Abraham Clearfield

The Mechanism Responsible for Extraordinary Cs Ion Selectivity in Crystalline Silicotitanate

Combining information from time-resolved X-ray and neutron scattering with theoretical calculations has revealed the elegant mechanism whereby hydrogen crystalline silicotitanate (H-CST; H2Ti2SiO7 x 1.5 H2O) achieves its remarkable ion-exchange selectivity for cesium. Rather than a simple ion-for-ion displacement reaction into favorable sites, which has been suggested by static structural studies of ion-exchanged variants of CST, Cs(+) exchange proceeds via a two-step process mediated by conformational changes in the framework. Similar to the case of ion channels in proteins, occupancy of the most favorable site does not occur until the first lever, cooperative repulsive interactions between water and the initial Cs-exchange site, repels a hydrogen lever on the silicotitanate framework. Here we show that these interactions induce a subtle conformational rearrangement in CST that unlocks the preferred Cs site and increases the overall capacity and selectivity for ion exchange.

Nanoencapsulation of Insulin into Zirconium Phosphate for Oral Delivery Applications

The encapsulation of insulin into different kinds of materials for noninvasive delivery is an important field of study because of the many drawbacks of painful needle and syringe delivery such as physiological stress, infection, and local hypertrophy, among others (Khafagy, E.-S.; et al. Adv. Drug Delivery Rev. 2007, 59 (15), 1521-1546). A stable, robust, nontoxic, and viable noninvasive carrier for insulin delivery is needed. We present a new approach for protein nanoencapsulation using layered zirconium phosphate (ZrP) nanoparticles produced without any preintercalator present. The use of ZrP without preintercalators produces a highly pure material, without any kinds of contaminants, such as the preintercalator, which can be noxious. Cytotoxicity cell viability in vitro experiments for the ZrP nanoparticles show that ZrP is not toxic, or harmful, in a biological environment, as previously reported for rats (Zhu, Z. Y.; et al. Mater. Sci. Forum 2009, 620-622, 307-310). Contrary to previous preintercalator-based methods, we show that insulin can be nanoencapsulated in ZrP if a highly hydrate phase of ZrP with an interlayer distance of 10.3 Å (10.3 Å-ZrP or θ-ZrP) is used as a precursor. The intercalation of insulin into ZrP produced a new insulin-intercalated ZrP phase with about a 27 A interlayer distance, as determined by X-ray powder diffraction, demonstrating a successful nanoencapsulation of the hormone. The in vitro release profile of the hormone after the intercalation was determined and circular dichroism was used to study the hormone stability upon intercalation and release. The insulin remains stable in the layered material, at room temperature, for a considerable amount of time, improving the shell life of the peptidic hormone. This type of material represents a strong candidate to developing a noninvasive insulin carrier for the treatment of diabetes mellitus.

Intercalation and Exfoliation: A Review on Morphology of Polymer Nanocomposites Reinforced by Inorganic Layer Structures

ABSTRACT This review aims to address the morphology issues of polymer nanocomposites based on inorganic layer structures. Depending upon the degree of dispersion of nano-sized layer structure, polymer composites can be divided into three main categories: microcomposites, intercalated nanocomposites, and exfoliated nanocomposites. The exfoliated nanocomposites can be subdivided into ordered and disordered exfoliations. An intermediate morphology between intercalation and exfoliation, known as partial exfoliation, is also commonly seen. The above differences in morphology lead to significant variations in physical and mechanical properties of polymer nanocomposites. This review uses epoxy/clay nanocomposites to illustrate the development and recent achievements in the syntheses and exfoliation of polymer nanocomposites. Herein, a recent study on an epoxy nanocomposite based on synthetic α-zirconium phosphate is reviewed. The potential impact of using the synthetic α -zirconium phosphate as a reinforcement phase to develop fundamental understanding of the physical and mechanical behavior of polymer nanocomposites is discussed.

Tin(IV) phosphonates: porous nanoparticles and pillared materials

Two types of tin(IV) phosphonates have been prepared; monophosphonates that form porous spherical aggregates and diphosphonates that form layered three-dimensional structures. The reaction of 4,4′-monophenyldiphosphonic acid with SnCl4·5H2O in a solvothermal reaction produces pillared layered porous materials with surface areas of 350–450 m2 g−1 and pore sizes in the 8–20 A range. The isotherms are type I in character. With biphenyldiphosphonic acid, preparation in water–alcohol solutions also yields porous pillared materials but the isotherms are type IV. The pores are somewhat larger and the pore distribution range is also larger. However, carrying out the reactions in DMSO–water or DMSO–alcohol, yields porous materials with type I isotherms. The use of spacer groups such as methyl, phenyl and phosphite were found to change the pore structure and/or to increase the surface area. The structure of these porous materials cannot be determined directly as they yield minimal X-ray powder patterns. However, the structure of Sn(O3PCH3)2 was determined from its powder pattern obtained from a sample treated hydrothermally at 220 °C for 30 days. The structure is indeed layered with pendant methyl groups forming a bilayer similar to the structure of zirconium phenylphosphonate. Using these structures to describe the porous pillared compounds led to a hypothesis of layer growth of these materials that explains their properties and their unique type of porosity. The usefulness of these porous materials resides in their ability to be functionalized to impart chemical reactivity.

