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Speciation and Crystal Chemistry of Iron(III) Chloride Hydrolyzed in the Presence of SiO4 Ligands. 3. Semilocal Scale Structure of the Aggregates Armand Masion,*,† Emmanuel Doelsch,† Je´roˆme Rose,† Ste´phane Moustier,† Jean Yves Bottero,† and Paul M. Bertsch‡ CEREGE (UMR 6635 CNRS Aix Marseille III), Europole Me´ diterrane´ en de l’Arbois, BP 80, 13545 Aix-en-Provence Cedex 04, France, and Savannah River Ecology Laboratory, AACES, University of Georgia, P.O. Drawer E, Aiken, South Carolina 29802 Received November 28, 2000. In Final Form: March 2, 2001 A series of fresh and 7 day aged Fe-Si samples have been investigated at the semilocal scale by smallangle X-ray scattering (SAXS). All the scattering curves are typical of amorphous aggregates. The subunits of the precipitates could not be determined precisely. However, the trend derived from SAXS modeling indicates that almost all the Fe is within subunits smaller than 7.5 Å. The high fractal dimension (Df) values of the aggregates ranging from about 2.0 to 2.7 are attributed to the presence of Si ligands. However, the value of Df as a function of pH and Si concentration varies linearly with the polymerization state of iron.
Introduction The structure of the hydrolysis products of Fe(III) is known to depend on the hydrolysis rate, the aging conditions, and the nature of the counterions. For instance, the crystalline oxyhydroxide phase obtained from ferric chloride is β-FeOOH (akaganeite) whereas ferric nitrate leads to R-FeOOH (goethite).1-3 The fresh hydrolysis products are amorphous or poorly ordered phases. Microscopy and scattering methods have been used to study the size and shape of the formed particles during the aging process. Small-angle X-ray scattering (SAXS) data showed that the freshly prepared aggregates are linear or have a branched structure (fractal dimension 1.7-2.0) depending on the hydrolysis ratio OH/Fe, the size of the subunit being either constant (16 Å; Fe24 polymer; Cl- counterion) or evolving with the hydrolysis ratio (7-13.5 Å; NO3counterion).4,5 At a larger scale of observation, the precipitates have a rodlike shape formed by the association of spherical clusters.1-3,6,7 The presence of complexing organic and inorganic ligands during the hydrolysis and subsequent aging has a significant influence on the structure of the precipitates in terms of loss of crystallinity of the formed phases (see, e.g., refs 8-10). By complexing of the growth sites, the ligands typically limit the Fe polymerization to the trimer * Corresponding author. Phone: (+33) 442 97 15 34. FAX: (+33) 442 97 15 59. E-mail:
[email protected]. † Europole Me ´ diterrane´en de l’Arbois. ‡ University of Georgia. (1) Murphy, P. J.; Mosner, A. M.; Quirk, J. P. J. Colloid Interface Sci. 1976, 56, 270-283. (2) Murphy, P. J.; Mosner, A. M.; Quirk, J. P. J. Colloid Interface Sci. 1976, 56, 284-297. (3) Murphy, P. J.; Mosner, A. M.; Quirk, J. P. J. Colloid Interface Sci. 1976, 56, 312-319. (4) Bottero, J. Y.; Tchoubar, D.; Arnaud, M.; Quienne, P. Langmuir 1991, 7, 1365-1369. (5) Tchoubar, D.; Bottero, J. Y.; Quienne, P.; Arnaud, M. Langmuir 1991, 7, 398-402. (6) Dousma, J.; DeBruyn, P. L. J. Colloid Interface Sci. 1976, 56, 527-539. (7) Dousma, J.; DeBruyn, P. L. J. Colloid Interface Sci. 1978, 64, 154-170. (8) Cornell, R. M.; Schwertmann, U. Clays Clay Miner. 1979, 27, 402-410.
