This paper introduces a new mathematical model that is used to compute either the interfacial tension of quiescent axisymmetric pendant/sessile drops and pendant/captive bubbles. This model consists of the Young-Laplace equation, that describes interface shape, together with suitable boundary conditions that guarantee a prescribed volume of drops/bubbles and a fixed position in the capillary. In order to solve the problem numerically, the Young-Laplace equation is discretized by using numerical differentiation and the numerical solutions are obtained applying the well-know Newton method. The paper contains a validation of the new methodology presented for what theoretical bubble/drops are used. Finally, some numerical results are presented for both drops and bubbles of water as well as several surfactant solutions to demonstrate the applicability, versatility and reproducibility of the proposed methodology.
Controlling S-nitrosothiol decomposition, with the consequent release of nitric oxide, is a topic of great research interest. The incorporation of nitrosomercaptopyridine (SNO+) into the cucurbit[7]uril cavity results in a large increase of its nitrosation equilibrium constant. This effect being a consequence of the preferential stabilization of organic cations by the formation of host : guest complexes with CB7 results in a drastic reduction of the SNO+ denitrosation rate constant. Moreover, SNO+ encapsulation also prevents its decomposition yielding disulfide and nitric oxide. The expulsion of SNO+ from the cucurbituril cavity through the application of a chemical stimulus (competitive binding) results in controlled nitric oxide release as was confirmed by using a NO selective electrode.
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 ß-CD is responsible for the change from the competitive complexation mechanism predominant with ß-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.
The host-guest chemistry between cucurbit[7]uril (CB7) and a series of bolaform (Bn) surfactants with different chain lengths, n = 12-22, was the target of our study. [3]Pseudorotaxanes are formed when the alkyl chain of the bolaform has more than 14 carbon atoms. In these cases, two CB7 molecules can be accommodated between the two head groups of the bolaform without addition of electrolytes to the medium. In the case of a bolaform with 12 carbon atoms, the electrostatic repulsion between the carbonyl groups of the CB7 molecules avoids the threading of a second CB7 molecule yielding a mixed structure formed by a [2]pseudorotaxane and an external host-guest complex. The assembly behavior was investigated using NMR spectroscopy, isothermal titration calorimetry (ITC), and kinetic measurements.
Supramolecular property systems composed of alkyltrimethylammonium surfactants and ß-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 ß-CD, we can quantitatively explain the observed behavior.
Cyclodextrins/chemistry , Micelles , Surface-Active Agents/chemistry , Hydrolysis
The host-guest assembly of CB7 with a series of alkyl(trimethyl)ammonium (C(n)TA(+)) surfactants of different chain lengths (n=6-18) has been studied. The complexation behaviour was investigated by NMR spectroscopy, isothermal titration calorimetry and kinetics measurements. The combined results of these techniques provided evidence for the formation of 1:1 inclusion and 2:1 external complexes in the cases of C(n)TA(+) with n=12-18. The binding constants for the 1:1 complexes are independent of the alkyl chain length of the surfactant, whereas a relationship between K(2:1) and the chain length of the surfactant was found for the 2:1 complexes.
To understand the analogies and differences between the cucurbituril and cyclodextrin cavities different solvolytic reactions have been studied in the presence of cucurbit[7]uril, CB7, and beta-CD or its methylated derivative, DM-beta-CD. Solvolysis of 1-bromoadamantane has been used as a test to evaluate the ability of the cavities to solvate the Br(-) leaving group. Obtained results show that in both cases the polarity inside the cavity is similar to that of a 70% ethanol:water mixture. Solvolysis of substituted benzoyl chlorides shows a great difference between the CB7 and DM-beta-CD cavity. Solvolysis of electron withdrawing substituted benzoyl chlorides (associative mechanism) is catalyzed by DM-beta-CD and inhibited by CB7. However, solvolysis of electron donating substituted benzoyl chlorides (dissociative mechanism) is catalyzed by CB7 and inhibited by DM-beta-CD. These experimental behaviors have been explained on the basis of different solvolytic mechanisms. Participation of the hydroxyl groups of the cyclodextrin as a nucleophile can explain the catalytic effect observed for solvolysis of benzoyl chlorides reacting by an associative mechanism. Solvolysis of benzoyl chlorides reacting by a dissociative mechanism is catalyzed by CB7 due to the ability of the CB7 cavity to stabilize the acylium ion developed in the transition state by electrostatic interactions.
