Change In Chemical Potential Research Articles

Thermodynamic Properties of the NaNO3+H2O System

New equations that describe the thermodynamic properties of the NaNO3+H2O system were obtained from previously published measurements for this system. The measured values included in the fitted equations spanned the range of temperatures of approximately 236–425 K for NaNO3(aq) and 16–548 K for NaNO3(cr). New equations and/or values for the following properties are given in the present work: (1) thermal properties of NaNO3(cr) from 0 K to near the lambda transition, 548.6 K, (2) the change in chemical potential for both NaNO3 and H2O in NaNO3(aq) as a function of temperature, and molality, valid from 236 to 425 K, and the molality range of 0 mol⋅kg−1 to the lesser of the saturation molality or 25 mol⋅kg−1, and (3) standard-state properties for the aqueous solution process.

Extraction of Thermodynamic Data from Ternary Diffusion Coefficients. Use of Precision Diffusion Measurements for Aqueous Lysozyme Chloride−NaCl at 25 °C To Determine the Change of Lysozyme Chloride Chemical Potential with Increasing NaCl Concentration Well into the Supersaturated Region

For ternary systems, we present a method for using measured values of the four ternary diffusion coefficients and the Onsager reciprocal relations to extract derivatives of solute chemical potentials with respect to solute molar concentrations. The method is applicable to systems in which the molar concentration of one solute is very small compared to that of the other, and also small enough that an inverse concentration dependence dominates certain activity coefficient derivatives. These conditions apply to a large number of aqueous systems involving macromolecules of biological interest. Unlike other techniques, the present method can be used to study undersaturated and supersaturated solutions. The approach is illustrated for the lysozyme chloride−NaCl−H2O system at 25 °C, using data reported here for pH 6.0 at 0.60 mM (8.6 mg/mL) lysozyme chloride and 0.25, 0.50, 0.65, 0.90, and 1.30 M (1.4, 2.8, 3.7, 5.1, and 7.2 wt %) NaCl concentrations, and our earlier data for pH 4.5 at the same concentrations. We use these solute chemical potential derivatives to compute the protein cation charge approximately, and to construct a function approximating the derivative of the lysozyme chloride chemical potential with respect to NaCl concentration, which we integrate over a range of NaCl concentrations. This provides the change of the lysozyme chloride chemical potential with NaCl concentration well into the supersaturated region, and hence provides the driving force for nucleation and crystal growth of lysozyme chloride as a function of the extent of supersaturation. We also compute the diffusion Onsager coefficients (Lij)0 for each composition at pH 4.5 and 6.0. Binary diffusion coefficients of aqueous lysozyme chloride at 0.89 mM (12.7 mg/mL) for pH values from 4.0 to 6.0, and at pH 6.0 for concentrations from 0.25 to 1.95 mM (3.6−27.9 mg/mL) are also reported.

Chemical Potential of Aromatic Compounds in Pure n-Alkanes Using Expanded Ensemble Monte Carlo Simulations

For environmental pollutants, the chemical potential, which provides a measure of the solubility of a solute, is important in determining the fate and transport of pollutants in various media, such as air, particulates, and soil. The chemical potential of benzene and naphthalene in pure n-alkane fluids at infinite dilution ({micro}{sub ex}{sup {infinity}}) is calculated using expanded ensemble Monte Carlo simulations. Simulations are carried out in an isobaric-isothermal ensemble at 1 atm pressure. The effect of temperature and carbon number of the solvent on {micro}{sub ex}{sup {infinity}} is studied. The results indicate that, within the statistical uncertainty of the simulations, there is no strong dependence of the chemical potential of benzene and naphthalene on the carbon number of the solvent between C{sub 6} and C{sub 24}. However, the chemical potential changes substantially with the temperature and the size of the solute. Good qualitative agreement is found between the simulation results and experiments.

Variational Principles for Describing Chemical Reactions: The Fukui Function and Chemical Hardness Revisited

The fundamental principles of density functional theory are applied to achieve a better understanding of various theoretical tools for describing chemical reactivity. Emphasis is given to the Fukui function, the central site reactivity index of density functional theory, which is approached through its own variational principle. A maximum hardness principle is then developed and discussed. To make contact with an earlier proof of a maximum hardness principle, changes in chemical potential are considered.

Promotion of catalytic reactions by electrochemical polarization

Electrochemical polarization of metal electrodes on solid electrolytes may result in a pronounced promotion of their catalytic activity. A phenomenological analysis of this promotion effect is developed from simple thermodynamic and kinetic arguments and is compared with existing models. The strong modification of catalytic activity is related to the electrochemically controlled change of the chemical potential of a mobile component at the electrode interface within the (solid state) galvanic cell. Empirical relations reported in the literature are interpreted in terms of this chemical potential change.

