Long Periodic Structure of a Room‐Temperature Ionic Liquid by High‐Pressure Small‐Angle X‐Ray Scattering and Wide‐Angle X‐Ray Scattering: 1‐Decyl‐3‐Methylimidazolium Chloride

The Bragg reflections of 1‐decyl‐3‐methylimidazolium chloride ([C 10 mim][Cl]), a room‐temperature ionic liquid, are observed in a lowly scattered wavevector ( q ) region using high‐pressure (HP) small‐angle X‐ray scattering methods. The HP crystal of [C 10 mim][Cl] was characterized by an extremely long periodic structure. The peak position at the lowest q (1.4 nm −1 ) was different from that of the prepeak observed in the liquid state (2.3 nm −1 ). Simultaneously, Bragg reflections at high‐ q were detected using HP wide‐angle X‐ray scattering. The longest lattice constant was estimated to be 4.3 nm using structural analysis. The crystal structure of HP differed from that of the low‐temperature (LT) crystal and the LT liquid crystal. With increasing pressure, Bragg reflections in the high‐ q component became much broader, and were accompanied by phase transition, although those in the low‐ q component were observed to be relatively sharp.