Partial molar negative value

The partial molar volumes at infinite dilution of the amides show a consistent volume decrease for the amides on transfer into the aqueous phase, which becomes greater with increasing size of the substituents and amounts to about 2.5 cc. per mole per pair of methylenic groups in the case of the acetamide series. This value may be compared with that found in the alcohol series (6), where a more negative value of about 2.5 cc. per mole per methylenic group is found, and also to similar de-... 

Negative values ofN —N0, the electrolyte effect on the association numbers of water, are called the structure-breaker effect. One can speak of negative hydration31. The estimation of the hydration numbers by spectroscopic or solubility methods gives only an approximation of the sum effect. The spectra of the H-bond bands show in second approximation distinct differences between the ion effects on the H-bonds7 ). — The partial molar volume Vx of water in electrolyte solutions is negative in all solutions but the series of -values corresponds to the Hofmeister ion series too. The negative V1 volume indicates an electrostriction effect around the ions. 

Values of V are generally small except near 62 bar where V reaches a minimum value of -9 L/mol. This is approximately one-third of that expected theoretically for electrostriction by Oj. Clearly there is a volume decrease in the reaction but the lack of agreement with theory indicates either that the hill volume change is not obtained until the reaction is complete, or that Oj has a significant cluster around it i.e. the partial molar volume of neutral O2 in xenon is negative. 

Hiis condensation or contraction is the cause of the highly negative partial molar volumes, which become more pronounced as the isothermal compressibility increases. These clusters are bound by van der Waals forces and are thus very different from other types of aggregates such as clathrates which are bound by specific chemical forces. Their size can reach values on the order of 100 molecules, which means that they extend over many coordination shells. While the partial molar volume, a macroscopic property, provides evidence of clustering, more detailed information has been obtained recently using spectroscopic techniques which probe solute-solvent interactions directly. 

Most prominent and well investigated is the group of donor-substituted cyclopropanes that readily react with electron-deficient dienophiles. Rate increase in polar solvents, the negative value of the partial molar volume and trapping reactions with methanol indicate a stepwise reaction via dipolar intermediates 2 for singly alkoxy-substituted cyclopropanes 1. 

The partial molar volume depends on temperature, pressure, and the composition of the system. The physical sense of the partial molar volume is the volume increase caused by the addition of 1 mol of component to an amount of solution such that the composition of the solution at constant temperature and pressure does not change. The partial molar volume can thus attain a negative value also. 

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