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 gamma(o/w), maximum water solubility omega(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, 2K + K-bar, with the different chain lengths, and the values were all in the range 0.40k(B)T to 4.0k(B)T. This rigidity increased with both surfactant and alcohol carbon number, so that 2K + K-bar similar to C2.4-3. With di-C-14-PC in n-hexane to n-decane the rigidity was around similar to 3k(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).