Hydrodynamic voltammetry at the liquid|liquid interface: facilitated ion transfer and the rotating diffusion cell

A new approach to the voltammetric investigation of facilitated ion transfer processes is reported. The technique uses a rotating diffusion cell approach to induce laminar flow in the organic phase of a liquid|liquid electrochemical cell. The interface between two immiscible electrolyte solutions (ITIES) was stabilised against rotation with either γ-alumina or a track-etched polyester membrane. The resultant voltammetry is shown to be consistent with the Koutecký–Levich equation enabling kinetic parameters associated with facilitated transfer of sodium by dibenzo-18-crown-6 across the water|1,2-dichloroethane interface to be evaluated. In particular, the use of the more hydrophilic alumina membrane permits the uncertainties regarding the use of the membrane-stabilised ITIES, namely the interfacial position, to be eliminated.     

Retention of alkali,alkaline earth and transition metals on an itaconic acid cation-exchange column. Eluent pH,ionic strength and temperature effects upon selectivity

The unusual selectivity of a methylene succinic (itaconic) acid modified polymeric column was investigated for the separation of alkali, alkaline earth, transition and heavy metals employing non-chelating inorganic eluents. The retention of selected metal ions on the column was investigated with simple HNO3 eluents and eluents prepared from KNO3 and KCl salts of varying pH (adjusted using HNO3). From these studies both the effect of eluent ionic strength and pH upon retention was evaluated for the itaconic acid stationary phase. The results obtained showed that despite slow exchange kinetics causing poor efficiencies, acceptable baseline separations of selected alkaline earth and transitions could be obtained under optimum conditions (the baseline separation of Mg(II), Ca(II), Mn(II), Cd(II), Zn(II) and Co(II) was possible using a 15 mM KNO3-5 mM KCl eluent at pH 3.50 in under 25 min). The use of an simple ionic strength step gradient was shown that facilitated the addition of Pb(II) to the above group of metal ions. An investigation into the effect of temperature upon peak efficiency and retention showed increased column temperature could be used to improve the resolution of closely eluting metal ions such as Ca(II) and Sr(II) and Ca(II) and Mn(II).     

Supramolecular structures of coronene and alkane acids at the Au(111)-solution interface: a scanning tunneling microscopy study

Scanning tunneling microscopy (STM) is utilized to study the solution-solid interface formed between Au(111) and solutions of coronene in hexanoic, heptanoic, and octanoic acids. In all three cases adsorbed coronene is observed and lays flat on the metal surface. Heptanoic and hexanoic acid solutions produce a hexagonal symmetry monolayer. For the heptanoic and hexanoic cases, dipole-image dipole interactions and H bonding stabilize a surface structure in which 12 acid molecules surround each coronene and produce a coronene spacing of 1.45 nm. In the case of octanoic acid as solvent, the incorporation of the solvent into the monolayer is not as strongly favored. The coronene spacing can range from close-packed (1.2 nm) with no solvent presumed present in the monolayer, to 1.50 nm with up to 12 solvent molecules surrounding each coronene. The close-packed regions have hexagonal symmetry, as do those with the largest (1.5 nm) spacing. Heptanoic acid solutions give the clearest STM images and are associated with the most stable two-component monolayer. The present paper demonstrates that non-covalent interactions at the solution-metal interface can lead to complex multicomponent monolayer structures.     