stage and thus induce the formation of amorphous aggregates, whose usually high density (fractal dimension significantly higher than 2) depends on the nature and concentration of the ligand (see, e.g. refs 11-14). To date, the case of the potentially strongly complexing SiO4 ligands has mainly been addressed in terms of the structure of heated and/or aged phases. In this context, it has been demonstrated that their presence inhibited the formation of goethite and/or akaganeite.9,15-21 For example, the crystallization of goethite is considerably hindered, the size of the formed particles being negatively correlated to the initial Si concentration, whereas the effects of Si on the formation of akaganeite are less drastic.9 This difference was attributed to difference of the polymerization state of Si set by the pH of the initial solutions. Other authors examined the structure of goethite formed in acidic and alkaline conditions: at low pH, the goethite crystals never exceeded about 10 nm in size, whereas at high pH, depending on the Si concentration crystals up to several hundred nanometers were observed.20 However, little is known about the mechanisms (9) Kandori, K.; Uchida, S.; Katoaka, S.; Ishikawa, T. J. Mater. Sci. 1992, 27, 719-728. (10) He, Q. H.; Leppard, G. G.; Paige, C. R.; Snodgrass, W. J. Water Res. 1996, 30, 1345-1352. (11) Rose, J.; Manceau, A.; Bottero, J. Y.; Masion, A.; Garcia, F. Langmuir 1996, 12, 6701-6707. (12) Rose, J.; Flanck, A. M.; Masion, A.; Bottero, J. Y.; Elmerich, P. Langmuir 1997, 13, 1827-1834. (13) Rose, J.; Vilge´, A.; Olivie-Lauquet, G.; Masion, A.; Frechou, C.; Bottero, J. Y. Colloids Surf. A: Physicochem. Eng. Aspects 1998, 136, 11-19. (14) Vilge´-Ritter, A.; Rose, J.; Masion, A.; Bottero, J. Y.; Laine´, J. M. Colloids Surf. A: Physicochem. Eng. Aspects 1999, 147, 297-308. (15) Anderson, P. R.; Benjamin, M. M. Environ. Sci. Technol. 1985, 19, 1048-1053. (16) Vempati, R. K.; Loeppert, R. H. Clays Clay Miner. 1989, 37, 273-279. (17) Hansen, H. C. B.; Raben-Lange, B.; Raulund-Rasmussen, K.; Borggaard, O. K. Soil Sci. 1994, 158, 40-46. (18) Hansen, H. C. B.; Wetche, T. P.; Raulund-Rasmusen, K.; Borggaard, O. K. Clay Miner. 1994, 29, 341-350. (19) Swelund, P. J.; Webster, J. G. Water Res. 1999, 33, 3413-3422. (20) Glasauer, S.; Friedl, J.; Schwertmann, U. J. Colloid Interface Sci. 1999, 216, 106-115. (21) Glasauer, S. M.; Hug, P.; Weidler, P. G.; Gehring, A. U. Clays Clay Miner. 2000, 48, 51-56.
