Chemical equilibrium model for the thermodynamic properties of mixed aqueous micellar systems: Application to thermodynamic functions of transfer

Mixed micelles can be formed in water between various pairs of hydrophobic solutes such as surfactants, alcohols and hydrocarbons. These systems can often be studied through the thermodynamic functions of transfer of one of the solutes, usually kept near infinite dilution, from water to an aqueous solution of the other solute. When mixed micelles are formed, these functions change significantly, and often go through extrema, in the region where the binary system micellizes or undergoes some microphase transition.Three main effects are responsible for the observed trends: pair-wise interactions between both solutes in the monomeric form, a distribution of the reference solute between the aqueous and micellar phases and a shift in the monomer-micelle equilibrium in the vicinity of the reference solute. Simple equations can be derived for these three effects which can account for the sign and magnitude of the observed trends using parameters which are derived for the most part from the two binary systems.     

Glass capillary columns for separation of free organic acids

Glass capillary columns have been prepared without acidic additive in the stationary phase, from which free organic acids elute as sharp and symmetrical peaks. The required surface in the borosilicate glass capillary was generated by a combination of leaching with aqueous HCl and deposition of colloidal silica particles; it can be coated with stationary phases have a broad range of polarity. Aqueous samples containing free organic acids can also be analyzed in such columns in an isothermal mode.     

Application of a chemical equilibrium model to thermodynamic functions of transfer of alcohols from water to aqueous surfactants

In ternary aqueous solutions, hydrophobic solutes such as alcohols tend to aggregate with surfactants to form mixed micelles. These systems can be studied by meas of the functions of transfer of hydrophobic solutes from water to aqueous solutions of surfactant. These thermodynamic functions often go through extrema in the critical micellar concentration (CMC) region of the surfactant. A simple model based on interactions between surfactant and hydrophobic solute monomers, on the distribution of the hydrophobic solute between water and the micelles and on the shift in the CMC induced by the hydrophobic solute, can simulate the magnitude and trends of the transfer functions using parameters which are mostly derived from the binary systems. In order to check the model more quantitatively, volumes and heat capacities of transfer of alcohols from water to aqueous solutions of a nonionic surfactant, octyldimethylamine oxide, were measured. A quantitative agreement was achieved with three adjustable parameters. Good fits are also obtained for the transfers to the ionic surfactants, octylamine hydrobromide and sodium dodecylsulfate. When the equilibrium displacement contribution is small, the distribution constants and the partial molar properties of the alcohols in the micellar phase agree well with the parameters obtained with similar models.     

A new method for the determination of mobile phase volume in normal and reversed-phase liquid chromatography

Summary A new method for the determination of the mobile phase volume (V _m) in liquid chromatography is presented based on the model regarding the retention of ionic solutes in the presence of eluent electrolytes. TheV _m value can be determined by measuring the retention volumes of two ions that have the same charge in two eluent electrolyte systems. Compared with the methods using isotopically labelled eluent components or inorganic salts asV _m markers, the method presented is proved to give more reasonableV _m values for both normal and reversed-phase liquid chromatography. As well as in binary mixed solvent systems, theV _m values in single solvent systems can be determined by this method.     

The scaled-particle theory and solvent isotope effects on the thermodynamic properties of inert solutes in water and methanol

The scaled-particle theory has been applied to the calculation of the thermodynamic changes associated with the formation of a cavity in several isotopic varieties of liquid water and methanol. From these results, the thermodynamic functions for the transfer of a cavity (or a hard-sphere solute) have been computed for the following solvent pairs: H₂OD₂O, H₂OH₂ ¹⁸O, H₂ ¹⁸OD₂ ¹⁸O, D₂OD₂ ¹⁸O, CH₃OHCH₃OD. For the last two of these solvents, density measurements required for the calculations were carried out as a function of temperature. The calculated deuterium solvent isotope effect on the heats and entropies of hard-sphere solutes in water is much greater than the¹⁸O isotope effect; the former also exhibits a more pronounced temperature dependence. The transfer functions computed for hard-sphere solutes are compared to experimental data on the transfer of various solutes from H₂O to D₂O and from CH₃OH to CH₃OD. In most of the cases examined, the cavity effect accounts for a large part of the transfer quantities measured for rare gases, hydrocarbons, and solutes containing a significant hydrocarbon substituent.     

