No abstract

A systematic approach is outlined for optimization of enantiomeric separations in free solution capillary electrophoresis using chiral mobile-phase additives. Maximum electrophoretic mobility difference between the enantiomers occurs when the concentration of free selector is equal to the reciprocal of the average binding constant. General equations and data analysis methods are presented to relate mobilities to equilibrium constants in simple and competitive binding equilibria and used to determine thermodynamic parameters for host-guest complexation of tioconazole enantiomers with a range of cyclodextrin selectors. Selectivities are found to be in the reverse order of binding constants in the series dimethyl-beta-cyclodextrin (K1 = 6.9 x 10(3) M-1, alpha = 1.10) to hydroxypropyl-beta-cyclodextrin (K1 = 0.72 x 10(3) M-1, alpha = 1.29). For beta-cyclodextrin (K1 = 1.32 x 10(3)M-1, alpha = 1.20), delta H zero provides the dominant contribution to binding but delta delta H zero and T delta delta S zero terms give comparable contributions to the selectivity. Addition of alcohol does not affect the selectivity, but allows displacement of the optimum separation conditions to higher cyclodextrin concentration through either competitive binding (with cyclohexanol) or preferential solvation of reactants (with methanol).

Highly mesoporous (Brunauer–Emmett–Teller surface area, SBET > 200 m2 g−1; mesopore volume > 1 cm3 g−1) carbonaceous materials are prepared in a truly sustainable manner, from the naturally occurring polysaccharide alginic acid. This approach yields large mesoporous materials (pore diameter > 14 nm) significantly without the use of a template or carbonization catalyst. The direct thermal decomposition of mesoporous forms of the acidic polysaccharide allows for an extremely flexible material preparation strategy. Materials can be prepared at any desired carbonization temperature (e.g., 200–1000 °C), possessing similar textural properties, but progressively presenting more uniform surface functionality through this temperature range, from more oxygenated surfaces at low temperatures to increasingly aromatic/graphitic‐like surfaces. The high‐temperature material (i.e., 1000 °C), while predominantly amorphous, presents some short range (turbostratic) ordering, providing sufficiently polarizable surfaces on which to perform challenging liquid phase separations of polar sugar analytes.

Mixtures of gelatin and maltodextrin in aqueous solution have been quenched to temperatures at which they are initially miscible but where gelatin ordering is initiated. In many cases phase separation was observed to occur after a significant time delay, and the dependence of these delays on quench temperature and biopolymer concentration has been studied in detail using turbidity measurements and confocal laser scanning microscopy (CLSM). Furthermore, by observing the optical rotation (OR) and turbidity of the system simultaneously, the gelatin helix content and the time delay before the onset of phase separation were monitored concurrently. The observed delay times were found to correspond to the time taken for the development of a certain degree of gelatin ordering, which drives the separation process. A further consequence of gelatin ordering is the viscosifying of the solution and, at sufficient concentrations, the formation of a gel. Therefore, rheological measurements have been used in addition to turbidity measurements and CLSM in order to monitor further the structural development of the systems. A comparison of the data obtained from these techniques shows that while the development of a certain elasticity will trap the system morphology, this elasticity is not directly related to that found at the gel point. At low maltodextrin concentrations, where phase separation was not detected by turbidity, transmission electron microscopy (TEM) has been used to examine the microstructure on a smaller length scale.