10.1021/la001650j CCC: $20.00 © 2001 American Chemical Society Published on Web 07/07/2001
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leading to these phase, mainly because of the lack of detailed knowledge concerning the initial state, i.e., nature and structure freshly precipitated Fe-Si systems. A recent investigation by Fe K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy focused the first steps of the hydrolysis of Fe-Si systems which are likely to control the pathway and kinetics of subsequent aging.22 In this local scale study, all the Fe-Si precipitates were found to be poorly crystalline. The effect of Si was evident from the hindered Fe polymerization. Nevertheless, this hindrance is not as drastic as in the case of other complexing ligands and does not follow a linear trend with the Si concentration. The proportion of corner linkages between Fe atoms decreased sharply with increasing Si/Fe molar ratio. The growth of Fe clusters is threedimensional for Si/Fe < 1.0, whereas the predominance of edge sharing between Fe atoms indicates a twodimensional growth for Si/Fe > 1.0. The systems at Si/Fe ) 1.0, where the polymerization of Fe reaches a minimum, represent a crossover between the two growth regimes. An FTIR and 29Si NMR study of the same samples23 gave some insight into the status of Si within the solids and allowed ascertaining the presence of Fe-O-Si linkages within the aggregates. Polymerization of Si within the aggregates became significant for systems with Si/Fe g 1.0 and was shown to be highly dependent on the pH. The aim of the present study is to extend the structural investigation of these systems to the semilocal scale, i.e., from several angstroms to several hundred angstroms, by using SAXS. The samples chosen for this work are freshly prepared Fe-Si systems with varying pH and Si/ Fe ratios corresponding to the three- and two-dimensional growth mechanisms as well as the crossover. The comparison of the results with those previously obtained on Fe systems in the absence and the presence of complexing ligands will allow assessment of the influence of Si on the aggregation behavior. The evolution of the structure of these aggregates with time is also examined on a subset of aged samples (pH ) 7). Materials and Methods Sample Preparation. Samples for SAXS analysis were prepared as described previously.22 Briefly, FeCl3‚6H2O and the appropriate amount of tetraethyl orthosilicate (TEOS) were added to 10 mL of 1 M HCl and bidistilled water so as to obtain a final Fe concentration of 0.2 M and Si/Fe molar ratios of 0.5, 1.0, and 2.0. These mixtures were then base hydrolyzed (10 M NaOH) until pH values of 3, 7, and 10 were reached. A separate set of samples at pH ) 7 was allowed to age 7 days in suspension at room temperature before analysis by SAXS and EXAFS. Small-Angle X-ray Scattering. SAXS curves were obtained on the D24 beamline of the LURE synchrotron (Orsay, France), DCI storage ring (E ) 1.85 GeV, I ) 320 mA). The wavelength λ was set to 1.89 Å to avoid the fluorescence of Fe. Two sampledetector distances were used covering Q ranges from 0.009 to 0.085 Å-1 and from 0.050 to 0.420 Å-1. Q is the wave vector modulus and is equal to 4π(sin θ)/λ, where 2θ is the scattering angle. The recording time was 5000 s for each sample. Standard background correction and smoothing procedures were applied to the scattering curves. Normalized curves were obtained by calculating In following eq 1:24
In(Q) )
IQ PO
(1)
(22) Doelsch, E.; Rose, J.; Masion, A.; Bottero, J. Y.; Nahon, D.; Bertsch, P. M. Langmuir 2000, 16, 4726-4731. (23) Doelsch, E.; Stone, W. E. E.; Petit, S.; Masion, A.; Rose, J.; Bottero, J. Y.; Nahon, D. Langmuir 2001, 17, 1399-1405. (24) Porod, G. In Small angle X-ray scattering; Glatter, O., Kratky, O., Eds.; Academic Press: London, 1982; pp 17-51.
