Electric field-induced transient birefringence and light scattering of synthetic liposomes.

The dynamics of electric field-induced transient birefringence Deltan(t) and light scattering (detected as turbidity) of 190 nm diameter unilamellar vesicles of dioleoylphosphatidylcholine are investigated as a function of applied field strength E, length of the square pulse Deltat, lipid concentration, mean hydrodynamic diameter , ionic strength, and temperature. Generally, induced birefringence exclusively is observed at low lipid concentration and below certain threshold values of E and Deltat, whereas concomitant induced turbidity appears at high lipid concentration and above thresholds values of E and Deltat. Turbidity is monitored through the change in transmitted intensity DeltaS parallel(t) and DeltaS perpendicular(t) of light polarized parallel and perpendicular to the applied field E. The field-induced structural changes are reflected in double-exponential forward relaxation and triple-exponential reverse relaxation of the positive birefringence, and in non-exponential relaxations of DeltaS parallel (t) and DeltaS perpendicular(t). Under the field, the associated physical events are interpreted as elongation of the spherical bilayer shells in the direction of E, linear chain formation (pearling) of the induced dipolar liposomes parallel to E, and partial fusion of adjoining vesicles within the chains. Under conditions where electroporation can be detected, pore opening succeeds the elongation of the vesicles. After termination of the field, the vesicles return to their original time average spherical shape, the oriented chains randomize and disintegrate, and the fused structures are converted either to unilamellar or multilamellar vesicles.

Atomic-level molecular modeling of the nonionic surfactant Triton X-100: the OPE9 component in vacuum and water.

The commercially available nonionic surfactant Triton X-100 is a mixture of polyoxyethylene tert-octylphenyl ethers (OPEn) with an average of n = 9.5 oxyethylene (OE) units in the molecules, and the population maximum at n = 9. Thus, the OPEn = 9 component was chosen to be studied by atomic level molecular modeling, using second-generation force fields. The 1,000 conformers generated via random sampling of torsional angles around single bonds yielded 11 clusters based on geometrical similarity. Representatives of geometrically distinctly different clusters with significant populations were chosen from a narrow energy range around the most probable energy to be analyzed for interaction with water. The effect of water on the conformation of the OE chain was found to be modest, similar to the situation that had been reported earlier for the anionic surfactant Aerosol-OT (AOT). The number of bound water molecules is strongly dependent on the conformation of the OE chain and is affected by electrostatic as well as steric effects. Unlike the case of AOT, for which the length of the hydrophobic tail was found to govern the size of reverse micelles in CCl4, the size of reverse micelles of OPEn = 9 cannot be predicted from the dimensions of the hydrophilic tail.

Electric double layer dynamics by high-field electroluminescence in aqueous solution (Destriau effect).

Transient luminescence induced by high electric fields (>2.5 x 10(4) V/cm) was observed in aqueous solutions of 6,8-dihydroxypyrene-1,3-disulfonic acid. The light emission takes place both at the leading and the trailing edges of the perturbing electric square pulse. It occurs only during the variation of the applied field and disappears with a time constant of 0.5 musec at constant field. In solid state phosphors, where this phenomenon has been reported previously, it is known as the Destriau effect. The mechanisms of excitation and emission can be explained by the electron injection model, in which the time constant of the luminescence decay is associated with the rate of formation of the space charge. This space charge appears as a separation of positive and negative charges into a double layer stabilized by the applied potential.