Vapor-Phase Dipole Moment Values from Solution Measurements

to dichloroacetate. The degree of counterion dissociation is largest for decylammonium acetate and therefore the negative entropy contribution is reduced to a higher degree when increasing the surfactant concentration. It is demonstrated that the Fourier transform NMR self-dif- fusion technique gives direct and easy access to information on organic counterion binding to micelles even in the presence of several counterions. The degree of counterion binding, the cmc, and the free amphiphile concentration above cmc were demonstrated to be distinctly different for decylammonium acetate, chloroacetate, and dichloroacetate. In a competitive situation, an amplification of the differences in ion binding was demonstrated. An equation proposed by H. Mechetti to describe the permittivity of very dilute polar/nonpolar solutions as a function of concentration was used to calculate the molecular dipole moment of the solutes. The procedure is very simple, and the results are in good agreement with those obtained through more elaborate known methods. Furthermore, an adequate choice of solvents leads to results that are in very good agreement with the corresponding vapor-phase values.


Introduction
The molecular dipole moment, as a very useful parameter for structural studies, should ideally be calculated from vapor-phase measurements, to avoid any kind of intermolecular interactions. This is only possible with compounds that have a vapor pressure high enough to allow them to be handled as vapors in the measuring devices. Therefore, the number of compounds that can be studied is limited.
To overcome this problem, calculations have been proposed based on measurements made on very dilute solutions of the candidate substance in nonpolar solvents. It was assumed that under these conditions there would be no solute/solute or solute/solvent interactions. The first equation proposed by Peter J. W. Debye1 in 1921 was followed by others that have been extensively described and discussed in the literature.2 The initial success of such equations was attributed to the absence of interactions. This appeared to be confirmed by the linear behavior, at very high dilutions, of permittivity (<), density (d) (or specific volume V), and the refractive index («) as a function of concentration. However, comparing solution and vapor-phase measurements on low-temperature boiling compounds, we found discrepancies in some cases as high as 30% (chloroform3), although in general they are between 8% and 15% (water, nitromethane, methanol, trichloroethane, and acetone4. This was originally attributed to a so-called solvent effect that has been carefully discussed over the years since 1934,5 under the more adequate denomination of solute/solvent interactions or solvation. 6 The latter appears to be the cause for the differences observed between vapor-phase and solution values. It is a known fact that for dilute binary liquid systems the permittivity (e) is a linear function of concentration. However, due to the above-mentioned interactions, this behavior cannot be described by a simple additive law. This was discussed in a previous paper,7 and as a result, eq I was proposed by H. Mechetti based on a model that proved adequate for the permittivity of dilute solutions of water and lower alcohols in nonpolar solvents: In this equation, is the permittivity of the pure solvent, e12 the permittivity of the solution, n2 the refractive index of the pure solute (squared), a2 the radius of the molecule calculated from the experimental specific volume (P), N2 the solute concentration expressed as the number of molecules per milliliter of solution, Uo the vapor-phase dipole moment of the solute molecule, k Boltzmann's constant, and T the temperature.
If <!, n2, V2, and c12 are measured, eq I can be rearranged to calculate µ0:  Furthermore, additional evidence was obtained experimentally for acetone, chloroform, and Me2SO.

Experimental Part
Spectrograde candidate substances were dissolved in solvents purified as reported elsewhere.40 Permittivity, specific volume, and refractive indices were also measured as described.40 Dilute solutions of concentrations between 0.0007 and 0.003 weight fraction) were prepared for the measurements. The results are listed in Table II II indicate that when eq II is used, there are a number of cases where agreement is very good, but with no regularity among solvents. Cyclohexane appears adequate for alcohols, acetone, and ethers, benzene for aromatics and water, and CC14 for halogenated compounds and Me2SO, thus suggesting a possible criterion for solvent selection.
Therefore, to a first approximation, when solute/solvent interactions are absent in highly dilute solutions, eq II gives calculated dipole moment values that agree well with vapor-phase measurements. However, as is the case with benzene as solvent,10 interactions have been established unequivocally in many binary systems.42 But their prediction and influence on measurements are still matters open to considerable discussion.

Conclusion
Although global applicability cannot be claimed for eq II, its simple experimental requirements provide a very convenient procedure to calculate molecular dipole moment values that are in good agreement with the results from vapor-phase measurements.
Acknowledgment. The continuing support of the Consejo Nacional de Investigaciones Científicas y Técnicas is gratefully acknowledged.