Internal pressures,
(∂U/∂V)T, have been obtained at 25� for the solvents dimethyl
sulphoxide, propylene carbonate, formamide, dimethylformamide, acetonitrile,
methanol and hexamethyl-phosphoramide by measurement
of the thermal pressure coefficients. Data in the literature suggest that
values of the internal pressure and the cohesive energy density, (ΔU/Vm),
are similar for a non-polar solvent. The observed divergence of the two
properties in the polar solvents under investigation is discussed in terms of
the nature of the solvent-solvent interactions.
Partial molal volumes at
25� are reported for some 1 : 1 electrolytes, and for triphenylmethane, in
dimethyl sulphoxide, N,N- dimethylformamide, acetonitrile, propylene carbonate,
formamide and hexamethylphosphoramide. The assumption that ΔV�tr(Ph4As+)
= ΔV�tr(BPh4-) was used to obtain ionic
partial molal volumes of transfer from water to the non-aqueous solvents.
Solvent compressibility appears to determine the partial molal volumes of non-electrolytes
and large hydrophobic ions in solution. Values of ΔV�tr for
cations and anions are discussed in terms of ion-solvent interactions,
solvent-solvent interactions and steric crowding of large solvent molecules
around the ions.
Internal pressures, (∂U/∂V)T, have been
obtained at 25�C for solutions of 25 solutes in water by measurement of the
thermal pressure coefficients. Values at 1.00 M solute concentration were
plotted against the molar volume of the solutes. Those solutes (electrolytes)
which appear above a line joining five solutes acting as 'non-interacting' homomorphs were judged to be structure breakers in water;
those solutes (lithium salts, tetraalkylammonium
salts, non-electrolytes) appearing below the line were termed structure makers.
These observations are in agreement with current knowledge, and they
substantiate the present interpretation of the physical significance of
internal pressures.
Partial molal
volumes, V�, at 25�C were measured for sucrose, ascorbic acid and aspirin in
the solvents water, formamide, N,N-dimethylformamide and methanol. The
dependence of V� on solvent compressibility enabled an estimate to be made of
the intrinsic volumes, Vint, of
the dissolved non-electrolytes. Effective pressures, Pe,
in aqueous electrolyte solutions were calculated from previously determined
internal pressure data. Substitution of Pe
into the Tait-Gibson equation yielded values of Ve, the volume contraction of water due to
electrostriction, which agree well with values obtained by other methods.
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