Thermodynamic Properties of Alkanes

TABLE 11.5. MM4 Entropies for Selected Alkanes (kcal/molK at 298K)  

It was expected, and found, that the entropies of hydrocarbons could be calculated to the accuracy shown in Table 11.5. This is a kind of independent test showing that aU of the statistical mechanics employed in MM4 is correctly implanented and the approximations adequate, so that the results shown are obtained. Entropies are cumulative from OK up to the temperature of interest (in this case 25 C). Any sizable systematic errors would be revealed in these results. 

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Thermodynamic properties of alkane chains 13 Thermodynamic properties of ring compounds 15 Enthalpy and entropy effects on ring closure 21... 

Hosoya, H., Gotoh, M., and Ikeda, S., Topological index and thermodynamic properties. 5. How can we explain the topological dependency of thermodynamic properties of alkanes with the topology of graphs , J. Chem. Inf. Comput. Sci., 39, 192-196, 1999. 

R. A. Alberty and C. A. Gehrig, Standard chemical thermodynamic properties of alkane isomer groups , J. Phys. Chem. Ref. Data, 13,1173 (1984). Tabulations of C°, S°, AfH° and Af G° for 0 < T (K) < 1500 for alkanes with 10 or fewer carbons. Also tabulated are group contributions and equilibrium fractions within alkane isomer groups. The values are updated from the study by D. W. Scott (1974). 

A single index can absorb a limited amount of structural information and often needs to be combined with information that other indices carry. Wiener used two descriptors, W and P (paths of length 3), to correlate selected thermodynamic properties of alkanes and structurally related amides, alcohols, and fatty acids. Selection of topological indices for regression analysis and other studies is always somewhat arbitrary, just as the selection of a coordinate system is in solving mathematical problems in calculus. The solution of problems, naturally, does not depend on the choice of coordinates, but solving of the problem may. Similarly, in structure-property-activity studies the relative magnitude of the computed result for two structures is not necessarily sensitive to the choice of descriptors, but the possibility of a simple interpretation of results may depend on the selection of descriptors. It is therefore desirable to use descriptors that are structurally related as this will facilitate comparison and interpretation of the results obtained for different molecules. We will review here several sets of structurally related molecular descriptors. 

Verlet, L. Computer experiments on classical fluids. I. Thermodynamical properties of Lennard-Jones molecules. Phys. Rev. 165 (1967) 98-103. Ryckaert, J.-P., Ciccotti,G., Berendsen, H.J.C. Numerical integration of the cartesian equations of motion of a system with constraints Molecular dynamics of n-alkanes. Comput. Phys. 23 (1977) 327-341. 

It was shown in the previous section that different relatively stable conformations of a given molecule can result from internal rotation of a particular functional group. The possibility of the existence of various conformers is of extreme importance in many applications. It should be noted, for example, that the biological activity of an organic molecule often depends on its confonfia-tion - in particular the relative orientation of a specific functional grtmp. As another example, the thermodynamic properties of, say, an alkane are directly related to the conformation of its carbon skeleton. In this context the industrial importance of /sooctane is well-known. 

Sugiyama, T., Takeuchi, T., Suzuki, Y. (1975) Thermodynamic properties of solute molecules at infinite dilution determined by gas-liquid chromatography. I. Intermolecular energies of w-alkane solutes in C28 - C36 w-alkane solvents. J. Chromatogr. 105,265-272. 

Traube s rule accommodates the balance between hydrophobicity and hydro-philicity. It has been extended somewhat and formalized with the development of quantitative methods to estimate the surface area of molecules based on their structures [19, 237]. The molecular surface area approach suggests that the number of water molecules that can be packed around the solute molecule plays an important role in the theoretical calculation of the thermodynamic properties of the solution. Hence, the molecular surface area of the solute is an important parameter in the theory. In compounds other than simple normal alkanes, the functional groups will tend to be more or less polar and thus relatively compatible with the polar water matrix [227,240]. Hence, the total surface area of the molecule can be subdivided into functional group surface area and hydro carbonaceous surface area . These quantities maybe determined for simple compounds as an additive function of constituent groups with subtractions made for the areas where intramolecular contact is made and thus no external surface is presented. 

Martinez, R., Gonzalez, J.A., de la Fuenta, LG., and Cobos, J.C. Thermodynamic properties of n-alkoxyethanols + organic solvent mixtures. XIV. Liquid-liquid equilibria of systems containing 2-(2-ethoxyethoxy)ethanol and selected alkanes. J. Chem. Eng. Data, 45(6) 1036-1039, 2000. 

