Alfredo Maestre

Association of caffeine in water and in aqueous solutions of sucrose

The association of caffeine in water and in aqueous sucrose solutions at 25 °C has been investigated by solubility, distribution, enthalpy of dilution and NMR chemical shift measurements. All the results are consistent with a model (the isodesmic model) in which the caffeine associates to form dimers, trimers etc and for which, the equilibrium constant, for each step, in any solvent mixture studied, is the same. Values are presented for thermodynamic quantities which represent the solution properties.

Limiting partial molar volumes of electrolytes in dimethylformamide–water mixtures at 298.15 K

Limiting partial molar volumes of electrolytes in several water-dimethyl sulfoxide mixtures, up to xDMSO= 0.1587, have been determined from density measurements at 298.15 K. A reference electrolyte assumption (TATB) has been used in order to obtain single-ion partial molar volumes of transfer from water to DMSO–water mixtures. The influence of DMSO on ΔtV′0i of alkali metals and halide ions is negligible in the highly water-rich region (up to xDMSO≈ 0.04) except for the I– ion. However, when the DMSO composition is greater than xDMSO≈ 0.04, the variation of ΔtV′0i for alkali metals is the reverse to that of halide ions. The results are discussed on the basis of ion–solvent and solvent–solvent interactions using Conways model.

Synthesis and physicochemical characterization of alkanedyil-α-ω-bis(dimethyldodecylammonium) bromide, 12-s-12,2Br-, surfactants with s=7, 9, 11 in aqueous medium.

In this work, three didodecyl dicationic dibromide dimeric surfactants 12-s-12,2Br(-), with different methylene spacer lengths (s=7, 9, and 11) were prepared and characterized and their properties compared to those of 12-s-12,2Br(-) surfactants with s=2, 3, 4, 5, 6, 8, 10, and 12. Information about the critical micelle concentration, the micellar ionization degree, the average aggregation number and the polarity of the interfacial region, and microviscosity of the micellar interior was obtained by using different techniques. Their surface activity was investigated by means of surface tension measurements. Micellization was also studied by using (1)H NMR and diffusion NMR (DOSY) spectroscopy as well as isothermal titration calorimetry. The values of the thermodynamic parameters show that the dimeric surfactants micellization is exothermic and driven by entropy. The occurrence of morphological transitions upon increasing surfactant concentration was studied, and the results indicate that the spacer length, s, plays a key role in the micellar growth of 12-s-12,2Br(-) aggregates. The value of s not only control the magnitude of C(*), the surfactant concentration above which the morphological transition from spherical micelles into elongated ones occurs, but also the sign of the enthalpy change accompanying the sphere-to-rod transition.

Setschenow coefficients for caffeine, theophylline and theobromine in aqueous electrolyte solutions

Setschenow coefficient measurements have been made on methylated xanthines (caffeine, theophylline and theobromine) in several inorganic and organic aqueous salt solutions at 298 K. In general, the order of salting effect is similar for the three solutes and can be rationalized as a consequence of structural salt–water interactions. Scaled particle theory and the Cross and McTigue equation have been employed to compare theoretical and experimental salting constants.

Enthalpies of dilution of some electrolytes in water at 298.15 K

Abstract The enthalpies of dilution of Ph 4 PCl, Ph 4 AsCl, NaPh 4 B and R 4 NBr (where R is ethyl, n -propyl, n -butyl and n -pentyl) have been determined by flow microcalorimetry at 298.15 K in water. From the experimental data, the relative apparent molar enthalpies φ L were calculated with the aid of an extended Debye—Huckel equation. A comparison with literature values for some R 4 NBr was made. Ph 4 PCl and Ph 4 AsCl show similar behavior, unlike that of NaPh 4 B, which may be a consequence of the effect of the Na + ion in relation to the Cl − ion.

Partial molal volumes for iron(II)-diimine complexes in water and in aqueous methanol

SummaryPartial molal volumes of the perchlorate or chloride salts of several tris-diimine-iron(II) cations have been determined in aqueous and methanol-water solutions by vibrating-tube densimetry. Partial molal volumes of the cations at infinite dilution have been estimated for aqueous solution, and compared with values for related cations. Trends of limiting partial molal volumes for perchlorate salts in methanol-water media are compared with trends in transfer chemical potentials for the respective cations; both are discussed in terms of ligand-dependent preferential solvation behaviour.

Thermodynamic functions for the transfer of 1-naphthoic acid from water to mixed aqueous solvents at 298 K

Values of free energies, ΔG⊖t, enthalpies, ΔH⊖t, and entropies, ΔS⊖t, of transfer of 1-naphthoic acid from water to water + ethanol, water + t-butyl alcohol and water + dimethyl sulphoxide (DMSO) mixtures at various mole fractions have been determined from solubility measurements at different temperatures. Variations of ΔG⊖t with xcosolvent in the water-rich alcoholic mixtures are governed by the sign of the entropy term. For water + DMSO mixtures ΔG⊖t is negative, but the variation of ΔH⊖t with xDMSO shows different behaviour from that of water + alcohol mixtures. We have calculated molecular pair-interaction parameters (gxy, hxy and TSxy) in water for the three systems studied; hxy > 0 for water + alcohol mixtures and hxy < 0 for water + DMSO mixtures. The results are discussed in terms of solute–cosolvent and solute–water interactions.

Enthalpy of interaction of some electrolytes with dimethyl sulfoxide in water at 298.15 K

Enthalpies of mixing in water of dimethyl sulfoxide with several alkali-metal halides, Ph 4 PCl and NaPh 4 B have been determined by flow microcalorimetry at 298.15 K. Enthalpic pair interaction coefficients, h NE , of the virial expansion of the excess enthalpy were calculated. The coefficients are positive for NaF, KF, Ph 4 PCl and NaPh 4 B, and negative for the remaining alkali-metal halides. A linear correlation between h NE and the anionic radius is observed. Gibbs energy and entropic pair interaction coefficients have been calculated from literature data. Single-ion pairwise coefficients have been obtained using the tetraphenylphosphonium tetraphenylborate reference electrolyte assumption.

Enthalpy of interaction of some electrolytes with 2-methyl-2-butanol in wate at 25°C

Enthalpies of mixing in water of 2-methyl-2-butanol with several electrolytes (alkali-metal halides, tetra-n-alkylammonium bromides, Ph4PCl, Ph4AsCl and NaPh4B) have been determined by flow microcalorimetry at 25°C. Enthalpic pair interaction coefficients, hNE, of the virial expansion of the excess enthalpy were calculated. All coefficients are positive and increase as the anionic size increases in anionic series of alkali-metal halides. A linear correlation between hNE and the number of carbon atoms in the apolar group of R4NBr is observed. For alkali-metal halides coefficients hNE depend mainly on the partial desolvation of halides, and for hydrophobic electrolytes both the partial desolvation of halide ions and hydrophobic interaction are the leading factors in the value of hNE.

Excess molar volumes and excess refractive indices of (cis-9-octadecenoic acid + n-hexane or cyclohexane or benzene or trichloroethene or tetrachloroethene) at 298.15 K

Abstract The excess molar volumes V m E and excess refractive indices n D E of { cis -9-octadecenoic (oleic) acid + n -hexane or cyclohexane or benzene or trichloroethene or tetrachloroethene}` have been determined as a function of mole fraction at atmospheric pressure and 298.15 K. The V m E are positive over the entire composition range in mixtures containing trichloroethene, negative for those with n -hexane or tetrachloroethene, and sigmoid for the others. The n D E s are positive for mixtures containing n -hexane and cyclohexane and negative for the others.