Mixture-model approach to the theory of classical fluids. III. Application to aqueous solutions of nonelectrolytes

An extension of the mixture-model approach to the theory of liquid water is developed to include aqueous solutions of nonelectrolytes. The Kirkwood-Buff theory of solution is employed to obtain a general and exact expression for the stabilization effect induced by the solute. This relation is applied, in the framework of a two-structure model, to obtain further insight into the molecular origin of some anomalous thermodynamic properties of aqueous solutions. The generalized continuous mixture-model formalism is also extended to solutions. It is demonstrated that current concepts such as structural changes in the solvent are strongly dependent on the particular classification procedure adopted to construct the mixture model.     

Effects of temperature and pH on the helicity of a peptide adsorbed to colloidal silica

The conformation of a cationic -helical peptide (DDDDAAAARRRRR) adsorbed to anionic colloidal silica has been investigated by circular dichroism (CD) spectroscopy as a function of temperature and pH in order to examine how the structure of an adsorbed molecule responds to two simultaneous perturbations. Increased temperature destabilizes the helicity of the peptide in solution, while pH changes alter the substrate surface charge and the corresponding strength of the interaction with the peptide. Near neutral pH, the helicity of the adsorbed peptide, which is determined from the intensity of the CD signal at 222 nm, decreases with increasing temperature, similarly to the temperature-dependent behavior observed for the peptide in aqueous solution. By contrast, at basic pH and a strongly negative surface charge, the helicity of the adsorbed peptide increases with temperature. In order to elucidate the origin of the reversal of the temperature dependence of helicity, a statistical model for the conformation of the adsorbed peptide has been formulated based on the Lifson–Roig model for the helix–coil transition of the peptide in solution. The model provides insight into the trends in fractional helicity and reveals that the temperature dependence of the helicity of the adsorbed peptide results from a competition between the intramolecular interactions that promote helicity and the intermolecular interactions with the surface. The statistical model also enables estimation of the free energy contributions from specific aspects of the adsorption process. Through identification of a connection between the conformation of adsorbed peptide and the interactions of the peptide with the surface, this work suggests a route for the control of adsorbate conformation through peptide and surface engineering.     

Solid-phase microextraction and gas chromatography–mass spectrometry for rapid characterisation of semi-hard cheeses

A headspace solid-phase microextraction (HS-SPME) method in combination with gas chromatography–mass spectrometry (GC–MS) has been used for extraction and identification of components of the volatile fraction of Provola dei Nebrodi, a typical semi-hard Sicilian cheese. Cheese samples from different producers and at different ripening stages have been examined. The effects of various conditions (e.g. sample volume, sample headspace volume, sample heating temperature, extraction time, etc.) on extraction efficiency were studied in order to optimise the technique. The technique used made it possible to identify 61 components: fatty acids from C₂ to C₁₄ and their esters, aldehydes, alcohols, methyl ketones, -lactones, aromatic compounds, hydrocarbons and terpenes. The main components were butanoic, hexanoic and octanoic acids. The linear free fatty acids (FFA) from C₂ to C₁₀ were quantified by using the standard addition method. Calibration curves constructed for the FFA spiked into cheese were highly linear with a correlation coefficient R²>0.998. Significant statistical differences (P0.05) were evident for the even-carbon-number fatty acids during ripening.     

Some results related with statistical convergence and Berezin symbols

The concept of discrete statistical Abel convergence is introduced. In terms of Berezin symbols we present necessary and sufficient condition under which a series with bounded sequence {an}_n?0 of complex numbers is discrete statistically Abel convergent. By using concept of statistical convergence we also give slight strengthening of a result of Gokhberg and Krein on compact operators.     

随机场熵率的计算和上界估计

提出了用信息论方法计算随机场熵率和估计其上界的新途径,并用此方法计算了若干实例．     

Mixing rules for binary Lennard–Jones chains: theory and Monte Carlo simulation

Theoretically-based van der Waals one-fluid (vdW1) mixing rules are derived for Lennard–Jones (LJ) chain mixtures. The rules provide equivalent one-fluid segment parameters for LJ size (σ) and energy () parameter as well as chain length (m) based on the parameters of the individual mixture components and the component mole fractions. The mixing rules are tested by performing Monte Carlo simulations of eight different binary mixtures and the equivalent vdW1 pure fluid, each at three densities. The simulations test the effects of changing LJ size parameter, LJ energy parameter and chain length individually and together. The effects of mole fraction and density are also examined. The mixing rules are tested for accuracy in predicting compressibility factors and radial distribution functions. It is found that the vdW1 rules provide excellent agreement when size and energy parameter are varied. Good agreement is found for mixtures with different chain lengths. The discrepancy is worst at very high densities when all component parameters are varied simultaneously.     

Fission probabilities across the π-nucleon delta resonance

Cross-sections for the π⁺-induced fission of ²⁰⁹Bi and ²³⁵U have been measured in small steps across the 3-3 resonance to search for any mechanisms that might signal a change in the reaction mechanism across a region where strong coupling of available energy into nuclear excitation can be expected. The bismuth data are found to be in good agreement with a statistical calculation not including new mechanisms. Received: 15 August 2000 / Accepted: 23 January 2001     

Effective conductivity in isotropic heterogeneous media using a strong-contrast statistical continuum theory

The strong-contrast formulation is used to predict the effective conductivity of a porous material. The distribution, shape and orientation of the two phases are taken into account using two- and three-point probability distribution functions. A new approximation for the three-point probability function appropriate for two-phase media is proposed and discussed. Computed results for the effective conductivity using the strong-contrast formulation are compared to the Voigt and the Hashin-Shtrikman upper-bound estimates. These results show that the predicted effective conductivity is lower than both Voigt and Hashin-Shtrikman bounds. Compared to previous results using the weak-contrast formulation, the strong-contrast formulation seems to provide a better estimate for the effect of the microstructure on the conductivity.     

Canonical ensemble for static elastic structures with random microstructures

A constant-(N,V,T,θ) ensemble is proposed to describe the elastostatics of random solid structures. Within the harmonic approximation, the energy of such a solid structure is the sum of a thermal and a strain component without mutual interaction. Systems in this ensemble thus draw energies from two separate baths: one thermal and the other mechanical. A mechanical entropy and an effective temperature (θ) can then be defined on the same rigorous basis as the thermal entropy and the Kelvin temperature (T). This ensemble approach can be used to calculate the properties of solid structures sharing similar microstructural randomness.     

Objective uncertainty relation with classical background in a statistical model

We show within a statistical model of quantization reported in the previous work based on Hamilton–Jacobi theory with a random constraint that the statistics of fluctuations of the actual trajectories around the classical trajectories in velocity and position spaces satisfy a reciprocal uncertainty relation. The relation is objective (observation independent) and implies the standard quantum mechanical uncertainty relation.