Characterization of Chemical Reactions from the Profiles of Energy, Chemical Potential, and Hardness

A theoretical model relating the kinetic, mechanistic, and thermodynamics aspects of a chemical reaction is presented. The model is based upon the consistency between the principle of maximum hardness and the Hammond postulate, and it is used to obtain insights about the reaction mechanisms and to define rate constants in terms of the chemical potential and hardness of activation. An important result is that when both electronic properties are allowed to vary along the reaction coordinate, it is found that the activation process is controlled by the change in chemical potential while the relaxation one is controlled by the change in hardness.

Influence of uniaxial stress on the lamellar spacing of eutectics

Directional solidification of lamellar eutectic structures submitted to uniaxial stress is investigated. In the spirit of an approximation first used by Jackson and Hunt, we calculate the stress tensor for a two-dimensional crystal with triangular surface, using a Fourier expansion of the Airy function. The effect of the resulting change in chemical potential is introduced into the standard model for directional solidification. This calculation is motivated by an observation, made recently [I. Cantat, K. Kassner, C. Misbah, and H. M\"uller-Krumbhaar, Phys. Rev. E 58, 6027 (1998)], that the thermal gradient produces similar effects as a strong gravitational field in the case of dilute-alloy solidification. Therefore, the coupling between the Grinfeld and the Mullins-Sekerka instabilities becomes strong, as the critical wavelength of the former instability gets reduced to a value close to that of the latter. Analogously, in the case of eutectics, the characteristic length scale of the Grinfeld instability should be reduced to a size not extremely far from typical lamellar spacings. Following Jackson and Hunt, we assume the selected wavelength to be determined by the minimum undercooling criterion and compute its shift due to the external stress. In addition, we find that in general the volume fraction of the two solid phases is changed by uniaxial stress. Implications for experiments on eutectics are discussed.

Thermodynamic Properties of the KCl+H2O System

The thermodynamic properties of the KCl+H2O system were examined in order to provide: (1) an improved equation for the osmotic coefficient as a function of molality and temperature for purposes of isopiestic measurements, (2) a determination of the thermodynamic properties of the standard-state solution process, and (3) a test of the accuracy of the enthalpy of solution values for KCl(cr), a calorimetric standard. New equations that describe the thermodynamic properties of the KCl+H2O system were obtained from previously published measurements for this system. The measured values included in the fitted equations spanned the range of temperature from approximately 260 to 420 K for KCl(aq) and 1.5 K to 1033.7 K for KCl(cr). New equations and/or values for the following properties are given in the present work: (1) thermal properties of KCl(cr) from 0 K to the melting point, 1045 K, (2) the change in chemical potential for both KCl and H2O in KCl(aq) as a function of temperature, and molality, valid from 260 to 420 K, and (3) standard-state properties for the aqueous solution process. The effect of heat treatment on the determination of enthalpy of solution values was also examined. This examination indicated that the NIST recommendation of heating Standard Reference Material 1655, potassium chloride, at 800 K for a minimum of 4 h prior to its use in calorimeter calibration, should probably not be followed. The heat treatment recommended by the NIST certificate appears, at this point, to have a higher probability of corrupting the sample than improving the accuracy of the measurement.

Silicate esters with additives as temporary protectives on steel surfaces

A series of tetra-alkoxysilanes and dimethyldialkoxysilanes (silicate esters) was prepared by reacting SiCl4 and (CH3 )2 SiCl2 respectively with corresponding alcohols. The performance of these silicate esters as temporary protectives for mild steel surfaces has been studied by assessing the time to appearance of the first rust spot on the specimen surface, electro chemical potential changes, and the percentage protection given by these esters to steel plates under different conditions using natural sea water containing 1%Na2 SO4 with various additives including thiourea and nicotinic acid. The corrosion resistance imparted by dimethyldialkoxysilanes is greater than that imparted by tetra-alkoxysilanes. However, for both series of silicate esters, those which have high molecular weight and branching in the alkyl fraction confer better corrosion resistance than those with low branching and a linear structure. Similar measurements were made for steel coated with silicone fluid and liquid paraffin and for uncoated steel for the purpose of comparison.

Kev Ion Beam Induced Surface Modification of Sic Hydrogen Sensor

AbstractSilicon carbide, a wide-bandgap semiconductor, is currently used to fabricate an efficient high temperature hydrogen sensor. When a palladium coating is applied on the exposed surface of silicon carbide, the chemical reaction between palladium and hydrogen produces a detectable change in the surface chemical potential. Rather than applying a palladium film, we have implanted palladium ions into the silicon face of 6H, n-type SiC samples. The implantation energies and fluences, as well as the results obtained by monitoring the current through the sample in the presence of hydrogen are included below. The exposure to hydrogen of this kind of sensor while monitoring the current flow with respect to time, has revealed a completely different behavior than the samples that have Pd deposited as a surface layer.

