The phase transition behavior and structure of an ionic liquid crystal (ILC), 1,3 detetradecyl imidazolium bromide [C(14)H(29)Im C14H29'] Br-, with double alkyl chain lengths and binary mixtures of 4-cyano-4'-pentylbiphenyl (5CB) with [C(14)Im C-14] Br- were studied by wide-angle X-ray scattering, differential scanning calorimetry, and polarizing optical microscopy. These ILCs can form a high-ordered mesophase smectic-A beside the plastic crystalline phases and also the isotropic state because of the formation of a three-dimensional network by noncovalent bonding. The phase behaviors of binary mixtures show a different phase transition compared to each pure compound. Also, the smectic (Sm-A) to I phase transition temperatures in binary mixtures were reduced significantly compared to the pure ones. A metastable phase was found to appear via a supercooled Sm-A phase on the cooling scan rate in a rich ratio of ILCs in binary mixtures. The modified regular solution theory was used to describe the nature of interaction in binary mixtures. The proposed model provided a good description of the asymmetric thermodynamic behavior in the binary system of 5CB and [C(14)H(29)Im C14H29] Br- mixtures.