Molecular excluded

It should be said that only since Cowan and Fickett s report have good experimental detonation pressure data been available for use in arriving at best-fit parameters. Also, the covolume factors (ki) have been deliberately normalized but not set equal to molecular excluded volumes. In this normalization, rather cogent argu-... 

Modeling of diffusion of liquids is more complex due to the steric effect of molecular excluding inside the pores. A distribution coefficient between the pore and free liquid may be defined by (Kerkhof and Geboers,... 

One of the determinants of the equUlbrium state of a system of many particles is the randomness or disorder of the system. According to the second law of thermodynamics, the term entropy is used to denote such a randomness in a system. In an ideal solution it is assiimed that the solute molecules are of the same order of size as the solvent molecules so chat the distribution of solute molecules in the solvent is entirely random as is seen in the case of true solutions. But this cannot hold true for macromolecular solutions because of the high solute to solvent size ratio. In these solutions the centre of each molecule is excluded from a volume of solution because of the high solute to solvent size ratio. In these solutions the centre of each molecule is excluded from a volume of solution because it is temporarily occupied by some part of another macromolecule. The departure from ideality thus depends on this molecular excluded... 

Solvent-excluded surfaces correlate with the molecular or Connolly surfaces (there is some confusion in the literature). The definition simply proceeds from another point of view. In this c ase, one assumes to be inside a molecaile and examines how the molecule secs the surrounding solvent molecules. The surface where the probe sphere does not intersect the molecular volume is determined. Thus, the SES embodies the solvent-excluded volume, which is the sum of the van der Waals volume and the interstitial (re-entrant) volume (Figures 2-119. 2-120). 

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Richmond T J 1984. Solvent Accessible Surface Area and Excluded Volume in Proteins. Journal oj Molecular Biology 178 63-88. 

The solvent-excluded volume is a molecular volume calculation that finds the volume of space which a given solvent cannot reach. This is done by determining the surface created by running a spherical probe over a hard sphere model of molecule. The size of the probe sphere is based on the size of the solvent molecule. 

Excluding enantiomers there are three isomeric cyclopropanedicarboxyhc acids Two of them A and B are constitutional isomers of each other and each forms a cyclic anhydnde on being heated The third diacid C does not form a cyclic anhydride C is a constitutional isomer of A and a stereoisomer of B Identify A B and C Construct molecular models of the cyclic anhy dndes formed on heating A and B Why doesn t C form a cyclic anhydride" ... 

Whether the beads representing subchains are imbedded in an array of small molecules or one of other polymer chains changes the friction factor in Eq. (2.47), but otherwise makes no difference in the model. This excludes chain entanglement effects and limits applicability to M < M., the threshold molecular weight for entanglements. 

Throughout this section we have used mostly p and u to describe the distribution of molecular weights. It should be remembered that these quantities are defined in terms of various concentrations and therefore change as the reactions proceed. Accordingly, the results presented here are most simply applied at the start of the polymerization reaction when the initial concentrations of monomer and initiator can be used to evaluate p or u. The termination constants are known to decrease with the extent of conversion of monomer to polymer, and this effect also complicates the picture at high conversions. Note, also, that chain transfer has been excluded from consideration in this section, as elsewhere in the chapter. We shall consider chain transfer reactions in the next section. 

Such a coil is said to be nondraining, since the interior of its domain is unaffected by the flow. We anticipate using Eq. (1.58) to describe the molecular weight dependence of In view of this, we replace rg by (rg ) and attach a subscript 0 to the latter as a reminder that, under 0 conditions, solvent and excluded-volume effects cancel to give a true value. With these ideas in mind, the volume fraction of the nondraining coil is written... 

A particularly useful property of the PX monomer is its enthalpy of formation. Conventional means of obtaining this value, such as through its heat of combustion, are, of course, excluded by its reactivity. An experimental attempt was made to obtain this measure of chemical reactivity with the help of ion cyclotron resonance a value of 209 17 kJ/mol (50 4 kcal/mol) was obtained (10). Unfortunately, the technique suffers from lack of resolution in addition to experimental imprecision. It is perhaps better to rely on molecular orbital calculations for the formation enthalpy. Using a semiempirical molecular orbital technique, which is tuned to give good values for heat of formation on experimentally accessible compounds, the heat of formation of /5-xylylene has been computed to be 234.8 kj/mol (56.1 kcal/mol) (11). 

An example of unique selectivity is provided by the use of 5A molecular sieves for the separation of linear hydrocarbons from branched and cycHc types. In this system only the linear molecules can enter the pores others are completely excluded because of their larger cross section. Thus the selectivity for linear molecules with respect to other types is infinite. In the more usual case, all the feed components access the selective pores, but some components of the mixture are adsorbed more strongly than others. A selectivity between the different components that can be used to accomplish separation is thus established. 

Poly(ethylene oxide)s [25372-68-3] are made by condensation of ethylene oxide with a basic catalyst. In order to achieve a very high molecular weight, water and other compounds that can act as chain terminators must be rigorously excluded. Polymers up to a molecular weight of 8 million are available commercially in the form of dry powders (27). These must be dissolved carefliUy using similar techniques to those used for dry polyacrylamides. Poly(ethylene oxide)s precipitate from water solutions just below the boiling point (see Polyethers, ethylene oxide polymers). 

Hydrocarbons, compounds of carbon and hydrogen, are stmcturally classified as aromatic and aliphatic the latter includes alkanes (paraffins), alkenes (olefins), alkynes (acetylenes), and cycloparaffins. An example of a low molecular weight paraffin is methane [74-82-8], of an olefin, ethylene [74-85-1], of a cycloparaffin, cyclopentane [287-92-3], and of an aromatic, benzene [71-43-2]. Cmde petroleum oils [8002-05-9], which span a range of molecular weights of these compounds, excluding the very reactive olefins, have been classified according to their content as paraffinic, cycloparaffinic (naphthenic), or aromatic. The hydrocarbon class of terpenes is not discussed here. Terpenes, such as turpentine [8006-64-2] are found widely distributed in plants, and consist of repeating isoprene [78-79-5] units (see Isoprene Terpenoids). 

Highly pure / -hexane can be produced by adsorption on molecular sieves (qv) (see Adsorption, liquid separation) (43). The pores admit normal paraffins but exclude isoparaffins, cycloparaffins, and aromatics. The normal paraffins are recovered by changing the temperature and/or pressure of the system or by elution with a Hquid that can be easily separated from / -hexane by distillation. Other than ben2ene, commercial hexanes also may contain small concentrations of olefins (qv) and compounds of sulfur, oxygen, and chlorine. These compounds caimot be tolerated in some chemical and solvent appHcations. In such cases, the commercial hexanes must be purified by hydrogenation. 

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