| 1. |
- Cabaleiro-Lago, Celia, et al.
(författare)
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Effects of zwitterionic vesicles on the reactivity of benzoyl chlorides
- 2006
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Ingår i: Journal of Physical Chemistry B. - Univ Vigo, Dept Phys Chem, Fac Chem, Vigo 36310, Spain. Univ Santiago de Compostela, Fac Chem, Dept Phys Chem, Santiago De Compostela 15782, Spain. : AMER CHEMICAL SOC. - 1520-6106 .- 1520-5207. ; 110:16, s. 8524-8530
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Tidskriftsartikel (refereegranskat)abstract
- A systematic study on the solvolysis reaction of substituted benzoyl chlorides in the presence of zwitterionic vesicles of dipalmitoyl phosphatidylcholine (DPPC) has been performed. Size, shape, surface charge, and polarity of the interface of the vesicular aggregates were determined using various techniques. The application of the pseudophase formalism allowed us to obtain the thermodynamic and kinetic coefficients characteristic of the reaction. The effects of vesicular aggregates on the solvolysis of benzoyl chlorides, which are known to be sensitive to the physical properties of the medium, depend on the nature of the substrate. For benzoyl chlorides with electron-donating groups, which react predominantly through a dissociative mechanism which is strongly affected by medium properties, the rate constant decreases as the vesicle concentration increases. On the other hand, for benzoyl chlorides with electron-withdrawing groups, which react mainly via an associative pathway, DPPC vesicles catalyze the solvolysis reaction.
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| 2. |
- Cabaleiro-Lago, Celia, et al.
(författare)
-
Fully uncomplexed cyclodextrin in mixed systems of vesicle-cyclodextrin : solvolysis of benzoyl chlorides.
- 2009
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Ingår i: Journal of Physical Chemistry B. - 1520-6106 .- 1520-5207. ; 113:19, s. 6749-6755
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Tidskriftsartikel (refereegranskat)abstract
- In this contribution the influence of beta-cyclodextrin (CD) on the behavior of aqueous systems containing vesicles of dipalmitoyl phosphatidyl choline (DPPC) has been studied by determining the kinetics of the solvolysis reaction of substituted benzoyl chlorides whose solvolysis reactivity entails a high sensitivity on media properties. The application of the pseudophase formalism allowed us to obtain the thermodynamic and kinetic coefficients characteristic of the reaction, which are essentially independent of the concentration of CD. We were able to determine the percentages of uncomplexed cyclodextrin in equilibrium with the vesicular system which were in all cases compatible with 100%. The obtained results led us to conclude that the properties of DPPC vesicles are not affected by the presence of CD in the medium and there is no type of interaction between the CD and the vesicular surfactant monomers and, therefore, all cyclodextrin is present in the mixed system as uncomplexed cyclodextrin.
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| 3. |
- Cabaleiro-Lago, Celia, et al.
(författare)
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In search of fully uncomplexed cyclodextrin in the presence of micellar aggregates
- 2006
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Ingår i: Journal of Physical Chemistry B. - Univ Vigo, Fac Quim, Dept Quim Fis, Vigo 36310, Spain. Univ Santiago, Fac Quim, Dept Quim Fis, Santiago 15782, Spain. : AMER CHEMICAL SOC. - 1520-6106 .- 1520-5207. ; 110:32, s. 15831-15838
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Tidskriftsartikel (refereegranskat)abstract
- The chemical behavior of beta-cyclodextrin/nonionic surfactant mixed systems has been investigated using the basic hydrolysis of N-methyl-N-nitroso-p-toluenesulfonamide as a chemical probe. The experimental results prove that at the cmc, there are significant quantities of uncomplexed beta-CD in equilibrium with the micellar aggregates. In contrast to the expected situation, the percentage of uncomplexed beta-CD in equilibrium with the micellar system increases on increasing the hydrophobicity of the surfactant molecule. This behavior is due to the existence of two simultaneous processes: complexation of surfactant monomers by cyclodextrin and the process of self-assembly to form micellar aggregates. The autoaggregation of surfactant monomers is expected to be more important than the complexation process in this mixed system. Varying the hydrophobicity of the surfactant monomer enabled us to determine that the percentages of uncomplexed cyclodextrin in equilibrium with the micellar system were in the range of 5-95%.
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| 4. |
- Cabaleiro-Lago, Celia, et al.
