P. Rodríguez-Dafonte

Self-Aggregation Properties of Ionic Liquid 1,3-Didecyl-2-methylimidazolium Chloride in Aqueous Solution: From Spheres to Cylinders to Bilayers

The self-aggregation behavior of the double-chained ionic liquid (IL) 1,3-didecyl-2-methylimidazolium chloride ([C10C10mim]Cl) in aqueous solution has been investigated with a number of different experimental techniques. Two cmc values (cmc1 and cmc2) are obtained from conductivity measurements. The fraction of neutralized charge on the micellar surface suggests that cmc1 corresponds to the formation of spherical micelles and cmc2 to the transition from spherical to cylindrical micelles. Data obtained from fluorescence spectroscopy (using pyrene and Nile red as chemical probes), fluorescence anisotropy (using rhodamine B as probe), and chemical shift (1)H NMR (in D2O) provide a picture that is also consistent with a sphere-to-cylinder transition. This structural change is further confirmed by diffusion-ordered NMR spectroscopy (DOSY), from the self-diffusion coefficients for surfactant unimer and aggregates. Furthermore, a third evolution from cylindrical micelles to bilayer aggregates is proposed from the analysis of diffusion coefficients at high surfactant concentration ([IL] > 0.2 M). Phase scanning experiments performed with polarized light microscopy clearly demonstrate the presence of a lamellar liquid crystalline phase at very high IL concentration, thus confirming the coexistence of bilayer structures with elongated micelles, found at lower concentration. Additionally, [C10C10mim]Cl micelles are proposed as novel reaction media, as evidenced by the solvolysis reaction of 4-methoxybenzenesulfonyl chloride (MBSC).

Competition between surfactant micellization and complexation by cyclodextrin

Supramolecular property systems composed of alkyltrimethylammonium surfactants and β-cyclodextrin were studied by means of a chemical probe. Solvolysis of 4-methoxybenzenesulfonyl chloride (MBSC) was used in the mixed systems with the aim of being able to determine the concentration of uncomplexed cyclodextrin in equilibrium with the micellar system. The surfactants used enabled us to vary the length of the hydrocarbon chain between 6 and 18 carbon atoms. In all cases the existence of a significant concentration of uncomplexed CD was observable in equilibrium with the micellar system. The percentage of uncomplexed cyclodextrin increases both on increasing and decreasing the surfactant alkyl chain length, being minimal for alkyl chains between 10-12 carbon atoms. This behavior is a consequence of two simultaneous processes: complexation of surfactant monomers by the cyclodextrin and surfactant self-assembly to form micellar aggregates. By using Gibbs free energies for micellization and surfactant complexation by β-CD, we can quantitatively explain the observed behavior.

Evidence for complexes of different stoichiometries between organic solvents and cyclodextrins

The influence of the organic solvent on the acid and basic hydrolysis of N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) in the presence of alpha- and beta-cyclodextrins has been studied. The observed rate constant was found to decrease through the formation of an unreactive complex between MNTS and the cyclodextrins. In the presence of dioxane, acetonitrile or DMSO, the inhibitory effect of beta-CD decreased on increasing the proportion of organic cosolvent as a result of a competitive reaction involving the formation of an inclusion complex between beta-CD and the cosolvent. The disparate size of the organic solvent molecules resulted in stoichiometric differences between the complexes; the beta-CD-dioxane and beta-CD-DMSO complexes were 1 : 1 whereas the beta-CD-acetonitrile complex was 1 : 2. The basic and acid hydrolysis of MNTS in the presence of alpha-CD showed a different behavior; thus, the reaction gave both 1 : 1 and 2 : 1 alpha-CD-MNTS complexes, of which only the former was reactive. This result was due to the smaller cavity size of alpha-CD and the consequent decreased penetration of MNTS into the cavity in comparison to beta-CD. The acid hydrolysis of MNTS in the presence of alpha-CD also revealed decreased penetration of MNTS into the cyclodextrin cavity, as evidenced by the bound substrate undergoing acid hydrolysis. In addition, the acid hydrolysis of MNTS in the presence of acetonitrile containing alpha-CD gave 1 : 1 alpha-CD-acetonitrile inclusion complexes, which is consistent with a both a reduced cavity size and previously reported data.

Polarity of the interface in ionic liquid in oil microemulsions.

Ionic liquid based microemulsions were characterized by absorption solvatochromic shifts, (1)H NMR and kinetic measurements in order to investigate the properties of the ionic liquid within the restricted geometry provided by microemulsions and the interactions of the ionic liquid with the interface. Experimental results show a significant difference between the interfaces of normal water and the new ionic liquid microemulsions. Absorption solvatochromic shift experiments and kinetic studies on the aminolysis of 4-nitrophenyl laurate by n-decylamine show that the polarity at the interface of the ionic liquid in oil microemulsions (IL/O) is higher than at the interface of water in oil microemulsions (W/O) despite the fact that the polarity of [bmim][BF(4)(-)] is lower than the polarity of water. (1)H NMR experiments showed that an increase in the ionic liquid content of the microemulsion led to an increase in the interaction between [bmim][BF(4)(-)] and TX-100. The reason for the higher polarity of the microemulsions with the ionic liquid can be explained in terms of the incorporation of higher levels of the ionic liquid at the interface of the microemulsions, as compared to water in the traditional systems.

