Elisabeth Bardez

Photoinduced biprotonic transfer in 4-methylumbelliferone

Abstract Steady-state and time-resolved fluorescence studies of 4-methylumbelliferone in acidic ethanol—water and 1-butanol—water mixtures reveal that one water molecule is involved in the rate-limiting step of the phototautomerization process. It is shown that the intermediate species cannot be the cationic form as stated by other authors, but is most likely the anionic form. Moreover, it is concluded that the phototautomer is not zwitterionic but exists only in the neutral keto form. A biprotonic transfer mechanism is proposed.

Absorption and fluorescence probing of the interface of aerosol OT reversed micelles and microemulsions

Absorption spectra, emission spectra, lifetime, and emission anisotropy of 2-naphthol have been measured in solutions of sodium bis(2-ethylhexyl)sulfosuccinate (AOT) in heptane. In the absence of water, the changes in absorption spectrum obtained on increasing AOT concentration, allowed us to determine the binding constant between 2-naphthol and AOT and this was found to be 335 ± 10. Water solubilization in AOT reversed micelles induces a decrease in the binding constant and/or a modification of the probe environment in the reversed micelles, according to the concentration of the surfactant. When this concentration is greater than 8 × 10−2M, 2-naphthol remains almost fully bound. Then, all the experimental data show that this probe is localized at the interface (where it is hydrogen-bonded to AOT polar head groups) and that the effects of solubilized water on the spectral characteristics and lifetime can be interpreted in terms of increased polarity of the probe microenvironment. A concomitant increase in mobility of the probe, as revealed by fluorescence polarization experiments, shows that the interface becomes more and more loose. The experimental data are consistent with an interface reorganization in the micellar domain (water-to-surfactant molar ratio < 10) with a concomitant water penetration along the surfactant head groups, in contradistinction to a “hard interface” picture.

Counterion control of reactivity in anionic reverse micellar aggregates

The sodium counterion of the surfactant AOT (sodium bis-(2-ethylhexyl) sulfosuccinate) has been replaced by calcium, magnesium, zinc and aluminum ions. In reverse micellar aggregates of these surfactants in heptane, acidity induced by these cations, at low water content, is revealed by an acid-sensitive optical probe (2′,7′-dichlorofluorescein). The magnitude of this effect parallels the hydrolysis rate of triethylorthobenzoate. A strong catalytic effect is observed with zinc and aluminum counterions. The acidity effect of the counterions follows the order: Na+ < Ca 2+ ≈ Mg2+ < Zn2+ < Al3+.

Hydrogen-bond effects induced by alcohol on the structure and dynamics of ionic reverse micelles

Abstract In this work we report how the structure of Zn(AOT)2/H2O/d-cyclohexane microemulsions is affected by the presence of alcohol (normal pentanol) molecules as cosurfactant. The systems are investigated by small-angle neutron scattering (SANS) and by Fourier transform IR (FTIR) spectroscopy. The SANS technique allows the study of the evolution of the size, the shape and the possible correlation length of concentration fluctuations as a function of water and alcohol content. Moreover FTIR spectra, in the OH stretching region, is quite sensitive to the structural changes of the water, induced by the polarization effects of the polar heads of the micelle surface and the positive counterions and by the presence of the alcohol.

Structural properties of micellar solutions

Abstract The results of a small angle neutron scattering experiment are reported in which the structural properties of a novel water/AOT/cyclohexane reverse micelle system is investigated. The metallic ions bound to AOT are Al 3+ and Zn 2+ . Spectra were collected at different water contents and at the maximum surfactant concentration. A quantitative evaluation of the shape and size of the droplets formed is performed, and a comparison among similar shapes is presented.

Cation-Responsive Fluorescent Sensors

Various examples of charge-transfer fluorescent probes for cation sensing are described. These probes are based on cation control of charge transfer in a conjugated donor-acceptor fluorophore, the bound cation interacting with either the electron-donating group or the electron-withdrawing group. Special attention is paid to the understanding of cation-induced photophysical changes in a crowned merocyanine, crown-ether-linked coumarins, 8-quinolinol and its derivatives. It is emphasized that when the cation interacts with the electron-donating group, there is a photodisruption of the interaction between the cation and this group, leading to possible photorelease of the cation. Optical sensors based on the entrapment of fluorescent indicators of pH (eosin Y) and potassium (PBFI) in sol-gel thin films are also presented.

Behavior of “proton sponge” in water-containing reverse micelles

Abstract Proton sponge (bis(1,8-dimethylamino)naphthalene) is solubilized in its neutral form in heptane and in its protonated form in the aqueous core of reverse micelles of AOT (sodium bis(2-ethylhexyl) sulfosuccinate). Partition between the heptane phase and the water pools has been quantitatively analyzed by spectrophotometry at various concentrations of proton sponge and as a function of water content of the micelles. The results show that the pseudophase model is valid and reveals a strong solubilization effect due to the presence of surfactant. In addition, proton sponge permits determination “in situ” of the acidity in the water pool.

Chapter 39 - Excited-state proton transfer dynamics of 6-hydroxyquinoline in acidic and alkaline aqueous solutions

The amphoteric 6-hydroxyquinoline (6HQ) molecule has a weak acidic hydroxyl function (p K A = 9.2) and a weak basic imine function (p K A = 5.1) in the ground-state [1] . A cationic (quinolinium) form 6HQ(C), an anionic (quinolinate) form 6HQ(A), and a neutral form 6HQ(N) are thus predominantly present in acidic, alkaline, and neutral aqueous solutions, respectively, according to the following acid-base equilibria: Figure options Figure options Download full-size image Download as PowerPoint slide 6HQ(N), 6HQ(C), 6HQ(A), and 6HQ(T) absorb at 326, 344, 358, and 408 nm, and fluoresce at 380, 450, 490, and 585 nm, respectively [1] xa0andxa0 [2] . The excited state proton transfer dynamics is almost unexplored. Recent investigations by picosecond time-resolved fluorescence [3] xa0andxa0 [4] led to opposed conclusions concerning the chronology of the double proton transfer process in neutral water. We present a sub-picosecond transient absorption investigation of the photoinduced tautomerization of 6HQ in acidic and alkaline aqueous solutions.

Growth processes in ionic microemulsions by SANS

Small-angle neutron-scattering results on novel reversed water-in-oil micellar aggregates made of Aluminium, Gallium, Scandium and Indium salts of bis(2-ethylhexyl) sulfoccinic acid are presented. The different polarization effects of the various counter ions on the confined water are confirmed by the metal-cation dependence of the micellar aggregate shape and size, quantitatively evaluated in this experiment for increasing values of the water-to-surfactant anion ratio w.