Gel to liquid crystal transitions for vesicles in aqueous solutions prepared using mixtures of sodium dialkylphosphates (R1O)(R2O)PO2–Na+ and (R3O)2PO2–Na+, where R1= C10H21, R2= C14H29 or C18H37 and R3= C12H25, C14H29, C16H33 or C18H37

Abstract

The scans recorded by differential scanning microcalorimetry (DSC) for aqueous solutions containing vesicles prepared from mixtures of two sodium dialkylphosphates are complicated where the first surfactant anion (R¹O)(R²O)PO₂^–Na⁺ has alkyl groups R¹ and R² with different chain lengths, and where the second surfactant anion (R³O)₂PO₂^– has two alkyl groups R³ with the same chain length. For mixed solutions where R¹= C₁₀H₂₁, R²= C₁₄H₂₉ or C₁₈H₃₇ and R³= C₁₂H₂₅, C₁₄H₂₉, C₁₆H₃₃ or C₁₈H₃₇, the DSC traces can be understood in geometric terms. Where the chains can be assembled into bilayers with modest mismatch, the DSC traces show well resolved features. With increase in mismatch, the complexities of the DSC traces are consistent with the presence of bilayers in which the domains differ in composition. Poor chain packing and consequent weak intravesicular van der Waals forces between the alkyl chains favour low temperatures for gel to liquid crystal transitions.