The effect of guest molecular architecture and host crystallinity upon the mechanism of the intercalation reaction

The intercalation process of alpha-zirconium phosphate (alpha-ZrP) was investigated by using two alpha-ZrP samples with different levels of crystallinity and two structurally different intercalating molecules, i.e., linear hexylamine and non-planar cyclohexylamine. The results show that the intercalation energy barrier, which is affected by both host alpha-ZrP and guest intercalating molecules, has a significant effect on the intercalation process. When the intercalation energy barrier is relatively low, the interlayer distance of alpha-ZrP expands continuously with increasing amount of intercalating molecules. When the energy barrier reaches a certain level, the interlayer distance expansion becomes stepwise. The observed differences in the intercalation process correspond well with the geometric arrangement of the intercalated molecules inside the gallery of alpha-ZrP.

Synthesis and Characterization of Protonated Zirconium Trisilicate and Its Exchange Phases with Strontium

A partially protonated form of the mineral umbite has been prepared by ion exchange of K2ZrSi3O9 x H2O with acetic acid. The protonated phase, compound 1, is assigned the formula H1.45K0.55ZrSi3O9 x 2 H2O and crystallizes in the space group P2(1)/c with unit cell dimensions of a = 7.1002(2), b = 10.1163(3), c = 13.1742(5), and beta = 91.181(1) degrees. The characteristic building blocks of the acid phase are almost identical to those of the parent compound. The framework is composed of polymeric chains of trisilicate groups linked by zirconium atoms, resulting in zeolite-type channels. When viewed down the a axis, two unique ion-exchange channels can be seen. Site 1 is marked by a 12-membered ring and contains 2 cations. Site 2, a 16-membered ring, contains 4 water molecules. Compound 2, consists of a mixed Sr2+ and K+ phase synthesized from 1 by ion exchange with Sr(NO3)2. Compound 2 has the formula K0.34Sr0.83ZrSi3O9 x 1.8 H2O and crystallizes in the same space group P2(1)/c. It has cell dimensions of a = 7.1386(3), b = 10.2304(4), c = 13.1522(4), and beta = 90.222(1) degrees. The Sr2+ cations are distributed evenly among the two exchange sites, showing no preference for either cavity. Compound 3 is the fully substituted Sr phase, SrZrSi3O9 x 2 H2O, and retains the same space group as that of the previous two compounds having unit cell dimensions of a = 7.1425(5), b = 10.2108(8), c = 13.0693(6), and beta = 90.283(1) degrees. The strontium cations show a slight affinity for ion-exchange site 2, having a higher occupancy of 0.535, while site 1 is occupied by the remainder of the Sr2+ cations with an occupancy of 0.465. Batch uptake studies demonstrate a selectivity series among alkaline earth cations of Ba2+ > Sr2+ > Ca2+ > Mg2+.

In situ X-ray diffraction study of cesium exchange in synthetic umbite.

The exchange of Cs(+) into H(1.22)K(0.84)ZrSi(3)O(9)·2.16H(2)O (umbite-(HK)) was followed in situ using time-resolved X-ray diffraction at the National Synchrotron Light Source. The umbite framework (space group P2(1)/c with cell dimensions of a = 7.2814(3) Å, b = 10.4201(4) Å, c = 13.4529(7) Å, and β = 90.53(1)°) consists of wollastonite-like silicate chains linked by isolated zirconia octahedra. Within umbite-(HK) there are two unique ion exchange sites in the tunnels running parallel to the a-axis. Exchange Site 1 is marked by 8 member-ring (MR) windows in the bc-plane and contains K(+) cations. Exchange Site 2 is marked by a larger 8-MR channel parallel to [100], and contains H(2)O molecules. The occupancy of the Cs(+) cations through these channels was modeled by Rietveld structure refinements of the diffraction data and demonstrated that there is a two-step exchange process. The incoming Cs(+) ions populated the larger 8-MR channel (Exchange Site 2) first and then migrated into the smaller 8-MR channel. During the exchange process a structural change occurs, transforming the exchanger from monoclinic P2(1)/c to orthorhombic P2(1)2(1)2(1). This structural change occurs when Cs(+) occupancy in the small cavity becomes greater than 0.50. The final in situ ion exchange diffraction pattern was refined to yield umbite-(CsK) with the molecular formula H(0.18)K(0.45)Cs(1.37)ZrSi(3)O(9)·0.98H(2)O and possessing an orthorhombic unit cell with dimensions a = 10.6668(8) Å, b = 13.5821(11) Å, c = 7.3946(6) Å. Solid state (133)Cs MAS NMR showed there is only a slight difference between the two cavities electronically. Valence bond sums for the completely occupied Exchange Site 1 demonstrate that Cs-O bonds of up to 3.8 Å contribute to the coordination of the Cs(+) cation.

Crystal Growth and Ion Exchange in Titanium Silicates

In situ experiments, whether carried out in-house or at synchrotron sources, can provide valuable information on the nucleation and subsequent growth of crystals and on the mechanism of growth as well as mechanisms of phase changes and ion-exchange phenomena. This chapter describes the types of x-ray detectors, in situ cells, and detectors used in such studies. The procedures are illustrated by a study of the preparation of a tunnel-structured sodium titanium silicate, the partially niobium framework phase, and the mechanism of ion exchange as revealed by time-resolved x-ray data.