Publications

Abstract Cationic liposomes have been extensively studied from the experimental and theoretical standpoints, motivated both by their fundamental interest and by potential applications in drug delivery and gene therapy. However, a detailed understanding of the nature of interactions within mixed bilayers containing cationic gemini surfactants is still lacking. This work focuses on the structural and dynamic properties of DODAB membranes in the presence of dicationic gemini surfactants. A thermodynamic characterization of the phase transitions in the mixed systems has been carried out by differential scanning calorimetry, while insight into the molecular interactions in the bilayer has been provided by molecular dynamics. For this purpose, variations in the gemini spacer and tail length, as well as in the respective molar fraction, have been included in both experimental and simulation studies. The results indicate that the influence of cationic gemini surfactants upon the thermotropic behavior and degree of order of DODAB structures is controlled by a complex interplay between charge density, conformation and hydrophobic effects, for which a detailed rationale is provided.

Abstract Dicationic alkylammonium bromide gemini surfactants represent a class of amphiphiles potentially effective as skin permeation enhancers. However, only a limited number of studies has been dedicated to the evaluation of the respective cytotoxicity, and none directed to skin irritation endpoints. Supported on a cell viability study, the cytotoxicity of gemini surfactants of variable tail and spacer length was assessed. For this purpose, keratinocyte cells from human skin (NCTC 2544 cell line), frequently used as a model for skin irritation, were employed. The impact of the different gemini surfactants on the permeability and morphology of model vesicles was additionally investigated by measuring the leakage of calcein fluorescent dye and analyzing the NMR spectra of P-31, respectively. Detail on the interaction of gemini molecules with model membranes was also provided by a systematic differential scanning calorimetry (DSC) and molecular dynamics (MD) simulation. An irreversible impact on the viability of the NCTC 2544 cell line was observed for gemini concentrations higher than 25 mM, while no cytotoxicity was found for any of the surfactants in a concentration range up to 10 mM. A higher cytotoxicity was also found for gemini surfactants presenting longer spacer and shorter tails. The same trend was obtained in the calorimetric and permeability studies, with the gemini of longest spacer promoting the highest degree of membrane destabilization. Additional structural and dynamical characterization of the various systems, obtained by P-31 NMR and MD, provide some insight on the relationship between the architecture of gemini surfactants and the respective perturbation mechanism.

Abstract The vapor pressures of condensed phases of methyl p-methylbenzoate and methyl p-(dimethylamino)benzoate were measured in the temperature ranges (269.3 to 357.0) K and (325.0 to 390.4) K, respectively, using a static method. The Knudsen mass-loss effusion technique was also used to study the vapor pressures as function of temperature of the crystals of methyl p-(dimethylamino)benzoate in the pressure range (0.1 to 1) Pa. The results obtained for each compound enabled the determination of the standard molar entropies and enthalpies of sublimation and of vaporization at T = 298.15 K as well as phase diagram representations of the (p,T) experimental data. The temperatures and molar enthalpies of fusion were determined using differential scanning calorimetry and were compared with the values derived from the vapor pressure measurements. The enthalpies of the intermolecular hydrogen bonds O-H center dot center dot center dot O in the crystalline phase of the parent substituted benzoic acids were calculated.

Abstract Calorimetric measurements are expected to provide useful data regarding the relative stability of alpha- versus beta-amino acid isomers, which, in turn, may help us to understand why nature chose alpha- instead of beta-amino acids for the formation of the biomolecules that are essential constituents of life on earth. The present study is a combination of the experimental determination of the enthalpy of formation of N-benzyl-beta-alanine, and high-level ab initio calculations of its molecular structure. The experimentally determined standard molar enthalpy of formation of N-benzyl-beta-alanine in gaseous phase at T = 298.15 K is (298.8 +/- 4.8) kJ center dot mol(-1), whereas its G3(MP2)//B3LYP-calculated enthalpy of formation is -303.7 kJ center dot mol(-1). This value is in very good agreement with the experimental one. Although the combustion experiments of N-benzyl-alpha-alanine were unsuccessful, its calculated enthalpy of formation is -310.7 kJ center dot mol(-1); thus, comparison with the corresponding experimental enthalpy of formation of N-benzyl-beta-alanine, -(298.8 +/- 4.8) kJ/mol, is in line with the concept that the more branched amino acid (alpha-alanine) is intrinsically more stable than the linear beta-amino acid, beta-alanine.

