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

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 T_m. The recorded T_ms 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 T_m 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.