 3.58 ( 0.02 to 3.62 ( 0.02 Å. Furthermore, irrespective of the nominal compositions of the solutions, chemical analysis performed on freeze-dried powders (Table 2) shows that the majority of powders of domain C have about the same molar ratio: RS ) Zr/SO42- ) 3 and RCl ) Zr/Cl- ) 3. On the other hand, the molar ratio of freeze-dried samples of domain D changes with the nominal composition of the initial solution. These structural and chemical results indicate that the same defined compound is formed within the domain of thermoreversible solutions (domain C). This compound already studied10 for solutions with RS ) 3 and RCl ) 0.75 was identified as an analogue of the basic zirconium sulfate Zr18O4(OH)38.8(OSO3)12.6‚33H2O,13 which can be described as a folded sheet of zirconium atoms linked via oxygen bridges capped on the surface by the sulfate ligands. Relevant nanostructural features of suspensions were revealed by SAXS measurements. Figure 5 shows the log-log plot of the SAXS intensity I(q) as a function of the modulus of the scattering vector q for typical

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samples of domains C (RS ) 2.5 and 3, RCl ) 0.75) and D (RS ) 15, RCl ) 0.83). Curves related to thermoreversible suspensions present two power law regimes, i.e., fractal11 and Porod regimes at intermediate and large q values (q > 0.3 Å-1), respectively. The constancy of the crossover between fractal and Porod regimes (q ≈ 0.3 Å-1) indicates that the size of the primary particles (a) is invariant with the composition in sulfate and equals ∼6 Å. Furthermore, the slope of -4 observed in the q range higher than 0.3 Å-1 indicates that the primary particles present a smooth and well-defined surface.7,21 At low q range (q < 0.03 Å-1) a third regime is observed, in which the scattering intensity is almost constant, characterizing noninteracting scatters. In this case we can assume a system formed of isolated fractal aggregates embedded in a matrix of constant electronic density, in which the scattered intensity I(q) is usually approximated by the product21

I(q) ) NP(q) S(q)

(2)

where N is the number of scattering objects per unit volume, P(q) is the scattering function of a single isolated particle, and S(q) is the structure function that accounts for the short-range spatial correlation between the particles. A particular case of a correlated system is that in which primary particles build up fractal objects. The structure function of this system is given by the Teixeira equation:22

{

S(q) ) C 1 +

DΓ(D - 1) D

(qa) [1 + (1/q2ξ2)](D-1)/2 sin[(D - 1) tan-1(qξ)]

}

(3)

in which C is a constant, D is the fractal dimension, ξ is the correlation length of fractal objects, a is the size of the primary particles, and Γ is the Gamma-function. Assuming a , ξ, the single-particle scattering function, P(q), can be considered as a constant. As shown in Figure 5, the theoretical structure functions given by eq 3, and displayed by continuous lines, fit very well the experimental data corresponding to thermoreversible samples (domain C). The fitting of eq 3 to the experimental SAXS curve yielded the values of ξ and D. The values of the correlation length (ξ) are 30 and 25 Å for samples with RS )2.5 and 3, respectively. The fractal dimensionality (D) is equal to 2.2 whatever the molar ratios RS and RCl. On the other hand, the values of the correlation length for a given RS increase from 17 to 37 Å when the RCl ratio decreases from 3.7 to 0.14. This feature suggests that the growth of the fractal aggregate is hindered by the presence of Cl- ions and favored by the increase of SO42- content. For thermostable suspensions of domain D, the SAXS curves exhibit a peak characterized by a decrease of the scattering intensity at low q values. As the q value of the maximum shifts to smaller values as the global zirconium concentration decreases, we attributed this peak to an interference effect in the X-ray scattering features due to the presence of spatially correlated scatters. To determine the effective structure function (21) Craievich, A. F. Mater. Res. 2002, 1, 1. (22) Teixeira, J. J. Appl. Crystallogr. 1988, 21, 781.

