Excess Enthalpy Function Research Articles

Theoretical analysis of superadiabatic combustion for non-stationary filtration combustion by excess enthalpy function.

The superadiabatic combustion for non-stationary filtration combustion is analytically studied. The non-dimensional excess enthalpy function (H) equation is theoretically derived based on a one-dimensional, two-temperature model. In contrast to the H equation for the stationary filtration combustion, a new term, which takes into account the effect of non-dimensional combustion wave speed, is included in the H equation for transient filtration combustion. The governing equations with boundary conditions are solved by commercial software Fluent. The predictions show that the maximum non-dimensional gas and solid temperatures in the flame zone are greater than 3 for equivalence ratio of 0.15. An examination of the four source terms in the H equation indicates that the thermal conductivity ratio between the solid and gas phases is the dominant one among the four terms and basically determines H distribution. For lean premixed combustion in porous media, the superadiabatic combustion effect is more pronounced for the lower .

Maximum superadiabatic temperature for stabilized flames within porous inert media

This work analyzes the superadiabatic temperature for laminar stationary lean premixed flames within porous inert media. The analysis is based on the excess enthalpy function applied to the one-dimensional volume-averaged equations. This formulation, with results obtained in a previous work, allows for the construction of an analytical solution valid over a large range of equivalence ratios. The model reveals the existence of a maximum non-dimensional superadiabatic temperature at a precisely determined equivalence ratio and connects previous works for near-stoichiometric and ultra-lean mixtures.

Excess Enthalpies of Mixing in Phospholipid-Additive Membranes

Isothermal titration calorimetry (ITC) allows the measurement of composition-dependent mixing heats of amphiphiles. A number of experimental protocols are now established to measure molecular transfer heats between, for example, micellar and lamellar aggregates. This study deals with the principle understanding of the physical effects contributing to the ITC data. The physical state of the mixture is described in terms of the molar excess enthalpy as a function of its composition hE(X). A relation is derived between this system property and the observable heat per mole of titrant (qobs )a sqobs ) (Xsyr - X)(@hE/@X) + hE(X) - hE(Xsyr) with X and Xsyr being the mole fractions of one chosen component within the mixed aggregates in the sample cell and in the injection syringe, respectively. According to this differential equation, one may derive information about the second and further derivatives (i.e., the curvature) of the excess enthalpy function. This can serve to construct the hE(X) plot based on the ITC data. We emphasize that for aggregates mixing nonideally (which must be considered rather the rule than the exception) one has to carefully distinguish between observed mixing heats and enthalpic state of the mixture. The formalism is presented at the example of mixtures of the phospholipid POPC and detergents of the type C12EOn with n ) 3-6. For instance, the system C12EO3/POPC was found to show an extremely asymmetric mixing enthalpy function with an attractive part (i.e., hE < 0) for low and a repulsive one for higher detergent contents in the mixed membranes. Such excess enthalpy functions could be modeled by a polynomial equation and discussed in terms of cooperative interactions between the molecules.

Head-on quenching of a premixed flame by a cold wall

We examine the premixed flame (PF) quenching problem, where an initially steadily propagating flame front impinges on a cold isothermal wall. We employ the simplest possible kinetics, R → P with a reaction order of unity, and we restrict ourselves to unit Lewis number. We formulate the problem in terms of the excess enthalpy function, H = τ + Y − 1. Interpretations are provided for the inflection points and maxima of various functional behaviors: for instance, flame arrest occurs at the inflection point of Y(0, t) whereas extinction occurs at the maximum of the cold wall H-gradient. The numerical solution code FRAPPE integrates formulae obtained from a simplified set of the conservation equations. Our results compare well qualitatively and quantitatively with the full numerical solutions of the original equations.

Excess enthalpy surfaces for n-heptane + carboxylic acid, amylamine and n-octanol mixtures by the nrtl model

The temperature and composition dependences of the excess enthalpy, h E , have been calculated using the nrtl model for mixtures of n-heptane with the hydrogen-bonded liquids of acetic acid, propionic acid, amylamine and n-octanol. Reduced and partial molar excess enthalpies were also calculated. The temperature-dependent parameters of the nrtl model, estimated directly from h E data for more than one isotherm, were used in the calculations. The overall deviations of all the experimental data points fall in the range 0.5–6.5% which shows highly satisfactory correlation of h E data with the model. The temperature range for the mixtures is 285.15–330.15 K. The surfaces of excess enthalpy functions facilitate better understanding of the thermodynamic properties of the hydrogen-bonded mixtures. The nrtl model is a reliable thermodynamic model for studying the hydrogen-bonded mixtures in a qualitative and quantitative manner.

