Optical investigation of motion of short chain liquid molecules

Abstract

An experimental study has been made of the molecular motions, the molecular optical anisotropy and the molecular magneto-optic anisotropy of short chain flexible n-alkanes and n-alcohols in neat liquids using the techniques of integrated and spectral depolarized light scattering and magnetic field induced birefringence (Cotton–Mouton effect).

The depolarized scattering spectra have been analysed to give high and low frequency contributions 〈γ²〉_f and 〈γ²〉_s to the integrated molecular optical anisotropy 〈γ²〉. The times τ_f and τ_s characteristic of the collisional and reorientational motion have also been obtained. Existing gas state data have been compared with 〈γ²〉_s and show that for these flexible molecules 〈γ²〉_s does not represent the intrinsic optical anisotropy of the molecular chain but includes strong inter- and intra-molecular effects. The variation of 〈γ²〉_f with the number of surface atoms per molecule suggests that 〈γ²〉_f originates from an intermolecular collisional process. τ_s is commensurate with a whole-molecular rotary diffusion, largely being controlled by the molecular volume and the shear viscosity of the liquid. τ_f appears to be independent of the above factors.

The magnetic birefringence results indicate that the high frequency processes which cause 〈γ²〉_f also contribute to the magneto-optic parameter 〈γ²_mo〉 so that the magnetic birefringence also has a partly collisional origin. Reasons for this are given.

The chain length dependences of 〈γ²〉, 〈γ²〉_s and 〈γ²_mo〉 are all compatible with that given by an isolated linear all-trans alkane or alcohol molecule in the liquid, which is known to be not the case. This chain length dependence is discussed and explanations for it suggested in terms of strong intramolecular interactions and correlations along the chain molecule.