Figure 5. log-log plot of the experimental SAXS curves (symbols) and fitting for the structure functions given by eq 3 (thin continuous lines) for thermoreversible samples with RS ) 3.0 and 2.5 and the thermostable one with RS ) 15. The inset shows the structure function determined from experimental SAXS curves corresponding to thermostable samples prepared with different RS values and RCl fixed at 0.83.

from the experimental I(q) (S(q) ) I(q)/NP(q), eq 2), the single-particle scattering function, P(q), needs to be known. Due to the rather isotropic shape of the oligomeric species identified previously as the primary particles forming the colloidal suspensions,10 we have assumed for P(q) the theoretical function of monodispersed spherical particles.21 In the high q range the interference effects are negligible, so S(q) ≈ 1 and then I(q) ∝ P(q). Therefore, in the large q range the theoretical function P(q) was directly fit to the experimental I(q) curve. For all suspensions of domain D the best fitting was achieved by adjusting the radius of the spherical particles at ∼7 Å. Thus, the structure function S(q) was calculated from eq 2, and the curves are shown in the inset of Figure 5. The S(q) function exhibits a broad peak in which the position of the maximum corresponds to the most probable pair correlation between particles in reciprocal space. Taking into account the peak position and the volumetric fraction of particles in suspension, the radius of scatters was estimated at about 12 Å, indicating that the suspensions are formed by aggregates of small primary particles (radius of ∼7 Å). The radius of the primary particles is in reasonable agreement with the radius estimated from the unit cell parameters (C66-P63, a ) b ) 33.770 Å, c ) 15.522 Å, number of molecules by unit cell 613) of the Zr18O4(OH)38.8(OSO3)12.6‚33H2O molecule. 3.4. Interfacial Properties. The results reported above do not exhibit remarkable differences in the shortrange structure of samples belonging to the thermoreversible and thermostable domains, suggesting that the different behaviors dealing with gelation are related to surface properties. Unfortunately, measurements of

Sol-Gel Transition of a Zirconyl Aqueous Precursor

the ζ potential during heating of the as-prepared suspensions were hindered by the etching of the Pd electrodes observed above 50 °C due to the high acidity of the medium. Consequently, measurements were carried out at room temperature (25 °C) using the freeze-dried powder redispersed in NaCl aqueous solution. The measurements of the ζ potential as a function of pH have shown a dependence of the curves on the ionic strength (NaCl content). The isoelectric point (iep) is shifted to lower pH values, typically from 6 to 5 as the NaCl concentration increases from 0.005 to 0.05 mol‚L-1. The absence of a common intersection point in ζ×pH curves indicates specific adsorption effects of this electrolyte. Furthermore, we observed a release of chloride and sulfate ions as the freeze-dried powders were redispersed in acidified Milli-Q water (pH 1.6). Then, we carried out ζ potential measurements at constant pH (near the value of the as-prepared suspension, pH 1.6) as a function of the ionic strength monitored by the addition of NaNO3. This electrolyte is well-known to be a nonactive electrolyte for ZrO2.23 Thus we assume that the specific adsorption of Na+ and NO3- ions on the surface of Zr18O4(OH)38.8(OSO3)12.6‚33H2O-like primary particles does not occur. The amounts of free chloride and sulfate ions in solution were measured after the equilibrium (18 h) at 25 and 80 °C. Parts a-c of Figure 6 display the evolution of the ζ potential, and the concentration of free sulfate and chloride ions in solution before and after heating as a function of the ionic strength (I) for redispersed powders corresponding to the thermoreversible and thermostable samples, respectively. For clarity we present first the results obtained before heating for each sample. For the thermostable sample (RS ) 15, RCl ) 0.83), there is a small decrease of the ζ potential as a function of the ionic strength. Although this behavior is expected for nonactive electrolytes, the concentrations of free sulfate and chloride ions decrease as the nominal ionic strength increases until 0.02 mol‚L-1. The concentration of chloride ions in high ionic strength solution (>0.02 mol‚L-1) is 20 times higher than that of sulfate ions. This high capacity of chloride release is in good agreement with the nominal molar ratio [Cl]/[SO4] ) 21 used in the synthesis of this thermostable sol. This feature suggests that Cl- species are essentially not chemically bonded to zirconium atoms, but adsorbed in the precipitate. For the thermoreversible sample (RS ) 2.5), a decrease of the ζ potential with a reversion of charge from positive to negative (RC+/-) as the ionic strength increases is observed. The RC+/- occurs for ionic strength near 0.03 mol‚L-1. At the negative region of the ζ potential (I > 0.03 mol‚L-1) the concentration of sulfate ions in solution decreases drastically, to values smaller than the detection limit, while that of chloride ions shows a small tendency to decrease. This indicates that the negative values of the ζ potential are associated with the specific adsorption of sulfate ions. At the positive region of the ζ potential (I < 0.03 mol‚L-1) the dependence of the concentration of anionic species in solution on the ionic strength is complex, showing an out of phase wave trend, so that, as the sulfate concentration (23) Franks, G. V.; Johnson, S. B. Scales, P. J.; Boger, D. V.; Healy, T. W. Langmuir 1999, 15, 4411.

