Sublimation free energy

Qne promising approach to predicting aqueous solubility is to calculate it directly from theory and/or molecular simulation. In this chapter, we will discuss the different molecular theory and simulation-based approaches that have been used to calculate the intrinsic aqueous solubility of drug-like molecules. We also devote some of this chapter to the computation of other thermodynamic parameters that are required for the prediction of solubility such as hydration and sublimation free energies. Moreover, since a lack of accurate experimental data is currently a limiting factor in developing and testing all classes of computational solvent models, we also discuss methods to measure solubility and the extent and reliability of available data. 

The sublimation free energy data that is reported in the published literature for organic crystals is often given relative to a latm standard state in the gas phase (0G ), while hydration free energies are more commonly given relative to a molar standmd state in the gas phase Sublimation free energy data reported as 2G", can be... 

Relatively few thermodynamic studies have been performed on compounds involving Th, U and Pu with noble metals. Most of the previous work has involved electrochemical cell determinations of free energies of formation, hence little has been published concerning the sublimation behavior of actinide intermetallics. 

FIGURE 7.26 For some substances and at certain pressures, the molar Gibbs free energy of the liquid phase might never lie lower than those of the other two phases. For such substances, the liquid is never the stable phase and, at constant pressure, the solid sublimes when the temperature is raised to the point of intersection of the solid and vapor lines. 

The standard free energy of formation of a gaseous metal ion, AGj (M+, g), can be viewed as the sum of the standard free energy of sublimation, AGj (M , g), and the free energy of ionization, AG j. The standard entropy of a monatomic gas is very nearly equal to the standard entropy of its corresponding monatomic gaseous... 

In Table 5.1 the surface structures of ordered metal monolayers adsorbed on metal surfaces are listed. For each substrate, the crystallographic structure, the distance between nearest neighbors, and the heat of sublimation (that is proportional to the surface free energy) are given. For each metal adsorbate the identical information is provided along with the technique of deposition and all the ordered surface structures that form with increasing coverage. 

Standard Enthalpies, Entropies and Gibbs Free Energies of Sublimation of... 

At temperatures of about 4000°K., the free energy of formation of acetylene from its elements approaches zero, and the equilibrium yield of acetylene is appreciable. The system is complicated, however, by other reactions and phase changes which occur at these high temperatures. Carbon sublimes at about 4000°K., various species of carbon Ci, C2, and Ca are formed, and dissociation of molecular hydrogen occurs. 

Aihara70 has measured the vapor pressure of pure solid NMA over the temperature range of 15 to 30 °C. From these data he has calculated values (298.1 K) of the free energy (20.6 kJ mol-1), enthalpy (54.1 kJ mol-1) and entropy (112 J mol-1 K-1) of sublimation for NMA. It should, however, be noted that the value determined for the enthalpy of sublimation at 298.1 K is slightly smaller than the calculated value of the enthalpy of vaporization of liquid NMA at 100 °C (cf. Section IV-b). 

Entropy and Enthalpy of Sublimation. Since we have a system of only one component, the chemical potentials for I2 in crystalline and gaseous forms, given in Eqs. (33) and (25), respectively, are equivalent to the molar Gibbs free energies Gs and Gg, aside from an additive constant. The entropies of the two phases can be obtained by differentiating with respect to temperature. The expressions obtained are... 

Other experimental and theoretical methods have been developed for the determination of the heat of sublimation of solid iodine these too are suitable for undergraduate laboratory experiments or variations on this experiment. Henderson and Robarts have employed a photometer incorporating a He-Ne gas laser, the beam from which (attenuated by a CUSO4 solution) has a wavelength of 632.8 nm, in a hot band near the long-wavelength toe of the absorption band shown in Fig. 3. Stafford has proposed a thermodynamic treatment in which a free-energy function ifef), related to entropy, is used in calculations based on the third law of thermodynamics. In this method either heat capacity data or spectroscopic data are used, and as in the present statistical mechanical treatment, the heat of sublimation can be obtained from a measurement of the vapor pressure at only one temperature. 

Evidently AGS,in = AGfa, + AG, v. The free energy of solution, AG i , is obtained from the molar solubility s, ACJ = —RT In s the quantity AGi aii is the lattice energy of the crystal, obtainable as the sublimation energy. AG i is often small because the interaction energy within the solid is comparable with the energy of interaction between solute and solvent. We will return to solvation energies... 

---