Denitrosation of N-Nitrososulfonamide as Chemical Probe for Determination of Binding Constants to Cyclodextrins

The influence of β-CD concentration on the acid hydrolysis of N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) has been studied in the presence and absence of different alcohol concentrations. The rate of the denitrosation reaction in bulk water decrease as the β-CD concentration increases due to MNTS complexation in the CD cavity and the reaction taking place exclusively outside the cyclodextrin. Changes in this inhibition due to the presence of β-CD allow us to obtain the binding constants of different alcohols to the cyclodextrin. These binding constants are in very good agreement with those determined in the bibliography by other methods.

Different kinetic behaviors for unimolecular and bimolecular ester hydrolysis reactions in strongly acidic microemulsions.

Replacing the counterion in sodium bis(2-ethylhexyl)sulfosuccinate with H(+) allows strongly acidic microemulsions to be obtained. These systems are the only known colloidal medium in which it is possible to reach local concentrations of acid, expressed as Hammett acidity function (H(0)), lower than H(0) = -0.2, which corresponds to a concentration of acid above 1 M in aqueous solution. In the present work, there has been analyzed the influence of this type of microemulsion on the acid hydrolysis of two esters derived from picolinic acid: 4-nitrophenylpicolinate (NPP) and 2,4-dinitrophenylpicolinate (DNPP). The reaction rate for NPP and DNPP increases up to 16 times on increasing the size of the aqueous nanocore of the microemulsion, which supposes an experimental behavior opposed to the one observed for the hydrolysis of nitrophenylacetate (NPA). The key to this differentiated behavior of NPP and DNPP resides in the fact that the rate-determining step for the acid hydrolysis mechanism is the water addition to the protonated ester. The reaction rate increases on increasing the nucleophilicity of water; that is, on increasing W (W = [H(2)O]/[surfactant]). Therefore, the acid hydrolysis of esters in strongly acidic microemulsions presents an A2 mechanism when reactivity increases with W, and an A1 mechanism if it decreases with W.

Mechanism of the deprotonation reaction of alkyl benzyl ethers with n-butyllithium.

Kinetic study of the α-lithiation of benzyl methyl ether (BME) by nBuLi has revealed that increasing the concentration of the organolithium compound does not necessarily increase the reactivity, and this is a consequence of the reactivities of the different nBuLi aggregates present in solution. We propose a dimer-based mechanism, in which a pre-complexation step is a key process for substrates bearing a donor oxygen atom that can interact with the lithium cation to form mixed dimers. For these studies, we have developed a system based on UV/Vis spectroscopy that allows kinetic measurements to be conducted at -80 °C under argon.

Kinetic Study of [2]Pseudorotaxane Formation with an Asymmetrical Thread

Kinetic and thermodynamic studies on cyclodextrin (CD)-based [2]pseudorotaxane formation have been carried out by a combination of NMR and calorimetric techniques using bolaform surfactants as axles. Experimental evidence of the formation of an external complex between the trimethylammonium head groups of the axle and the external hydrogen atoms of α-cyclodextrin (α-CD) is reported. Inclusion of this external complex in the reaction pathway allows us to explain the kinetic behavior as well as the nonlinear dependence of the observed rate constant on CD concentrations. The equilibrium constant for [2]pseudorotaxane formation is strongly affected by the spacer length of the axle. This effect is a consequence of increasing rotaxane stability because the threading rate constant is almost independent of the spacer length, but dethreading strongly decreases on increasing the axle size. Using a nonsymmetrical axle with tripropyl and trimethylammonium cations precludes CD threading by the large head side. CDs will thread this asymmetrical bolaform by both their wide and narrow sides, yielding two isomeric [2]pseudorotaxanes. Threading by the wide side of the CD is 60% more favorable than that by the narrow one, but dethreading rate constants are the same for both isomers.

γ-Cyclodextrin modulates the chemical reactivity by multiple complexation

Multiple complexation by γ-CD has been studied by self-diffusion coefficients (DOSY) and chemical kinetics experiments in which 4-methoxybenzenesulfonyl chloride (MBSC) solvolysis was used as a chemical probe. The addition of a surfactant as a third component to the reaction mixture induced a very complex reactivity pattern that was explained on the basis of multiple complexation phenomena and surfactant self-assembly to form micelles. A cooperative effect that yielded a ternary complex formed by cyclodextrin-surfactant-MBSC was observed. The larger cavity of γ-CD in comparison with β-CD is responsible for the change from the competitive complexation mechanism predominant with β-CD to a cooperative/competitive mixed mechanism operating for the larger derivative. The cavity size in γ-CD is large enough to bind two surfactant alkyl chains with a cooperative effect. Water molecules released by the formation of 1:1 host-guest complexes made the cavity more hydrophobic and promoted further inclusion. A reduction in the available volume of the cavity should be considered on binding a second guest.

Displacement assay methodology for pseudorotaxane formation in the millisecond time-scale

Abstract Rotaxanes, formed by an axis through the cavity of a macrocycle, are promising systems for the construction of molecular machines. A very limited number of experimental techniques are available for mechanistic studies since only mechanical bonds are formed, being NMR one of the most widely used. The major inconvenience derived from NMR use is the time-scale for threading/dethreading processes lasting a few minutes in the case of faster processes. In the present manuscript, we report the application of a new kinetic methodology based on a displacement assay for cyclodextrin-based pseudorotaxane formation. By coupling a very fast (microseconds time-scale) binding/dissociation of nitrophenol to α-CD with a dicationic axle threading/dethreading process, we have been able to study kinetic processes taking place in the millisecond time-scale.