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Figure 6. Effect of ionic strength on the (a) ζ potential and concentration of sulfate and chloride ions released in solution after equilibrium at 25 and 80 °C for the (b) thermoreversible samples (RS ) 2.5 and 3.0, both with RCl ) 3.75) and (c) thermostable sample (RS ) 15 and RCl ) 0.83). The arrows in (a) indicate schematically the position of samples prepared with different RCl values.

increases that of chloride decreases, and vice versa. This indicates that the adsorption-desorption equilibrium of one anionic species is affected by the concentration of the other anion in solution that can be associated with an ionic exchange process between Cl- and SO42species. A similar wave trend is observed after heating at 80 °C. However, in the low ionic strength solution (I < 0.03 mol‚L-1) the heating at 80 °C favors the sulfate adsorption and the release of chloride ions, and the opposite behavior is verified for I > 0.03 mol‚L-1. An analogous behavior was observed for the sample with RS ) 3, but its overall dependence on the ionic strength is less abrupt, leading to an RC+/- for I = 0.2 mol‚L-1 (not shown). The above results are not directly correlated to the thermoreversible behavior since it deals with the interfacial characteristics of redispersed powders at room

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Figure 7. Raman spectra corresponding to the thermostable sample (25 °C) and thermoreversible sample (RS ) 3.0 and RCl ) 3.75) measured at 25 °C and during heating at 50 °C and cooling back to 25 °C. For comparison purposes, the intensity of the room-temperature spectrum of the thermostable sample was multiplied by a factor of 5.

temperature. But we assume that similar anionic adsorption-desorption and ionic exchange phenomena could occur by increasing the temperature of the asprepared thermoreversible sol. To check this hypothesis, we carried out in situ Raman spectroscopy measurements during heating and cooling of the as-prepared thermoreversible and thermostable sols. The corresponding Raman spectra in the region of the sulfate anion, υ1 band, are shown in Figure 7. Room-temperature sols exhibit an intense narrow band centered at ∼1004 cm-1 corresponding to complexed sulfate anions.24 Besides the strong decrease of the band intensity for the thermostable sample compared to the thermoreversible one, indicating a lesser amount of total sulfate anions in the former, the main difference between both samples arises from the presence of an additional resolved shoulder at ∼982 cm-1 for the room-temperature thermoreversible sol. This shoulder is ascribed to free sulfate anion.24 During heating, this shoulder disappears and the overall band narrows; the reverse behavior is observed during cooling. These results demonstrate that the capacity of adsorption of sulfate ions increases in thermoreversible samples with heating and decreases with cooling. 4. Discussion Due to the structural and interfacial results presented above, we propose that the dual property of sulfate ions to change the charge density of the particle surface either by adsorption or by ion exchange is mainly responsible for the difference in the thermoinduced behavior observed for suspensions of domains C and D. We will first discuss the thermoreversible behavior of gelation in domain C and thermostability of domain D samples. Then, we will examine the evolution of thermoreversible gelation characteristics induced by RS and RCl changes. 4.1. Domain C. As evidenced by elemental chemical analysis of freeze-dried samples, powders with a welldefined composition can be formed in domain C provided (24) Rull, F. Ohtaki, H. Spectrochim. Acta, A 1997, 53, 643.