Thermoresponsive N,N-dialkylacrylamide copolymer blends as DNA sieving matrices with a thermally tunable mesh size

In an earlier study we showed that a blend of thermoresponsive and nonthermoresponsive hydroxyalkylcelluloses could be used to create a thermally tunable polymer network for double-stranded (ds) DNA separation. Here, we show the generality of this approach using a family of polymers suited to a wider range of DNA separations: a blended mixture of N,N-dialkylacrylamide copolymers with different thermoresponsive behaviors. A mixture of 47% w/w N,N-diethylacrylamide (DEA)/53% w/w N,N-dimethylacrylamide (DMA) (DEA47; thermoresponsive, transition temperature = 55 degrees C in water) and 30% w/w DEA/70% w/w DMA (DEA30; nonthermoresponsive, transition temperature > 85 degrees C in water) copolymers in the ratio of 1:5 w/w DEA47:DEA30 was used to separate a dsDNA restriction digest (PhiX174-HaeIII). We investigated the effects of changing mesh size on dsDNA separation, as controlled by temperature. We observed good DNA separation performance with the copolymer blend at temperatures ranging from 25 degrees C to 48 degrees C. The separation selectivity was evaluated quantitatively for certain DNA fragment pairs as a function of temperature. The results were compared with those obtained with a control matrix consisting only of the nonthermoresponsive DEA30. Different DNA fragment pairs of various sizes show distinct temperature-dependent selectivities. Over the same temperature range, no significant temperature dependence of selectivity is observed for these DNA fragment pairs in the nonthermoresponsive control matrix. Overall, the results show similar trends in the temperature dependency of separation selectivity to what was previously observed in hydroxyalkylcellulose blends, for the same DNA fragment pairs. Finally, we showed that a ramped temperature scheme enables improved separation in the blended copolymer matrix for both small and large DNA fragments, simultaneously in a single capillary electrophoresis (CE) run.     

The effect of macromolecular architecture in nanomaterials: a comparison of site isolation in porphyrin core dendrimers and their isomeric linear analogues

The influence of macromolecular architecture on the physical properties of polymeric materials has been studied by comparing poly(benzyl ether) dendrons with their exact linear analogues. The results clearly confirm the anticipation that dendrimers are unique when compared to other architectures. Physical properties, from hydrodynamic volume to crystallinity, were shown to be different, and in a comparative study of core encapsulation in macromolecules of different architecture, energy transduction from the polymer backbone to a porphyrin core was shown to be different for dendrimers as compared to that of isomeric four- or eight-arm star polymers. Fluorescence excitation revealed strong, morphology dependent intramolecular energy transfer in the three macromolecular isomers investigated. Even at high generations, the dendrimers exhibited the most efficient energy transfer, thereby indicating that the dendritic architecture affords superior site isolation to the central porphyrin it surrounds.     

The use of 90°-1,3-butadiene as a reference structure for the evaluation of stabilization energies for benzene and other conjugated cyclic hydrocarbons

Reactions are described that employ 90°-1,3-butadiene as a reference structure for the evaluation of the stabilization energyof the benzenoid and other cyclic conjugated hydrocarbons. The unique benefits of this rotamer of butadiene as a reference molecule within the homodesmotic conceptual framework are discussed. Experimental stabilization energies are presented for a number of cyclic hydrocarbons.     

Fast ion chromatography of inorganic anions and cations on a lysine bonded porous silica monolith

A 0.46 cm x 10.0 cm silica monolith column was modified through the in situ covalent attachment of lysine (2,6-diaminohexanoic acid) groups. Due to the zwitterionic nature of the resultant stationary phase, the modified monolithic column contained both cation and anion exchange capacity. In the case of cation exchange, the capacity was found to be relatively low at between 5 and 6.5 micromoles Me2+ per column. However, as expected, the lysine monolith exhibited a higher anion exchange capacity at 12-13 micromoles A- per column (at pH 3.0), which was found to be dependent upon column pH, due to the dissociation of the weak acid carboxylic acid groups. High-performance separations of transition metal cations and inorganic anions were achieved using the modified monolith, with the effects of eluent concentration, pH and flow rate evaluated. Using elevated flow rates of up to 5 mL/min the separation of nitrite, bromate, bromide, nitrate, iodide and thiocyanate was possible in approximately 100 s with peak efficiencies of between 50 and 100,000 N/m and retention time %RSD of under 0.3%.     