Figure 1. Radial distribution functions (RDF) for 7 day aged samples at pH ) 7.0. where PO is the so-called invariant and is equal to
PO )
∫ Q I(Q) dQ
1 2π2
∞
2
0
(2)
The normalized curves were connected to cover the total Q range from 0.009 to 0.420 Å-1 (which corresponds to real space distances from 7.5 to 350 Å). EXAFS Spectroscopy. Fe K-edge EXAFS spectra were recorded in the transmission mode on beamline 23A2 of the NSLS synchrotron (E ) 2.5 GeV, I ) 300 mA) (BNL, Upton, NY), and on beamline D42 of the LURE synchrotron (DCI ring). EXAFS data reduction was accomplished according to a procedure described previously.25 A Kaiser window was used for Fourier transforms. Thus obtained radial distribution functions (RDF) are uncorrected for phase shift, leading to a shift of the peaks by 0.3-0.4 Å compared to crystallographic distances. The modeling of experimental spectra was carried out by using amplitude and phase shift functions determined experimentally from γ-FeOOH for the Fe-Fe atomic pair and from andradite for the Fe-O atomic pair. These references were used to determine the electron mean free path length and ∆E0 which were then kept fixed for our unknown samples. The uncertainties on R, the distance between two atoms, and N, the number of atoms, were (0.01 Å and 10%, respectively.26
Results and Discussion Local Scale Analysis of the Aged Samples. In a recent study we described the local structure derived from EXAFS data for a number of freshly prepared Fe-Si systems, including those chosen for the present study.22 To discuss the semilocal structure of the aged samples on the same basis as the fresh ones, the Fe K-edge EXAFS results obtained for the 7 day old pH ) 7 systems will be briefly presented in this section. A detailed analysis of the effect of aging on the local structure of Fe-Si precipitates will be the subject of a separate paper. The radial distribution functions (RDF) display two peaks originating from Fe-backscatterer interactions (Figure 1). The first peak at about 1.5 Å (distance uncorrected for (25) Manceau, A.; Calas, G. Clay Miner. 1986, 21, 341-360. (26) Theo, B. K. EXAFS: Basic principles and data analysis; SpringerVerlag: Berlin, 1986; 349 pp.
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Table 1. Structural Parameters for Fe (Backscatterer in the Second Coordination Sphere) Contributions Derived from EXAFS Analysis for Si/Fe ) 0.5, 1.0, and 2.0, pH ) 7, after 7 days of Aging Fe-Fe1 shell
Fe-Fe2 shell
Fe-Fe3 shell
Fe-Fe4 shell
sample
R window (Å)
Ra
Nb
σc
R
N
s
R
N
s
R
N
s
Qd
Si/Fe ) 0.5 Si/Fe ) 1 Si/Fe ) 2
2.20-3.61 2.20-3.67 2.20-3.56
2.98 2.99 2.98
1.43 1.43 1.64
0.09 0.07 0.08
3.12 3.15 3.12
0.82 0.64 0.90
0.06 0.06 0.04
3.44 3.46 3.50
0.85 0.65 0.61
0.11 0.09 0.09
3.89
0.18
0.05
0.07 0.09 0.04
a R: distance between the two atoms of each atomic pair. b N: number of atoms in shell of iron. c σ: Debye-Waller factor (disorder parameter; Å2). d Q ) ∑[(k3χtheo) - (k3χexp)]2/(k3χexp)2.
Figure 2. Typical shape of the scattering curves (log-log) for Fe-Si systems.
phase shift) corresponds to the contribution of O, OH, or H2O in the first coordination sphere of Fe atoms. The peaks at higher distances (the ones we will focus on in the present study) indicate the presence of atoms in the next nearest shells of iron. As already observed for the fresh samples,22 the width of the second peak between 2.2 and 3.4 Å tends to decrease with increasing Si/Fe (Figure 1). The RDF for the Si/Fe ) 1.0 sample displays an additional peak at about 3.6 Å (see arrow on Figure 1). As shown previously, it is very difficult to detect the presence of neighboring Si with Fe K-edge EXAFS and the quality of the fit is not affected whether a Si shell is included in the calculations.22,27 Therefore, only Fe shells were considered for the analysis of the next nearest coordination spheres. The modeling included three Fe-Fe contributions (four in the case of Si/Fe ) 1.0) so as to accurately fit the experimental spectra (Table 1). The Fe-Fe contributions correspond to edge sharing linkages (≈3.00 and 3.10 Å), double corner linkages (≈3.45 Å), and single corner linkages (≈3.90 Å). Qualitative Analysis of the Scattering Curves. Figure 2 displays the typical shape of log I vs log Q plots of the scattering curves obtained for our Fe-Si systems. The experimental curves could not be fitted by the scattering of homogeneous particles (spheres, platelets, needles, ...). The absence of correlation peaks indicates that there are no characteristic distances within the samples. This confirms the amorphous and disordered nature of the aggregates pointed out earlier by XRD:22 Aside from small quantities of poorly crystallized akaganeite at pH e 5, no ordered phase has been detected on the diffraction diagrams. For all the systems, no asymptotical behavior at low Q (Figure 2) has been observed. Therefore, the size of the aggregates exceeds the detection limit of our experimental setup, viz., 350 Å. The absence of a Porod region at high Q (characterized by a -4 slope and indicative of the surface and/or interface of the particles) reveals that the smallest subunits building the aggregates are smaller than our detection limit, i.e., 7.5 Å. The slopes of the scattering curves in this region (Q > (27) Manceau, A.; Ildefonse, P.; Hazemann, J. L.; Flank, A. M.; Gallup, D. Clays Clay Miner. 1995, 43, 304-317.
