Electro-Responsive Surface Composition and Kinetics of an Ionic Liquid in a Polar Oil.

The quartz crystal microbalance (QCM) has been used to study how the interfacial layer of an ionic liquid dissolved in a polar oil at low weight percentages responds to changes in applied potential. The changes in surface composition at the QCM gold surface depend on both the magnitude and sign of the applied potential. The time-resolved response indicates that the relaxation kinetics are limited by the diffusion of ions in the interfacial region and not in the bulk, since there is no concentration dependence. The measured mass changes cannot be explained only in terms of simple ion exchange; the relative molecular volumes of the ions and the density changes in response to ion exclusion must be considered. The relaxation behavior of the potential between the electrodes upon disconnecting the applied potential is more complex than that observed for pure ionic liquids, but a measure of the surface charge can be extracted from the exponential decay when the rapid initial potential drop is accounted for. The adsorbed film at the gold surface consists predominantly of ionic liquid despite the low concentration, which is unsurprising given the surtactant-like structures of (some of) the ionic liquid ions. Changes in response to potential correspond to changes in the relative numbers of cations and anions, rather than a change in the oil composition. No evidence for an electric field induced change in viscosity is observed. This work shows conclusively that electric potentials can be used to control the surface composition, even in an oil-based system, and paves the way for other ion solvent studies.

Intrinsically disordered protein as carbon nanotube dispersant: How dynamic interactions lead to excellent colloidal stability.

The rich pool of protein conformations combined with the dimensions and properties of carbon nanotubes create new possibilities in functional materials and nanomedicine. Here, the intrinsically disordered protein α-synuclein is explored as a dispersant of single-walled carbon nanotubes (SWNTs) in water. We use a range of spectroscopic methods to quantify the amount of dispersed SWNT and to elucidate the binding mode of α-synuclein to SWNT. The dispersion ability of α-synuclein is good even with mild sonication and the obtained dispersion is very stable over time. The whole polypeptide chain is involved in the interaction accompanied by a fraction of the chain changing into a helical structure upon binding. Similar to other dispersants, we observe that only a small fraction (15-20%) of α-synuclein is adsorbed on the SWNT surface with an average residence time below 10 ms.

Block Copolymers as Dispersants for Single-Walled Carbon Nanotubes: Modes of Surface Attachment and Role of Block Polydispersity.

When using amphiphilic polymers to exfoliate and disperse carbon nanotubes in water, the balance between the hydrophobic and hydrophilic moieties is critical and nontrivial. Here, we investigate the mode of surface attachment of a triblock copolymer, Pluronics F127, composed of a central hydrophobic polypropylene oxide block flanked by hydrophilic polyethylene oxide blocks, onto single-walled carbon nanotubes (SWNTs). Crucially, we analyze the composition in dispersant of both the as-obtained dispersion (the supernatant) and the precipitate-containing undispersed materials. For this, we combine the carefully obtained data from 1H NMR peak intensities and self-diffusion and thermogravimetric analysis. The molecular motions behind the observed NMR features are clarified. We find that the hydrophobic blocks attach to the dispersed SWNT surface and remain significantly immobilized leading to 1H NMR signal loss. On the other hand, the hydrophilic blocks remain highly mobile and thus readily detectable by NMR. The dispersant is shown to possess significant block polydispersity that has a large effect on dispersibility. Polymers with large hydrophobic blocks adsorb on the surface of the carbonaceous particles that precipitate, indicating that although a larger hydrophobic block is good for enhancing adsorption, it may be less effective in dispersing the tubes. A model is also proposed that consistently explains our observations in SWNT dispersions and some contradicting findings obtained previously in carbon nanohorn dispersions. Overall, our findings help elucidating the molecular picture of the dispersion process for SWNTs and are of interest when looking for more effective (i.e., well-balanced) polymeric dispersants.

Complexing Cations by Poly(ethylene oxide): Binding Site and Binding Mode.