Low‐density extraction solvent based solvent‐terminated dispersive liquid–liquid microextraction for quantitative determination of ionizable pesticides in environmental waters

A rapid, efficient, and new solvent terminated dispersive liquid–liquid microextraction technique coupled with HPLC was developed for selective extraction and analysis of s‐triazine herbicides from environmental water samples. Important parameters influencing the extraction process including type and volume of extraction and disperser solvent, extraction time, sample pH, ionic strength, and extraction temperature were successfully optimized. Under the optimal conditions, there are excellent linear relationships between the analytical results and concentration in the range of 10–400 mg/L for atrazine, propazine, prometryn, and terbutryn. LOD and LOQ ranged from 0.60 to 2.33 μg/L and 2.0 to 7.7 μg/L, respectively. Performance of the analytical technique was evaluated by carrying out the repeatability and reproducibility analyses that were ranged from 2.86 to 5.66% and 4.64 to 5.89% for 100 μg/L of each target analyte, respectively. The proposed method has been successfully applied to the analysis of real water samples and acceptable relative recoveries over the range of 65.93–101.46%, with RSDs ≤ 8.80%, were obtained. The overall results have been compared with the literature values. Thus, the method developed could efficiently be used for selective extraction of the target analytes from complex matrices, particularly environmental waters.     

Excess enthalpies of aqueous solutions of polyols at 25°

The excess enthalpies of aqueous solutions of nine polyols were determined at 25°C and reported in the virial form. The most interesting and new feature of this family of solutes is that the sign of the enthalpic pair interaction coefficients h_xx is positive for the first members of the series and negative for the higher homologues. Other points are the large differences found among the values of h_xx for stereoisomers, whereas pairs of enantiomers show the same values within experimental errors. An application of the group additivity method is also discussed.     

Adsorption of octadecyltrimethylammonium chloride and adsolubilization on to cellulosic fibers

The coadsorption of different organic solutes on cellulosic fibers treated with octadecyltrimethylammonium chloride (ODTMA) has been studied. In the absence of ODTMA cellulosic fibers had little tendency to retain organic compounds. The enhanced solute incorporation was ascribed to the adsolubilization of these compounds on the aggregated domains of the adsorbed surfactant molecules at the solid/liquid interface. The specific shape of solute coadsorption isotherms indicated that the adsolubilization process may be regarded as a partition phenomenon between the aqueous bulk phase and the adsorbed surfactant aggregates. The decrease in solute uptake at the cellulose/water interface above the CMC of ODTMA was ascribed to micelle formation in the bulk solution and to the ensuing micellar solubilization of organic solutes. Preliminary experiments performed using vertical fixed bed columns showed that modified cellulosic fibers can be conveniently used as substrate for treating organic pollutants.     

Critical parameters in a supported liquid membrane extraction technique for ionizable organic compounds with a stagnant acceptor phase

The reviews cover important critical parameters that are often optimized in a supported liquid membrane extraction technique in both flat sheet and hollow fibre designs for ionizable organic molecules. Understanding of these parameters can enable one to predict the behavior of the compound before hand and thus reduce the number of optimization experiments. Moreover, less number of experiments can be also generated using statistical techniques which are now becoming more commonly used. Supported liquid membrane extraction optimal parameters such as the conditions of the pH of the acceptor and donor phases should easily be fixed from the pKa values of the compounds. Other parameters, including the polarity of the compound can help to predict the partitioning into the membrane and the behavior of the compound. The influence of parameters such as temperature on the mass transfer in supported liquid membrane depends on the design of the module, experimental design and type of mass transfer controlling the extraction process.     

Abraham model correlations for transfer of neutral molecules and ions to sulfolane

Data have been compiled from the published literature on the partition coefficients of solutes and vapors into anhydrous sulfolane. The logarithms of the water-to-sulfolane partition coefficients, log P, and gas-to-sulfolane partition coefficients, log K, were correlated with the Abraham solvation parameter model. The derived correlations described the observed log P and log K values for solutes dissolved in sulfolane to within average standard deviations of 0.14 log units or less. The log P correlation was extended to include the partition of ions by inclusion of a cation-solvent and an anion-solvent term.