Thermodynamic determining factors of the positive electrode potential of lithium batteries

Thermodynamic considerations have been made to extract the determining factors for the positive electrode potential in the lithium batteries in which the lithium-transition metal–oxygen compounds are used as the positive electrode. Interpretation has been made by dividing the contributions to the lithium chemical potential change for the electrode reaction into two terms; the stabilization energy of the double oxide from the constituent oxide and the valence stability of lithium and the transition metals, in other words, the redox term. The major factor in the ternary Li–M–O system has been found to be the redox term of the transition metal oxides; in other words, the equilibrium oxygen partial pressure of the redox equilibria of the transition metal oxide. This explains why LiCoO 2 or LiNiO 2 have large potentials. In the quaternary Li–M–X–O system, the stabilization energy of Li(XO m) n becomes an additional term. This term leads to high potentials in Fe 3(SO 4) 2 or LiMVO 4(M=Ni, Co).

Protein cross talking through osmotic work: the free energy of formation of the MgADP-myosin complexes at the muscle protein osmotic pressure

A method is presented to determine the energy of formation of the myosin-ADP complexes at the muscle protein osmotic pressure. It is found that, at 18 kP, the putative protein osmotic pressure in skeletal muscle, the increase of MgADP from 0.05 to 2 mmolal, increases the free energy of myosin-ADP and of myosin-(ADP) 2 by 0.756 and by 9.85 kJ/mol, respectively, and decreases the free energy of myosin by 8.34 kJ erg/mol. It is pointed out that the local changes of water chemical potential, induced by the binding of MgADP to myosin, can be sensed by other structures of the contractile machinery, which per se may even be insensitive to MgADP. Cross talking between macromolecules can thus be achieved by changes of the water chemical potential.

Influence of grain boundaries on hydrogen transport in polycrystalline silicon

The effect of the microscopic structure of polycrystalline silicon on hydrogen transport was investigated by H and D diffusion experiments. Poly-Si composed of columnar grains and grown by low pressure chemical vapor deposition (LPCVD-grown) exhibits enhanced diffusion compared to solid-state crystallized (SSC) poly-Si. The data can be explained by a two-level model used to explain diffusion in amorphous silicon. Clear evidence for deep H traps was only found for SSC poly-Si. LPCVD-grown poly-Si exhibited little evidence of deep trapping. Shallow transport traps were found to be about 0.5 eV and ∼1.6 eV below the transport level, respectively. The H chemical potential changes about 0.2 eV in response to an increase of the H concentration from mid-10 18 to ≈10 20 cm −3.

Coarsening of Self-Assembled Ge Quantum Dots on Si(001)

The size distribution of self-assembled heteroepitaxial islands is critical to their application as quantum dots in novel devices. In situ, real time UHV transmission electron microscopy studies of Ge island growth on Si(001) show that island coarsening occurs even during growth. With increasing volume, a shape transition from pyramids to domes gives rise to an abrupt change in chemical potential. This leads to a bifurcation in the size distribution and ultimately to a narrow size range. Simulations of coarsening in the presence of a shape transition are in good agreement with experimental results.

Salting out and coupled diffusion in aqueous dioxane and aqueous propan-2-ol mixed solvents

Taylor dispersion and differential refractometry are used to measure ternary interdiffusion coefficients (Dik) for 13 different water + dioxane(1) + electrolyte(2) systems at 25 °C. The diffusion of the dilute electrolytes in the water + dioxane (50 mass%) mixed solvent is accurately described by the pseudo-binary equation J2 = −D22∇C2. Because the gradient in the concentration of each electrolyte drives a measurable coupled flow of dioxane, the dioxane flux is described by the ternary equation J1 = −D11∇C1 − D12∇C2. Vapour pressure data are used to calculate the changes in the dioxane chemical potential (µ1) with added electrolyte, and hence the thermodynamic driving force for the coupled flux of dioxane produced by the electrolyte gradient. Values of ∂µ1/∂C2 range from −0.90 kJ d mol−2 for sodium tetraphenylboron (dioxane “salted in”) to +1.78 kJ d mol−2 for NaOH (dioxane “salted out”), in close correlation with D12 values ranging from −0.82 × 10−5 for NaPh4B (dioxane diffuses toward sodium tetraphenylboron) to +1.56 × 10−5 cm2 s−1 for NaOH (dioxane diffuses away from NaOH). For comparison with these results, ternary Dik coefficients are reported for the electrolytes in water + propan-2-ol (50.00 mass%) mixtures.

