Self-assembled lyotropic liquid crystals from natural surfactant: A study on their structural, rheological and antimicrobial behaviour

Abstract

Soft self-assembled systems with smart functions have been explored over the past decades for diverse applications. However, the toxicity of synthetic surfactant based self-assembled systems and their stability always remain a point of concern. There is always a great quest to find an alternative to overcome such shortcomings of self-assembled systems. Hence, in this context, we made an effort to engineer self-assembled lyotropic liquid crystals (LLCs) using different concentrations of the medicinal plant Glycyrrhiza glabra L. in the nonaqueous polar media of glycerol, ethylene glycol and formamide. These nonaqueous solvents with considerable polarity, high cohesive energy, dipole moment and hydrogen bonding ability, not only offer the perfect environment for self-assembly, but also provide longtime stability. Nematic lyotropic ordering is obtained for studied systems at different concentrations of Glycyrrhiza glabra L./ nonaqueous solvents as confirmed via polarising optical microscopy (POM) and X-ray diffraction (XRD) analysis. Electrostatic and hydrogen bonding interactions among polar solvents and amphiphilic Glycyrrhiza glabra L. molecules could be responsible for nematic ordering. FTIR spectroscopy is employed to confirm the hydrogen bonding in LLCs. The prepared LLCs are studied through static, dynamic rheology under the shear range 1–100 s-1 to explore their fluidic behaviour and establish correlation with structural parameters. The studied LLC systems from three solvents exhibit non-Newtonian shear thinning behaviour at all concentrations. The dynamic rheology tests indicate the elastic behaviour of all LLC systems. LLCs derived from glycerol exhibit soft-solid gel-like behaviour, while weak gel-type behaviour is observed for ethylene glycol and formamide-based LLCs. Considering the medicinal value of prepared LLCs, their antibacterial activity has been checked against the gram-positive bacteria Bacillus subtilis. LLC phases show strong antibacterial activity, about 2–3 times better than that of the standard drug streptomycin used for this study.