J. C. Mejuto

Enhancement of copper and cadmium adsorption on kaolin by the presence of humic acids.

The competitive adsorption equilibrium isotherms of Cu2+ and Cd2+ on kaolin have been measured at 298 K, in the presence and the absence of humic acids (HAs). HAs were found to enhance the metal adsorption capacity of mineral surfaces, in particular kaolin. This enhancement was also observed in the competitive adsorption of copper and cadmium on kaolin and kaolin-HA complex. This competitive adsorption shows that the presence of Cd2+ has not an important effect on Cu2+ adsorption, whereas a dramatic decrease is observed on the adsorption of Cd2+ in the presence of Cu2+. The Freundlich isotherm equation was found to provide an excellent fit to the experimental data. These results were compared with the independent adsorption of both heavy metals.

Microemulsions as microreactors in physical organic chemistry

Microemulsions are very versatile reaction media which nowadays find many applications, ranging from nanoparticle templating to preparative organic chemistry. The thermodynamically stable and microheterogeneous nature of microemulsions, used as reaction media, induces drastic changes in the reagent concentrations, and this can be specifically used for tuning the reaction rates. In particular, amphiphilic organic molecules can accumulate and orient at the oil-water interface, inducing regiospecificity in organic reactions. In this review, we will show the recent tendencies of the use of microemulsions as organic reaction media.

Evaluation of atmospheric Poaceae pollen concentration using a neural network applied to a coastal Atlantic climate region

In the South of Europe an important percentage of population suffers pollen allergies, being the Poaceae pollen the major source. One of aerobiologys objectives is to develop statistical models enabling the short- and long-term prediction of atmospheric pollen concentrations to take preventative measures to protect allergic patients from the severity of the atmospheric pollen season. The implementation of a computational model based on supervised MLP neural network was applied for the prediction of the atmospheric Poaceae pollen concentration. There is a good correlation between the values predicted by the ANN for the training cases in comparison with the real pollen concentrations. A high coefficient of linear regression (R(2)) of 0.9696 was obtained. The accuracy of the neural network developed was tested with data from 2006 and 2007, which was not taken into account to establish the aforementioned models. Neural networks provided us a good tool to forecasting allergenic airborne pollen concentration helping the automation of the prediction system in the aerobiological information diffusion to the population suffering from allergic problems.

Interaction of Hg(II) with kaolin-humic acid complexes

Abstract The adsorption and desorption of Hg(II) by humic acid (HA) previously adsorbed on kaolin was studied. In the range of HA concentration investigated (0.0 -26.9 mg g-1), the Hg(II) adsorption capacity of kaolin at pH 4 is enhanced by the presence of HA. For the complexes with the highest HA concentration and for low Hg(II) initial concentrations, adsorption was lower, i.e. as HA concentration on the complexes increases, Hg(II) equilibrium concentration also increases. This behaviour is due to the increasing presence of dissolved organic matter as the HA concentration on the complexes increases. The dissolved organic matter is able to form a soluble complex with Hg, thus decreasing adsorption. Hg(II) adsorption from a 2.5 × 10-5MHg(II) solution was influenced by pH. For kaolin, a pHmax (pH where maximum adsorption occurs) of 4.5 was observed. At pH values >pHmax retention decreased with increasing pH. This same behaviour was observed for the kaolin-HA complex containing the lowest HA concentration (6.6 mg g-1). For the other kaolin-HA complexes there was little effect of pH on Hg(II) adsorption between pH 2.5 and pH 6.5. The presence of HA increased the adsorption of Hg(II) on kaolin all along the pH range studied. Desorption experiments showed that the amount of Hg(II) desorbed was quite low (<1%) for all the HA and Hg(II) concentration range studied, except for the kaolin at acid pH (pH 2.5) where the Hg(II) released was >50% of Hg(II) previously adsorbed. The presence of HA dramatically reduced this percentage of desorption to values of <3%, indicating reduced risk of toxicity problems in surface and subsurface waters. The addition of Cu(II) did not favour any Hg(II) desorption, even though Cu exhibits a strong affinity for organic matter.

Thermodynamics of sodium dodecyl sulphate-salicylic acid based micellar systems and their potential use in fruits postharvest

Micellar systems have excellent food applications due to their capability to solubilise a large range of hydrophilic and hydrophobic substances. In this work, the mixed micelle formation between the ionic surfactant sodium dodecyl sulphate (SDS) and the phenolic acid salicylic acid have been studied at several temperatures in aqueous solution. The critical micelle concentration and the micellization degree were determined by conductometric techniques and the experimental data used to calculate several useful thermodynamic parameters, like standard free energy, enthalpy and entropy of micelle formation. Salicylic acid helps the micellization of SDS, both by increasing the additive concentration at a constant temperature and by increasing temperature at a constant concentration of additive. The formation of micelles of SDS in the presence of salicylic acid was a thermodynamically spontaneous process, and is also entropically controlled. Salicylic acid plays the role of a stabilizer, and gives a pathway to control the three-dimensional water matrix structure. The driving force of the micellization process is provided by the hydrophobic interactions. The isostructural temperature was found to be 307.5 K for the mixed micellar system. This article explores the use of SDS-salicylic acid based micellar systems for their potential use in fruits postharvest.

