Thermochimica Acta 511 (2010) 27–31 Contents lists available at ScienceDirect Thermochimica Acta journal homepage: www.elsevier.com/locate/tca Effect of intermolecular hydrogen-bonding and terminal substituents on the mesophase behavior of binary mixtures of dissimilarly-substituted benzoic acids Refaat I. Nessim a,∗ , Maher I. Nessim b a Department of Chemistry, Faculty of Science, Cairo University, Cairo 00202, Egypt b Department of Analysis and Evaluation, Egyptian Petroleum Research Institute, Egypt article info Article history: Received 10 April 2010 Received in revised form 4 July 2010 Accepted 20 July 2010 Available online 19 August 2010 Keywords: Alkoxy benzoic acid Smectic C Nematic phase abstract Binary mixtures of two types of 4-substituted benzoic acids, covering the whole composition range, were prepared by melting both components together, stirring to give an intimate blend, and then cooling to room temperature. The mixtures prepared were characterized for their mesophase behavior by differ- ential scanning calorimetry (DSC) and polarized-optical microscopy (POM). The first type of the acid is substituted with a small compact polar group that cover a wide range of polarity. The other type of the acid is 4-alkoxy benzoic acid bearing an alkoxy (OC n H 2n+1 ) group with varying chain length (n = 6–16). Binary phase diagrams were constructed whereby all phases observed were identified by POM. The smectic C (SmC) mesophase was observed in all mixtures investigated, while the nematic phase (N) was observed, together with the SmC phase, only in mixtures possessing the acid homologues with n ≤ 12 carbons. Complex formation was also supported by FT-IR spectroscopic measurements. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Usually, at least two rings are required to enable the genera- tion of liquid crystal phases; however, 4-alkoxy (or alkyl) benzoic acids are the only examples of one-ring mesogens. In a single molecule, the actual molecular species is certainly not long or lath- like; however, dimerization through hydrogen-bonding creates a long lath-like structure (1) with a three-ring core and two flexible terminal chains [1]. Benzoic acid derivatives were the most frequently employed components for the formation of LC materials through such hydrogen–bonding interaction [2]. Wing polar groups are usually used in the modification of a liquid crystal material. Since calamitic (rod-like) molecule has two ends, the wing groups may be simi- lar or different. Many terminal units have been employed in the generation of liquid crystals, but the most successful route [3] is to use either a fairly long, straight hydrocarbon (usually alkyl or alkoxy), or small polar substituent (e.g. CH 3 , CH 3 O, CN, NO 2 , Cl, ∗ Corresponding author. Tel.: +20 101649558; fax: +20 24591403. E-mail address: refaat nessim@hotmail.com (R.I. Nessim). etc.). The role of these groups is to act either as a flexible extension to the core or as a dipolar moiety to introduce anisotropy in phys- ical properties. The option to choose dissimilar wing groups in the case of 4-substituted benzoic acids is not allowed, since we come with either an acid B, (I a–f ), substituted with a small compact polar terminal group (X) to give the non-mesomorphic dimer (2), Or the mesomorphic benzoic acid (In) substituted with 4-alkoxy group (1). Acid B...I a , X = CH 3 O; I b , X = CH 3 ; I c , X = Cl; I d , X = Br; I e , X = CN; I f , X = NO 2 . Acid A...I6, n = 6; I8, n = 8; I10, n = 10; I12, n = 12; I16, n = 16. 0040-6031/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tca.2010.07.020