ABSTRACT
In this contribution, the influence of the adsorption of dodecylsulfate, an anionic surfactant, on the stability of colloidal aqueous dispersions containing ceria (CeO2) nanoparticles (NPs) was investigated using zetametry, UV-visible spectrophotometry, and potentiometry involving an ionic surfactant-selective electrode (ISSE). In particular, thanks to absorbance follow-ups carried out as a function of time, aqueous dispersions containing a given loading of CeO2 NPs were found simultaneously to stabilize more quickly with time and to adopt a higher opacity and a more pronounced light-yellow color as the sodium dodecylsulfate (SDS) concentration increased. Knowing that this absorbance was attributed undoubtedly to CeO2 NPs, the fact that the measured absorbance is lower for a higher amount of CeO2 NPs in suspension, as revealed by a higher opacity of the studied dispersions, is somewhat counterintuitive. Besides the higher opacity of the dispersions, a shield effect of the adsorbed SDS layer toward UV-visible light may also explain this observation. The adsorption of dodecylsulfate on CeO2 NPs was indeed demonstrated using zetametry measurements in the presence of SDS and the potentiometric method combined with an ISSE. This latter method did not only allow the accurate determination of impoverishment in freely diffusing dodecylsulfate (DS) anions resulting from DS adsorption on CeO2 NPs but it also showed that this latter obeys a Freundlich isotherm.
ABSTRACT
We report a systematic study of the effect of the cationic chain length and degree of hydrogen bonding on several equilibrium and transport properties of the first members of the alkylammonium nitrate protic ionic liquids (PILs) family (ethylammonium, propylammonium, and butylammonium nitrate) in the temperature range between 10 and 40 °C. These properties were observed by means of several experimental techniques, including density, surface tension, refractometry, viscosimetry, and conductimetry. The dilatation coefficients and compressibilities, as well as the Rao coefficients, were calculated, and an increase of these magnitudes with alkyl chain length was detected. Moreover, the surface entropies and enthalpies of the studied PILs were analyzed, and the temperature dependence of the surface tension was observed to be describable by means of a harmonic oscillator model with surface energies and critical temperatures that are increasing functions of the cationic chain length. Moreover, the refractive indexes were measured and the thermo-optic coefficient and Abbe numbers were calculated, and the contribution of the electrostrictive part seemed to dominate the temperature dependence of the electric polarization. The electric conductivity and the viscosity were measured and the influence of the degree of hydrogen bonding in the supercooled liquid region analyzed. Hysteresis loops were detected in freezing-melting cycles and the effect of the length of the alkyl chain of the cation on the size of the loop analyzed, showing that longer chains lead to a narrowing of the supercooled region. The temperature dependence of the conductivity was studied in the Vogel-Fulcher-Tamman (VFT) framework and the fragility indices, the effective activation energies, and the Vogel temperatures obtained. A high-temperature Arrhenius analysis was also performed, and the activation energies of conductivity and viscosity were calculated, showing that these transport processes are governed by two distinct mechanisms. The exponents of the fractional Walden rule for the different compounds were obtained. Finally, the ionicities and fragilities of the studied PILs were analyzed, proving that all the studied PILs are subionic and fragile liquids, with propylammonium nitrate showing the lowest fragility and the greater ionicity of all the studied compounds.
ABSTRACT
The existence and properties of mesoscopic self-assembly structures formed by surfactants in protic ionic liquid solutions are reported. Micellar aggregates of n-alkyltrimethylammonium (n = 10, 12, 14, 16) chlorides and bromides and of n-alkylpyridinium (n = 12, 16) chlorides in ethylammonium nitrate and propylammonium nitrate were observed by means of several experimental techniques, including surface tension, transmission electron micrography, dynamic light scattering, and potentiometry using surfactant-selective electrodes. The effect of the alkyl chain length of both solute and solvent molecules on the critical micelle concentration is discussed, and a Stauff-Klevens law is seen to apply to surfactant solutions in both protic ionic liquids. The counterion role is also a matter of study in the case of alkyltrimethylammonium-based surfactants, and the presently reported evidence suggests that the place of the surfactant counterion in the Hoffmeister's series could determine its effect on micellization in IL solution. The size distribution of the aggregates is also analyzed together with the Gibbs free energies of micellization and the minimum surface area per monomer in all of the studied cases. All of the hereby reported evidence suggests that the negative entropic contribution arising from the release of the solvent layer upon micellization is also the driving force of conventional surfactant self-association in protic ionic liquids.
