The Mechanisms of Iron(III) (Oxyhydr)oxide Nucleation

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SCHECK, Johanna, 2017. The Mechanisms of Iron(III) (Oxyhydr)oxide Nucleation [Dissertation]. Konstanz: University of Konstanz

@phdthesis{Scheck2017Mecha-39255, title={The Mechanisms of Iron(III) (Oxyhydr)oxide Nucleation}, year={2017}, author={Scheck, Johanna}, address={Konstanz}, school={Universität Konstanz} }

2017-06-13T10:56:28Z Scheck, Johanna 2017-06-13T10:56:28Z Scheck, Johanna The Mechanisms of Iron(III) (Oxyhydr)oxide Nucleation terms-of-use This work provides an extensive study of the formation of solid iron(III) (oxyhydr)oxides from aqueous solution. The phase separation process was examined, characterized and placed within a physical-chemical framework. A mechanistic understanding of precipitation pathways and the underlying chemical reactions enables us to direct and control the process itself. This is especially interesting when it comes to the question of how the reaction can be altered, i.e., how ions and additives influence this mechanism and thus the structure and properties of the products. A focus is put on the early stages of the nucleation pathway, as the species occurring are the fundamental precursors for the product of the synthesis.<br />For the investigation of the species occurring at the different stages, an experimental setup was designed, and tailored for the very specific requirements of the iron(III) (oxyhydr)oxide system. Hedström and Schneider suggested that any complex, that is, oligomeric or polymeric reaction products, except the monomeric ones that are formed in the basic pre-equilibria involving iron(III) and different ligands, result from heterogeneous nucleation processes. The titration setup used herein provides the homogenous and reproducible reaction conditions by slow mixing of very dilute solutions that are required to experimentally address this long-standing hypothesis. It also provides access to the distinct pre- and post-nucleation stages of the reaction pathway. For the analysis of these stages, two general strategies were applied. First, real-time in situ analytics were used for monitoring changes in the reaction solution and assigning them to the different stages. Second, samples were drawn at time points that had also been characterized with the real-time analytics, and investigated with a number of additional offline analytical techniques. The latter had to be optimized especially for the examination of the early stages of the precipitation pathway. On the one hand, the occurring species are very difficult to analyze due to their small size and their highly dynamic nature, while on the other hand, isolation effects have to be minimized to obtain information on the solution state of the species. The requirements for the analysis of the system were met by comparison with results from in situ analyses or reducing isolation and/or drying effects, for instance, via cryo methods.<br />The reaction is carried out at low, constant pH levels (pH 2.0 - 2.85). The uptake of hydroxide ions upon hydrolysis leads to a net generation of protons that requires titration of NaOH solution in order to maintain a constant pH level. Utilizing the titration setup, the NaOH consumption was recorded and it was shown that the reaction involves two distinct, basic stages. In the first stage, the reaction is in a pre-nucleation equilibrium, not proceeding without further increase of the iron(III) concentration. Upon entering the second stage, however, iron(III) is continuously hydrolyzed, even if the addition of iron(III) is stopped. There are different chemical reactions that can take place during hydrolysis — ligand exchange, olation and oxolation. The exchange of aquo- or chloro- by hydroxo-ligands from the iron(III) core occurs in the equilibrium reaction stage, and can be measured directly via the base consumption during the titration experiment. The constant ratio between added iron(III) and reacted hydroxide shows that 40% of the iron(III) ions in solution carry a single hydroxo-ligand. A second possible reaction is the olation process. Here, multiple iron(III) cores bridge via hydroxo-ligands. This process cannot be detected with the titration setup alone, as no further protons are generated that would require pH counter-titration. As olation processes result in polynuclear species, it was possible to confirm their presence by means of cryo-TEM (cryogenic transmission electron microscopy), SAXS (small angle x-ray scattering) and AUC (analytical ultracentrifugation). Rather polydisperse, 1 - 2 nm sized, spherical species were detected in the pre-nucleation equilibrium stage. Their size, being significantly larger than what is expected for mononuclear species, proves the formation of olation clusters during the earliest stage of the hydrolysis reaction according to the notions of Flory polycondensation. These are built up of the Fe(OH)<sup>2+</sup> complexes, whereas the charge of the olation clusters is likely balanced by counter ions. The logarithm of the equilibrium constant K<sub>2</sub> for the formation of these complexes is approximately log K<sub>2 </sub>= 11. Complementary AUC experiments show that the polynuclear olation complexes quantitatively contain the bound iron(III) ions determined by titration, that is, the equilibrium constant K<sub>2 </sub>represents an average for hydroxo-ligand exchange and olation in all associated states in the clusters. This proves that the olation clusters are thermodynamically stable (ΔG = -RT lnK<sub>2 </sub>= -62.8 kJ/mol). Closer examination of the olation clusters with SAXS and cryo-TEM reveal that the small species exhibit non-aggregative behavior due to the — if anything — repulsive interactions. With the sizes obtained from these methods and the sedimentation coefficient of 0.3 S obtained from the AUC experiments, the density of the clusters was determined. It was found to be within 1.055 - 1.16 g/mL, i.e., significantly lower than the value of 3.96 g/mL reported for ferrihydrite. This points toward a high water content within the cluster structure.