Controlling diffusion in sol-gel derived monoliths

Redox probes were trapped within a silica monolith prepared in part with organoalkoxysilanes containing a quaternary ammonium functional group. The diffusion coefficients of the entrapped molecules were measured as the gels were slowly dried using chronoamperometry and cyclic voltammetry with ultramicroelectrodes. Gel-entrapped cobalt(II) tris(bipyridine) (Co(bpy)(3)(2+)) diffuses at rates similar to that measured in the sols by incorporating a small amount of the positively charged functional group in the matrix. In comparison, the diffusion coefficient of gel-entrapped ferricyanide (Fe(CN)(6)(3-)) drops an order of magnitude relative to its value in the sol soon after gelation. These results demonstrate the ease at which diffusion in hydrated gels can be easily controlled by simply changing the charge on the walls of the silica host.     

Permselectivities and ion-exchange properties of organically modified sol—gel electrodes

Sol—gel films prepared from organosilanes containing acidic/basic sites have been investigated as permselective and ion-exchange coatings for electroanalytical and bioanalytical investigations. When a glassy carbon electrode was modified with a sol—gel film fabricated from 3-aminopropyl-methyl-diethoxysilane (silane—NH₂), excellent permselectivity and anion-exchange properties were obtained. For a pH 7.4, 1 mM potassium ferricyanide solution, an eight-fold increase in current was observed after the electrode was immersed in solution for 10min whereas complete suppression of the electrochemical response for ruthenium hexaamine and methyl viologen was observed. Sol—gel films fabricated from trimethoxysilylpropyl ethylenediamine triacetic acid (silane—(COOH)₃) exhibited nearly complementary behavior. An approximately 10-fold increase in current was observed for pH 7.4, 1 mM ruthenium hexaammine solutions and complete suppression of the potassium ferricyanide response was observed. These ion-exchange properties can be attributed to the strong electrostatic interactions between the acid/basic functional group in the matrix and the highly charged analyte molecules. When glassy carbon electrodes were modified with diethyl-(triethoxysilypropyl) malonate (silane—(COOEt)₂), the films did not exhibit distinct ion-exchange properties but rather suppressed the reduction of potassium ferricyanide. The observed permselectivity results from the electron dense carbonyl group and/or hydrolyzed ester functionalities in the film.     

A Titration Microcalorimetric Study of the Effects of Halide Counterions on Vesicle-Forming Aggregation in Aqueous Solution of Branched-Chain Alkylpyridinium Surfactants

Titration microcalorimetry is used to study the influences of iodide, bromide, and chloride counterions on the aggregation of vesicle-forming 1-methyl-4-(2-pentylheptyl)pyridinium halide surfactants. Formation of vesicles by these surfactants was characterised using transmission electron microscopy. When the counterion is changed at 303 K through the series iodide, bromide, to chloride, the critical vesicular concentration (cvc) increases and the enthalpy of vesicle formation changes from exo- to endothermic. With increase in temperature to 333 K, vesicle formation becomes strongly exothermic. Increasing the temperature leads to a decrease in enthalpy and entropy of vesicle formation for all three surfactants. However the standard Gibbs energy for vesicle formation is, perhaps surprisingly, largely unaffected by an increase in temperature, as a consequence of a compensating change in both standard entropy and standard enthalpy of vesicle formation. Interestingly, standard isobaric heat capacities of vesicle formation are negative, large in magnitude but not strikingly dependent on the counterion. We conclude that the driving force for vesicle formation can be understood in terms of overlap of the thermally labile hydrophobic hydration shells of the alkyl chains. Copyright 2000 Academic Press.