ABSTRACT
A theoretical model for ionic liquids (ILs) based on a pseudoreticular structural model for the bulk mixture is reported. The original Bahe-Varela pseudolattice theory of concentrated ionic solutions is modified and the short-range interactions modeled by a Lennard-Jones potential. In this framework, the surface tension of the pure IL is calculated and the correct dependence of this magnitude on the density of the liquid, as provided by the parachor, is recovered. The anions in the mixture are assumed to form a continuum structureless neutralizing background, and that the organic cations and water molecules are placed in the nodes of the pseudolattice. The surface pressure of IL-water mixtures is calculated using a localized model for the adsorption of particles in the surface of the mixture and a mean-field Bragg-Williams approximation for the chemical potential of the adsorbed particles in the pseudolattice. The theoretical predictions are tested with experimental data of several ionic liquid aqueous mixtures.
ABSTRACT
Molten salts at room temperature and their mixtures with water or molecular solvents are excellent candidates for future replacement of most of organic solvents used in many industrial processes. To make this possible and to allow efficient application, it is necessary to determine physico-chemical parameters (such as the acidity scales) for these reaction media. This work follows a study of the autoprotolysis constants (K(s)) of water-1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF(4)) mixtures at 298.15K over the composition range of 0-77.43 vol.% bmimBF(4) [I. Bou Malham, P. Letellier, M. Turmine, Talanta 72 (2007) 155-164]. In this second analysis, we determined the values of the dissociation constant (pK(a)) of various conjugate acid-base pairs for the same water-bmimBF(4) mixtures, to establish acidity scales for each medium. These data can be used to produce proton buffer solutions and thus to control the acidity level of water-ionic liquid (IL) mixtures. We compare the values of pK(a) for acid-base pairs in water-bmimBF(4) mixtures with published values for water-methanol mixtures.
ABSTRACT
The structure of aqueous solutions of a prototype ionic liquid, the short alkyl chain 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim][BF4]) has been investigated by small-angle neutron scattering. Concentration fluctuations and Kirkwood-Buff integrals have been calculated, and the results are in good agreement with corresponding data calculated herein from vapor pressure measurements. The large concentration fluctuations and Kirkwood-Buff integral values indicate that the system is in the vicinity of phase separation, which is known to occur some 20 K below room temperature, at a salt mole fraction of around 0.075.
ABSTRACT
The autoprotolysis constants (K(s)) of water - 1-butyl-3-methylimidazolium tetrafluoroborate (bmimBF(4)) mixtures were determined at 298K over the composition range of 0 to 77.43vol.% bmimBF(4) using potentiometric method with a glass electrode. A slight increase in the autoprotolysis constant was observed when the salt was added to the water. The value of the ionic product of the medium then decreases as the bmimBF(4) content increases from about 20vol.%. The acid-base properties of these media were perfectly described by Bahe's approaches that were completed by Varela et al. concerning structured electrolyte solutions with large short-range interactions.
ABSTRACT
We studied the adsorption of a cationic surfactant, dodecyltrimethylammonium bromide (DTABr), onto laponite in water-methanol and water-DMSO mixtures at 298 K. Adsorption isotherms were determined by potentiometric methods using DTA(+) and Br(-)-selective electrodes. All of the isotherm shapes in the mixture were classical. Activity coefficients were determined for the transfer between water and hydroorganic mixtures. Comparison of the adsorption isotherms obtained for different surfactant chemical potentials showed that at higher concentrations of free surfactant, the aggregation state does not depend on the nature of the surrounding media. A study of the influence of the ionic strength showed that there is a common intersection point in each hydroorganic mixture. This point corresponds to the charge compensation point and depends on the nature of the surrounding medium.
ABSTRACT
The adsorption of a solute on a solid can be followed by contact angle measurements of a drop of the solution on the solid. The Gibbs isotherm model can be used for quantitative interpretation of wettability variations. Its use in linking the wettability to the adsorption isotherm involves assimilating the Gibbs' planes to the surfaces themselves. Within this framework, these interpretations lead to the conclusion that adsorption of surface-active agents is greater on solid-vapor interfaces than on solid-liquid interfaces, for hydrophilic solids. This is not the only approach. Thermodynamics allows other formalisms, the conclusions of which can be completely different. We present a thermodynamic approach which explicitly reveals relationships between surface tensions and contents of surfaces, without referring to the Gibbs' plane. This permits us to explain the behavior of a drop of surfactant solution put on hydrophilic or hydrophobic solids with conclusions different from those reached using the Gibbs approach. We show that all these thermodynamic approaches are linked; they do not dismiss one another but give different views of the same phenomenon. Copyright 2000 Academic Press.
ABSTRACT
Titration is most often associated with the idea of a stoichiometric reaction. Generally, it is not considered possible to titrate a compound against a reagent unless the titration reaction is near total and if the result is not a product of well-defined stoichiometry. In this work, we illustrate that accurate titration is possible with compounds and reagents that interact to form an association of undefined stoichiometry. Our model is the potentiometric titration of nonionic surfactant with cationic surfactant using a cationic surfactant-selective electrode. The result of this reaction is mixed micelles, the composition of which depends on the concentrations of the two surfactants in solution.