0.240 Å-1) range from -0.12 to -1.15. This means that, locally, the subunits are either “isolated” or arranged in a quasilinear fashion. The same feature of the scattering curves at high Q has been observed previously with Alligand and Fe-ligand systems.14,28-32 In those studies, based on results of independent methods, the speciation of Al and Fe in the precipitated phase could be determined or refined by modeling the outermost part of the scattering curves. For the present Fe-Si systems, the lack of precise knowledge of the nature and structure of the Fe species (and thus their precursors) prevented the modeling procedure28 from being performed accurately. However, simulations based on crude models in which the Fe species were tentatively represented by spheres with radii between 2 and 15 Å suggested that more than 99% of Fe is within subunits whose size is smaller than 7.5 Å. This trend is consistent with the absence of a Porod region. Nevertheless, no reliable data concerning size and proportions of Fe species can be derived from these simulations. Structure of the Aggregates. The fractal dimension Df is the most convenient tool to describe the structure of amorphous phases. It is easily derived from SAXS measurements. For an aggregate, the scattered intensity is24
I(Q) ) I0(Q) S(Q)
(3)
where I0 is the scattering by the subunits and S(Q) is the interference function describing the arrangement of the subunits within the aggregate. For a fractal aggregate, S(Q) scales as Q-Df.33 Thus, a rigorous determination of Df implies the knowledge of the subunits. In the present case, the subunits are not known precisely and therefore I0 cannot be computed. However, this does not prevent determination of the fractal dimension of the precipitates. Since I0 corresponds to the scattering of the subunits and since these subunits are very small in our case, the contribution of I0(Q) to I(Q) is mainly felt at high Q values. In the Q range we used for the determination of the fractal dimension (0.009 Å-1 < Q < 0.075 Å-1), I0 is expected to be quasiconstant, and thus the slopes of log I(Q) vs log Q and log S(Q) vs log Q should be almost equal. Furthermore, a comparison of the “true” fractal dimension (i.e., based on S(Q)) and the “apparent” fractal dimension (i.e., based on I(Q)) for aggregates similar to those studied here showed that the difference between these numbers is less than 0.15.31 For simplicity’s sake, “fractal dimension” and “Df“ (28) Masion, A.; Tchoubar, D.; Bottero, J. Y.; Thomas, F.; Villie´ras, F. Langmuir 1994, 10, 4344-4348. (29) Masion, A.; Bottero, J. Y.; Thomas, F.; Tchoubar, D. Langmuir 1994, 10, 4349-4352. (30) Masion, A.; Rose, J.; Bottero, J. Y.; Tchoubar, D.; Elmerich, P. Langmuir 1997, 13, 3882-3885. (31) Masion, A.; Rose, J.; Bottero, J. Y.; Tchoubar, D.; Garcia, F. Langmuir 1997, 13, 3886-3889. (32) Masion, A.; Vilge´-Ritter, A.; Rose, J.; Stone, W. E. E.; Teppen, B. J.; Rybacki, D.; Bottero, J. Y. Environ. Sci. Technol. 2000, 34, 32423246. (33) Vicsek, T. Fractal growth phenomena; World Scientific: London, 1989; 355 pp.
