Ethosome-like catanionic vesicles are vesicular carriers prepared by using ion-pair amphiphiles (IPAs) and relatively high concentrations of ethanol through a semispontaneous process for dermal drug delivery.In this work, the effects of ethanol and cholesterol on the polymer configuration and vesicular membrane rigidity (or hydrophobicity) and therefore the gelation of the as-prepared ethosome-like catanionic vesicles by water-soluble polycations with and without hydrophobic modification (HMP + and P + ), respectively, were systematically studied for the decyltrimethylammonium-dodecylsulfate (DeTMA-DS) IPA system. The experimental results revealed that the configuration of the polymer chain in aqueous ethanol solution may vary significantly with the dielectric constant of mixed solvents.Moreover, while the vesicular membrane rigidity of the ethosome-like catanionic vesicle was slightly deteriorated by ethanol, it was greatly enhanced by cholesterol. Phase maps and rheological properties of the mixtures of ethosome-like catanionic vesicles and polymers indicated that the interaction between vesicles and polymer HMP + decreases and increases with the increasing ethanol concentration and cholesterol content, respectively. Both ethanol concentration and cholesterol content, however, showed little effect on the interaction between vesicles and polymer P + . These findings lead to the conclusion that the gelation of ethosome-like catanionic vesicles by polymer molecules is driven by the hydrophobic more than the electrostatic interactions. Possible mechanisms of ethanol and cholesterol effects on gelation of the ethosome-like catanionic vesicles by the polycations were also proposed based on the experimental results.
This work aims at understanding the ethanol effects on the gelation behavior of ethosomes with the consideration of encapsulating a hydrophobic material. Gelation of the empty ethosomes by positively charged watersoluble polymers with and without hydrophobic modification, respectively, was systematically studied first for the effects of solution dielectric constant and gelator concentration on the phase map and rheological property of the mixtures. A comparison of the gelation behavior of empty and α-tocopherol acetate (α-TA)-encapsulated ethosomes sheds light on the possible influence of encapsulated hydrophobic material itself on the interaction between ethosomal lipid bilayers and gelator molecules. The experimental results revealed that ethosomes with an optimized amount of ethanol could result in a reasonable lifetime and encapsulation efficiency of more than 90 %. This is due to the effects of the solution dielectric constant on the formability of liposome and the partition of the hydrophobic material (α-TA) between ethosomal lipid bilayer and bulk phase. Moreover, the phase map and rheological property of the ethosome/polymer mixtures were found to be affected by the configuration of the polymer chain in aqueous ethanol solution. That is, the driving interactions between ethosomal lipid bilayers and gelator molecules were dominated by the hydrophobic material more than the electrostatic association. Finally, inclusion of a hydrophobic material, such as α-TA, in the ethosomes had less influence on the gelation behavior of the ethosomes with water-soluble polymers.
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