Phase diagrams and phase structures of identical and mixed chain lithium di-n-alkyl phosphate-water binary systems. Asymmetric molecular shape effect

Lithium salts of di-n-pentyl (DPP), n-butyl(n-hexyl) (BHP), n-propyl(n-hexyl) (PHP) and ethyl(n-octyl) (EOP) phosphates were synthesized and the phase diagrams of the lithium phosphate-water binary systems were determined. The phase diagrams of the DPP-, BHP- and PHP-water systems contain three regions (I, II and III) in common, which correspond to a homogeneous transparent one-phase solution, and lyotropic liquid crystalline and coagel phases, respectively. However, the EOP-H2O system contains an additional hard gel phase (region IV). P-31 NMR spectra suggest that region I is a monomer double left right arrow micelle equilibrium phase and region II is a lamellar phase. X-ray diffraction results show that for the DPP-, BHP- and PHP-water systems the two n-alkyl chains are closely packed in the lamellar phase in a manner which alternatively combines short and long chains, while in EOP-water system the two long chains are loosely packed. Furthermore, it may be assumed from P-31 NMR spectra and x-ray diffraction results that region IV in the EOP-water system is a cubic phase. Thermotropic properties for these DAP-water systems were also investigated by DSC temperature profile curves. From the Delta H variation upon the II --> I thermal transition, we assumed that stability of the aggregate structure in the liquid crystalline state increases in the order EOP < PHP < BHP < DPP. Thus, we have found that thermotropic properties for a series of DAP-water binary systems are closely correlated with the extent of asymmetric molecular shape in DAP.