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that the initial nominal composition allows this stoichiometry to be satisfied, i.e, RCl e 0.75 and RS e 3. Thus, we assume that, as RCl and RS decrease, the particles are in equilibrium with an increasing content of free anions in solution. Taking into account the EXAFS results, we assume the structure of this compound analogous to that of the basic zirconium sulfate Zr18O4(OH)38.8(OSO3)12.6‚33H2O,13,10 so that the primary particle in the sol can be roughly modeled as a ZrO2like core surrounded at the surface by bridging hydroxyl and sulfate groups. On the basis of this structural model, the experimental data presented in Figure 6 can be explained as follows. In the highly acidic water solution, like that used to prepare the thermoreversible sol, the surface hydroxyl and SO42- groups react with H+, creating a positively charged surface. To maintain the system electroneutrality, a cloud of counterions (ions of charge opposite that of the surface) surrounds the particles. As particles are pushed together by the Brownian motion, the concentration of counterions increases in the space between particles, giving rise to a repulsive potential due to the osmotic pressure. This is known as the electrical double-layer repulsion.15,20 When the salt concentration is increased, the extent of the ionic cloud is reduced, allowing the particles to come closer before repulsion is observed; then the van der Waals attraction becomes dominant. When enough salt is added, the counterion cloud collapses to a finite minimum extent, determined in many cases by the charge-to-size ratio of the ion, and the particle surface charge is nearly neutralized by the thin cloud of counterions.15,20 These circumstances allow the particle surfaces to approach at very close distance; the gap between surfaces is filled with a very concentrated solution of counterion. Then both specific adsorption of counterions and physical aggregation can occur. The opposite change of the free chloride and sulfate ion amounts in solution verified as the addition of salt (NaNO3) increases until 0.03 mol‚L-1 (Figure 6b) indicates that the specific adsorption is followed by an ionic exchange between Cl- and SO42-. A further increase of the salt concentration led to complete neutralization of the positively charged surface and the RC+/-. The decrease of the sulfate ion amount in solution verified as the addition of salt (NaNO3) increases until 0.03 mol‚L-1 (Figure 6b) indicates that the SO42- ions are involved in this process of specific adsorption. To correlate the interfacial feature of particles discussed above with the thermoreversible sol-gel transition, it is necessary to take into account that the increase in temperature favors the increase of the capacity of SO42- specific adsorption, while that of Clions decreases (Figures 6 and 7). As evidenced by the ζ potential results, this increase in the SO42- adsorption capacity leads to a change in the magnitude of the surface charge and to an RC+/-. Near the RC+/- conditions the electrical double-layer repulsion potential is weak, allowing the close approach between particles, physical aggregation, and the consequent gelation. Till now we have been able to understand why the occurrence of the above phenomenon causes gelation, but the reasons why it occurs in a reversible way by cycling the temperature are not immediately apparent.