Thermodynamic strategies for stabilizing intermediate states of proteins

This paper presents three theorems pertaining to thermodynamic properties of the intermediate (e.g., molten globule) state of proteins exhibiting such a conformation in the presence of GuHCl or urea. The theorems are proved for the three-state case using the denaturant binding model and the linear extrapolation model; their utility is illustrated via applications to examples in the literature. Theorem One states that the denaturant activity that maximizes the population of a partly folded conformation is at any temperature independent of the Gibbs free energy difference between the intermediate and native states. This result holds for both models of protein-denaturant interaction. The second theorem claims that the population maximum is independent of the denaturant association constant for the denaturant binding model. Theorem Three, which also applies to both models considered here, states that at the temperatures corresponding to the extrema in the population of the intermediate, the enthalpy change of the intermediate is equal to the excess enthalpy function, an experimentally accessible quantity. In the absence of denaturant, the enthalpy change of the intermediate state at the population extrema can be written as a function of the thermodynamic parameters of the unfolded state alone. These results, which can be applied to systems of any number of states under certain conditions, should aid in the optimization of conditions employed for experimental studies of partly organized states of proteins.

Binary common-ion alkali halide mixtures thermodynamic analysis of solid-liquid phase diagrams i. systems with negligible solid miscibility application of the extxd/sivamin method

Solid-liquid phase diagrams of 15 binary common-ion alkali halide mixtures, showing negligible solid miscibility, have been analyzed using the EXTXD/SIVAMIN method. The principles of and the philosophy behind the method, which permits the simultaneous derivation of the excess enthalpy and excess entropy functions from a single TX phase diagram, are discussed. The significance of the results is investigated by recalculating the phase diagrams and by comparing the excess enthalpy functions with experimental heats of mixing.

Statistical mechanical deconvolution of thermal transitions in macromolecules. I. Theory and application to homogeneous systems

AbstractThe theoretical basis for the statistical mechanical deconvolution of a thermally induced macromolecular melting profile is presented. It is demonstrated that all the thermodynamic quantities characterizing a multistate macromolecular transition can be obtained from the average excess enthalpy function, 〈ΔH〉, of the system, without any assumption of the particular model or mechanism of the reaction.Experimentally, 〈ΔH〉 is obtained from scanning calorimetric data by direct integration of the excess apparent molar heat capacity function, ΦCp. Once 〈ΔH〉 is known as a continuous function of the temperature, the partition function, Q, of the system can be calculated by means of the equation From the partition function all the thermodynamic quantities of the system can be obtained. It is shown that the number of discrete macroscopic energy states, the enthalpy and entropy changes between them, and the relative population of each state as a function of temperature can be calculated in a recursive form.

Statistical mechanical deconvolution of thermal transitions in macromolecules. III. Application to double‐stranded to single‐stranded transitions of nucleic acids

AbstractThe statistical mechanical deconvolution theory for macromolecular conformational transitions is extended to the case of nucleic acids transitions involving strand separation. It is demonstrated that the partition function, Q, as well as all the relevant thermodynamic quantities of the system, can be calculated from experimental scanning calorimetric data. In particular, it is shown that important thermodynamic parameters such as the size of the average cooperative unit during strand separation, the mean helical segment length, and the mean coil‐segment length can be calculated from the average excess enthalpy function 〈ΔH〉. The theory is applied to the double‐stranded to single‐stranded transition of the system poly(A)·poly(U) at different salt concentrations. It is shown that the mean helical segment length is a monotonically decreasing function of the temperature well before strand separation occurs. On the other hand, the mean coil segment length remains practically constant until temperatures very close to Tm. Both experimental findings clearly indicate that the unfolding of poly(A)·poly(U) proceeds through the formation of many short helical sequences. The cooperative unit for the strand separation is calculated to be about 120 base pairs and essentially independent of the salt concentration. The existence of a minimum helical segment length of 10 ± 2 base pairs within the double‐stranded form is calculated.