Surfactant-encapsulated Clusters Research Articles

Direct experimental 31P 2D DOSY NMR evidence for oligomerization of transition-metal substituted polyoxotungstates in nonpolar solvents

Transition-metal substituted polyoxotungstates (TMSPOTs) dissolved in toluene, where they form surfactant encapsulated clusters (SEC), have been recognized as potential catalysts for carbon dioxide activation. Therefore, their detailed structures in toluene are of great interest. This paper reports the first direct experimental evidence, based on 31-P 2D DOSY NMR, for the oligomerization of TMSPOTs in dry toluene. Two kinds of experiments are described in this paper. First, the 31-P 2D DOSY NMR data in aqueous solutions confirmed the reliability of 31-P 2D DOSY NMR for the determination of diffusion coefficients for the phospho-polyoxotungstates (P-POT) of various shapes. Second, 31-P 2D DOSY NMR for toluene solutions of tetraheptylammonium (THA) salts of cobalt- and nickel- substituted Keggin POTs (PW11Co and PW11Ni) and Weakley Co-POTs (P2W18Co4) showed the existence in the solution of polyoxotungstates with various sizes. For Keggin POTs (with one exchangeable water molecule) the slower and faster diffusing species were identified as the dimers and monomers, respectively. For Weakley POTs, with two exchangeable water molecules, the slower diffusing species is much greater than a dimer. The experimental results are in agreement with the results determined computationally using DFT method.

Complex-decorated surfactant-encapsulated clusters (CD-SECs) as novel multidynamic hybrid materials.

Generation of well-defined organic-inorganic hybrid materials with controllable size and morphology is challenging and an active area of modern research in view of their unique properties and thus multifunctional applications. Specifically polyoxometalates (POMs) were recognized as a very promising group for the construction of those systems, nonetheless there are domains where the profound understanding of hierarchical mutual interactions and assembly are lacking. We present an efficient approach towards the synthesis of a novel group of POM-based nanocomposites that we name Complex-Decorated Surfactant Encapsulated-Clusters (CD-SECs). In the investigated system the organic surfactant may behave as a metal-coordinating agent, thus allowing for derivatization of the synthesized SECs via utilization of the non-covalent interactions. We demonstrate possibilities and limitations of three types of hybrid systems (H1-H3) generated via seven distinct constructing approaches (routes A-G). These systems have the potential to exhibit multiresponsive functions depending on the nature of their building blocks and could find many potential applications in biology or materials science.

Secondary dialkylammonium salt/crown ether [2]pseudorotaxanes as nanostructured platforms for proton transport.

Multivalent secondary dialkylammonium salt/crown ether [2]pseudorotaxane, supramolecular polymer networks have been obtained by mixing surfactant-encapsulated clusters with dibenzo[24]crown-8 groups and star polymers end functionalized with dibenzylammonium ions. This induces remarkable enhancements and rational control of proton conductivity of the supramolecular networks.

Thermotropic liquid crystals built from organic–inorganic hybrid polyoxometalates and a simple cationic surfactant

By ion metathesis reactions, the dimethyldioctadecylammonium (DODA+) was used as the surfactant to encapsulate organic–inorganic hybrid polyoxometalates (POMs) (NH4)6{Mo2VO4[(Mo2VIO6)NH3CH2CH2CH2C(O)(PO3)2]2}·10H2O (POM-1) and (NH4)8{Mo2VO4[(Mo2VIO6)CH3C(O)(PO3)2]2}·14H2O (POM-2), resulting in two surfactant-encapsulated clusters (SECs) with characteristic thermotropic liquid-crystalline behavior. The chemical composition of the obtained SECs was determined by IR spectra, elemental analysis, TG analysis and NMR spectra. Also, polarized optical microscopy, differential scanning calorimetry (DSC), variable temperature X-ray diffraction (VT-XRD) and TEM of the samples were also performed, which revealed that both SECs show thermotropic liquid crystal behavior.

Activity of Keggin and Dawson polyoxometalates toward model cell membrane

We present direct information showing that polyoxometalates (POMs) show destructive activity toward the model cell membrane, via the rapid adsorption of POMs on the vesicle surface. The crucial step in causing the vesicle destruction is the structural change into stable POM–lipid conjugates, which are known as surfactant-encapsulated clusters.

