Quaternary water-in-oil microemulsion of a cationic surfactant (cetyltrimethylammonium bromide, CTAB), n-hexane, water, and n-pentanol has been investigated using conductivity, quasi-elastic light scattering, nearinfrared absorption spectroscopy, and pulsed field gradient spin-echo NMR measurements. The conductivity behavior shows features characteristic of the migration of charged droplets. Consequently, using the charge fluctuation model, the conductivity data were correlated with the droplet radius obtained from self-diffusion coefficients for different obstruction factors. Conductivity and self-diffusion measurements were found to be self-consistent for spherical droplets with hard-sphere interactions. Comparison between collective diffusion and self-diffusion coefficients fully supports this conclusion. The average head-group area of CTAB, the amount of water free in the organic bulk, and the fraction of alcohol present into the aggregates were evaluated together with the thickness of both the interfacial film and the bound water layer providing a full microscopic picture of the system. IntroductionMicroemulsions are transparent, isotropic, thermodynamically stable dispersions of oil and water, stabilized by surfactant molecules. 1-3 Four-component systems of surfactant, cosurfactant (generally a short chain linear alcohol), oil, and water have many important features and are the most studied microemulsion systems. The reason for the significance of these systems is that the introduction of cosurfactant greatly extends the isotropic solution region, especially with single-chain ionic surfactants. Microemulsions based on the cationic surfactant cetyltrimethylammonium bromide (CTAB) have been extensively used as host for different enzymes. 4 These systems offer the possibility to compare the enzymatic activities performed in a cationic microemulsion with those performed in the wellknown systems AOT/hydrocarbon/water. 5 Furthermore, the system CTAB/n-pentanol/n-hexane/water can be a useful tool to investigate the properties of anionic polyelectrolytes such as nucleic acids, 6 since the system is cationic, is transparent in the UV region (avoiding the limitation imposed by the use of chloroform which is often employed as cosolvent for CTABbased microemulsions 7 ), and can solubilize high quantities of water, up to 80 molecules of water per surfactant molecule. These characteristics are at the basis of two recent papers, where this microemulsion was used as a microreactor to perform the self-replication of oligonucleotides. 8,9 Despite such widespread interest in CTAB water-in-oil (w/ o) microemulsions, little is known about their microstructure. This prevents one from completely understanding the basic mechanism of the phenomena taking place inside them. The problem of the structure of a quaternary microemulsion is not an easy task to afford, as can be deduced by the fact that several studies, making use of a wide range of experimental techniques, gave a small contribution toward a reliable picture of these syst...

The microstructure of the quaternary water-in-oil microemulsion CTAB/water/n-pentanol/n-hexane has been investigated by means of the pulsed gradient spin-echo NMR technique over a wide range of composition. The composition of the continuous organic phase and of the interfacial phase has been determined through the analysis of the n-pentanol self-diffusion coefficient. The size of the reverse aggregates has been evaluated from the CTAB self-diffusion coefficient. The correlation of the reverse micellar size with interfacial composition has therefore been possible. Results coming from both water dilution lines and interface dilution lines have been analyzed according to suitable models. A "master plot", i.e., a graphical representation that allows us to display the data collected at all the possible compositions of the four components system, is also proposed.

A pulsed gradient spin-echo FT 1 H NMR study on the system soybean lecithin/water/perdeuterated cyclohexane is presented. The self-diffusion coefficient of water, D w , was measured as a function of the water content (W 0 ) and was found to show a bell-shaped trend. The composition at which the maximum in D w occurs is the same at which the viscosity is the highest. We rationalize the rising part of the plot in terms of a water diffusion inside wormlike reverse micelles which increase their length upon increasing W 0 . On the contrary, the descending part indicates a rod-to-sphere evolution of the aggregate's shape. This interpretation is supported by measurement on the headgroup rotation by dielectric spectroscopy. The composition at which a structural transition occurs and the location of the phase boundary can be predicted by a geometrical model in which the effective packing parameter is equal to 1.58. This value is in agreement with small-angle neutron scattering data in the literature.

