Ultrasonic studies of N-formylmorpholine + water mixtures

Abstract

N-formylmorpholine (NFM)+ water mixtures are used to extract aromatics from petroleum feed-stocks. In this paper possible conformational and structural causes of this specific solvation are investigated using ultrasonic attenuation and velocity measurement complemented by ¹³C n.m.r., density and viscosity observations. The conformational studies suggest that gross rearrangements of the morpholine ring are relatively slow (below 60 MHz) but that inversion at the nitrogen atom is faster, with lower activation energy, than in other morpholine analogues. The effect is magnified by the presence of water, and it is suggested that this greater ‘planarity’ of aqueous NFM permits the stronger interaction with aromatic rings.

Structural information is inferred mainly from excess volumes and adiabatic compressibilities of mixing. The observations suggest that when NFM is added to water it first introduces hydrogen bonding and ordering of water molecules not bound into an extended ice I tetrahedral structure; then at higher concentrations it disrupts this structure to a less open, but no less compressible, hydrogen-bonded array of ‘quasi-planar’ NFM capable of absorbing aromatic rings.