Abstract A novel volatile microemulsion formed by the catanionic surfactant hexadecyltrimethylammonium octylsulfonate (TA(16)So(8)), heptane and water has been explored as a template for producing nanoparticles of hydrophobic organic materials. Butylated hydroxytoluene (BHT) was employed as the model hydrophobic substance. First, the oil-in-water microemulsion was formed, containing TA16So8 as the single emulsifier and BHT dispersed in the volatile microphase. Microstructure characterization by self-diffusion NMR revealed that BHT was indeed incorporated into the oil droplets and that the mean diameter of the main droplet population was 30 nm, larger than in the BHT-free microemulsion. Next, a rapid solvent and water removal by freeze drying allowed converting the microemulsion droplets into nanoparticles in the form of a dry, fine powder. This powder was freely dispersible in water to yield a stable suspension of amorphous BHT particles with a mean size of 19 nm and zeta-potential of +37 mV. The solid nanoparticles in the aqueous dispersion were thus smaller than the initial microemulsion droplets. For comparison, a conventional o/w microemulsion composed of CTAB and sec-butanol was also tested as a template for BHT particle formation by the same process, and it was found that it yielded crystalline particles of micrometre size. On the basis of our results, we anticipate the catanionic microemulsion method to be an efficient one for producing size-controlled, water-dispersible nanoparticles of other hydrophobic organic materials.

Abstract This work reports the standard (p degrees = 0.1 MPa) molar enthalpies of formation, in the gaseous phase, Delta(f)H degrees(m)(g), of 2,4-, 2,6-, and 3,5-dibromophenol, at T = 298.15 K, respectively, as (59.6 +/- 2.6) kJ . mol(-1), (49.1 +/- 2.2) kJ . mol(-1) and (39.5 +/- 2.0) kJ . mol(-1). These experimental values were derived from the measurements of the standard molar enthalpies of formation, in the crystalline phase, Delta(f)H degrees(m), (2, 4-dibromophenol, Cr) = -(140.9 +/- 2.1) kJ . mol(-1), Delta(f)H degrees(m) (2, 6-dibromophenol, cr) = -(132.5 +/- 1.6) kJ . mol(-1) and Delta fH degrees(m) (3, 5-dibromophenol, cr) = -(134.5 +/- 1.7) kJ . mol(-1), at the same reference temperature, achieved from the standard molar enthalpies of combustion, in oxygen, to yield CO(2)(g) and HBr.600H(2)O(I), measured by rotating-bomb combustion calorimetry, together with measurements of the standard molar enthalpies of sublimation, at T = 298.15 K, as Delta(g)(cr)H degrees(m) (2, 4-dibromophenol) = (81.3 +/- 1.5) kJ . mol(-1), Delta(g)(cr)H degrees(m) (2, 6-dibromophenol) = (83.4 +/- 1.5) kJ . mol(-1) and Delta(g)(cr)H degrees(m) (3, 5-dibromophenol) (94.3 +/- 1.8) kJ . mol(-1), obtained using the Calvet high temperature vacuum sublimation technique. The standard molar enthalpy of sublimation, at T = 298.15 K, for the 3,5-dibromophenol was also determined from the temperature-vapour pressure dependence, obtained by the Knudsen mass loss effusion method, Delta(g)(cr)H degrees(m) (3.5-dibromophenol) = (95.0 +/- 1.1) kJ . mol(-1). For this isomer it is also reported the standard (p degrees = 0.1 MPa) molar entropy and Gibbs energy of sublimation, at T = 298.15 K. The experimental values of the gas-phase enthalpies of formation of each compound were compared with estimates using the empirical scheme developed by Cox and with the calculated values based on density functional theory calculations using the B3LYP hybrid exchange-correlation energy functional at the 6-311++G(d,p) basis set. These two methodologies were also used to estimate the enthalpies of formation in the gas-phase of the 2,3-, 2,5-, and 3,4-dibromophenol.