Two Structures Toward Understanding Evolution from Surfactant-Polyoxometalate Lamellae to Surfactant-Encapsulated Polyoxometalates

Surfactant-POM (polyoxometalate) phases are fascinating in both their self-assembly behavior and their utility as catalysts, probes, and photochromic, electrochromic, and magnetic devices. Well-ordered lamellar phases are formed when the surfactant:POM ratio is 4:1 or 2:1, and these have been described in great detail from single-crystal X-ray diffraction studies. However, the surfactant-encapsulated clusters (SECs) with much larger surfactant:POM ratios do not form single-crystals readily. Thus less is known about their structural detail, and the evolution from the well-ordered lamellar phases to the SECs with increasing surfactant:POM ratio has not been detailed. We present here two structures that have resulted from an investigation of understanding the evolution of the surfactant-POM lamellar phase as the surfactant:POM ratio increases. [H2SiMo12O40][CH3CN]2[C16H33N(CH3)3]4 (monoclinic #4, P21 a = 12.636(2) A, b = 20.577(4) A, c = 22.364(4) A, β = 93.394(4)°) holds true to the preference of 4:1 surfac...

Isolated and Linear Arrays of Surfactant-Encapsulated Polyoxometalate Clusters on Graphite

We report on the self-assembly of several surfactant-encapsulated clusters (SECs) on the basal plane of graphite consisting of the doughnut-shaped tungstophosphate anion [Na(H2O)P5W30O110] covered by a hydrophobic shell of surfactants. Well-ordered rodlike structures are observed using scanning force microscopy. No such ordering is observed if the surfactant methyltrioctadecylammonium is used for encapsulation, suggesting that the density of alkyl chains around the polyoxometalate cluster is an important factor in determining the order of SEC assemblies on graphite. Coadsorption of tetratetracontane (n-C44H90) and (DODA)14[Na(H2O)P5W30O110] results in single, isolated SECs on a buffer layer of tetratetracontane, as determined by scanning tunneling microscopy.

Structural characterization of dimethyldioctadecylammonium-encapsulated terbium-substituted heteropolyoxotungatates in solid, Langmuir–Blodgett and solvent-casting films

We synthesized a series of dimethyldioctadecylammonium (DODA)-encapsulated terbium-substituted heteropolyoxotungstates: (DODA) 11[Tb(PW 11O 39) 2]·4H 2O (SEC-1), (DODA) 13[Tb(SiW 11O 39) 2]·5H 2O (SEC-2) and (DODA) 13H 2[Tb(BW 11O 39) 2]·11H 2O (SEC-3). Their structures were characterized by elemental analysis, thermogravimetric analysis (TGA), X-ray diffraction, transmission, reflection–absorption (RA) and polarized Fourier transform infrared (FT-IR) spectroscopy. These hybrids can form stable Langmuir monolayers at air–water interface and the molecular areas at collapsed point increase according to the order of SEC-1 < SEC-2 ≈ SEC-3, which corresponds to the numbers of DODA adsorbed on the surfaces of POM-1, POM-2 and POM-3, respectively. The polyoxometalates (POMs) in surfactant-encapsulated clusters (SECs) are organized with their long-axis parallel to the substrate in Langmuir–Blodgett (LB) films, as well as in casting films. DODA alkyl chains are well-ordered in solid and oriented to the normal of substrate with tilting angles of 29° and 32° in LB and casting films, respectively. The calculated layer distances of the LB and casting films according to the IR spectra are well consistent with the experimental data obtained by their X-ray diffraction. In addition, the short layer spacing in casting film is proposed to derive from interdigitation of DODA alkyl chains comparing with the long one in LB film. These layer spacings are in good agreement with the two structures in bulk solid, respectively. Furthermore, we found that the percentage of the interdigitiation structure in solid is related to the numbers of DODA covered on the POM surface.

Investigation on Solvent Casting Films of Surfactant-Encapsulated Clusters

The surfactant-encapsulated cluster (SEC) composed of a hydrophobic dimethyl dioctadecyl ammonium (DODA) shell and an encapsulated hydrophilic polyoxoanion core can form casting films. The structure of the casting film is influenced by evaporation rates of organic solvent. When the casting films are prepared by slow evaporation of chloroform, the alkyl chains are considered to possess a partial interdigitation, and the interdigitated length is 1.6 nm. The casting film structure is characterized by scanning force microscopy (SFM), Fourier transformation infrared (FT-IR), wide-angle X-ray diffraction, and differential scanning calorimetry (DSC).