Properties of Phosphocholine Microemulsions and the Film Rigidity Model

Properties of Winsor II water-in-oil microemulsions formed by water, synthetic dichain phosphocholines (di-C n -PC's), medium chain length alcohols, and the oils isooctane or n -alkanes are reported. The effects of the chain length of each separate component were investigated, covering di-C 8 -PC to di-C 18 -PC, pentanol to octanol, and n -hexane to n -hexadecane. For each system the oil−water interfacial tension γ o/w, maximum water solubility w max, water core radius R c, droplet polydispersity p, and mean PC headgroup area a h were measured using surface light scattering (SLS), spinning drop tensiometry (SDT), and small-angle neutron scattering (SANS). These parameters were used to follow the evolution of the mean film rigidity, 2 K + K bar, with the different chain lengths, and the values were all in the range 0.40 k B T to 4.0 k B T . This rigidity increased with both surfactant and alcohol carbon number, so that 2 K + K bar ∼ C 2.4 - 3 . With di-C 14 -PC in n -hexane to n -decane the rigidity was around ∼3 k B T, and above this alkane chain length it decreased smoothly. These changes are shown to be consistent with the chain packing statistical model for film rigidities of Szleifer et al. ( J. Chem. Phys. 1990, 92, 6800).