C. La Mesa

Interactions between single-walled carbon nanotubes and lysozyme.

Dispersions of single-walled and non-associated carbon nanotubes in aqueous lysozyme solution were investigated by analyzing the stabilizing effect of both protein concentration and pH. It was inferred that the medium pH, which significantly modifies the protein net charge and (presumably) conformation, modulates the mutual interactions with carbon nanotubes. At fixed pH, in addition, the formation of protein/nanotube complexes scales with increasing lysozyme concentration. Electrophoretic mobility, dielectric relaxation and circular dichroism were used to determine the above features. According to circular dichroism, lysozyme adsorbed onto nanotubes could essentially retain its native conformation, but the significant amount of free protein does not allow drawing definitive conclusions on this regard. The state of charge and charge distribution around nanotubes was inferred by combining electrophoretic mobility and dielectric relaxation methods. The former gives information on changes in the surface charge density of the complexes, the latter on modifications in the electrical double layer thickness around them. Such results are complementary each other and univocally indicate that some LYS molecules take part to binding. Above a critical protein/nanotube mass ratio, depletion phenomena were observed. They counteract the stabilization mechanism, with subsequent nanotube/nanotube aggregation and phase separation. Protein-based depletion phenomena are similar to formerly reported effects, observed in aqueous surfactant systems containing carbon nanotubes.

Bile salts form lyotropic liquid crystals

A reinvestigation of the phase diagrams relative to some conjugated and non-conjugated bile salts in water has demonstrated the formation of lyotropic liquid crystalline phases, in contradiction with generally accepted statements. The phase behaviour is complex and the phase diagrams are unusual, compared to most surfactants and lipids. In particular, coexistence of liquid crystalline phases with crystals has been observed. The formation of liquid crystalline phases requires very long equilibration times and the thermal stability of the lyotropic phases is moderate. The observed structure is tentatively assumed to be of the reverse hexagonal type. Structural relations with currently accepted models for the organisation of bile salts into micelles and solid form have been found.

Solution properties of octyl-β-D-glucoside. Part 2: Thermodynamics of micelle formation

Solution properties of octyl-β-D-glucoside have been examined in a wide temperature and concentration range, by means of freezing point depression, volumetric and calorimetric methods. Partial molal quantities and the variations consequent to micellization have been evaluated and discussed. Strong support to the hypothesis of a growth of the micellar aggregates was inferred from heat capacity findings.

Solution properties of octyl β-D glucoside. Part 1: Aggregate size. shape and hydration

Aqueous solutions of octyl-β-D-glucoside have been examined in a wide concentration range. By combining information from density, viscosity and dielectric permittivity, the transition from molecular to micellar state has been evidentiated. Classical equations for the viscosity colloidal dispersions have been reexamined and applied to get information on micelle shape. A combined analysis of dielectric and viscosity findings indicates a slight and continuous dependence of micellar shape on surfactant content. The role of hydration on micellar stability is significant.

POLYMER-SURFACTANT INTERACTIONS

Aqueous solutions containing poly(vinyl-pyrrolidone) and sodium caprylate, or poly(vinyl-pyrrolidone) and tetraethylammonium perfluorooctanesulfonate,respectively, have been investigated by volumetric, ionic conductivity and surface tension methods. The presence of an interaction region has been determined from conductivity and surface tension. The width of such a region depends on the amount of polymer in the mixture,temperature, surfactant content and added electrolyte (NaCl). The observed behaviour was explained in terms of the combined effects played by the alkyl-chain hydophobicity, polar head group(s) and counter-ions.An approximate solution to a mass action model for the binding of surfactants onto polymers has been introduced. It allows determining the width of the interaction region as a function of polymer mass percent in the mixture.

Micellization and surface activity ofn-alkyldimethylaminopropanesulfonates in aqueous solutions

Surface tension experiments were performed on severaln-alkyldimethylaminopropanesulfonate aqueous solutions at 15°, 25°, and 35°C. The critical micellar concentrations have been obtained and the surface properties calculated. The enthalpic and entropic contributions to either micellization and adsorption processes have been discussed. The observed properties have been related to the zwitterionic character of these surfactants.

MICELLE AGGREGATION NUMBERS FROM COLLIGATIVE PROPERTIES

An equilibrium model has been developed to derive micelle aggregation numbers n from colligative properties of surfactant solutions, with special regard to freezing point depression. The introduction of a mass action approach in the solute activity equation allows to get aggregation numbers close to those obtained from light scattering. The model can be applied with small formal changes to ionic and nonionic surfactants.

Water self-diffusion in micellar solutions and in lyotropic mesophases of the system water/Triton TX-100

AbstractStudies by Pulsed Field Gradient NMR (PFG-NMR) methods and other physico-chemical experiments have been used to clarify the processes connected with water self-diffusion in mixtures formed by water and Triton TX-100. In micellar solutions the solvent diffusive trend is related to micelle hydration and, to a much less extent, to micelle size and shape. Hydration numbers from PFG-NMR are close to those obtained by viscosity experiments. In solution phases of the reversed kind, water in oil, water self-diffusion data suggest that aqueous domains are large and bicontinuous. Water self-diffusion in the hexagonal lyotropic mesophase has been interpreted by introducing a geometrydependent contraint,

Catanionic vesicles and DNA complexes: a strategy towards novel gene delivery systems

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