(författare)
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Reactivity of benzoyl chlorides in nonionic microemulsions : Potential application as indicators of system properties
- 2005
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Ingår i: Journal of Physical Chemistry B. - Univ Vigo, Fac Chem, Dept Phys Chem, Vigo 36310, Spain. Univ Santiago de Compostela, Fac Chem, Dept Phys Chem, Santiago De Compostela 15782, Spain. : AMER CHEMICAL SOC. - 1520-6106 .- 1520-5207. ; 109:47, s. 22614-22622
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Tidskriftsartikel (refereegranskat)abstract
- The solvolysis reactivity of benzoyl chlorides entails a high sensitivity on medium properties. A systematic study of the reaction of a series of these substrates, varying the electron-withdrawing character of the substituent, has been performed in nonionic microemulsions. The kinetic effects due to variation of microemulsion compositions can be assigned to modifications in system properties, to be precise, to modifications in interface properties. Microemulsion properties that are obtained from kinetic analysis of solvolysis show a good agreement with the characterization of the microemulsion that was made via H-1 NMR and solvatochromic fluorescence probes. Benzoyl chlorides with electron-donating groups react through a dissociative mechanism, whereas electron-withdrawing groups favor an associative mechanism. A comparative analysis of reactivity between the different substrates at the interface shows a variation in the contributions of both reaction pathways, associative and dissociative, to the whole reaction mechanism. The confined media shift the point where the mechanism changes from an associative to a dissociative pathway, far away from the turning point in water. Furthermore, the change in mechanism can be modulated by modification of the microemulsion composition.
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| 5. |
- Cabaleiro-Lago, Celia, et al.
(författare)
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Spectrophotometric study of metal-ligand reactions in isooctane/Brij30/water nonionic microemulsions
- 2007
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Ingår i: Colloids and Surfaces A. - Univ Vigo, Dept Quim Fis, Fac Quim, Vigo 36310, Spain. Univ Santiago Compostela, Fac Chem, Dept Chem Phys, Santiago 15782, Spain. : ELSEVIER SCIENCE BV. - 0927-7757 .- 1873-4359. ; 295:1-3, s. 49-54
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Tidskriftsartikel (refereegranskat)abstract
- The complexation between Ni2+ and pyridine-2-azo-p-dimethylaniline (PADA) was studied in isooctane/polyoxyethylenglycol dodecyl ether (Brij30)/water microemulsions at 25 degrees C. The apparent complexation constant depends on microemulsion composition. The proposed model, which takes into account the heterogeneity of the system at the microscopic scale, allows us to determine the distribution constants of both reactants and the complexation constant at the interface. The complex is less stable in microemulsion than in water due to a more efficient hydration of the nickel hexahydrate coordination complex that arises from the interaction between the polar head group of the surfactant and the interfacial water. (c) 2006 Elsevier B.V. All rights reserved.
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| 6. |
- Cabaleiro-Lago, Celia, et al.
(författare)
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The effect of changing the microstructure of a microemulsion on chemical reactivity
- 2007
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Ingår i: Langmuir. - Univ Santiago de Compostela, Fac Quim, Dept Quim Fis, Santiago De Compostela 15782, Spain. Univ Vigo, Fac Quim, Dept Quim Fis, Vigo 36310, Spain. : AMER CHEMICAL SOC. - 0743-7463 .- 1520-5827. ; 23:19, s. 9586-9595
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Tidskriftsartikel (refereegranskat)abstract
- A kinetic study was carried out on various solvolytic reactions in water/NH4OT/isooctane microemulsions. The NH4OT surfactant is a derivative of the sodium salt of bis(2-ethylhexyl) sulfosuccinate (NaOT or AOT), where the Na+ counterion has been replaced by NH4+. The counterion substitution effects the phase diagram of the system, and therefore, NH4OT-based microemulsions with high water content reaching values of W = 350 (W = [H2O]/[NH4OT]) can be obtained. The presence of high W values suggests a transition in the microemulsion microstructure from water-in-oil (w/o) to oil-in-water (o/w), as was confirmed by conductivity and H-1 NMR self-diffusion measurements. The interpretation of the kinetic studies in terms of pseudophase formalism allows us to analyze the effect of the microemulsion on chemical reactivity, regardless of its microstructure. It has been confirmed that the values of the solvolytic rate constants at the interphase of oil-in-water microemulsions are similar to those obtained for aqueous SDS systems, showing that the hydration degree of the interphase of the oil-in-water microemulsions is independent of W. The influence of the surfactant counterion on the solvolytic rate constants was analyzed by comparing HOT-, NaOT-, and NH4OT-based microemulsions. An important influence on the rate constants caused by the changes in the structural properties of water has been observed as was confirmed by the water H-1 NMR signals.
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