The interaction of doxorubicin (DX) with model polynucleotides poly(dG-dC)·poly(dG-dC) (polyGC), poly(dA-dT)·poly(dA-dT) (polyAT), and calf thymus DNA has been studied by several spectroscopic techniques in phosphate buffer aqueous solutions. UV-vis, circular dichroism, and fluorescence spectroscopic data confirm that intercalation is the prevailing mode of interaction, and also reveal that the interaction with AT-rich regions leads to the transfer of excitation energy to DX not previously documented in the literature. Moreover, the DX affinity for AT sites has been found to be on the same order of magnitude as that reported for GC sites.

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The kinetics of charge recombination between the primary photoxidized donor (P(+)) and the secondary reduced quinone acceptor (Q(B)(-)) have been studied in reaction centers (RCs) from the purple photosynthetic bacterium Rhodobacter sphaeroides incorporated into lecithin vesicles containing large ubiquinone pools over the temperature range 275 K = (50 +/- 15) nm). Following these premises, we describe the kinetics of P(+)Q(B)(-) recombination with a truncated cumulant expansion and relate it to P(Q) and to the free energy changes for Q(A)(-)Q(B) --> Q(A)Q(B)(-) electron transfer (DeltaG(AB)(o)) and for quinone binding (DeltaG(bind)(o)) at Q(B). The model accounts well for the temperature and quinone dependence of the charge recombination kinetics, yielding DeltaG(AB)(o) = -7.67 +/- 0.05 kJ mol(-1) and DeltaG(bind)(o) = -14.6 +/- 0.6 kJ mol(-1) at 298 K.

We report on the effect of 1-pentanol loading on the phase behaviour of mixtures of CTAB/water/n-hexane at fixed mole ratios 1:80:47. The cosurfactant induces changes in the interfacial film curvature. By increasing the pentanol/CTAB mole ratio, the system evolves from oil-in-water to water-in-oil structures. For very large 1-pentanol loading some water is expelled from the reverse micelles resulting in a L(2) plus water equilibrium (emulsification failure). In the range of compositions investigated most of the phase equilibria reveals the coexistence of liquid crystals and hexane/pentanol solutions. In these cases, we estimate the interface composition by assuming a constant 1-pentanol concentration in all the oil domains, and by tacking into account the alcohol solubility in water. In the case of single-phase sample the interfacial composition was determined by means of turbidimetric titration. The adsorption of cosurfactant at the interface of direct micelles, planar lamellae, bicontinuous microemulsions, and spherical reverse micelles follows the same adsorption isotherm (independently from the curvature of the interface). Moreover, the results obtained unambiguously show that the interface composition dictates the spontaneous curvature of interfacial film. Actually positive, null, and negative curvatures correspond to different compositions of the interfacial film. Once the influence of cosurfactant on the spontaneous curvature of the interface is understood, the appearance of the emulsification failure upon pentanol loading can be rationalised within the framework of the flexible surface model

Tetrakis-2,3-[5,6-di-(2-pyridyl)pyrazino]porphyrazinatopalladium(II) [Py 8TPyzPzPd] ( 1) and the corresponding pentapalladated species [(PdCl 2) 4Py 8TPyzPzPd] ( 2), dissolved (c approximately 10 (-5)-10 (-6) M) in preacidified dimethylformamide ([HCl] approximately 10 (-4) M), behave as potent photosensitizing agents for the production of singlet oxygen, (1)O 2, with Phi Delta values of 0.89 +/- 0.04 and 0.78 +/- 0.05, respectively. The related octacation [(2-Mepy) 8TPyzPzPd] (8+) ( 3), examined under similar experimental conditions, exhibits lower Phi Delta values, that is, 0.29 +/- 0.02 (as an iodide salt) and 0.32 +/- 0.02 (as a chloride salt). In view of the very high values of Phi Delta, the photophysics of complexes 1 and 2 has been studied by means of pump and probe experiments using ns laser pulses at 532 nm as excitation source. Both complexes behave like reverse saturable absorbers at 440 nm because of triplet excited-state absorption. The lifetimes of the triplet excited states are 65 and 96 ns for the penta- and mononuclear species, respectively. Fluorescence quantum yields (Phi f) are approximately 0.1% for both 1 and 2. Such low Phi f values for the two complexes are consistent with the high efficiency of triplet excited-state formation and the measured high yields of (1)O 2. Time-dependent density-functional theory (TDDFT) calculations of the lowest singlet and triplet excited states of the mono- and pentapalladated species help to rationalize the photophysical behavior and the relevant activity of the complexes as photosensitizers for the (1)O 2 ( (1)Delta g) generation.