Publications

Abstract Self-organization in aqueous systems based on ionic surfactants, and their mixtures, can be broadly understood by a balance between the packing properties of the surfactants and double-layer electrostatic interactions. While the equilibrium properties of micellar systems have been extensively studied and are understood, those of bilayer systems are less well characterized. Double-chained and pseudodouble-chained (or catanionic) surfactants are among the amphiphiles which typically form bilayer structures, such as lamellar liquid-crystalline phases and vesicles. In the past 10-15 years, an experimental effort has been made to get deeper insight into their aggregation patterns. With the double-chained amphiphiles, by changing counterion, adding salt or adding anionic surfactant, there are possibilities to depart from the bilayer aggregate in a controlled manner. This is demonstrated by several studies on the didodecyldimethylammonium bromide surfactant. Mixtures of cationic and anionic surfactants yield the catanionics, surfactants of the swelling type, and also show a rich phase behavior per se. A variety of liquid-crystalline phases and, in dilute regimes, equilibrium vesicles and different micellar shapes are often encountered. Phase diagrams and detailed structural studies, based on several techniques (NMR, microscopy and scattering methods), have been reported, as well as theoretical studies. The main features and conclusions emerging from such investigations are presented.

Abstract A reinvestigation of the phase diagrams relative to some conjugated and non-conjugated bile salts in water has demonstrated the formation of lyotropic liquid crystalline phases, in contradiction with generally accepted statements. The phase behaviour is complex and the phase diagrams are unusual, compared to most surfactants and lipids. In particular, coexistence of liquid crystalline phases with crystals has been observed. The formation of liquid crystalline phases requires very long equilibration times and the thermal stability of the lyotropic phases is moderate. The observed structure is tentatively assumed to be of the reverse hexagonal type. Structural relations with currently accepted models for the organisation of bile salts into micelles and solid form have been found.

Abstract The standard (p(o) = 0.1 MPa) molar enthalpies of formation for crystalline 2-chloro-4-nitroaniline and 2-chloro-5-nitroaniline were derived from the standard molar enthalpies of combustion, in oxygen, at the T = 298.15 K, measured by rotating bomb combustion calorimetry. The Knudsen mass-loss effusion technique was used to measure the vapour pressures of the crystals as function of temperature, and the standard molar enthalpies of sublimation, at T = 298.15 K, were derived by the Clausius-Clapeyron equation. Direct measurements of the standard molar enthalpies of sublimation, using Calvet microcalorimetry, for 2-chloro-4-nitroaniline and 2-chloro-5-nitroaniline confirmed the values derived from the Knudsen technique. [GRAPHICS] Additionally, standard enthalpies of formation were estimated by employing two different methodologies. One is based on the Cox scheme and the other one, much more accurate, is based on first-principles calculations. The theoretical calculations were performed at the AMI, B3LYP/6-31+G** and BP86/6-31+G** levels of theory. Estimated values are in good agreement with the reported experimental numbers.

Abstract The complexes between gamma-cyclodextrin and lanthanide (111) chelates of the polyazamacrocycles DOTA (DOTA equivalent to 1,4,7,10-tertraazacyclododecane-1,4,7,10-tetraacetate) and DOTP (DOTP equivalent to 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetramethylenephosphonate) have been thought out to enhance the potential of such chelates as contrast agents for MRI. Given the actual demand for the design of new contrast agents, we thought it worthwhile to confirm previous results for the equilibrium constant K obtained by one of us by NMR on the DOTP complex, as well as to determine K for a new one with DOTA. Further, we wanted to study and quantify the interactions present in these complexes, with a view to improve them in newly designed complexes. The interactions between gamma-cyclodextrin and the lanthanide (III)-polyazamacrocyclic chelates, [Tm(DOTP)](5-), and [Gd(DOTA)](-) were then studied by isothermal calorimetry (ITC) and molecular dynamics. The calorimetric experiments can be interpreted by considering that in both cases there is a weak association, characterized by low values for the equilibrium constant as well as for the molar enthalpy change for complex formation, at T = 298.15 K. The K value for the complex with DOTP obtained now by ITC is of the same order of magnitude of the one determined previously by NMR. Further, the complex formation seems rather insensitive to the macrocycle, as the values now obtained by ITC for the DOTA complex are very similar to the ones obtained for the DOTP complex. We have also carried out molecular dynamics simulations on these very same inclusion complexes, which provided quantitative data on the interactions present, as well as a plausible explanation for the data obtained, leading to the proposal of possible solutions to improve the modelling of new contrast agents on a host-guest basis.