Abstract The standard (p degrees = 0.1 MPa) molar enthalpies of formation, in the crystalline state, of the 1- and 2-cyanonaphthalene were derived from the standard molar energies of combustion, in oxygen, at T = 298.15 K, measured by static-bomb combustion calorimetry. Vapor pressure measurements at different temperatures, using the Knudsen mass loss effusion technique, enabled the determination of the enthalpy, entropy, and Gibbs energy of sublimation, at T = 298.15 K, for both isomers. The standard molar enthalpies of sublimation, at T = 298.15 K. for 1- and 2-cyanonaphthalene, were also measured by high-temperature Calvet microcalorimetry. [GRAPHICS] Combining these two experimental values, the gas-phase standard molar enthalpies. at T = 298.15 K, were derived and compared with those estimated by employing two different methodologies: one based on the Cox scheme and the other one based on G3MP2B3 calculations. The calculated values show a good agreement with the experimental values obtained in this work.

Abstract This paper reports a combined experimental and computational thermochemical study of 4-benzyloxyphenol. Static bomb combustion calorimetry and Knudsen mass-loss effusion technique were used to determine the standard (p(o) = 0.1 MPa) molar enthalpy of combustion, Delta H-c(m)o = -(6580.1 +/- 1.8) kJ. mol(-1), and of sublimation, Delta H-g(cr)m(o)= (131.0 +/- 0.9) kJ . mol(-1), respectively, from which the standard (p(o) = 0.1 MPa) molar enthalpy of formation, in the gaseous phase, at T = 298.15 K, Delta H-t(m)o = -(119.5 +/- 2.7) kJ . mol(-1) were derived. For comparison purposes, the gas-phase enthalpy of formation of this compound was estimated by G3(MP2)//B3LYP calculations, using a set of gas-phase working reactions; the results are in excellent agreement with experimental data. G3(MP2)//B3LYP computations were also extended to the calculation of the gas-phase enthalpies of formation of the 2- and 3-benzyloxyphenol isomers. Furthermore, this composite approach was also used to obtain information about the gas-phase acidities, gas-phase basicities, proton and electron affinities, adiabatic ionization enthalpies and, finally, O-H bond dissociation enthalpies.

Abstract The present work reports the values of the standard (p degrees = 0.1 MPa) molar enthalpies of formation in the condensed phase of the three isomers of monoiodophenol derived from the standard molar energies of combustion, in oxygen, to yield CO(2)(g), I(2)(cr)., and H(2)O(1), at T = 298.15 K, measured by rotating-bomb combustion calorimetry, as well as the values of the standard molar enthalpies of sublimation, at T = 298.15 K, determined using high-temperature Calvet microcalorimetry. Combining the former two experimental quantities, the standard molar enthalpies of formation in the gaseous phase were derived, at T = 298.15 K: Delta(f)H(m)degrees (2-iodophenol, g) = -(15.3 +/- 2.0) kJ.mol(-1) Delta(f)H(m)degrees(3-iodophenol, g) = -(7.2 +/- 2.1) kJ.mol(-1), and Delta(f)H(m)degrees(4-iodophenol, g) = -(14.3 +/- 2.3) kJ.mol(-1). The experimental values of the gas-phase enthalpies of formation of each compound were compared with estimates using the empirical scheme developed by Cox and with the calculated values based on high-level density functional theory calculations using the B3LYP hybrid exchange-correlation energy functional at the 6-311G(d,p) basis set.