Twin boundaries ind-wave superconductors

Twin boundaries in orthorhombic d-wave superconductors are investigated numerically using the Bogoliubov-deGennes formalism within the context of an extended Hubbard model. The twin boundaries are represented by tetragonal regions of variable width, with a reduced chemical potential. For sufficiently large twin boundary width and change in chemical potential, an induced s-wave component may break time-reversal symmetry at a low temperature. This temperature, and the magnitude of the complex component, are found to depend strongly on electron density. The results are compared with recent tunneling measurements.

Computer Simulations on the Free Energies and Phase Diagrams of Asymmetrically Interacting Blends

We consider binary polymer blends where the energetic interactions between the three different pairs of monomers are not equal to each other. From computer simulations in the isothermal−isobaric ensemble, we show that these disparate energetic interactions yield different packing behavior even at the level of the pure components. This results in the pure materials having unequal densities and cohesive energies. In the context of mixtures we have considered two cases. In the system termed “attractive” we have employed Lennard−Jones interactions which favor the formation of dissimilar 1−2 contacts. The residual chemical potential changes on mixing for both components, independently, appear to vary parabolically with system composition. If one chose to model these results with the Flory model [or alternately, the polymer analog of the regular solution theory], then the chemical potential changes on mixing for each component would appear to be describable by a composition independent value of the χ parameter....

Probing the Fermi surfaces of coupled double quantum wells in the presence of an in-plane magnetic field

The anticrossing of the dispersion relations of two two-dimensional electron gases (2DEGs) in an in-plane magnetic field results in distorted Fermi surfaces, leading to density of states and effective mass changes. We have used two different techniques to probe such changes with both the in-plane field and carrier density being systematically altered. The first technique uses a fixed perpendicular field to form Landau levels. By following the evolution of these Landau levels with in-plane field, carrier density and temperature, we are able to determine changes in the subband populations and effective masses. The second technique uses a third conducting layer as a detector of chemical potential changes in the coupled electron gases. This method also allows the determination of effective mass changes in an in-plane field. Calculations of the dispersion curves have been made, illustrating the effect of the parallel field upon subband occupancy and effective mass. These calculations are compared with the experimental data from both methods and are found to be in good agreement.

Compressibility studies of two-dimensional electron gases

Double quantum well structures have been used to study the compressibility of two-dimensional electron gases. By using independent contacts to the two layers we are able to measure accurately the carrier density changes of the lower layer (which acts as a detector) in response to altering the top layer density (whose compressibility we are probing). The carrier density changes are related to the chemical potential changes in the top layer which is proportional to the inverse compressibility. Comparison with a model which solves the Poisson and Schrödinger equations in a self-consistent manner provides good qualitative agreement with the experimental results, with the compressibility changing from positive at high carrier densities to negative at low densities when the energy changes due to interactions dominate the chemical potential changes. Samples with different well widths have also been studied, these show that the compressibility varies with well width. When analysing this data we find the Hartree contribution is significant and cannot be ignored.

Mechanism of the stabilization of ribonuclease A by sorbitol: preferential hydration is greater for the denatured then for the native protein.

The effect of interactions of sorbitol with ribonuclease A (RNase A) and the resulting stabilization of structure was examined in parallel thermal unfolding and preferential binding studies with the application of multicomponent thermodynamic theory. The protein was stabilized by sorbitol both at pH 2.0 and pH 5.5 as the transition temperature, Tm, was increased. The enthalpy of the thermal denaturation had a small dependence on sorbitol concentration, which was reflected in the values of the standard free energy change of denaturation, delta delta G(o) = delta G(o) (sorbitol) - delta G(o)(water). Measurements of preferential interactions at 48 degrees C at pH 5.5, where protein is native, and pH 2.0 where it is denatured, showed that sorbitol is preferentially excluded from the denatured protein up to 40%, but becomes preferentially bound to native protein above 20% sorbitol. The chemical potential change on transferring the denatured RNase A from water to sorbitol solution is larger than that for the native protein, delta mu(2D) > delta mu(2N), which is consistent with the effect of sorbitol on the free energy change of denaturation. The conformity of these results to the thermodynamic expression of the effect of a co-solvent on denaturation, delta G(o)(W) + delta mu(D)(2)delta G(o)(S) + delta mu(2D), indicates that the stabilization of the protein by sorbitol can be fully accounted for by weak thermodynamic interactions at the protein surface that involve water reversible co-solvent exchange at thermodynamically non-neutral sites. The protein structure stabilizing action of sorbitol is driven by stronger exclusion from the unfolded protein than from the native structure.