Weak molecular interactions between different molecules in a liquid state give rise to several breaking points on the curves of various physical properties vs composition at simple molar ratios. This fact has been examined in the previous papers for various kinds of physical properties such as dielectric constant, viscosity, optical refractivity and refractive dispersion. Through these investigations, it has been cleared out that the observation of the polarizability of a liquid mixture by the method of dielectric measurements is most effective for the detection of the weak molecular interactions and that of the refractive dispersion is more effective than that of simple refractivity measurements.
In the present paper, the method of infrared absorption was introduced and examined with related to the measurements of the refractive dispersion. However, especially in the case of weak interactions, the shifts of wave-number of the absorption spectra were very small, so it was very difficult to estimate qualitatively the shifts of these spectra excepting the pyridine-dioxane system.
The very interesting result obtained in the pyridine-dioxane system was that the curve of wave-number vs weight-fraction of dioxane showed a breaking point and this fact showed a remarkable difference from that of observed in the ordinal solvent effects as noted by Naofumi Oi in the case of benzene-phenol mixture. The curve expressed the relation between the composition which is ordinally expressed by the weight-fraction of j component (wj) and the shift of wave-number of CH-extra-planer bending vibration of 700 to 750 <cm>^<-1> of pyridine molecule clearly showed a breaking point at a fixed molar ratio, and this composition of the mixture approximately coincides with that of obtained from refractive dispersion measurements.