The binding of K+ and Ba2+ cations to short poly(ethylene oxide) (PEO) chains with ca. 4-25 monomeric units in methanol was studied by determining the effective charge of the polymer through a combination of electrophoretic NMR and diffusion NMR experiments. These cations were previously found to bind to long PEO chains in a similar strong manner. In addition, 1H chemical shift and longitudinal spin relaxation rate changes upon binding were quantified. For both systems, binding was stronger for the short chains than that for the longer chains, which is attributed mainly to interactions between bound ions. For K+ ions, the equilibrium binding constant of a cation to a binding site was measured. For both cations, the binding site was estimated to consist of ca. six monomeric units that coordinated with the respective ions. For the systems with barium, a significant fraction of the bound ions are (BaAnion)+ ion pairs. This leads to a strong anion effect in the effective charge of the oligomers acquired upon barium ion binding. For K+, the coordinating oligomer segment remains rather mobile and individual oligomers exchange rapidly (≪s) between their free and ion-complexing states. In contrast, segmental dynamics slows significantly for the oligomer section that coordinates with the barium species, and for individual oligomers, binding and nonbinding sections do not exchange on the time scale of seconds. Hence, oligomers also exchange slowly (>s) between their free and barium complexing states.

Polymer-drug interactions and wetting of solid dispersions.

We demonstrate the ability of drugs to influence the wetting of solid dispersion tablets in unexpected ways. Five model drugs of different water solubility and ability to interact with the involved polymers were incorporated in hydrophilic polymer matrices, made of either hydroxypropyl methylcellulose (HPMC) or polyvinyl pyrrolidone (PVP). The physical mixtures of all combinations of drug and polymer presented surface hydrophobicities, as measured by the equilibrium advancing contact angle of water, which are expected for materials that do not influence the interactions of each other with water. However, the solid dispersions containing HPMC deviated from this regular behaviour and displayed contact angles below those of the pure compounds involved, either drug or polymer. This behaviour is explained by changed surface exposure of HPMC side groups, as a result of changes in intermolecular hydrogen bonds. In addition to water contact angle measurements, we employed NMR imaging to monitor the time course of water ingress and swelling.

Slow recrystallization of tripalmitoylglycerol from MCT oil observed by 2H NMR.

The crystallization and recrystallization of fats have a significant impact on the properties and quality of many food products. While crystallization has been the subject of a number of studies using pure triacylglycerols (TAG), recrystallization in similarly pure systems is rarely studied. In this work, perdeuterated tripalmitoylglycerol ( (2)H-PPP) was dissolved in medium chain triacylglycerol oil (MCT) to yield a saturated solution. The solution was heated to cause partial melting of the solid and dissolution of the molten fraction of (2)H-PPP in MCT and was then cooled to the original temperature to induce recrystallization from the supersaturated solution. (2)H NMR was used to monitor the disappearance of (2)H-PPP from the solution and showed that recrystallization occurred in two steps. The first step was rapid, taking place over a few minutes, and accounted for more than two-thirds of the total recrystallization. The second step was much slower, taking place over a remarkably long timescale of hours to days. It is proposed that dissolution occurs from all parts of the crystals, leaving an etched and pitted surface. The first step of crystallization is the infilling of these pits, while the second step is the continued growth on the smoothed crystal faces.

Combinatorial multinuclear NMR and X-ray diffraction studies of uranium(VI)-nucleotide complexes.

The complex formation of uranium(VI) with four nucleotides, adenosine- (AMP), guanosine- (GMP), uridine- (UMP), and cytidine-monophosphate (CMP), has been studied in the alkaline pH range (8.5-12) by (1)H, (31)P, (13)C, and (17)O NMR spectroscopy, providing spectral integral, chemical shift, homo- and heteronuclear coupling, and diffusion coefficient data. We find that two and only two complexes are formed with all ligands in the investigated pH region independently of the total uranium(VI) and ligand concentrations. Although the coordination of the 5'-phosphate group and the 2'- and 3'-hydroxyl groups of the sugar unit to the uranyl ions is similar to that proposed earlier ("Feldman complex"), the number and the structures of the complexes are different. The uranium-to-nucleotide ratio is 6:4 in one of the complexes and 3:3 in the other one, as unambiguously determined by a combinatorial approach using a systematic variation of the ratio of two ligands in ternary uranium(VI)-nucleotide systems. The structure of the 3:3 complex has been determined by single-crystal diffraction as well, and the results confirm the structure proposed by NMR in aqueous solution. The results have important implications on the synthesis of oligonucleotides.