Mixtures of ionic liquids with similar molar volumes form regular solutions and obey the cross-square rules for electrolyte mixtures

► Mixtures of ionic liquids with similar molar volumes form regular solutions. ► These mixtures also obey the cross-square rules for the mixing of electrolytes. ► Different molar volume mixtures exhibit a demixing region in the phase diagram. ► The demixing suggests that ionic liquids could freeze by spinodal decomposition.