Lecithin microemulsion gels: A NMR study of molecular mobility based on line widths

Microemulsion gels obtained from solutions of lecithin in organic solvents (in particular isooctane) in the presence of a small amount of water are studied by NMR line width measurements. The ‘H, 13C, and slP resonances of the lecithin molecule are investigated as a function of added water w, (w, = [HzO]/ [LEC]). The line width of the 31P resonance increases sharply at the w, value that induces the maximal viscosity (w, = 3), and similar to the viscosity, the 1H resonance of the -N+(CH& polar head, as well as the l3C resonances of the -CH2CH2N+ group, shows a maximum in their line width (i.e. a minimal molecular mobility) around w, = 3. Conversely, at this w, the mobility of water does not appear to be particularly restricted; actually an effect is noticed only by the addition of the first molecule, which induces a significant increase of mobility both in the polar head of the phospholipid moiety and in the water itself. More in general, the molecular details obtained by NMR data are compared with the data obtained by other chemical physical techniques that are only sensitive to the macroscopic molecular structure. The bulk of data indicates that (i) the highly flexible, transient network constituting the gel consists of entangled polymer rodlike lecithin reverse micelles, (ii) such a network structure is formed continuously upon addition of water rather than in an all-or-nothing transition, (iii) solvent and water mobility do not correlate with macroscopic gel viscosity, whereas there is a clear correlation between viscosity and stiffening of the phosphorus atom, (iv) during water addition and gel formation significant change in mobility of specific lecithin groups occur, and (v) in particular, the first added water molecule induces a conformational change of the lecithin polar head.