Publications

Abstract A new dissolution microcalorimeter that can measure enthalpies of dissolution of slightly soluble solids was developed by Ingemar Wadso at Lund University, and the prototype as well as the commercial vessel were tested in our laboratory. Recently we did report the testing of the prototype and we are now extending the previous investigation to measurements with an organic solvent in the commercial vessel. The instrument performance was found to be as good with formamide as with water. The vessel was calibrated chemically (dissolution of KCl in water) and electrically, by means of a permanent and an insertion heater. The results obtained from the three methods are compared and discussed. The enthalpy of dissolution of a-cyclodextrin, dry and hydrated with six water molecules, was determined in water and in formamide. The results are discussed in terms of the difference between water and formamide as regarding dissolution and binding to the cyclodextrin molecule. 2004 Elsevier B.V. All rights reserved.

Abstract Type S thermocouples were assembled in situ by applying high intensity electric are discharges to the contact junction of two platinum (Pt) and Pt-10% rhodium (Pt-10% Rh) wires, inserted on a silica capillary. The electrically insulated thermocouples built in this way were afterwards employed to estimate the temperature of an optical fiber subjected to arc discharges. For typical values of the arc discharge parameters used to arc-induce long-period fiber gratings (electric current I = 9 mA and arc duration t = 1 s), a capillary peak temperature value of 1420 degreesC +/- 400 degreesC was obtained by extrapolation of the experimental data for the limit situation of having a thermocouple with negligible diameter. The temperature profiles in the capillary and in an optical fiber were calculated based on a heat transfer model implemented by a finite element algorithm and fitted to the experimental temperature distribution in the Pt and Pt-10% Rh wires. The correspondent peak temperatures computed for the capillary and for the fiber were 1450 degreesC and 1320 degreesC, respectively. A good agreement between the capillary temperature values determined graphically and numerically was obtained.

Abstract The thermodynamics of the interaction between a hydrophobically modified cationic polyelectrolyte and an anionic surfactant (sodium dodecyl sulfate, SDS) has been investigated by microcalorimetry, conductivity, and UV-vis spectrophotometry. The polyelectrolyte employed was a newly synthesized polymer (D40OCT30) based on dextran having pendant N-(2-hydroxypropyl)-N,N-dimethyl-N-octylammonium chloride groups randomly distributed along the polymer backbone with a degree of substitution (DS) of 28.1%. The interaction between D40OCT30 and SDS was found to be very strong because of the introduction of ionic and hydrophobic moieties on the backbone of the dextran polymer. The aggregation concentration of polyelectrolyte-SDS complex (CAC(complex)) was derived from the curves of variation of the observed enthalpy, solution conductivity, and optical dispersion with SDS concentrations. The results show that these values obtained from different methods are coincident and increase with D40OCT30 concentration. A mechanism of interaction is proposed and discussed in detail in the text. The total interaction enthalpies were derived from the observed enthalpy curves. The results indicate that the total interaction process is entropy-driven. From the calorimetric and turbidity measurements, the partial phase diagram that describes the dependence of the phase boundary on polymer alkyl side chain concentration is also deduced.

Abstract The self-aggregation of polyelectrolytes having N-alkyl-N,N-dimethyl-N-(2-hydroxypropyl)ammonium chloride pendant groups (alkyl = octyl, dodecyl, or cetyl) randomly distributed along a polysaccharide backbone (dextran) was studied by steady-state fluorescence techniques using several free fluorescent probes or pyrene-labeled polymers and by viscometry. The onset, offset, and highest values of the fluorescence response of N-phenylnaphthylamine (NPN), pyrene (Py), and 1,6-diphenyl-3,5,6-hexatriene (DPH) were corroborated with NPN and DPH anisotropy and quenching experiments to describe the dynamic of hydrophobic microdomain formation and microdomain characteristics. The start of the aggregation process (critical aggregation concentration, cac) and the microdomain characteristics such as polarity, microviscosity, size, and number strongly depend on the alkyl chain length and the degree of substitution with cationic pendant groups. Fluorescence experiments with pyrene-labeled polymers and viscosity data suggest that microdomains are mainly formed by intramolecular hydrophobic associations, except for the polymers carrying octyl groups, where some intermolecular associations were revealed.

