Phase sensitive molecular dynamics of self-assembly glycolipid thin films: A dielectric spectroscopy investigation

Abstract

Glycolipid, found commonly in membranes, is also a liquid crystal material which can self-assemble without the presence of a solvent. Here, the dielectric and conductivity properties of three synthetic glycolipid thin films in different thermotropic liquid crystal phases were investigated over a frequency and temperature range of (10(-2)-10(6) Hz) and (303-463 K), respectively. The observed relaxation processes distinguish between the different phases (smectic A, columnar/hexagonal, and bicontinuous cubic Q) and the glycolipid molecular structures. Large dielectric responses were observed in the columnar and bicontinuous cubic phases of the longer branched alkyl chain glycolipids. Glycolipids with the shortest branched alkyl chain experience the most restricted self-assembly dynamic process over the broad temperature range studied compared to the longer ones. A high frequency dielectric absorption (Process I) was observed in all samples. This is related to the dynamics of the hydrogen bond network from the sugar group. An additional low-frequency mechanism (Process II) with a large dielectric strength was observed due to the internal dynamics of the self-assembly organization. Phase sensitive domain heterogeneity in the bicontinuous cubic phase was related to the diffusion of charge carriers. The microscopic features of charge hopping were modelled using the random walk scheme, and two charge carrier hopping lengths were estimated for two glycolipid systems. For Process I, the hopping length is comparable to the hydrogen bond and is related to the dynamics of the hydrogen bond network. Additionally, that for Process II is comparable to the bilayer spacing, hence confirming that this low-frequency mechanism is associated with the internal dynamics within the phase.

Dry Thermotropic Glycolipid Self-Assembly:A Review

“…These relaxations exhibit Arrhenius-type temperature dependence. Similar type of activation energy, E a was observed in the cubic bicontinuous phase as well as in the columnar phase by Velayutham et al 31 .…”

“…Since the number of the HBs is smaller for glucosides than that of the maltosides the relaxation for 31,32 . Typically, maltosides have an order of magnitude lower relaxation frequency than that of the glucosides.…”

“…However, since the GL has no helical conformation and spontaneous electric polarization, they suggested that the direction of the tilt is not correlated between the bilayers. In another study, the diffusion process in the thermotropic phases was investigated using the dielectric analysis 31 . Analyzing the relaxation process using Einstein-Smoluchowski relation and the random walk model, the charge carrier hopping distances were calculated 31 .…”

“…In another study, the diffusion process in the thermotropic phases was investigated using the dielectric analysis 31 . Analyzing the relaxation process using Einstein-Smoluchowski relation and the random walk model, the charge carrier hopping distances were calculated 31 . The calculated hopping length was approximately similar to the hydrogen bond length 2 to 3.5 Å and the bilayer spacing.…”

“…Therefore, in this review the lyotropic nomenclature is preferred for the dry thermotropic GL assemblies. Recent studies of dry GL thermotropic phases lamellar, hexagonal and bicontinuous cubic include molecular dynamics using dielectric spectroscopy 31,32 , which established the tilted organization of bilayer 33,34 and possible exploitation of the ferroelectric property 35 . Some fundamental ideas have emerged from thermotropic studies of GLs such as the fact that the cubic phases may be induced by mixing the hexagonal and lamellar phases 36 .…”