Thermochimica Acta 439 (2005) 142–150 Phase behaviour of binary mixtures of LC compounds of different molecular structure and comparable molecular length I. 4-(4 ′ -Substituted phenylazo)phenyl-4 ′′ -alkoxybenzoates and 4-(4 ′ -substituted phenyl carbonyloxy)phenyl-4 ′′ -alkoxybenzoates Refaat I. Nessim Department of Chemistry, Faculty of Science, University of Cairo, Giza, Egypt Received 24 June 2004; received in revised form 27 December 2004; accepted 28 December 2004 Abstract Phase diagrams were constructed for the various binary systems made from compounds of structurally different mesogens; one component is from the five homologous series 4-(4 ′ -substituted phenylazo)phenyl-4 ′′ -alkoxybenzoates (I a–e ) and the other from the similarly substituted homologous series 4-(4 ′ -substituted phenyl carbonyloxy)phenyl-4 ′′ -alkoxy benzoates (II a–e ). Transition temperatures of all mixtures prepared were measured by differential scanning calorimetry and identified by polarized light microscopy. Polarity and polarizability of the investigated compounds could be varied by changing the substituent X (CH 3 O, CH 3 , Cl, CN and NO 2 ), which covers a wide range of polarity, and the alkoxy-chain length, which varied between 6, 8, 14, or 16 carbons. © 2005 Published by Elsevier B.V. Keywords: Binary phase diagrams; Arylazo phenyl arylates; Unsymmetrical bis-1,4-phenylene arylates 1. Introduction Substantial impetus has been provided for the systematic investigation of the relationships between molecular struc- ture and liquid crystallinity. Since no single material yet discovered has a satisfactory nematic range for device appli- cation, all currently marketed devices contain mixtures of liquid crystals. Trial-and-error attempts to find eutectics in homologous series or using compounds of differing molecu- lar structure have met with reasonable success [1]. In view of the growing upsurge of interest in room-temperature liquid crystalline materials, we are continuing our works towards investigating mixtures of liquid crystals of structurally dis- similar structures. The ester group was selected as a basic linkage, in both components, since it offers stability advan- tage over other linking groups. E-mail address: refaat nessim@hotmail.com. It has been previously shown [2,3] that introducing a third phenylene ring into a two-ring liquid crystalline compound enhances both solid and mesophase stabilities, i.e. both melt- ing (T m ) and clearing (T C ) temperatures. Thus, introducing the phenylazo [2] or the phenyl carbonyloxy [3] groups into the phenyl benzoate molecule, to give compounds I and II, respectively, has shown to raise both T m and T C . We have reported binary systems of structurally similar mesogens, where both components of a system bear different X but of the same alkoxy group [2,4–7] or they are homologues of the same series that bear the same substituent, X [8]. The phase diagrams obtained, in most of the cases, did not show eutectic points which was attributed to the close similarity in the solid crystalline structure of nearly all members of one and the same series of compounds (I or II). Continuing our research in this field, we report here binary systems consist- ing of the structurally dissimilar mesogens (I and II) but both components bear the same substituents X and R. 0040-6031/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.tca.2004.12.027