Abstract The standard (p°=0.1 MPa) molar enthalpies of formation of the crystalline complexes bis[N-(N″,N″-diethylaminothiocarbonyl) benzamidinato]nickel(II), {Ni(datb)2}, and bis[N-(N″,N″- diethylaminothiocarbonyl)-N′-phenylbenzamidinato]nickel(II), {Ni(datpb)2}, were determined, at T=298.15 K, by high precision solution-reaction calorimetry. ΔfHm°(cr)/ (kJ·mol-1)Ni(datb)26.5 ± 7.9Ni(datpb) 2285.1 ± 11.3 From the obtained results, the metal-ligand exchange enthalpies in the crystalline phase were derived. The enthalpy of a hypothetical metal-ligand exchange reaction in the crystalline phase was derived, thus allowing a discussion of the energetics of complexation in comparison with known crystal-structural parameters.

Abstract The standard (pdegrees = 0.1 MPa) molar enthalpies of formation of crystalline thianthrene and monothiodibenzoylmethane, HDBMS, at T = 298.15 K were measured by rotating-bomb calorimetry and the standard molar enthalpies of sublimation at T = 298.15 K of dibenzoylmethane (HDBMO) and monothiodibenzoylmethane (HDBMS) were measured by microcalorimetry. [GRAPHIC] From the present results it could be shown that, in these molecules, the intramolecular hydrogen bond energy (O-H...O), is ca. 20 kJ . mol(-1) larger than for (S-H...O).

Abstract Quasielastic neutron scattering measurements were performed in hydrated samples of ds-DNA and ss-DNA. The samples were hydrated in a high relative humidity atmosphere, and their final water content was 0.559 g H2O/g ds-DNA and 0.434 g H2O/g ss-DNA. The measurements were performed at 8 and 5.2 Angstrom for the ds-DNA sample, and at 5.2 Angstrom for the ss-DNA sample. The temperature was in both cases 298 K. Analysis of the obtained data indicates that in the ds-DNA sample we can distinguish two types of protons-those belonging to water molecules strongly attached to the ds-DNA surface and another fraction belonging to water that diffuses isotropically in a sphere of radius 2.8 Angstrom, with a local diffusion coefficient of 2.2 x 10(-5) cm(2) s(-1). For ss-DNA, on the other hand, no indication was found of motionally restricted or confined water. Further, the fraction of protons strongly attached to the ds-DNA surface corresponds to 0.16 g H2O/g ds-DNA, which equals the amount of water that is released by ds-DNA upon thermal denaturation, as studied by one of us (G.M.) by differential scanning calorimetry. This value also equals the difference between the critical hydration values of ds-DNA and ss-DNA, also determined by DSC. These results represent, thus, a completely independent measurement of water characteristics and behavior in ds- and ss-DNA at critical hydration values, and therefore substantiate the previous suggestions/conclusions of the results obtained by calorimetry.

Abstract The standard (p(0) = 0.1 MPa) molar enthalpies of formation, at T = 298.15 K, in the gaseous phase of two Schiff bases, N,N'-bis(salicylaldehydo)ethylenediimine and N.N'-bis(salicylaldehydo)tetramethylenediimine, were determined from their enthalpies of combustion and sublimation, obtained by static bomb calorimetry in oxygen and by the Knudsen effusion technique, respectively. The enthalpies of fusion of both compounds have been determined by differential scanning calorimetry.

Abstract The state of the art of voltammetric and amperometric methods used in the study and determination of pesticides in crops, food, phytopharmaceutical products, and environmental samples is reviewed. The main structural groups of pesticides, i.e., triazines, organophosphates, organochlorides, nitrocompounds, carbamates, thiocarbamates, sulfonylureas, and bipyridinium compounds are considered with some degradation products. The advantages, drawbacks, and trends in the development of voltammetric and amperometric methods for study and determination of pesticides in these samples are discussed.