Sol-Gel Transition of a Zirconyl Aqueous Precursor

As the adsorption and ionic exchange phenomena are both dependent on the specificity of the adsorbentadsorbate couple, some possible variation of characteristic properties of particles and of ions in solution must be considered to explain the thermoreversible sol-gel transition. Previous work on zirconia powders23 dispersed in water solution with monovalent anions has shown that the strength of the particle network created by collapsing the electrical double layer with counterions follows the anti-Hofmeister sequence IO3- > BrO3- > Cl- > NO3- > ClO4-. Other properties of ionic solutions, such as conductivity, viscosity, diffusion, and adsorption of ions, also follow this (or the opposite) sequence.25 This sequence can be rationalized in terms of the effect of the electric field at the surface of the ions (determined mainly by the charge-to-size ratio) upon the structure of water in their sphere of influence. Ions with a large electric field at their surface tend to increase the structure of vicinal water (known as structure-makers), while ions with a small electric field at their surface tend to decrease the structure of vicinal water (structurebreaking).25 The structure-maker ions have a greater hydration enthalpy and a greater number of water molecules in their hydration shell and produce an aqueous solution with a higher viscosity, lower conductivities, and a lower self-diffusion coefficient. Early studies26,27 indicate that the concept of the structuremaking ability of ions in solution is applied also for the solid-water interface, and it is generally observed that solids having an iep at pH < 4, such as silica particles, are structure-breakers, while those with high iep surfaces such as zirconia (iep at pH 8.2) are structuremakers. Moreover, structure-maker surfaces preferentially adsorb structure-maker ions, and the structurebreaker surfaces preferentially adsorb structure-breaker ions.27 The Cl- ions are generally classed as structurebreakers;23,25 nevertheless, their ability to change the vicinal water structure is very weak as compared with that of the other halogen ions. For instance, the B coefficient of viscosity (nonlinear coefficient of the Jones-Dole relationship between the viscosity of a solution and the electrolyte concentration) for a Cl-/ water solution measured at 25 °C is close to zero (-0.007 L‚mol-1), while for Br-, I-, and SO42- the B coefficient values are -0.032, 0.080, and 0.201 L‚mol-1, respectively.25 Thus, SO42- ions are structure-makers, while Cl- ions can be considered as borderline, neither significantly breaking nor significantly making water structure. In this framework, a slight change in the pH, temperature, and loading of secondary ionic species in solution can alter the magnitude and the nature (breaker/ maker) of Cl- ion effects on the water structure. Unfortunately, we did not find data about the effect of pH and temperature on the ability of an ionic solute to change the water structure. However, as the hydration enthalpy for Cl- ions is relatively low (-332 kJ‚mol-1),25 upon heating, its degree of hydration should decrease (because hydration is exothermic) softly (because of the low hydration enthalpy). SO42- ions have a large hydra(25) Marcus, Y. Ion Properties; Marcel Dekker: New York, 1997. (26) Berube, Y. G.; De Bruyn, P. L. J. Colloid Interface Sci. 1968, 28, 92. (27) Dumont, F.; Warlus, J.; Watillon, A. J. Colloid Interface Sci. 1990, 138, 543.

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tion enthalpy (-909 kJ‚mol-1);25 thus, upon heating, the degree of hydration should decrease sharply, due to the large values of the hydration enthalpy. We did not have similar data for sulfate-modified zirconyl compounds such as that studied here, but the same overall feature is expected for sulfate groups present at the particlewater interface. In this framework, the ability of sulfate groups to structure the hydration layer at the surface of particles decreases upon heating. As a consequence, a decrease of the thickness of the surface hydration layer and of the hydrated size of the free sulfate ion is expected, making easy their specific adsorption. This phenomenon can occur in a reversible way, leading to changes in the magnitude of the surface charge, allowing a close approach between particles so that gelation can occur by physical aggregation. 4.2. Domain D. As evidenced by ζ potential measurements, the surface charge of samples belonging to domain D changes very slowly with electrolyte loading, remaining at positive values. This can be well understood keeping in mind that samples of domain D are characterized by the lowest sulfate concentration of the gel formation diagram (Figure 1). The short-range structural characteristics of the primary particles do not change extensively for samples of domains C and D, indicating that the core of primary particles obtained in these domains have almost the same structure, but the amount of the SO42- groups at the particle surface can be lower for samples of domain D than those of domain C (Table 2). Furthermore, the amount of free chloride ions released in solution is almost 4 times higher than that of domain C samples (Figure 6c). The characteristics of both the solution and the surface do not allow the RC+/- point to be reached by increasing the temperature, which favors the thermal stability of the sol. 4.3. Effect of RS on the Thermoreversible Gelation. As evidenced in Figure 2, the critical temperature of gelation increases with RS. This could be well understood on the basis of the net surface charge of particles in solution. By increasing the RS ratio, the surface becomes more positively charged as evidenced by the ζ potential values (Figure 6a) measured without addition of salt (NaNO3). Then the electrostatic repulsion increases and makes difficult the aggregation as RS increases. As the structures of the primary particles and of the fractal aggregate do not change across domain C, we propose that the magnitude of the ζ potential is affected by the surface density or by the chemical state of the sulfate groups. Indeed the higher positive values of the ζ potential observed for samples with RS ) 3 indicate that the surface is more protonated and thus more acidic. Literature data concerning sulfated ZrO2 powders28,29 have shown that the bidentate sulfate groups have acidity higher than that of the monodentate species. It is noteworthy that mono- and bidentate sulfates are both present at the surface of highly polynuclear species such as Zr18O4(OH)38.8(OSO3)12.6‚33H2O.13 Thus, presumably the ratio between mono- and bidentate sulfate species present at the surface of particles could be slightly different across (28) Ardizzone, S. Bianchi, C. L. Signoreto M. Appl. Surf. Sci. 1998, 136, 213. (29) Ardizzone, S. Bianchi, C. Appl. Surf. Sci. 1999, 153, 63.

