Abstract:We report a new route for forming reverse wormlike micelles (i.e., long, flexible micellar chains) in nonpolar organic liquids such as cyclohexane and n-decane. This route involves the addition of a bile salt (e.g., sodium deoxycholate) in trace amounts to solutions of the phospholipid lecithin. Previous recipes for reverse wormlike micelles have usually required the addition of water to induce reverse micellar growth; here, we show that bile salts, due to their unique "facially amphiphilic" structure, can play a role analogous to that of water and promote the longitudinal aggregation of lecithin molecules into reverse micellar chains. The formation of transient entangled networks of these reverse micelles transforms low-viscosity lecithin organosols into strongly viscoelastic fluids. The zero-shear viscosity increases by more than 5 orders of magnitude, and it is the molar ratio of bile salt to lecithin that controls the viscosity enhancement. The growth of reverse wormlike micelles is also confirmed by small-angle neutron scattering (SANS) experiments on these fluids.

Wormlike micelles are flexible polymerlike chains formed by the self-assembly of amphiphilic molecules either in water ("normal" worms) or in oil ("reverse" worms). Normal and reverse worms have both been studied extensively and have generally been found to exhibit analogous rheological properties (e.g., Maxwell fluidlike behavior). Here, we report a hitherto unexplored difference between these two classes of micelles pertaining to the effect of temperature on their rheological properties. For normal worms, the plateau modulus remains constant as the sample is heated while the relaxation time exponentially decreases. For reverse worms, however, both the plateau modulus and the relaxation time decrease exponentially upon heating. Consequently, the zero-shear viscosity of reverse worms decreases more rapidly with temperature than for normal worms. To explain these differences, we propose that increasing the temperature weakens the driving force for micellization in reverse worms whereas it only accelerates the dynamics of surfactant exchange in normal worms.

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We describe the preparation of nanostructured polymeric materials by polymerizing a monomer within a scaffold composed of self-assembled nanofibrils. 1,3:2,4-Dibenzylidene sorbitol (DBS) is an inexpensive sugar derivative that can form nanofibrillar networks in a variety of organic solvents at relatively low concentrations. Here, we induce DBS nanofibrils in styrene and then thermally initiate the free-radical polymerization of the monomer. The polymerization proceeds without any evidence of macroscopic phase separation, ultimately yielding a transparent solid of polystyrene. Within this material, intact DBS nanofibrils (diameter 10-100 nm) are preserved, as shown by atomic force microscopy (AFM). The DBS fibrils can also be subsequently extracted from the polymer, leaving behind a network of nanoscale pores. The porosity of the resulting polymer has been characterized by the BET technique.