The boron-catalyzed polymerization of dimethylsulfoxonium methylide. A living polymethylene synthesis

Trialkyl and aryl organoboranes catalyze the polymerization of dimethylsulfoxonium methylide (1). The product of the polymerization is a tris-polymethylene organoborane. Oxidation affords linear telechelic alpha-hydroxy polymethylene. The polymer molecular weight was found to be directly proportional to the stoichiometric ratio of ylide/borane, and polydispersities as low as 1.01-1.03 have been realized. Although oligomeric polymethylene has been the most frequent synthetic target of this method, polymeric star organoboranes with molecular weights of 1.5 million have been produced. The average turnover frequency at 120 degrees C in 1,2,4,5-tetrachlorobenzene/toluene is estimated at >6 x 10(6) g of polymethylene (mol boron)(-1) h(-1). The mechanism of the polyhomologation reaction involves initial formation of a zwitterionic organoborane.ylide complex which breaks down in a rate-limiting 1,2-alkyl group migration with concomitant expulsion of a molecule of DMSO. The reaction was found to be first order in the borane catalyst and zero order in ylide. DMSO does not interfere with the reaction. The temperature dependence of the reaction rate yielded the following activation energy parameters (toluene, DeltaH(++) = 23.2 kcal/mol, DeltaS(++) = 12.6 cal deg/mol, DeltaG(++) = 19.5 kcal/mol; THF, DeltaH(++) = 26.5 kcal/mol, DeltaS(++) = 21.5 cal deg/mol, DeltaG(++) = 20.1 kcal/mol).     

Ion chromatographic analysis of cyanate in gold processing samples containing large concentrations of copper(I) and other metallocyanide complexes

An ion chromatographic (IC) method was developed for the determination of cyanate in gold cyanidation samples containing large concentrations of metallo-cyanide complexes. The analysis was performed on a Waters HC IC-Pak A anion-exchange column with an anthranilic acid eluent, with detection achieved using indirect UV at 355 nm. Two procedures were developed for removal of the metallo-cyanide complexes prior to the IC analysis. The first was a manual off-line method which used solid-phase extraction cartridges containing a strong anion-exchange resin to trap the complexes and to then enable determination of cyanate without interference. In the second approach, an automated on-line method was developed which used an anion-exchange guard column to trap the complexes and a column switching valve to allow backflushing of the cyanate from the guard column. This enabled the total analysis to be performed in a time of 10–14 min, depending on the sample composition. Finally, a comparison of results obtained by the standard Kjeldahl nitrogen method for cyanate and the IC method revealed an interference in the Kjeldahl method for samples containing large concentrations of Cu(I)-cyanide complexes.     

Zwitterionic ion chromatography with carboxybetaine surfactant-coated particle packed and monolithic type columns

Both particle packed (25 cm x 0.46 cm I.D. SUPELCOSIL 5 microm C18) and monolithic type (10 cm x 0.46 cm I.D. Merck Chromolith Performance C18) reversed-phase substrates were dynamically coated with a carboxybetaine type zwitterionic surfactant ((dodecyldimethyl-amino) acetic acid) and investigated as stationary phases for use in zwitterionic ion chromatography (ZIC). Investigations into eluent concentration and pH were carried out using KCl eluents containing 0.2 mM of the carboxybetaine surfactant to stabilise the column coatings. It was found that eluent concentration decreased anion retention whilst simultaneously increasing peak efficiencies, which may be due to the dissociation of intra- and inter-molecular salts of the carboxybetaine surfactant under higher ionic strength conditions. The Effect of eluent pH was an increase in anion retention with decreased eluent pH due to the increased protonation of the weak acid terminal group of the carboxybetaine, causing both a relative increase in the positive charge of the stationary phase and less repulsion of the anions by the dissociated weak acid group. The carboxybetaine-coated monolithic phase was applied to rapid anion separations using elevated flow rates and flow rate gradients.