<br />Upon leaving the pre-nucleation equilibrium stage, an increase in hydroxide consumption in the pH titration can be assigned to the onset of oxolation reactions. This reaction results in oxobridged iron(III) cores. It proceeds either via the release of a proton from a hydroxo-bridge formed by preceding olation, or via the condensation of two iron-cores each carrying a hydroxo-ligand. Both possible processes can be observed with the titration setup. In the first case, the release of the proton provides a direct measure, while in the case of condensation, two iron(III) cores need to be hydroxylated, instead of one for the pure olation process. The change in the reaction from olation to oxolation was also observed as a change in properties of the solution with a number of additional analytics, including cryo-TEM, SAXS, UV-Vis spectroscopy, turbidity, and conductivity measurements. The beginning of the second stage of the experiment is characterized by a curved increase in base consumption due to the onset of oxolation — compared to the linear gradient found in the pre-nucleation stage. Here, the reaction proceeds independent of the addition of iron(III), strongly suggesting that the second stage constitutes a post-nucleation, off-equilibrium stage. Cryo-TEM imaging reveals aggregation of post-nucleation clusters in the second stage, which have a similar size as the olation clusters present in the pre-nucleation equilibrium stage. The formation of larger connected structures via aggregation was further confirmed by a sudden increase in turbidity. Also, a band in the UV-Vis spectra at 485 nm occurs simultaneously, and was assigned to the formation of extended ferric species. Indeed, SAXS measurements reveal the development of attractive interactions between the post-nucleation clusters. All these findings point towards a change in speciation in the post-nucleation clusters, which arises from the change in reaction mechanism, i.e. the onset of oxolation within the olation pre-nucleation clusters. Conductivity measurements deliver insight into the state of the iron(III) ions. While the conductivity of the pre-nucleation solution is in accord with the theoretical values calculated from the known ion concentrations, the situation differs for the post-nucleation stages: With the onset of oxolation, the experiment yields lower concentrations than expected from the calculations. This can be explained with the disappearance of iron(III) ions from solution via uptake by a second phase.<br />All of this enables the mechanistic assignment of the phase separation event at the onset of the oxolation reaction. The olation clusters are built up from monomeric Fe(OH)<sup>2+</sup> complexes that are thermodynamically stable, entirely meet the criteria underlying the definition of pre-nucelation clusters (PNCs). The olation PNCs are highly dynamic due to the lability of the hydroxo bridges, and change connectivity and structure on timescales typical for rearrangements in solution. The olation PNCs are thus solutes, as there is no interface. The bridging with oxo-units upon the onset of oxolation within PNCs distinctly slows down their dynamics, due to stronger and less dynamic bonds, rendering them post-nucleation clusters. The as-formed new phase continuously dehydrates and the structure becomes more rigid. The interfacial surface between the postnucleation clusters and the solution is now characterized by a marked transition in dynamics, and the reduction of the unfavorable interfacial surface area drives the observed aggregative processes. These results highlight that nucleation is not governed primarily by a critical size, as classical nucleation theorie (CNT) suggests, but rather by the dynamics of the species that are forming at the distinct nucleation stages, that is, eventually, by the chemistry of the linkages within the clusters. Altogether, the results show that the formation of iron(III) (oxyhydr)oxides follows a route that is fully consistent with the notions of the so-called PNC pathway.