Scanning Transmission X-Ray Microscopy: A New Method for the Investigation of Aggregation in Silica

Beelen, Tpm Theo; Shi, WD; Morrison, G.C.; Garderen, van Hf; Browne, M. T.; Santen, van Ra Rutger; Pantos, E.

doi:10.1006/jcis.1996.4593

Cited by 9 publications

(5 citation statements)

References 12 publications

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“…This same phenomenon has been observed during drying of silica solutions. 46 This contrast at large scale between high and low PEG/silica ratio can also be observed in the aforementioned SEM and BET results: at low ratio the fractals are preserved (Figure 3a), at high ratio the micrometer-sized particles appear to be very homogeneous (Figure 3b), in accordance with the surface fractal dimension D s of 2.3 (D s ) 6 -(-slope)). 42 During drying and calcination the rough surface has been smoothened, probably driven by the principle of minimizing surface tension.…”

Section: Discussionsupporting

confidence: 69%

“…With a surplus of PEG no structural change is observed and the floc structure is preserved during heating, drying and calcination. With a surplus of silica, however, there is not enough PEG to maintain all the bonds between the aggregates, and a branched structure should be expected to appear, concentrating most of the inter-aggregate bonds in the center of the big floc and resulting in fractal dimensions of 2.7 and 2.5 after heating and drying/calcination, respectively. This same phenomenon has been observed during drying of silica solutions .…”

Section: Discussionmentioning

confidence: 99%

“…With a surplus of silica, however, there is not enough PEG to maintain all the bonds between the aggregates, and a branched structure should be expected to appear, concentrating most of the inter-aggregate bonds in the center of the big floc and resulting in fractal dimensions of 2.7 and 2.5 after heating and drying/calcination, respectively. This same phenomenon has been observed during drying of silica solutions . This contrast at large scale between high and low PEG/silica ratio can also be observed in the aforementioned SEM and BET results: at low ratio the fractals are preserved (Figure a), at high ratio the micrometer-sized particles appear to be very homogeneous (Figure b), in accordance with the surface fractal dimension D s of 2.3 ( D s = 6 − (−slope)) …”

Section: Discussionmentioning

confidence: 99%

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PEG-Mediated Silica Pore Formation Monitored in Situ by USAXS and SAXS: Systems with Properties Resembling Diatomaceous Silica

Sun¹,

Beelen²,

Santen³

et al. 2002

J. Phys. Chem. B

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Poly(ethylene glycol) (PEG) was employed as templating agent for the synthesis of porous silica. The effect of PEG chain length and of the PEG/silica ratio on textural properties (fractality, pore size, and pore distribution) were investigated by monitoring the development of silica-PEG intermediates using ultrasmall and smallangle X-ray scattering analysis with high-brilliance synchrotron radiation to obtain sufficient radiation intensity for dynamic results at the subminute scale. We show that even a simple structure-directing polymer, such as PEG, results in silicas with pores of diameters spanning a range of less than 2 nm up to 20 nm, depending on polymer molecule length and polymer/silica ratio. Flocculation may well be the most important distinction between silicas prepared with small and large PEG. In this view, small PEG600 gets encapsulated by silica and forms pools within the silica framework, whereas large PEG20,000 is entangled in a mass of silica spheres, making enclosure by silica or phase separation impossible. Both polymer chain length and the polymer/silica ratio govern the relative importance of flocculation, phase separation, and hydrophobic silica-PEG interactions steering the silica polymerization. Increase in hydrophobicity results in a larger surface area and a more uniform pore size distribution, an effect confirmed by scanning electron microscopy (SEM) and observations of physical adsorption of nitrogen gas (BET). Polymers, such as PEG, may well be an inexpensive and versatile substitute and model for polypeptides known as structure-directing agents in biomineralization if silicas resembling natural ones, notably the ones present in such a huge diversity in algae of the group of diatoms, are the focus of scientific attention, e.g., for biomimicking with a view on industrial applications.

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Section: Discussionsupporting

confidence: 69%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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PEG-Mediated Silica Pore Formation Monitored in Situ by USAXS and SAXS: Systems with Properties Resembling Diatomaceous Silica

Sun¹,

Beelen²,

Santen³

et al. 2002

J. Phys. Chem. B

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“…7.18b is a Vicsek fractal (Vicsek, 1992;Beelen et al, 1997), which is constructed by the endless addition of the same figure to the corner points of the original construct. This addition results in a threefold increase in the size of the construct, R f .…”

Section: Fractalsmentioning

confidence: 99%

Nucleation, Growth, and Aggregation of Mineral Phases: Mechanisms and Kinetic Controls

