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At least 1,333 records · Page 74Linked to original sources
PubMed2006

Titration of fatty acids solubilized in cationic and anionic micelles. Calorimetry and thermodynamic modeling.

The electrostatic properties of charged surfactant micelles are investigated through titrations of fatty acid probes solubilized in the micelles. The titration process is followed by means of calorimetric measurements and by determining the pH values as a function of added base. This approach yields a complete thermodynamic description of the titration process. In particular, we find that the process is endothermic at 298 K. This is contrary to the titration of carboxylic acids in water, where DeltaH is approximately 0. To identify the main effect underlying the difference in DeltaH between titration in a micelle and water, a thermodynamic model has been developed which focuses on the transfer properties of charged and uncharged species from bulk water to the surface of a micelle and which incorporates a dielectric discontinuity at the micellar surface. The model relies on the use of the Poisson-Boltzmann equation which is solved using a finite element method. Experimental results and the model calculations imply that the dielectric discontinuity at (or near) the micellar surface plays a major role and hence must be included when analyzing the titration behavior of an acid functionality at the surface of a charged micelle.

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PubMed2005

Nanoscale, phonon-coupled calorimetry with sub-attojoule/Kelvin resolution.

We have developed an ultrasensitive nanoscale calorimeter that enables heat capacity measurements upon minute, externally affixed (phonon-coupled) samples at low temperatures. For a 5 s measurement at 2 K, we demonstrate an unprecedented resolution of DeltaC approximately 0.5 aJ/K ( approximately 36 000 k(B)). This sensitivity is sufficient to enable heat capacity measurements upon zeptomole-scale samples or upon adsorbates with sub-monolayer coverage across the minute cross sections of these devices. We describe the fabrication and operation of these devices and demonstrate their sensitivity by measuring an adsorbed (4)He film with optimum resolution of approximately 3 x 10(-5) monolayers upon an active surface area of only approximately 1.2 x 10(-9) m(2).

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PubMed2005

Surprises in the design of anion receptors: calorimetry prevents false reasoning.

Supplementing bicyclic guanidinium anion receptors with four sec-carboxamido groups leads to enhanced affinity for oxoanions, however, for a different reason than originally planned. Calorimetric analysis reveals that better binding is due to higher association entropies rather than more negative enthalpies. Thus, molecular design following geometric and functional complementarity principles may misguide supramolecular constructions aimed at a unique host-guest binding mode, as required, e.g., by self-assembly or catalysis.

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PubMed2005

Isothermal reorganization of poly(ethylene terephthalate) revealed by fast calorimetry (1000 K s(-1); 5 ms).

Reorganization of semicrystalline polymers on heating is a fast process. For poly(ethylene terephthalate) (PET) heating rates of several thousand Kelvin per second are needed to prevent reorganization of unstable crystals. Utilizing a thin film vacuum gauge as a fast calorimeter we are able to extend the scanning rate range of commercial DSC's (microK s(-1) to 10 K s(-1)) to rates as high as 10000 K s(-1) on heating and cooling. Because of the fast equilibration time isothermal experiments can be performed after scanning at several thousand Kelvin per second. The dead time after such a quench is in the order of 10 ms and the time resolution is in the order of milliseconds. These ultra fast calorimeters allow us to study the kinetics of extremely fast processes in semicrystalline polymers like reorganization. For PET crystallized at 130 degrees C reorganization needs less than 40 ms between 150 degrees C and 200 degrees C. Isothermal reorganization at 223 degrees C is about two orders of magnitude faster than isothermal crystallization from the isotropic melt at the same temperature. The melt memory for the remaining structures needed for reorganization is removed 25 K above the equilibrium melting temperature of PET.

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PubMed2006

Picosecond calorimetry: time-resolved x-ray diffraction studies of liquid CH2Cl2.

Liquid phase time-resolved x-ray diffraction with 100 ps resolution has recently emerged as a powerful technique for probing the structural dynamics of transient photochemical species in solution. It is intrinsic to the method, however, that a structural signal is observed not only from the photochemical of interest but also from the embedding solvent matrix. To experimentally characterize the x-ray diffraction signal deriving from the solvent alone we performed time-resolved diffraction studies of a pure liquid sample over a time domain from -250 ps to 2.5 micros. Multiphoton excitation was used to rapidly heat liquid CH(2)Cl(2) using UV pulses of 100 fs duration. A significant x-ray diffraction signal is visible prior to the onset of thermal expansion, which characterizes a highly compressed superheated liquid. Liquid CH(2)Cl(2) then expands as a shock wave propagates through the sample and the temporal dependence of this phenomenon is in good agreement with theory. An unexpectedly slow initial release of energy into the liquid as heat is observed from multiphoton excited CH(2)Cl(2), revealing the presence of a metastable state of multiphoton excited CH(2)Cl(2).

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PubMed2004

Calorimetry at surfaces using high-resolution core-level photoemission.

We have developed a method to measure simultaneously the internal energy of bulk and the first layer atoms of a crystal. The internal energy of bulk and the surface atoms of lithium (110) have been evaluated from 22 K up to above the melting transition, applying the Debye model to the thermal broadening of the respective 1s photoemission lines. Our measurements clearly reveal two phase changes: the known liquid to solid transition and the surface melting, occurring 50 K below the bulk melting point.

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PubMed2005

Attojoule calorimetry of mesoscopic superconducting loops.

We report the first experimental evidence of nontrivial thermal behavior of the simplest mesoscopic system--a superconducting loop. By measuring the specific heat C of an array of 450,000 noninteracting aluminum loops with very high accuracy of approximately 20 fJ/K, we show that the loops go through a periodic sequence of phase transitions (with a period of an integer number of magnetic flux quanta) as the magnetic flux threading each loop is increased. The transitions are well described by the Ginzburg-Landau theory and are accompanied by discontinuities of C of only several thousands of Boltzmann constants kB.

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PubMed1984

Calorimetry of nitrogenase-mediated reductions in detached soybean nodules.

Heat evolved by isolated soybean (Glycine max cv Clark) nodules was measured to estimate more directly the metabolic cost associated with the symbiotic N(2) fixation system. A calorimeter constructed by modifying standard laboratory equipment allowed measurement on 1 gram of detached nodules under a controlled gas stream. Simultaneous gas balance and heat output determinations were made.There was major heat output by nodules for all of the nitrogenase substrates tested (H(+), N(2), N(2)O, and C(2)H(2)) further establishing the in vivo energy inefficiency of biological N(2) fixation. Exposure to a short burst of 100% O(2) partially inactivated nitrogenase to permit calculations of heat evolved per mole of substrate reduced. The specific rate of heat evolution for H(+) reductions was 171 +/- 6 kilocalories per mole H(2) evolved in an Ar-O(2) atmosphere, that for N(2) fixation was 784 +/- 26 kilocalories per mole H(2) evolved and N(2) fixed, and that for C(2)H(2) reduction was 250 +/- 12 kilocalories/mole C(2)H(4) formed. When the appropriate thermodynamic parameters are taken into account for the different substrates and products, a DeltaH' of -200 kilocalories per mole 2e(-) is shown to be associated with active transfer of electrons by the nitrogenase system. These values lead to a calculated N(2) fixation cost of 9.5 grams glucose per gram N(2) fixed or 3.8 grams C per gram N(2), which is in close agreement with earlier calculations based on nodular CO(2) production.

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