SYNTHESIS AND LIQUID CRYSTAL PHASE BEHAVIOUR OF BRANCHED CHAIN ALKYLGLYCOSIDES

Abstract

ABSTRACT Twenty branched chain alkylglycosides and their peracetylated derivatives have been synthesised and their structures elucidated and characterised by nuclear magnetic resonance (‘H and UC), infrared spectroscopy and high-resolution mass spectrometry. The head groups of these glycolipids are either B-glucose or B-maltose, while their tails are derived from branched chain alcohols (Guerbet alcohols) having 8, 12, 16, 20 and 24 carbon atoms. Their thermotropic phase behaviour is investigated using optical polarising microscopy, differential scanning calorimetry and small angle X- ray scattering. The glucoside compounds with shorter tails of 8 and 12 carbon atoms exhibit a smectic A phase only while the glucosides with longer tails of 16, 20 and 24 carbon atoms exhibit a columnar phase only. The smectic A phase is also exhibited by maltosides with 8, 12 and 16 carbon atoms in their tails. Thermotropic polymorphism (smectic A phase, cubic phase, and columnar phase) is observed for maltosides with longer tails of 20 and 24 carbon atoms. Moreover, the maltoside with 20 carbon atoms in its tail shows also two bicontinuous cubic phases (Im3m and Pn3m), a rare observation among a lkylglycoside c ompounds. A comparison 0 f the therrnotropic phase behaviour between the branched and straight chain alkylglycosides has been carried out. Compared to the straight chain alkylglycosides, the branched chain alkylglycosides have lower melting temperatures and a wider variety of liquid crystal phases. Lyotropic phase behaviour investigation by contact penetration with water shows a variety of phases for these glycolipids. All glucosides with 8, 12 and 16 carbon atoms in their tails exhibit micellar solution and fluid lamellar while those with 20 and 24 carbon atoms in their tails exhibit myelin texture. Maltoside compound that has the shortest tail shows lyotropic polymorphism behaviour (micellar solution, normal hexagonal, normal bicontinuous cubic and fluid lamellar phase). Maltosides with 12, 16, 20 and 24 carbon atoms in their tails exhibit (micellar solution and fluid lamellar), fluid lamellar, (fluid lamellar and myelin) and myelin texture, respectively. The phase behafiour of mixture systems have also been studied for maltoside compounds and explained qualitatively.