Enthalpic

In polymer solutions or blends, one of the most important thennodynamic parameters that can be calculated from the (neutron) scattering data is the enthalpic interaction parameter x between the components. Based on the Flory-Huggins theory [4T, 42], the scattering intensity from a polymer in a solution can be expressed as... 

The most direct effect of defects on tire properties of a material usually derive from altered ionic conductivity and diffusion properties. So-called superionic conductors materials which have an ionic conductivity comparable to that of molten salts. This h conductivity is due to the presence of defects, which can be introduced thermally or the presence of impurities. Diffusion affects important processes such as corrosion z catalysis. The specific heat capacity is also affected near the melting temperature the h capacity of a defective material is higher than for the equivalent ideal crystal. This refle the fact that the creation of defects is enthalpically unfavourable but is more than comp sated for by the increase in entropy, so leading to an overall decrease in the free energy... 

Lewis acid and the ligand. By contrast, soft-soft interactions are mainly enthalpic in origin and are characterised by a negative change in entropy" ... 

If one would ask a chemist not burdened with any knowledge about the peculiar thermodynamics that characterise hydrophobic hydration, what would happen upon transfer of a nonpolar molecule from the gas phase to water, he or she would probably predict that this process is entropy driven and enthalpically highly unfavourable. This opinion, he or she wo ild support with the suggestion that in order to create room for the nonpolar solute in the aqueous solution, hydrogen bonds between water molecules would have to be sacrificed. 

It is hardest to obtain precise values for the enthalpic values A, S, and G because they depend more heavily on high-energy states, which the system achieves infrequently. These functions depend on the actual value of Q, not just its derivatives. 

Monte Carlo simulations are an efficient way of predicting liquid structure, including the preferred orientation of liquid molecules near a surface. This is an efficient method because it is not necessary to compute energy derivatives, thus reducing the time required for each iteration. The statistical nature of these simulations ensures that both enthalpic and entropic effects are included. 

Basic Thermodynamics. Equilibrium-phase behavior of mixtures is governed by the free energy of mixing and how this quantity, consisting of enthalpic... 

T Lazaridis, G Archontis, M Karplus. Enthalpic contribution to protein stability Atom-based calculations and statistical mechanics. Adv Protein Chem 47 231-306, 1995. 

In the first category of solutions ( regular solutions ), it is the enthalpic contribution (the heat of mixing) which dominates the non-ideality, i.e. In such solutions, the characteristic intermolecular potentials between unlike species differ significantly from the average of the interactions between Uke species, i.e. 

ENTHALP, a computer software, was developed to calculate the heat of reaction at the system temperature. To run the program, the user must know the following ... 

The value of K is very sensitive to small errors in AH° and AS° and the equilibrium eonstant belongs to the reaetion equation N2 -1- 3H2 —> 2NH3. Table 2-1 shows the ealeulated results of the heats of reaetion at standard state (298 K) and at system temperature of 428 K using tlie software ENTHALP. 

The abscissa of the Mollier diagram is humidity x. The axis is provided with a pitched scale. A straight line is drawn with a 45° angle to the abscissa, and it is provided with an enthalpy scale hj ) according to the equation hj. - = X = 2501.6 X, kj/kg (Fig. 4.8). Consequently the enthalpic scale... 

As is known, SEC separations require interaction-free conditions. Therefore, the enthalpic contribution to the free energy term vanishes when no enthalpic interaction is postulated between analyte and sorbent ... 

In the case of no steric exclusion of the molecule from parts of the stationary phase, the retention can be described by the enthalpic term alone ... 

As known, SEC separates molecules and particles according to their hydro-dynamic volume in solution. In an ideal case, the SEC separation is based solely on entropy changes and is not accompanied with any enthalpic processes. In real systems, however, enthalpic interactions among components of the chromatographic system often play a nonnegligible role and affect the corresponding retention volumes (Vr) of samples. This is clearly evident from the elution behavior of small molecules, which depends rather strongly on their chemical nature and on the properties of eluent used. This is the case even for... 

For these reasons, it is very important to effectively suppress the enthalpic interactions in SEC so that Benoit s plot is valid for both calibration standards and characterized polymers. This is done by choosing non (inter) active column packings, adjusting the temperature of the experiment, and applying interaction (mainly adsorption) suppressing liquids (single or mixed) as eluents. 

If AG is equal to 0, the process is at equilibrium, and there is no net flow either in the forward or reverse direction. When AG = 0, A.S = H/T, and the enthalpic and entropic changes are exactly balanced. Any process with a nonzero AG proceeds spontaneously to a final state of lower free energy. If AG is negative, the process proceeds spontaneously in the direction written. If AG is positive, the reaction or process proceeds spontaneously in the reverse direction. (The sign and value of AG do not allow us to determine how fast the process will go.) If the process has a negative AG, it is said to be exergonic, whereas processes with positive AG values are endergonic. 

Enthalpic increments for a group additivity scheme in thermochemistry of pyridine, quinoline, and their substitution products 99PAC1257. 

An important part of the optimization process is the stabilization of the monomer-template assemblies by thermodynamic considerations (Fig. 6-11). The enthalpic and entropic contributions to the association will determine how the association will respond to changes in the polymerization temperature [18]. The change in free volume of interaction will determine how the association will respond to changes in polymerization pressure [82]. Finally, the solvent s interaction with the monomer-template assemblies relative to the free species indicates how well it will stabilize the monomer-template assemblies in solution [16]. Here each system must be optimized individually. Another option is simply to increase the concentration of the monomer or the template. In the former case, a problem is that the crosslinking as well as the potentially nonselective binding will increase simultaneously. In the... 

For more than two decades researchers have attempted to overcome the inadequacies of Flory s treatment in order to establish a model that will provide accurate predictions. Most of these research efforts can be grouped into two categories, i.e., attempts at corrections to the enthalpic or noncombinatorial part, and modifications to the entropic or combinatorial part of the Flory-Huggins theory. The more complex relationships derived by Huggins, Guggenheim, Stavermans, and others [53] required so many additional and poorly determined parameters that these approaches lack practical applications. A review of the more serious deficiencies... 

The three activation parameters, AG, AH, and AS decreased with polyelectrolyte addition. The decrease in AS suggests that the acceleration is due to the enthalpic loss. We recall that the acid hydrolyses of aliphatic esters with polymeric sulfonic acid was accompanied by decreases in AH and AS 97, 98 ... 

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