Molecular clustering in liquid fatty acids

Abstract

The variations in intensity and depolarisation factor of the transversely scattered light have been studied in detail in the case of the first four fatty acids, namely, formic acid, acetic acid, propionic acid and normal butyric acid, with the incident light in the three different states of polarisation, namely, unpolarised, horizontally polarised and vertically polarised. In the case of formic and acetic acids at the room temperature the depolarisation factor ρ _h was found to be about 90% when the incident light is polarised with vibrations horizontal. This observation furnishes for the first time definite experimental evidence for the existence of large molecular aggregates in these highly associated liquids, of size not small compared with the wave-length of light. ρ _h increases with rise in temperature and at about 90° C. it attains its limiting value of 100% in these two acids. The intensity of scattering also at first diminishes with rise in temperature in these two cases, contrary to the molecular theory of light scattering. It is shown that molecular association is distinctly the cause for the extraordinary behaviour of formic and acetic acids in the initial stages. Propionic and normal butyric acids do not show such anomalies.