Abstract A new batch microcalorimetric vessel for the determination of enthalpies of dissolution of small amounts of easily or slightly soluble solids was developed at University of Lund, Sweden and was tested at University of Porto, Portugal. The vessel forms part of a series of twin heat conduction microcalorimeters. In a series of consecutive dissolution steps up to four samples, each 0.1-3 mg, can be injected into a solvent chamber of the vessel, volume 20 ml. The high stability of the baseline allows solution experiments to be extended over several hours. All measurements reported were conducted at 298.15 K and with water as the solvent. The calorimeter was calibrated chemically by dissolution of potassium chloride. The performance of the instrument was further tested by measurements of the enthalpies of dissolution of acetanilide and adenine, 18.25 +/- 0.56 and 31.78 +/- 0.64 kJ mol(-1), respectively. No concentration dependence was found. The results are in good agreement with values in the literature.

Abstract A square-wave voltammetric (SWV) method and a flow injection analysis system with amperometric detection were developed for the determination of tramadol hydrochloride. The SWV method enables the determination of tramadol over the concentration range of 15-75 muM with a detection limit of 2.2 muM. Tramadol could be determined in concentrations between 9 and 50 muM at a sampling rate of 90 h(-1), with a detection limit of 1.7 muM using the flow injection system. The electrochemical methods developed were successfully applied to the determination of tramadol in pharmaceutical dosage forms, without any pre-treatment of the samples. Recovery trials were performed to assess the accuracy of the results; the values were between 97 and 102% for both methods.

Abstract The standard (pdegrees = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, were determined using static-bomb calorimetry for crystalline 6-methoxyquinoline N-oxide (6MeOQU-INO), as -(85.6 +/- 3.7) kJ . mol(-1) and liquid 6-methoxyquinoline (6MeOQUI), -(25.8 +/- 2.2) kJ . mol(-1). The standard molar enthalpies of sublimation/vaporization, at T = 298.15 K, were measured by microcalorimetry and from the enthalpies of formation of the gaseous compounds, the dissociation enthalpy D-m(0) of the (N+-O-) dative covalent bond was derived, as (269.2 +/- 5.0) kJ . mol(-1). @ 2003 Elsevier Science Ltd. All rights reserved.

Abstract Electrochemical oxidation of propanil in deuterated solutions was studied by cyclic, differential pulse, and square wave voltammetry using a glassy carbon microelectrode. The oxidation of propanil in deuterated acid solutions occurs at the nitrogen atom of the amide at a potential of +1.15 V vs Ag/AgCl. It was also found that, under the experimental conditions used, protonation at the oxygen atom of propanil occurs, leading to the appearance of another species in solution which oxidizes at +0.60 V. The anodic peak found at +0.79 V vs Ag/AgCl in deuterated basic solutions is related to the presence of an anionic species in which a negative charge is on the nitrogen atom. The electrochemical data were confirmed by the identification of all the species formed in acidic and basic deuterated solutions by means of NMR spectroscopy. The results are supported by electrochemical and spectroscopic studies of acetanilide in deuterated solutions.

Abstract An electrochemical glucose biosensor based on the assisted transfer of H+ across a gel/liquid interface was developed. The transducer element consisted of a composite membrane prepared by assembling a micromachined polyester film with a DBS-NPOE gel layer. The biocomponent, glucose oxidase (GOx), was attached to the composite membrane by adsorption. The glucose analysis was made following the gluconic acid production by the enzymatic reaction between glucose and glucose oxidase. The change in pH was monitored by measuring the current intensity due to the transfer of H+ across the interface between the organic gel and aqueous solution assisted by an ionophore present in the organic phase.