Bridged-Ring Compounds/chemistry , Imidazoles/chemistry , Macromolecular Substances/chemistry , Catalysis , Cyclodextrins , Models, Molecular , Solubility , Stereoisomerism , Structure-Activity Relationship
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.
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.
Benzoates/chemistry , Cyclodextrins/chemistry , Membranes, Artificial , Solvents/chemistry , 1,2-Dipalmitoylphosphatidylcholine/chemistry , Kinetics , Water/chemistry
A study was carried out on the influence of the n-alkyl acid addition on the electric percolation of AOT/iso-octane/water microemulsions ([AOT] = 0.5 M and W= [H(2)O]/[AOT] = 22.2). The observed influence has been explained taking into account the organic nature of these molecules and, hence, their capacity of disturbing the structure of the AOT-film. For these reasons, relationships with their molecular structure (chain length) were analysed.
Carboxylic Acids/chemistry , Succinates/chemistry , Electric Conductivity , Electrochemistry , Emulsions/chemistry , Octanes/chemistry , Surface Properties , Water/chemistry
The kinetics of the hydrolysis of 4-methoxybenzenesulfonyl chloride (MBSC) have been studied in mixed systems made up of surfactant, sodium dodecyl sulfate (SDS) or tetradecyltrimethylammonium bromide (TTABr), and cyclodextrin, beta-CD or SBE-beta-CD(Captisol). The use of SBE-beta-CD instead of beta-CD allowed us to indicate certain characteristics of the mixed cyclodextrin-surfactant system: (a) The percentage of uncomplexed cyclodextrin is higher for SBE-beta-CD than for beta-CD when we use SDS, but the opposite effect was observed when we use TTABr. This behavior can be explained by taking into account the increase in salinity when we add SBE-beta-CD, and the electrostatic forces between the SBE-beta-CD and the surfactant that have influence on the complexation. (b) The presence or even the charge of cyclodextrin has no effect on the properties of surfactant micelles once they have been formed; in particular, it does not alter K(s)(m) or k(m), parameters very sensitive to the micellar system structure. Therefore, we can conclude that for surfactants concentrations lower than the micellization point, the charge of cyclodextrin modifies the cyclodextrin-surfactant interactions but once the micelles have been formed there is no interaction between them and the cyclodextrins.
Cyclodextrins/chemistry , Sulfones/chemistry , Surface-Active Agents/chemistry , Anions/chemistry , Cations/chemistry , Hydrolysis , Kinetics , Micelles , Molecular Structure
An ion-exchange process has been used to prepare HOT from NaOT (sodium bis(2-ethylhexyl)sulfosuccinate), where the Na(+) counterion has been replaced by H(+). The acidity function, H(0), of the aqueous core of HOT-based microemulsions decreases with W from H(0) approximately 0.6 at W>20 to H(0)=-1.4 at W=2. On the basis of the H(0) acidity function of the aqueous core and the dependence of H(0) on acid concentration, we have been able to determine the degree of counterion binding (beta) in microemulsions with a value of beta approximately 0.92 which is practically independent of the water content of the system.
The catalytic effect of triethylene glycol dimethyl ether (glyme) on the butylaminolysis of 4-nitrophenylcaprate (NPC) in water/AOT/chlorobenzene microemulsions has been studied. Experimental results show the existence of four simultaneous reaction pathways. One of them takes place at the microemulsion interphase where the rate-determining step of butylaminolysis is the formation of the addition intermediate, T+/-. The locus of the other three pathways is the continuous medium of the microemulsion. These three pathways consist of the decomposition of the addition intermediate catalyzed by butylamine, by glyme, and by both of them. The kinetic model allows us to obtain the value of every rate and distribution constant involved in the overall reaction mechanism. We must emphasize that the reactions located in the continuous medium exhibit a kinetic behavior similar to the corresponding one found in pure chlorobenzene. On the basis of the pseudophase model, the percentage of reaction in each of the microdomains of the microemulsion has been calculated. Likewise, changes in the loci of reaction from the interphase to the continuous medium as a function of catalyst concentration have been proved.
