Homomolecular association

The values of homomolecular association constants of alcohols and acids are high, typieally in the range of 10 -1 O. Therefore eoncentration of the eomponent A is rather low, even at high content of solvate-inert component B, and may be neglected. 

The systems having components which interact by means of donor-acceptor bond (without proton transfer) belong to the same type of solvents (e.g., pyridine-chloracetyl, dimethylsulfoxide-tetrachloroethylene, etc.). Components of mixed solvents of such type are more or less associated in their individual states. Therefore, processes of heteromolecular association in such solvents occur along with processes of homomolecular association, which tend to decrease heteromolecular associations. 

Type II is determined by the negative deviations from isotherm [9.26]. This type corresponds to systems with weak heteromolecular interactions between components, but with strong homomolecular association of one of the components. The system formic acid-anisole is an example of this kind of isotherm. Furthermore, these isotherms are characteristic when non-associating components in pure state form heteromolecular associates with lower dipole moment, DM, then DM of both components. The average DM for such kind of interaction in mixed systems is lower than correspondent additive value for non-interacting system. The system 1,2-dichloroethane - n-butylbromide can be referenced as an example of this kind of mixed binary solvent. 

MIXED SOLVENT EFFECT ON THE POSITION OF EQUILIBRIUM OF HOMOMOLECULAR ASSOCIATION PROCESS... 

All questions this part deals with are considered based on the example of a special and wide-studied type of homomolecular association process, namely, monomer-dimer equilibrium ... 

One ean ealeulate, with the help of this equation, that in hexane atc = 0.1, half of the dissolved aeetie aeid is in dimer form (Kjj 1.5x10 ). In nitrobenzene solution (K im 10 ), only a third of analytieal eoneentration of aeid is in dimer form. For phenol these values are 12% and 2%, respectively. Therefore the composition variation of universal mixed solvents, for instance, using hexane-nitrobenzene binary solvent, is an effective method to control the molecular composition of dissolved compounds able to undergo homomolecular association, in particular, through H-bonding. 

Equation [9.120] changes to [9.121], if only EF and EL undergo homomolecular association ... 

The dependence of equilibrium constant for reaction [9.116] on e is similar in a concentration range from 0.5 to 1.0 molar fraction of aldehyde, if the degrees of homomolecular association for aldehydes and acetals are smaller than for alcohol and water. 

Similar dipole moments are presumably preferred because of competing homo- and hetero-molecular interactions. A strongly dipolar molecule, for example a zwitterion, is likely to interact strongly with other molecules of the same kind. Intermolecular interactions with a less polar molecule (one with a small dipole moment) are unlikely to compete successfully with the strong homomolecular association. This explanation suggests that molecules with... 

Type 1 isotherm eorresponds to the experimental isotherm with isotherms [8.1.24] or [8.1.26] of the liquid systems without interaction. The additive e inerease is eompensated by 8 decrease, due to homomolecular association process. The system diethyl ether-m-cresol illustrates this type of isotherm. 

Mixed solvent effect on the position of equilibrium of homomolecular association process... 

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