Benning

Waychunas

2008

Kinetics of Water-Rock Interaction

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The formation of any phase, whether natural or synthetic (Fig. 7.1), is usually a disequilibrium process that follows a series of steps until a thermodynamically stable state (equilibrium) is achieved. The first step in the process of creating a new solid phase from a supersaturated solution (either aqueous or solid) is called nucleation. A particle formed by the event of nucleation usually has a poorly ordered and often highly hydrated structure. This particle is metastable with respect to ordering into a well-defined phase, which can accompany growth of the particle. This process of initiation of a new phase is defined as a first order transition and can follow various pathways involving a host of mechanisms. One of these pathways occurs when individual nuclei coalesce into larger clusters, a process defined as aggregation, which itself can follow a series of different pathways. The new phase is thermodynamically defined when the growing nucleus or aggregate has distinct properties relative to its host matrix; for example, a well-defined crystal structure, composition and/or density. These processes depend on a plethora of chemical and physical parameters that control and strongly affect the formation of new nuclei, the growth of a new crystal, or the aggregation behavior of clusters, and it is these issues that will be the focus of this chapter. We will discuss the mechanisms and rates of each process as well as the methods of quantification or modeling from the point of view of existing theoretical understanding. Each step will be illustrated with natural examples or laboratory experimental quantifications. Complementary to the information in this chapter, a detailed analysis of the mechanisms and processes that govern dissolution of a phase are discussed in detail in Chap. 5 and more detailed information about molecular modeling approaches are outlined in Chap. 2.

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“…The USAXS spectrum of the xerogel revealed no transition points for defining the radius of gyration R g or primary particle size r p . Therefore, it can be concluded9, 14 that the aggregates were large ( R g >6000 nm with d =2π q −1 ) and composed of primary particles with r p ≪30 nm 9, 10…”

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confidence: 99%

Mesophases of (Bio)Polymer-Silica Particles Inspire a Model for Silica Biomineralization in Diatoms

Vrieling¹,

Beelen²,

Santen³

et al. 2002

Angew. Chem.

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Diatoms, the dominant component of the plant biomass in marine and freshwater environments, are best known for their species-specific cell-wall architecture of amorphous silica. [1,2] Synthetic silica-based materials are widely used for industrial purposes, but if nature could be mimicked innovative tailormade silicas could be produced that approach the architectures of diatom exoskeletons. [3±5] Silica formation in diatoms seems to be mediated by peptides and polyamines, [6±8] but how short-term processes control structure-directed silica deposition remains unknown. The size range of the siliceous units and the intracellular location of silica deposition prohibit in situ analysis, while delicate silica intermediates are altered or damaged during isolation and sample preparation prior to structural or biophysical analysis.Noninvasive X-ray scattering approaches are promising for the study of silica transformation in situ. [9,10] To gain insight into silica aggregation and transformation on much larger length scales (up to 6500 nm) in comparison to the wellknown silica chemistry on smaller length scales (< 10 nm), [11] we examined X-ray scattering of silica structures at ultrasmall angles (USAXS), which allows reliable statements regarding the geometry and interaction of scattering sources.Silica formation in diatoms was approximated by synthesizing silica from acidified water glass (tetraethyl orthosilicate is not a natural silica source) in the presence of structuredirecting agents; the concentration of dissolved silica and the pH of the solvent were comparable to conditions during silica deposition inside diatoms. [12] All syntheses were monitored in situ at room temperature and at 90 8C to accelerate the aging of silica; all end-points were compared with that of a template-free control. In the control (Figure 1 a), silica aggregates formed the expected xerogel by reaction-limited cluster ± cluster aggregation (RCCA). [13] The observed slope in the lg I ± lg q plot of the aged xerogel is À 2.58 (Figure 1 a), which corresponds to a mass-fractal dimension D m of 2.58. Surface fractals [D s , where D s 6 À (À slope)] are excluded, since they are only physically significant if 2 < D s < 3. [13] The USAXS spectrum of the xerogel revealed no transition points for defining the radius of gyration R g or primary particle size r p . Therefore, it can be concluded [9,14] that the aggregates were large (R g > 6000 nm with d 2p q À1 ) and composed of primary particles with r p ( 30 nm. [9,10] Porous silicas can be prepared when polymers are used as structure-directing agents. [15,16] In silica directed by poly-(ethylene imine) (PEI) distinct straight lg I ± lg q lines were present (bottom line Figure 1 b), with a transition point at q % 0.05 nm À1 . The slope at q > 0.05 nm À1 was À 2.63, which almost equals that of the above-mentioned xerogel. However, rather large fractal particles may have formed by interaction of silica with CÀCÀNH 2 branches of PEI. The size (R g ) of these fractal particles at q 0.05 is approximatel...

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Scanning Transmission X-Ray Microscopy: A New Method for the Investigation of Aggregation in Silica

Cited by 9 publications

References 12 publications

PEG-Mediated Silica Pore Formation Monitored in Situ by USAXS and SAXS: Systems with Properties Resembling Diatomaceous Silica

PEG-Mediated Silica Pore Formation Monitored in Situ by USAXS and SAXS: Systems with Properties Resembling Diatomaceous Silica

Nucleation, Growth, and Aggregation of Mineral Phases: Mechanisms and Kinetic Controls

Mesophases of (Bio)Polymer-Silica Particles Inspire a Model for Silica Biomineralization in Diatoms

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