Physicochemical studies of morpholinium based ionic liquid crystals and their interaction with cyclodextrins

Abstract

The various physicochemical properties of morpholinium based ionic liquid crystals (ILC) (N-methyl-N-tetradecylmorpholinium bromide (M14) and N-methyl-N-hexadecylmorpholinium bromide (M16)) in the absence and presence of β-cyclodextrin (β-CD) and hydroxypropyl-β-cyclodextrins (HPβ-CD) have been studied using conductance, surface tension, fluorescence and 1H NMR measurements. The various micellar, interfacial and thermodynamic parameters such as critical micelle concentration (cmc), degree of counterion binding (g), association constants (K), surface excess concentration (Гmax), minimum area per molecule (Amin), aggregation number (Nagg), interaction parameter (βm), standard Gibbs free energy of adsorption (ΔGadso) and standard Gibbs free energy of micellization (ΔGmo) have been evaluated for mixtures of M14/M16+ β-CD/HPβ-CD. Conductance study also shows that cyclodextrins (CDs) entraps in hydrophobic moieties of ILC to form inclusion complexes with stoichiometry of 1:1. The formation of inclusion complexes is confirmed by increase in cmc of synthesized ILC and changes in chemical shift of CDs and ILC protons in the presence of CDs. Forces responsible for the formation of inclusion complexes are van der Waals, hydrogen bonding, hydrophobic interactions, release of ring strain in cyclodextrin molecules and changes in solvent-surface tensions. The value of packing parameter (p>0.5) for pure ILC which suggest a flexible bilayer arrangement for ILC.