Modification of reactivity by changing microemulsion composition. Basic hydrolysis of nitrophenyl acetate in AOT/isooctane/water systems

A study was carried out on the basic hydrolysis of p-nitrophenyl acetate (NPA) in AOT/isooctane/water microemulsions keeping constant the hydroxide ion concentration referred to the droplet water. The obtained results show that the pseudofirst order rate constant, kobs, is approximately 100 times lower than the corresponding value in bulk water and increases together with the [AOT], keeping constant the molar ratio W, W = [H2O]/[AOT]. On experiments varying W, keeping constant [AOT], it can be observed that kobs decreases from W = 2 to W = 10 as W increases, reaching a minimum value for W ≅ 10, and then increasing again. The observed behavior is a consequence of two factors: the variation of hydroxide ion concentration referred to the total volume of the system, and the association of the NPA to the interface of the microemulsion. The experimental results can be explained quantitatively considering that the NPA is distributed throughout the three pseudophases of the system and that the reaction takes place solely in the aqueous microdroplet. The second order rate constant in the aqueous microdroplet of the microemulsion, kw2, increase as W decreases, showing a parallelism with the behavior observed in aqueous DMSO. The values of kw2 increase approximately 35 times from W = 40 to W = 2 due fundamentally to the partial desolvation of the hydroxide ions as the water content of the microemulsion decreases.

Organic Reactivity in Aot-Stabilized Microemulsions

Microemulsions are highly versatile reaction media, which currently find many applications. In this review, we shall describe recent trends in the use of microemulsions as organic reaction media, and present models for their functioning, in particular the pseudophase model. This model allows a quantitative explanation of organic reactivity in these microheterogeneous media.

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.

Comparative study of nitroso group transfer in colloidal aggregates: micelles, vesicles and microemulsions

A kinetic study was carried out on nitroso group transfer from N-methyl-N-nitroso-p-toluenesulfonamide (MNTS) to different secondary amines: morpholine (MOR), piperazine (PIP), dimethylamine (DMA) and piperidine (PIPER) in micelles of dodecyltrimethylammonium bromide (LTABr) and in vesicles of dioxadecyltrimethylammonium chloride (DODAC). Amine nucleophiles were chosen on the basis of their hydrophobicity and basicity. The observed rate constant, kobs, shows opposite behavior in micelles and vesicular systems. kobs for micelle systems decreases as the surfactant concentration increases. This behavior can be interpreted according to the distribution of the reagents among the different pseudophases of the system and the physicochemical properties of the latter. It has been observed that the product of the rate constant in the micellar pseudophase and the distribution constant of the amine, k2mKmR2NH, retains a sequence similar to the reactivity observed in water. The differences observed can be explained on the basis of the different hydrophobicity of the amines and consequently different values of KmR2NH. In all cases a catalytic effect on the addition of vesicles was observed reaching a limiting value of kobs. The kinetic behavior can be explained quantitatively on the basis of a single pseudophase model. k2vKvR2NH in the vesicular systems of DODAC displays a variation which is analogous with the micellar systems but approximately 35 times greater, which in turn indicates the greater hydrophobic character of the vesicles of DODAC by comparison with the micelles of LTABr. In AOT/isooctane/water microemulsions we have found similar behavior where the product is approximately 22 times lower than in vesicles, indicating that the polarity of the interface of the microemulsions is greater than that of the micelles of LTABr and smaller than that of DODAC vesicles. The comparative analysis of the reactivities in the interface of the microemulsion and in an aqueous medium shows that the reactive position in the interface changes as the hydrophobic character of the amine varies.

Degradation of carbofuran and carbofuran-derivatives in presence of humic substances under basic conditions

The influence of humic aggregates in water solution upon the chemical stability of carbofuran (CF) and the carbofuran-derivatives, 3-hydroxy-carbofuran (HCF) and 3-keto-carbofuran (KCF), has been investigated in basic media. An inhibition upon the basic hydrolysis of 3-hydroxy-carbofuran and 3-keto-carbofuran (≈ 1.7 and ≈ 1.5-fold, respectively) was observed and it was rationalized in terms of the micellar pseudophase model. Nevertheless, non-significant effect upon the carbofuran stability was found in the presence of humic substances. These behaviors have been compared with the corresponding ones in other synthetic colloidal aggregates.