New Classes of Self-Assembled Structures in Nonpolar Solvents

Abstract

Many researchers have investigated the self-assembly of amphiphilic molecules in water into characteristic structures such as micelles and vesicles. In comparison, amphiphilic self-assembly in nonpolar organic liquids, which can be referred to as "reverse" self-assembly, is much less studied. In this dissertation, we describe a variety of new reverse self-assembled structures formed from amphiphilic molecules. Especially, we focus on long reverse cylindrical structures that can induce high viscosity, and reverse vesicles, i.e., hollow spherical containers surrounded by reverse bilayers. We expect that these reverse structures may be useful for applications such as gelling agents for fuels and oils, hosts for enzymatic reactions, and controlled release.

In the first part of this study, we describe the effects of adding inorganic salts to solutions of lecithin in nonpolar solvents. Lecithin is a zwitterionic, mono-unsaturated phospholipid that by itself forms reverse spherical micelles. Salts can be dissolved in these solvents in the presence of lecithin. Interestingly, salts of multivalent cations like calcium (Ca2+), magnesium (Mg2+), lanthanum (La3+) and cerium (Ce3+) greatly increase the viscosity of lecithin sols and transform them into optically transparent organogels. In comparison, monovalent cations or transition-metal cations have negligible effect on reverse self assembly. Based on data from small-angle neutron scattering (SANS), we show that gelation is accompanied by a transition from spherical micelles to cylindrical micelles/filaments. The varying abilities of different cations to induce gelation is shown to correlate with their binding tendencies to the phosphocholine headgroups of lecithin.

Next, we describe a class of photorheological (PR) fluids based on a nonpolar solvent such as cyclohexane. The rheological properties of these fluids can be reversibly tuned by UV and visible light. In order to create such PR fluids, reverse wormlike micelles of lecithin + sodium deoxycholate (SDC) are doped with a photoresponsive compound, spiropyran (SP). Spiropyrans can be reversibly converted from a closed-form (SP) to an open-form (MC) by UV and visible light, respectively. Initially, the reverse micelles in the lecithin/SDC/SP system are long and entangled, which makes the solution highly viscous. When exposed to UV light, the viscosity of these micellar solutions drops by a factor of 10. Conversely, when exposed to visible light, the viscosity recovers to approximately its initial value. We have found that this cycle between high and low viscosity states can be repeated more than 10 times.

Finally, we describe a new route to forming bilayered structures such as reverse vesicles and lamellae in organic solvents such as cyclohexane. This involves the combination of a saturated phospholipid, dimyristoyl phosphatidyl choline (DMPC) with an inorganic salt having either a trivalent cation like gadolinium (Gd3+) or a divalent cation like calcium (Ca2+). We find that the addition of the salt to DMPC solutions leads to either cylindrical aggregates or bilayered aggregates depending on the concentration of the salt. The structural changes can be explained qualitatively in terms of changes in the molecular geometry (packing parameter) induced by the binding of cations to the headgroups of the phospholipid.