Phase transitions in the bilayers of vesicles formed from binary mixtures of symmetric di-n-alkylphosphates in aqueous solutions

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Abstract
Vesicles in aqueous solutions were prepared from binary equimolar mixtures of di-n-alkyl-phosphates (sodium and potassium), (R1O)2PO2 –M+ and (R2O)2PO2 –M+. When the number of carbon atoms in R1 and R2 differs by two and when R1 or R2= C12H25, C14H29, C16H33 and C18H37 the membranes undergo well defined gel to liquid crystal transitions at characteristic temperatures Tm. The recorded Tms are intermediate between the melting temperatures for vesicles prepared from the respective single di-n-alkylphosphates. Furthermore, the extrema recorded by differential scanning microcalorimetry show that the vesicle membrane is made up of domains that differ in composition. For those vesicles produced from di-n-alkylphosphates where the number of carbon atoms in R1 and R2 differs by more than two the plots recorded by the scanning microcalorimeter are complex. The scans show many extrema, suggesting that the bilayers are formed from many domains having different compositions. In all cases, the scan patterns are essentially repeated through several heat–cool–heat…cycles. The temperatures Tm are increased relative to those of the component surfactants when K+ and Na+ salts are mixed, showing that the counter cations play an important role in determining the thermotropic properties of the vesicles reflecting the importance of electrical interactions in determining the packing within the bilayers.

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