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Figure 3. Fractal dimension (Df) of the Fe-Si aggregates. (a) Fresh samples; (b) comparison of fresh and 7 day old systems at pH ) 7.0.
will hereafter refer to the “apparent” fractal dimension derived from log I vs log Q plots. Except Si/Fe ) 0.5, pH ) 3, all the samples have a fractal dimension larger or equal to 2.2 (Figure 3). These high values indicate the presence of dense aggregates. The Df values are significantly higher than those determined for flocs formed by hydrolysis of iron chloride or iron nitrate.4,5 For these systems, the highest fractal dimensions measured (samples at pH 7) did not exceed 2, the precipitates being formed by a cluster-cluster aggregation mechanism. In the present case, the Df values measured for Fe-Si aggregates are too high to correspond to this type of mechanism. Actually, they are similar to the Df values determined for other Fe-complexant (PO4, organic matter) systems. For all these systems, it was concluded that the ligands, by complexation of the ferric species, are responsible of the structuring of the solid phase, thus leading to dense structures.14,31 By analogy, it is very likely that the SiO4 ligands play a similar role in the structuring of the aggregates, even though the presence of Si has not been determined by Fe K-edge EXAFS. For Si/Fe ) 0.5 and 2.0, i.e., the systems before and after the crossover between the two- and three-dimensional growth regimes,22 the density of the aggregates increases with pH (Figure 3a). This evolution can be explained by a decrease of the polymerization of Si which (i) corresponds to an increased solubility of silica above pH ) 734 and (ii) has been experimentally observed on the same samples by FTIR and 29Si NMR.23 However, this mechanism does not explain the evolution of the Si/Fe ) 1.0 samples, where the opposite trend is observed, viz., a decrease of Df with increasing pH (Figure 3a). Since these samples correspond to the transition between two growth regimes, an atypical evolution of the structure is not necessarily surprising, but it questions the predominant influence of the SiO4 ligands on the variations of the fractal dimension of the aggregates. For the three Si/Fe ratios, the 7 day aging of the pH 7 samples leads to a more open structure compared to the fresh systems (Figure 3b). The lack of data on the status of Si within these phases does not allow proposing a mechanism. Effect of Fe Polymerization. Since variations of the Si speciation cannot always account for the evolution of the fractal dimension, it seems reasonable to examine the influence of the Fe speciation on Df. A closer look at the total number of Fe neighbors around each iron atom (Ntot) determined by EXAFS (Table 2) shows that this number increases with pH for Si/Fe ) 1.0 systems, whereas it (34) Iler, R. K. The chemistry of silica. Solubility, polymerisation, colloid and surface properties and biochemistry; John Wiley & Sons: New York, 1979.
Figure 4. Fractal dimension (Df) of the aggregates vs total number of neighbors (Ntot) determined by EXAFS. Open symbols, fresh samples; black symbols, aged samples. Table 2. Total Number of Fe Neighbors (NTot) in the Second Coordination Sphere of Iron Determined by Fe K Edge EXAFS for Fresh Fe-Si Systems (Data from Ref 22) and 7 day Aged pH ) 7 Samples sample pH ) 3 pH ) 7 pH ) 10 pH ) 7; aged pH ) 3 pH ) 7 pH ) 10 pH ) 7; aged pH ) 3 pH ) 7 pH ) 7; aged pH ) 10
Ntot Si/Fe ) 0.5 3.49 2.44 2.50 3.10 Si/Fe ) 1.0 2.00 2.49 3.22 2.90 Si/Fe ) 2.0 2.97 2.51 2.25 3.15
decreases for the two other ratios 0.5 and 2.0.22 In addition, for all three Si/Fe ratios, the aging of the pH ) 7 samples is accompanied by an increase of Ntot (Table 1). The plot of Df vs Ntot for all 12 samples studied shows a linear decrease of the fractal dimension for increasing Ntot which is quasi-identical for all three Si/Fe ratios (Figure 4). It appears that although the presence of SiO4 ligands leads to high values of the fractal dimension, its variations are mainly due to changes in the polymerization of iron, independent of the Si concentration. The effects of the Fe speciation on Df override those due to the changes in Si polymerization, and this all the more since SAXS is essentially sensitive to the presence of iron. At first sight, it may appear surprising that an increase of Fe neighbors in Fe clusters leads to less dense, more “porous” structures.
