Ionic liquid crystals of quaternary ammonium salts with a 2-hydroxypropoxy insertion group

Abstract

A 2-hydroxypropoxy group was inserted at the junction between the headgroup and the alkyl chain of quaternary ammonium salts. This group promotes a bent configuration for the alkyl chain relative to the ammonium headgroup, as compared to those without the 2-hydroxypropoxy group. At the same time, the insertion of this group weakens the N⁺–X⁻ ionic interactions and hydrogen bonding interactions within the ionic layers and, therefore, allows the compounds to have a wide temperature range of mesophase. The molecular structures were determined by single crystal X-ray diffraction, in order to have a better understanding of the influence of the 2-hydroxypropoxy group on the packing arrangement and the mesomorphic properties. Their mesomorphic properties were investigated by differential scanning calorimetry, polarizing optical microscopy, X-ray powder diffraction and rheological measurements. It's important to note that anions have a profound effect on the phase properties of these liquid-crystalline compounds, such as phase transition temperatures, crystallisation rate, layer spacing and structural strength.