<br />Iron(III) hydrolysis in the presence of chloride ions yields akaganéite, an iron oxyhydroxide mineral with a tunnel structure stabilized by the inclusion of chloride. Yet the interactions of this anion with the iron oxyhydroxide precursors occurring during the hydrolysis process, as well as its mechanistic role during the formation of a solid phase, are debated. The newly established picture of the nucleation pathway described above can be expanded and applied to this system in order to explain akaganéite formation and the mechanistic role of the chloride anion. It could be shown that the chloride ions bind as strong ligands to the complexes constituting the PNCs. Hydrolysis leads to the binding of hydroxo-ligands, which mainly takes place via the replacement of aquoligands, while the chloride ions remain in the clusters. From this observation it can be concluded that the binding strength of chloro-ligands exceeds that of aquo- and hydroxo-ligands, both of which bind with a similar strength to the iron(III) ions. As a consequence of the phase separation event, a release of chloride ions can be observed. During the oxolation reactions that are the molecular basis of the phase separation event, the chloro-ligands get progressively replaced by newly formed oxo-bridges, as these bonds are much stronger. Nevertheless, a significant amount of chloride ions remains within the structure, as due to their presence not only on the surface but also within the clusters and due to the ongoing densification via oxolation, a complete release is kinetically not possible. Instead, a microphase separation within the material is induced leading to the characteristic tunnel structure of the precipitate. Therefore, the chloride ions in the akaganéite structure must be considered as remnants from the early stages of precipitation, as they do not influence the basic mechanism of the subsequent hydrolysis reactions. As it is, the nucleation pathway of akaganéite can be described and understood within the general picture of the nucleation of iron(III) (oxyhydr)oxides via PNCs.<br />The experimental setup and the knowledge and understanding of the system can be applied to investigate new systems and the formation of novel phases. The addition of polyaspartic acid (pAsp) into the hydrolysis reaction of iron(III) solution yields an organic-inorganic composite material consisting of polydisperse spheres. Their sizes measured with DLS (200 - 1000 nm) and light microscopy (1.5 μm) are larger than those obtained with electron microscopy (20 - 1000 nm). The latter, however, reveals that the solid is built up from aggregates. Images taken with in situ AFM show no particles larger than 70 nm on the substrate. This is due to the weak attachment of the particles, especially of larger ones, i.e., these particles are removed by the AFM-tip during scanning. Generally, the size of the particles can be determined to range between 50 and 200 nm. The material is slightly positively charged, with a zeta potential of 17.1 (±4.3) mV. TGA measurements deliver insight into the composition and the material was found to contain 6 - 7% water and 49% polymer. Both accounts for the softness of the particle, which exhibit a Young’s modulus of 570.73 ± 25.41 kPa that was determined using distance-force measurements. Using cryo-TEM imaging, very small, seemingly crystalline particles were observed within the composite material. However, their existence could not be proven with Xray diffraction, which might be due to a rather small content of the crystalline phase.<br />The hybrid material was synthesized with the titration setup and thus an investigation of the formation mechanisms of the composite was possible. The polymer pAsp was shown to strongly affect the progress of iron(III) hydrolysis. The titration data revealed an increased hydroxide consumption in the presence of pAsp. The higher the pH value, the larger is the effect of the additive and the more hydroxide ions with respect to added iron(III) ions are consumed (OH<sup>-</sup> /Fe(III) ratio ~0.5 at pH 2.3 and ~1.0 at pH 2.7). At the same time, the reaction solution becomes visibly turbid as the composite material is formed. However, this is only the case if hydrolysis occurred in the first place. At a low pH value of 2.0 where no hydrolysis occurs in the absence of additive, no hydrolysis is observed in its presence either. Moreover, the solution shows no clouding and no composite material can be isolated. The impact of the polymer concentration and the molecular weight was investigated. It was shown that an increase in concentration leads to an increased effect and that thus pAsp is used up to capacity in the experiments. An increase in molecular weight of the polymer leads to an increase in the amount of formed material. Generally, the precipitation of the composite material happens at the very early stages of the reaction. In the hydrolysis reaction without pAsp, only olation PNCs are present at this stage. It can thus be concluded that the influence of the polymer relies on it interaction with PNCs. The adsorption of PNCs onto the pAsp and their subsequent promoted oxolation already at low concentrations due to their close proximity and a stabilization of the hydroxo-bridges was suggested as an underlying mechanism.<br />This thesis presents a novel and comprehensive model describing the precipitation pathway of iron(III) (oxyhydr)oxides in different systems. It explains how different parameters influence the formation of solid products and describes the underlying mechanisms and reactions. The mechanisms leading to well-known phases were investigated and successfully applied to the generation of a new organic-inorganic hybrid system. The developed model serves as a road map towards specific phases enabling a better orientation in the highly complex and wide-ranging world of iron(III) (oxyhydr)oxides. eng 2017

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