Modulation of physical properties of reverse hexagonal mesophases: A dielectric spectroscopy study

The structural, dynamic, and kinetic aspects of the H_II systems based on glycerol monooleate (GMO), phosphatidylcholine (PC), triacylglycerol (TAG), and water were investigated by dielectric spectroscopy in a frequency range of 10^-2-10⁶ Hz, and a temperature range of 290-320 K. Three distinct processes as well as a temperature-activated dc conductivity were detected and examined. These were assigned to the reorientation of the GMO polar heads, the tangential movement of counterions at the interface, the transport of TAGs through the lipids tails, and the ion mobility within the water cylinders. Upon addition of PC, the critical temperature (T0) of the dehydration of the GMO headgroups increased. The optimal concentration found for structural stabilization of the H_II mesophase was 10 wt% PC, since it imparted the strongest bonding at the interfacial layer and increased the association between the lipid tails. Within the H_II cluster, TAG percolated and shifted between the hexagonal rods themselves. The present study demonstrated the benefit of controlling the critical temperature of the H_II mesophase partial dehydration and softening, as well as the percolation of TAGs. These factors influence the diffusion mode of embedded drugs in the physiological temperature range.