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domain C, and this should play an additional role in the thermoreversible gelation. 4.4. Effect of RCl on the Thermoreversible Gelation. Considering a given RS value, we observe a nonmonotonic variation of Tc with RCl (Figure 2b). As the initial chloride content increases, Tc decreases from RCl ) 3.75 to RCl ) 0.75, then increases from RCl ) 0.75 to RCl ) 0.46, and remains essentially constant between RCl ) 0.46 and RCl ) 0.14. As already mentioned the suspensions of domain C are characterized by the same particles of similar composition and structural features dispersed in acidic media with increasing free chloride ions in solution as RCl decreases. This can be viewed as the same solid compound embedded in a medium with an increasing ionic strength, and consequently, the different samples should correspond to different points of the typical ζ potential curve reported for samples of domain C (Figure 6a). Due to the evolution of Tc with RCl, we propose a schematic location of these points as indicated by arrows in Figure 6a. As RCl decreases from 3.75 to 0.75, the extent of clouds of counterions is reduced (decrease of the Debye screening length), allowing close approach of particles. Thus, the physical aggregation is easier and the temperature at which gelation occurs is lower for RCl ) 0.75 than for RCl ) 3.75. The neat surface charge of the sample with RCl ) 0.75 is proposed to be positive with a value near the RC+/-. As the increase of the ionic strength favors the specific adsorption of sulfate, the density of the negatively charged surface increases with RCl the amount of free Cl- ions. Thus, we proposed that the samples with RCl ) 0.46 and 0.14 correspond to the conditions far from RC+/- in which the surfaces of the particles are negatively charged. In this context, the electric

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double-layer repulsion between particles is higher than for samples with RCl ) 0.75, and the gelation temperature increases. 5. Conclusions We have put into evidence the possibility to prepare a kinetically stable sol, thermoreversible gel, and precipitate by controlling the RS ) [Zr]/[SO42-] and RCl ) [Zr]/[Cl-] nominal molar ratios of the zirconyl chloride aqueous solution modified by sulfuric acid. Both kinetically stable and thermoreversible sols are formed by aggregates having a close packing and a fractal structure, respectively. Inner core oxo groups bridging together zirconium atoms surrounded on the surface by hydroxyl and sulfate ligands form the primary particles of these aggregates. Irrespective of the nominal value of RS and RCl, the same compound with a well-defined composition constitutes all studied samples of the thermorevesible gel domain. The specific adsorption of free sulfate ions favors the reversion of the particle surface charge of the sample presenting the thermoreversible sol-gel transition. The temperature-dependent ability of both the ion and surface to change the water structure in their sphere of influence and thus the capacity of ionic adsorption of particles determines the sol-gel transition temperature. Acknowledgment. This work has been financially supported by the FAPESP (Brazil) and by the CAPES/ COFECUB cooperation program between Brazil and France. We acknowledge F. Villain for use of the Raman facility and discussions. CM035258D