The partial pressure of water in the low water concentration region has been determined for ten binary waterorganic solvent systems at 25.0".Assuming Henry's law to be obeyed by individual molecular aggregates of water, w e have calculated association constants for water in the various solvents. Statistical evidence is given to support the conclusions that monomerdimer equilibrium prevails in some of the solvents and monomer-trimer equilibrium in others.WE have reported the solubility of water as a function of water activity in a number of organic solvents.1*2 Water exists primarily as monomer in cyclohexane, carbon tetrachloride, toluene, and benzene, whereas in 1,2-dichloroethane and 1,1,2,2-tetrachloroethane, in addition to monomers, polymers are present a t the higher water activities. The polymerisation of water in these solvents was interpreted as a monomer-trimer equilibrium; however, the assumption of monomertetramer equilibrium could equally well explain the results. I n a search for systems that would dissolve water in excess of O -O ~M , we extended our study to a series of solvents having one or more proton-accepting groups, to clarify the nature of water aggregates in non-aqueous media.3EXPERIMENTAL AND RESULTS
The vapor phase association of trifluoroacetic acid with acetone and with cyclopentanone has been investigated using a vapor density technique. Equilibrium constants and enthalpies of the heteroassociation reactions are reported.
Results for the volume change of mixing of acetic acid with carbon tetrachloride, benzene, and cyclohexane are given. Further, the dielectric constant has been determined for acetic acid in carbon tetrachloride from spectroscopic measurements of the association equilibrium. The volume changes are both positive and large. The dielectric behaviour of acetic acid in carbon tetrachloride parallels that of acetic acid in benzene and n-heptane previously reported. Our results taken together with published data indicate: (i) the dipole moment of acetic acid monomer in the gas phase is higher than has been previously reported, (ii) the dipole moment of acetic acid monomer in solutions rich in acetic acid is considerably greater than in the gas phase, possibly because of a contribution of the trans-configuration, and (iii) a strong attractive interaction exists between acetic acid monomer and acetic acid dimer.
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