Véronique Pimienta

Hydrogen peroxide evolution during V-UV photolysis of water

Hydrogen peroxide evolution during the vacuum-ultraviolet (V-UV, 172 nm) photolysis of water is considerably affected by the presence of oxalic acid (employed as a model water pollutant) and striking differences are observed in the absence and in the presence of dioxygen.

Partition and water/oil adsorption of some surfactants.

Adsorption isotherms have been determined at the water/oil interface for five biphasic systems involving surfactants (non-ionic and ionic) present in both phases at partition equilibrium. The systems studied were polyoxyethylene(23)lauryl ether (Brij35) in water/hexane and four ionic surfactants, hexadecyltrimethylammonium bromide (CTAB), and a series of three tetraalkylammonium dodecylsulfate (TEADS, TPADS, and TBADS) in water/CH 2Cl 2. Interfacial tension measurements performed at the water/air and water/oil interfaces provided all the necessary information for the determination of the adsorption parameters by taking partition into account. These measurements also allowed the comparison of the adsorption properties at both interfaces which showed an increase of the adsorption equilibrium constant and a decrease of the maximum surface concentration at the water/oil interface compared to water/air. The values of the critical aggregation concentration showed, in all cases, that only the surfactant dissolved in the aqueous phase contribute to the decrease of the water/oil interfacial tension. In the case of the four ionic surfactants, the critical aggregation concentration obtained in biphasic conditions were lowered because of the formation of mixed surfactant-CH 2Cl 2 aggregates.

Kinetic modelling of the photochromism and photodegradation of a spiro[indoline-naphthoxazine]

The photochromism of the 1,3,3-trimethylspiro[indoline-naphthoxazine] A has been studied under continuous monochromatic irradiation (313 and 365 nm) in liquid toluene solution at 278 K. The values of the main photochromic parameters (quantum yields and absorption coefficients) have been determined by kinetic modelling of absorbance (Abs) versus time curves recorded under continuous irradiation. We showed that the quantum yields of both photocoloration and reversible photochemical bleaching were wavelength-dependent. Under 365 nm irradiation, photodegradation processes that are not sensitive to the presence of oxygen were observed. The degradation occurs either from the direct irradiation of the closed spiro form A or from the thermal decomposition of the open photomerocyanine form B. Under our experimental conditions, photochemical decomposition of the open photomerocyanine form B was negligible.

Continuous irradiation and flash-photolysis studies of new[3H]naphtho[2,1-b]pyrans linked by covalent bonds to oligothiophene units. Effect of thiophene substituents on the photochromism

The influence of oligothiophene substituents on five new [3H]naphtho[2,1-b]pyrans has been investigated using flash photolysis and continuous irradiation techniques. Photochromic properties strongly depend on the number and position of the thienyl units. Thienyl substitution increases the UV/visible absorbance of the lower energy transition of both the naphthopyran and merocyanine isomers. The rate constants of the thermal fading processes also increase with the number of thiophenic entities. Although several minor photoisomers, whose relative amounts depend on the irradiation conditions could be involved, it has been assumed that under continuous monochromatic irradiation, the evolution of the photochromic reaction can be described by the formation of only one photoisomer. Under these conditions, quantum yields of photocolouration and molar absorption coefficients of the non-isolable photomerocyanine isomer were evaluated by photokinetic analysis. Flash-photolytic colourability can be interpreted using continuous irradiation parameters. A linear correlation has been established between experimental and calculated colourability. This result shows that similar photochromic reactions leading to similar photoisomers are triggered either by polychromatic flash photolysis or by continuous monochromatic irradiation.

Mass-spring model of a self-pulsating drop.

Self-pulsating sessile drops are a striking example of the richness of far-from-equilibrium liquid/liquid systems. The complex dynamics of such systems is still not fully understood, and simple models are required to grasp the mechanisms at stake. In this article, we present a simple mass-spring mechanical model of the highly regular drop pulsations observed in Pimienta, V.; Brost, M.; Kovalchuk, N.; Bresch, S.; Steinbock, O. Complex shapes and dynamics of dissolving drops of dichloromethane. Angew. Chem., Int. Ed. 2011, 50, 10728-10731. We introduce an effective time-dependent spreading coefficient that sums up all of the forces (due to evaporation, solubilization, surfactant transfer, coffee ring effect, solutal and thermal Marangoni flows, drop elasticity, etc.) that pull or push the edge of a dichloromethane liquid lens, and we show how to account for the periodic rim breakup. The model is examined and compared against experimental observations. The spreading parts of the pulsations are very rapid and cannot be explained by a constant positive spreading coefficient or superspreading.

Bistable photochemical reactions

Abstract In photochemistry, under continuous irradiation, kinetic non-linearity results from Beer-Lamberts law. In monophotonic or photoequilibrated systems, non-linearity is too weak in order to exhibit dynamic instabilities. On the other hand, when there is a biphotonic process, as in the so-called ABC system or a process of auto-inhibition, as in the self-quenching model, screen effects (internal filter) are sufficient to give rise to kinetic instabilities such as bistability or oscillations. Starting from a simple bistable photochemical system, the kinetic scheme of a sustained photochemical oscillator operating in an isothermal reactor closed to flux of matter is designed. From the experimental point of view, the photochemistry of the triphenylimidazyl dimer (TPID) in chloroform is described as an example of a photochemical bistable system in a CSTR. The mechanism is interpreted quantitatively by application of the laws and concepts of photochemical dynamics. A series of experimental systems are reviewed and discussed in order to highlight the non-linear nature of their kinetics. Our objective is to pave the way for further experimental demonstrations of bistable or oscillating systems in photochemistry.

