From Cyclodextrin Assemblies to Porous Materials by Silica Templating

Abstract

Since the discovery by Beck, Vartuli et al. [1,2] that ordered mesoporous silicas, such as MCM-41, can be obtained by surfactant templating, the development of new porous materials and porogens as well as the characterization of the resulting structures has become one of the most promising fields in modern materials chemistry. [3±5] Meanwhile, these observations have been extended to a number of techniques by which pore morphology, pore size, and wall material can be varied over a broad range. [6±14] One of the modifications of the original process relevant here is the ™nanocasting∫ introduced by Attard, Glyde, and Gˆltner. [15] This is a high-concentration synthesis in which the solidified inorganic compound is a copy of the original phase structure, that is its structure is predetermined by the selection of the template phase. They used lyotropic liquid-crystalline phases, derived from surfactants or-later-block copolymers, [15±17] as templates and obtained the material either as a homogeneous film or as a monolithic object. In addition, this technique also allows the structural characterization of fragile phase structures and transition states of polymers or surfactants by analyzing the well-contrasted solid replica instead of the original soft organic phases. [16,18] We extend this approach to supramolecular aggregates of nonpolar cyclodextrins (CDs) in aqueous solution as templates. Our aim is to exploit the use of CDs to generate porous materials, focussing on the fine details of the pore structure and the self-organization of this template. The self-assembly of cyclodextrins has been assumed previously, [19±23] but their organization could-to the best of our knowledge-not be determined in detail.CDs are cyclic oligosaccharides consisting of covalently linked glucose units (6 units a-CD, 7 units b-CD, 8-units g-CD); they are characterized by a hydrophilic exterior and a hydrophobic interior [24] the size of which is in the range of 1.5 ± 2 nm. The relatively large number of hydroxy groups per unit area should ideally mediate compatibility with an oxidic inorganic matrix through hydrogen bonding. In fact, this has been proven by three reports in which CDs were employed at low concentrations in a sol ± gel process to incorporate functional units such as dyes into a dense and nonporous silica. [25±27] So far, CD-containing solutions have been difficult to analyze by standard techniques, thus information about the assembly of CDs in water is still rather limited. We employ nanocasting to investigate the CD assemblies as templates for the silica pore in detail, and we also investigate the driving force for the self-assembly of the cyclodextrin molecules to ordered aggregates.In a solution of a hydrophilic sugar, such as a cyclodextrin, one would assume that it is moleculary dispersed in the aqueous phase. Nanocasting by a sol ± gel process should then result in a porous material in which each pore exhibits the size and shape of a CD molecule, and in which the pores are not mutually ordered. Depending on ...

Silica–Carbon Nanocomposites—A New Concept for the Design of Solar Absorbers

“…Just by heating, dehydration occurs, which results in the formation of carbon. Together with the recent observation that cyclodextrins (CDs), cyclic oligo-saccharides containing 6, 7, or 8 sugar units, can be employed as templates for the formation of porous silica materials with a bicontinuous and ªworm-typeº pore system, [26,28] this results in the possibility of a one-step synthesis towards the desired carbon±silica nanocomposites by high-temperature treatment under non-oxidative conditions as monoliths or continuous coatings.…”

Aggregation of Cyclodextrins: Fundamental Issues and Applications

“…In CD solutions, dynamic light scattering (DLS) [31,32] and transmission electron microscopy (TEM) [32] have provided reliable evidence of the aggregation phenomenon. The latter has been recognized by several research groups that identified aggregates of different shapes, including spherical and elongated particles, welded fibers, and rods [1,2,19,29].…”

“…The extensive development of CD-based formulations has resulted in a strongly increasing amount of studies, in which the exceptional features of CDs have been described (see e.g., [10][11][12][13][14][15][16]). In the last decades, CDs have also been used for functionalization of organic [17] and inorganic [18,19] materials and for the synthesis of nanosized and microsized aggregates [20][21][22]. This process is, in general, driven by hydrophobic interactions and is affected by a number Table 1.…”

Structure Characterization of Surfactant Assisted Polymer Thickeners by Silica Nanocasting

“…tion of formerly unknown structural details, [5] or to learn about the self-assembly of different cyclodextrin species to worm-like stacks in aqueous solutions. [6] Therefore, nanocasting has turned out to be a valuable analytical technique to learn about the structure of complex interacting polymer systems or colloids in water. Its main advantage is the possibility to analyze a delicate and fragile, not cuttable and badly contrasted complex fluid by its solidified and therefore permanent, well-contrasted and cuttable "hard-copy".…”