Water Diffusion and Headgroup Mobility in Polymer-like Reverse Micelles:  Evidence of a Sphere-to-Rod-to-Sphere Transition

Abstract

A pulsed gradient spin-echo FT 1 H NMR study on the system soybean lecithin/water/perdeuterated cyclohexane is presented. The self-diffusion coefficient of water, D w , was measured as a function of the water content (W 0 ) and was found to show a bell-shaped trend. The composition at which the maximum in D w occurs is the same at which the viscosity is the highest. We rationalize the rising part of the plot in terms of a water diffusion inside wormlike reverse micelles which increase their length upon increasing W 0 . On the contrary, the descending part indicates a rod-to-sphere evolution of the aggregate's shape. This interpretation is supported by measurement on the headgroup rotation by dielectric spectroscopy. The composition at which a structural transition occurs and the location of the phase boundary can be predicted by a geometrical model in which the effective packing parameter is equal to 1.58. This value is in agreement with small-angle neutron scattering data in the literature.

Phosphatidylcholine reverse micelles on the wrong track in molecular dynamics simulations of phospholipids in an organic solvent

“…Water-to-lipid ratio was selected to be w 0 = 7 which is located well in the range where DPPC (and lecithin) form wormlike RMs in cyclohexane. 5,7,8,10 The sensitivity of the results to the choice of aggregation number and water-to-lipid ratio was examined throughout by simulations of different size aggregates and also both smaller (w 0 = 5) and larger (w 0 = 11) water-to-lipid ratios. The studied water-to-lipid ratios fall all in the DPPC (and lecithin) gelling regime corresponding to wormlike RMs and experimental comparison data in this data range also exist, see, e.g., Refs.…”

“…5 Maximum water uptake in cyclohexane is around w 0 = 24; at higher water-tolipid ratios the system phase separates into a microemulsion phase and an excess water phase. 5 The organogel transition and the viscous characteristics of the solution, as well as, the size change in the amphiphilic aggregates can be characterized experimentally very well with, e.g., rheological measurements, 10 NMR self-diffusion, 5 and scattering techniques including light, 8 neutron, 7,9 and X-ray 6 scattering. This provides experimental comparison data for simulations at the level of basic structural characteristics.…”

“…In most cases, lecithin which is a mixture of PC lipids (exact composition depends on the source of origin) has been studied [5][6][7][8][9][10] but also organogels of single PC lipids such as 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) 10 have been examined. Regardless of the exact PC lipid composition, PC organogels are thermodynamically stable, optically clear, and isotropic solutions 11 of entangled wormlike reverse micelles (RMs) with similar viscosity characteristics.…”

“…10 In particular in cyclohexane, the addition of water triggers uniaxial growth of the initially spherical RMs 13,14 into disconnected wormlike RMs. 5,7 Maximum viscosity is attained around w 0 = 11 followed by a second shape transition to water-in-oil (w/o) microemulsion droplets at w 0 = 14. 5 Maximum water uptake in cyclohexane is around w 0 = 24; at higher water-tolipid ratios the system phase separates into a microemulsion phase and an excess water phase.…”

Nonionic Reverse Micelles near the Critical Point

“…temperature has different effect for normal and reverse micelles. In aqueous systems, temperature induces sphere-to-rodlike transition 30 34 , which has been explained to be caused due to dehydration of surfactant aqueous systems headgroups, but an opposite trend; rodto-sphere type transition, has been commonly observed with nonaqueous media 25,35,36 . It has been anticipated that temperature enhances the penetration tendency of solvent molecules into the lipophilic chain of surfactant and micellar curvature tends to be more curved at higher temperature.…”