Ester containing aggregates in the autocatalytic biphasic hydrolysis of ethyl alkanoate

Abstract Autocatalytic kinetics are observed during the biphasic alkaline hydrolysis of C-4, C-6 and C-8 (butanoate, hexanoate and octanaoate) ethyl esters. Ethanol and alkanoate anions are the products of the reaction. A kinetic model including spontaneous dissolution of the organic phase into the aqueous phase, aqueous hydrolysis of the ester, formation of ester containing aggregates and micellisation has been proposed. For C-4, taking into account salting-in and solvent effects is sufficient to reproduce the autocatalytic behaviour. For C-8, the major autocatalytic process is an ester containing aggregates (ECA) mediated phase transfer. For C-6, it appears that the role of ECA is double. Small sized ECA which are formed at the beginning of the reaction do not perform rapid phase transfer and trap surfactant molecules. Larger ECA which appear at the end of the reaction are the main phase transfer agent. Using an empirical formula to describe the size of the interface, the model is able to reproduce the effect of a variation of the volumic ratio of the two phases as well as the effect of stirring rate and non-emulsified experiments.

Marangoni-driven flower-like patterning of an evaporating drop spreading on a liquid surface

Drop motility at liquid surfaces is attracting growing interest because of its potential applications in microfluidics and artificial cell design. Here we report the unique highly ordered pattern that sets in when a millimeter-size drop of dichloromethane spreads on an aqueous substrate under the influence of surface tension, both phases containing a surfactant. Evaporation induces a Marangoni flow that forces the development of a marked rim at the periphery of the spreading film. At some point this rim breaks up, giving rise to a ring of droplets, which modifies the aqueous phase properties in such a way that the film recoils. The process repeats itself, yielding regular large-amplitude pulsations. Wrinkles form at the film surface due to an evaporative instability. During the dewetting stage, they emit equally spaced radial strings of droplets which, combined with those previously expelled from the rim, make the top view of the system resemble a flower.In liquid–liquid systems, Marangoni effects induced by surface tension gradients may result in the formation of peculiar self-assembled patterns. Wodlei et al. utilize this effect to draw a ‘flower’ by letting an oil droplet evaporate on an aqueous substrate in the presence of a cationic surfactant.

Photorelease of nitric oxide (NO) on ruthenium nitrosyl complexes with phenyl substituted terpyridines

We investigated the influence of the nature of the substituent on the 4′-position of terpyridine ligands and the geometry around the metal ion on the photosensitization of ruthenium nitrosyl complexes. This was achieved by synthesizing a series of [Ru(R-Phtpy)Cl2(NO)]+ complexes based on tridentate terpyridine ligands with a substituted phenyl ring on the central pyridine (R = NO2, H, Br, OMe). The properties of both cis-(Cl,Cl)[Ru(R-Phtpy)Cl2(NO)]+ and trans-(Cl,Cl)[Ru(R-Phtpy)Cl2(NO)]+ isomers are presented. The optimized geometries and UV-Vis spectra were fully computed by DFT calculations and are in agreement with the experimental data. Quantum yields of NO release were obtained by numerical modeling and parameter optimization on the time evolution of the electronic spectra during irradiation. On irradiation with visible light in MeCN both cis(Cl,Cl)- and trans(Cl,Cl)-[Ru(MeO-Phtpy)Cl2(NO)](PF6) undergo facile photorelease of NO with the concomitant formation of the photoproduct as the solvate. Unambiguous characterization of the photoproduct for R = OMe was achieved by X-ray analysis and revealed that both isomers lead to trans(Cl,Cl)-[Ru(MeO-Phtpy)Cl2(MeCN)](PF6) as the unique photoproduct. The calculation of the relaxed potential energy surface confirmed an internal rearrangement of the cis(Cl,Cl) isomer to the trans(Cl,Cl) one after NO release.

Autocatalysis in liquid/liquid surfactant transfer

Equilibrium and transfer properties of two surfactants (a non-ionic one, Brij5, and a cationic one, CTAB) were studied in water/oil biphasic systems. Water/air surface tension measurements provide information on both the partition coefficient and also on the type of aggregates formed: classical micelles or swollen micelles (oil-in-water microemulsion). A clear correlation is established between the type of aggregates and the kinetics recorded during transfer of the surfactant from the organic to the aqueous phase. Indeed, an accelerating kinetics is obtained for every system leading to the formation of swollen micelles. This is the first example of autocatalytic dynamics observed in biphasic conditions for a non-reactive system. This non-linear behavior, correlated to the Critical Aggregation Concentration, confirms the contribution of an interface mediated pathway, known as the “budding mechanism”, to the formation of microemulsion for both non-ionic and ionic surfactants.