Methylene Blue (MB + ) is a sensitizer that has been used for a variety of applications including energy conversion and photodynamic therapy (PDT). Although its photochemical properties in isotropic solution are well established, its effect in vivo and in restricted reaction environments is somewhat erratic. In order to understand its photochemical behavior when it interacts with biomolecules, in particular with membranes, MB + properties were studied in sodium dodecyl sulfate (SDS) and cetyl trimethylammonium bromide (CTAB) solutions. Because of an electrostatic attraction, SDS and MB + form complexes, changing the properties of both the micelles and the MB + solutions. Surface tension measurements show that the c.m.c. of SDS decreases from $7 mM to $70 mM when the MB + concentration increases from 0 to 45 mM. Above the c.m.c., binding of MB + in the micelle pseudo-phase causes the formation of aggregates (mostly dimers) as attested by the increase in the absorption at 580 nm and the decrease in fluorescence emission. The extent of dimer formation is dependent on the relative concentrations of MB + and SDS. In the presence of excess of SDS, MB + is mainly in the monomer form and at low SDS concentration dimers are favored. Such effect, which was not observed in CTAB micelles, was modeled qualitatively by considering that MB + molecules partition to the micelle pseudophase which favors or disfavors dimers as a function of its volume. MB + transient species were characterized by laser flash photolysis and NIR emission showing the presence of triplets and subsequently singlet oxygen at high SDS concentration and semi-reduced and semi-oxidized MB + radicals at low SDS concentration. Therefore it was shown that, depending on the ground state MB + monomer/dimer equilibrium, induced by the micelles, the photochemical properties of MB + can be shifted from a Type II (energy transfer to oxygen forming singlet oxygen) to a Type I mechanism (electron transfer forming the semi-reduced and the semi-oxidized radicals of MB + ).

Properties of the ground and excited states of methylene blue (MB) were studied in negatively charged vesicles, normal and reverse micelles and sodium chloride solutions. All these systems induce dimer formation as attested by the appearance of the dimer band in the absorption spectra (lamdaD approximately 600 nm). In reverse micelles the dimerization constant (KD) corrected for the aqueous pseudophase volume fraction is two-three orders of magnitude smaller than KD of MB in water, and it does not change when W0 is increased from 0.5 to 10. Differences in the fluorescence intensity as a function of dimer-monomer ratio as well as in the resonance light scattering spectra indicate that distinct types of dimers are induced in sodium dodecyl sulfate (SDS) micelles and aerosol-OT (sodium dioctyl sulfoxinate, AOT) reversed micelles. The properties of the photoinduced transient species of MB in these systems were studied by time-resolved near infrared (NIR) emission (efficiency of singlet oxygen generation), by laser flash photolysis (transient spectra, yield and decay rate of triplets) and by thermal lensing (amount of heat deposited in the medium). The competition between electron transfer (dye*-dye) and energy transfer (dye*-O2) reactions was accessed as a function of the dimer-monomer ratio. The lower yield of electron transfer observed for dimers in AOT reverse micelles and intact vesicles compared with SDS micelles and frozen vesicles at similar dimer-monomer ratios is related with the different types of aggregates induced by each interface.

Properties of the ground and excited states of methylene blue (MB) were studied in negatively charged vesicles, normal and reverse micelles and sodium chloride solutions. All these systems induce dimer formation as attested by the appearance of the dimer band in the absorption spectra (l D~6 00 nm). In reverse micelles the dimerization constant (K D ) corrected for the aqueous pseudophase volume fraction is two-three orders of magnitude smaller than K D of MB in water, and it does not change when W 0 is increased from 0.5 to 10. Differences in the fluorescence intensity as a function of dimer-monomer ratio as well as in the resonance light scattering spectra indicate that distinct types of dimers are induced in sodium dodecyl sulfate (SDS) micelles and aerosol-OT (sodium dioctyl sulfoxinate, AOT) reversed micelles. The properties of the photoinduced transient species of MB in these systems were studied by time-resolved near infrared (NIR) emission (efficiency of singlet oxygen generation), by laser flash photolysis (transient spectra, yield and decay rate of triplets) and by thermal lensing (amount of heat deposited in the medium). The competition between electron transfer (dye*dye) and energy transfer (dye*-O 2 ) reactions was accessed as a function of the dimer-monomer ratio. The lower yield of electron transfer observed for dimers in AOT reverse micelles and intact vesicles compared with SDS micelles and frozen vesicles at similar dimer-monomer ratios is related with the different types of aggregates induced by each interface. Scheme 6. Photophysical pathways of MB, where k f , k nr and k isc are the fluorescence, nonradiative and intersystem crossing constants, respectively. Pathways 2 and 5 are reactions 2 and 5 shown in scheme 4. The other symbols are the same as those of Scheme 4.466 Divinomar Severino et al.

Marangoni convection was induced by laser heating in shallow layers of solutions of crystal violet in ethylene glycol and water. Single- and double-beam instrumentations were developed for monitoring the deformation generated on the solution surfaces and to detect the influence of surfactants. In the single-beam configuration, the depression of the liquid surface acted as a converging lens, focusing the reflected beam, and the magnitude of the lens effect was measured as in a classical thermal lens experiment. In the double-beam configuration, the depression of the surface was induced by a pump beam, and the deflection of a probe beam reflected on the surface near the deformed region was monitored by a position sensor detector. The influence of surfactants as hexadecanol, DPPC, and cardiolipin was studied by forming monolayers on the surface of the solutions. The results showed that the lens effect, as well as the deflection of the probe beam, can be suppressed by a small amount of surfactant deposited on the solutions surfaces. The high sensitivity of the instruments enabled the detection of the point at which the suppression first occurs, which is close to the G−LE transition of the monolayers. In all cases the surface healing was explained taking into account the Gibbs−Marangoni effect, and the possible correlations with other phenomena involving surface deformations were briefly discussed.

The motion of wine drops observed on the internal walls of a glass, usually called "tears of wine", is perhaps the oldest known example of surface tension driven flows.