A kinetic study was carried out on the acid hydrolysis of 4-nitrophenylacetate and 4-nitrophenyllaurate in water/HOT/isooctane microemulsions. The substitution of Na+ in the sodium salt of bis(2-ethylhexyl)sulfosuccinate by H+ has permitted us to obtain a functionalized surfactant (HOT) and, consequently, strongly acid microemulsions. The use of HOT-based microemulsions allows us to reach concentrations of H+ in the aqueous core corresponding to a Hammett acidity function of H0 = -2. The rate constant at the interface and the distribution constants of the carboxylic esters throughout the different microenvironments of the microemulsion have been quantified by application of the pseudophase formalism. The results obtained show that the hydrolysis rate constant at the interface increases as the water content of the system decreases. The correlation of the rate constants at the interface of the microemulsion with the Hammett acidity function, H0 (on the basis of the Bunnett-Olsen criterion), has allowed us to confirm that the hydrolysis process takes place via an A2 mechanism for high water contents and through an A1 mechanism for values of W
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 1H 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 1H NMR signals.
Replacing the counterion in sodium bis(2-ethylhexyl)sulfosuccinate (NaOT, usually known as AOT or Aerosol OT) with H+ (HOT) allows strongly acidic microemulsions to be obtained through the effect of a change in the solvation mechanism of the surfactant, where the Na+...OH2 interaction is displaced by a stronger H+...OH2 interaction. This raises the proportion of water bound to the counterion, which is reflected in the FT-IR spectrum for water trapped in the microemulsion and the 1H NMR spectrum for the hydrogen atoms in the water molecules. In NaOT microemulsions, the resonance signal for hydrogen atoms in the water molecules increases from delta approximately 3.9 ppm at W = 2 (with W = [H2O]/[NaOT]) to delta approximately 4.8 ppm at W = 50. In HOT microemulsions, the disparate strength of Na+...OH2 and H+...OH2 interactions results in a decrease in the resonance signal for the hydrogen atoms in the water molecules from delta approximately 8.6 ppm at W = 2 to delta approximately 4.9 ppm at W = 50. These changes in the physical properties of water alter chemical reactivity in a way that is clearly apparent in solvolytic processes in NaOT and HOT microemulsions. Thus, the rate constants of reactions involving an associative mechanism increase with decreasing W in NaOT microemulsions, but decrease with decreasing W in HOT microemulsions. The disparate behavior is a result of a decreased nucleophilicity of interfacial water in HOT microemulsions relative to NaOT microemulsions. For a dissociative process the rate constants are greater in HOT microemulsions than in NaOT ones, and increase with increasing W in both types of microemulsions, which can be ascribed to an increased electrophilicity of interfacial water in HOT microemulsions.
The influence of acidity upon the pyridine-2-azo-p-dimethylaniline (PADA) absorption spectrum has been studied. The obtained results allowed us to calculate the acidity constants of PADA. The spectra resolution method has been used to determinate the constants. The absorption spectrum was decomposed in two sub-bands for the neutral form of the indicator, one for the monoprotonated molecule and another more for the diprotonated structure. The quantitative analysis of relative areas variation with the medium acidity allows us to obtain the equilibrium constants of PADA prolongation. The obtained values are in good agreement with the values reported in the literature.
Aniline Compounds/chemistry , Pyridines/chemistry , Spectroscopy, Near-Infrared , Hydrogen-Ion Concentration , Kinetics , Spectrophotometry, Ultraviolet
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%.
Micelles , beta-Cyclodextrins/chemistry , Hydrolysis , Molecular Structure , Sulfonamides/chemistry , Surface Tension , Surface-Active Agents/chemistry
A study was carried out on the solvolysis reactions of different benzoyl halides in microemulsions of water/NH4DEHP/isooctane, where NH4DEHP is ammonium bis(2-ethylhexyl) phosphate. Because of the low solubility of benzoyl halides in water, they are distributed between the continuous medium and the interface of the microemulsion, where the reaction takes place. The application of the pseudophase model has allowed us to obtain the distribution constants and the rate constants at the interface for the benzoyl halides. Reaction mechanisms and the changes in these mechanisms in terms of the water content of the microemulsion have been determined on the basis of kinetic data. The influence of the substituent and the leaving group on the reaction rate has been investigated. A comparison of kinetic results with those previously obtained in water/AOT/isooctane microemulsions allows a kinetic evaluation of the change in the microemulsion properties with the surfactant.