Heintz, A., Kulikov, D.V., and Verevkin, S.R, Thermodynamic properties of mixtures containing ionic liquids. 1. Activity coefficients at infinite dilution of alkanes, alkenes, and alkylbenzenes in 4-methyl-M-butylpyridinium tetrafluo-roborate using gas-liquid chromatography, /. Chem. Eng. Data, 46,1526,2001. 

Sugamiya, K. Kuwahara, N. Kaneko, M., "Thermodynamic Properties of Moderately Concentrated Solutions of Poly(dimethylsiloxane) in n-Alkanes," Macromolecules, 7, 66 (1974). 

Wilhoit R. C. Marsh, K. N. Hong, X. Gadalla, N. Frenkel, M. Thermodynamic Properties of Organic Compounds and Their Mixtures, Subvolume B. Densities of Aliphatic Hydrocarbons Alkanes, Landolt-Bomstein, Group IV. Physical Chemistry, Vol. 8, Berlin Springer-Verlag, (1996). Also Subvolumes C-F. 

Recently Guermouche and Vergnaud measured the activity coefficients of normal alkanes in squalane at various temperatures in order to determine the excess thermodynamic properties of mixing. Unfortunately the results, obtained from conventional atmospheric g.l.c. (pi 180 kPa), do not always agree with the static and medium-pressure g.l.c. results quoted by Cruickshank et al. ... 

E. S. Domalski and E. D. Hearing, Estimation of the thermodynamic properties of hydrocarbons at 298.15 , J. Phys. Chem. Ref. Data, 17, 1637 (1988). Application of group additivity techniques to calculated thermodynamic properties for liquid and solid phases at 298 K for straight chain (C1-C20) and branched alkanes, cycloalkanes and alkylcycloalkanes, and other compounds. Group values for AfH°, C° and S° at 298 K are tabulated for gas, liquid and solid phase. Corrections for gauche interactions in gas-phase branched alkanes are reevaluated using a questionable, and certainly more complex, scheme. Sources of experimental data are not indicated, and seem to include previously estimated values as well as experimentally measured ones. 

Some of the computed interfacial tensions have been included in Figures 2a and b (solid lines). In the first figure the results of the calculations for octane are presented. These results do not show a large deviation from the experiments, an effect that has been observed for the PR equation of state as well. This is not very surprising since both the PR equation and APACT are able to describe the thermodynamic properties of the alkanes accurately. 

The HT voltammetry with gold electrodes was also recently used to measure the surface partitioning constant of a soluble, redox-active surfactant at the air/water interface [25]. Malec and coworkers modified the surface of gold electrodes by self-assembly of short alkane chain thiols in order to mimic the thermodynamic properties of the air/water interface. They relied on the fact that the surface tensions of the air/water interface and of the liquid alkane/water interface are similar [8]. Indeed, the HT measurements of the Gibbs monolayer formation constant were in agreement with their surface tensiometry and Brewster angle microscopic measurements [25]. 

While the theory of solubility parameters is limited in resx)ect of the thermodynamic properties of hydrocarbon mixtures, Williamson s variant for n-alkanes as well as similar modifications for other homologous series of hydrocarbons are useful for correlation purposes. 

Figure 7.2 Molar enthalpies of mixing for n-hexane + n-hexadecane. (From Orwoll, R.A. and Flory, RJ., Thermodynamic properties of binary mixtures of n-alkanes, /. Am. Chem. Soc., 89, 6814, 1967. With permission.)...

The two monomers of major interest, styrene and ethylene, are well known and details can be found on all aspects of their technology elsewhere. Poly(ethylene-co-styrene) is primarily produced via solution polymerization techniques using metallocene catalyst/co-catalyst systems, analogous to the production of copolymers of ethylene with a-olefin monomers. Solvents that can be employed include ethyl-benzene, toluene, cyclohexane, and mixed alkanes (such as ISO PAR E, available from Exxon). The thermodynamic properties of poly(ethylene-co-styrene), including solvent interactions and solubility parameter assessments, are important factors in relation to polymer manufacture and processing, and have been reported by Hamedi and co-workers (41). 

An early report on computational estimation of the thermodynamic properties of (difluoramino)alkanes, outside of the context of the American and Russian research programs, was by inel (Bogazi i University, Turkey), who estimated heats of formation, entropies, and bond dissociation energies of several simple difluoramines [141,142], derived from pubhshed experimental data on perfluorinated amines and thermochemical relationships. 

J. H. van der Waals, Thermodynamic Properties of Mixtures of Alkanes Differing in Chain Length , D. B. Centen, Amsterdam, 1950. 

D. C. Scott and J. P. McCullough, The Chemical Thermodynamic Properties of Hydrocarbons and Related Substances. Properties of 100 Linear Alkane Thiols, Sulphides, and Symmetrical Disulphides in the Ideal Gas State from 0 to 1000 K , U.S. Bureau of Mines, Bulletin 395, 1961. 

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