GEL TO LIQUID-CRYSTAL TRANSITIONS FOR VESICLES IN AQUEOUS-SOLUTIONS PREPARED USING MIXTURES OF SODIUM DIALKYLPHOSPHATES (R(1)O)(R(2)O)PO2(-)NA+ AND(R(3)O)(2)PO2(-)NA+ WHERE R(1)=C10H21, R(2)=C14H29 OR C18H37 AND R(3)=C12H25, C14H29, C16H33 OR C18H37

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 (R(1)O)(R(2)O)PO2-Na+ has alkyl groups R(1) and R(2) with different chain lengths, and where the second surfactant anion (R(3)O)(2)PO2- has two alkyl groups R(3) with the same chain length. For mixed solutions where R(1) = C10H21, R(2) = C14H29 or C18H37 and R(3) = C(12)H(2)5, 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.