Characterization of Lipid-Based Lyotropic Liquid Crystal and Effects of Guest Molecules on Its Microstructure: a Systematic Review

Liquid crystals (LCs) are conventionally divided into thermotropic or lyotropic, based on the organization and sequence of the controlled molecular system. Lipid-based lyotropic liquid crystal (LLC), such as lamellar (L alpha), bicontinuous cubic (Q(II)), or hexagonal (H-II) phases, have attracted wide interest in the last few decades due to their practical potential in diverse applications and notable structural complexity. Various guest molecules, such as biopharmaceuticals, chemicals, and additives, can be solubilized in either aqueous or oily phase. And the LLC microstructure can be altered to affect the rate of drug release eventually. To utilize these microstructural variations to adjust the drug release in drug delivery system (DDS), it is crucial to understand the structure variations of the LLC caused by different types of guest molecules. Therefore, in this article, we review the effect of guest molecules on lipid-based LLC microstructures. In particular, we focus on the different characterization methods to evaluate this change caused by guest substances, such as polarized light microscopy (PLM), small-angle X-ray scattering (SAXS), Fourier-transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), self-diffusion nuclear magnetic resonance (SD-NMR), and so on.