Hydrolyzed Iron(III) Chloride
Figure 5. Aggregation of a model system. (a) Initial state (r ) 1); (b) final state (r ) 1.65).
However, if one considers nine spheres with a radius of 1, the “aggregation” of these units to form two new ones by conserving the same total volume leads to two spheres with a radius of 1.65 (Figure 5). At the same scale of observation, the final aggregate is less compact than the initial one. It is obvious though that, at a smaller scale, this polymerization induces a densification. This densification is not observed on the present scattering curves since it occurs beyond the experimental detection limit (≈100% Fe in units smaller than 7.5 Å; see above). The scale of observation is also responsible for the apparent contradiction between the fractal dimension and the growth regime for the Si/Fe ) 0.5 and Si/Fe ) 2.0 samples: For the lower Si concentration the growth of Fe clusters has been shown to be threedimensional whereas it is two-dimensional for high Si concentrations.22 Consequently, the fact that the fractal dimensions for Si/Fe ) 2.0 are higher than for Si/Fe ) 0.5 (Figure 3a) appears inconsistent with this result. However, this difference in growth mechanism has been determined at the local scale, and thus concerns Fe clusters forming the subunits, which are not accessible with our experimental setup. Therefore, the fractal dimensions measured at a larger scale do not undermine the trends observed by EXAFS. The present results in restating more precisely previous findings about the structure of systems consisting of hydrolyzable cations (e.g., Al, Fe) and complexing ligands: 14,29-32,35 In these studies, the usually high value of the fractal dimension and its variations as a function of pH and/or ligand concentration were found to be dependent on modifications of the speciation and/or
Langmuir, Vol. 17, No. 16, 2001 4757
structure of the ligands. However, in all these systems, the speciation of Al and Fe consisted of very small species (typically mono-, di-, and trimers) and did not evolve significantly with the pH or ligand concentration. The scattering unit being constant, any modification of the “cement” between these units naturally affected the structure of the whole aggregate. When, as in the present case, the speciation of the metal cation also evolves with the experimental conditions, the structure of the aggregate is no longer under exclusive control of the ligands and it can evolve in a fashion opposite that imposed by the chemistry of the complexants. The general trend that emerges from this is that, if the ligands are sufficiently complexing to limit the polymerization of the metal cation to the stage of small oligomers, whatever the experimental conditions, the structure of the aggregates and its evolution will essentially be under control of these ligands. At the opposite, if the ligands allow significant changes of the speciation of the metal cation, which is the case of the SiO4 ligands, their presence seems to result in the formation of aggregates with high Df, the evolution of the structure however being strongly dependent on the status of the metal cation. Acknowledgment. This work was partially supported by CNRS-NSF Collaboration Agreement No. 7383. This research was carried out in part at the National Synchrotron Light Source, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences under Contract No. DE-AC02-98CH10886. P.M.B. was supported by Financial Assistance Award No. DEFC09-96SR18546 from the U.S. Department of Energy to the University of Georgia Research Foundation. The authors wish to thank the personnel at LURE and NSLS for their assistance during the experiments, and especially C. Bourgaux (D24, LURE), A. Traverse and F. Bouamrane (D42, LURE), and J. Woicik (X23A2, NSLS). LA001650J (35) Masion, A.; Thomas, F.; Tchoubar, D.; Bottero, J. Y.; Tekely, P. Langmuir 1994, 10, 4353-4356.
