Cross-polarisation equilibrium

Equilibrium Phase Behaviour. Phase studies were performed using approximately 10 g samples of oil-surfactant mixture diluted sequentially by the weighed addition of water. The initial binary mixture contained 5-70 w/w surfactant at 5 intervals. Phase boundaries were determined to + 0.5 water. The ternary mixtures in Pyrex glass tubes fitted with PTFE lined caps were equilibrated to the required temperature (20-65 0.1°C) for 2 hours and then thoroughly mixed for 5 minutes using a Fisons orbitsil whirlimixer. The tubes were then returned to the waterbath and left undisturbed for 48 hours before identification of the phase type using a crossed polarised viewer and an optical microscope. 

A number of observations should be made here. Firstly, the typical rate of cross polarisation for carbons in organic materials is such as to require contact times of between 1 and 10 ms for complete equilibrium to be achieved. For nuclei with no directly bonded protons the longer times are appropriate. Secondly, having carried out one contact and acquired an FID (total time required T + aq possible in principle to... 

When a bicontinuous cubic lipid-water phase is mechanically fragmented in the presence of a liposomal dispersion or of certain micellar solutions e.g. bile salt solution), a dispersion can be formed with high kinetic stability. In the polarising microscope it is sometimes possible to see an outer birefringent layer with radial symmetry (showing an extinction cross like that exhibited by a liposome). However, the core of these structures is isotropic. Such dispersions are formed in ternary systems, in a region where the cubic phase coexists in equilibrium with water and the L(x phase. The dispersion is due to a localisation of the La phase outside cubic particles. The structure has been confirmed by electron microscopy by Landh and Buchheim [15], and is shown in Fig. 5.4. It is natural to term these novel structures "cubosomes". They are an example of supra self-assembly. 

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