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43543-GB4
Amphiphilic Catenanes
Our efforts to prepare high aspect ratio (fibrous) materials has continued to grow as has our understanding of these phenomena. We have synthesized and explored the utility of several amphiphilic catenanes as well as two related non-catenated amphiphiles. Several new biscationic, single-chained amphiphiles (prepared in one step from an intermediate in the catenane synthesis) exhibit surface activity and the ability to form needle-like fibers in an aqueous environment. In addition, a series of propargylic alcohols can be induced to form crystals or gels, as controlled by the nature of the liquid phase.
One undergraduate (Nicholas Henrich, JMU '08) has completed the synthesis of three different amphiphilic catenanes, each bearing two or four hydrophobic chains on a central polar catenane. Compound 1 has two linear chains [–OC14H19], one on each of the cyclophane two phenylenes. 2 has two branched chains [–O(CH2)2CH(CH3)(CH2)3CH(CH3)2] on the same phenylene of the cyclophane, and 3 has four of these branched chains, two on each of the two phenylenes of the cyclophane. These molecules are designed to aggregate into fibers in non-polar media due to their amphiphilic nature, and p-p stacking of aromatic rings. Slow addition of a non-polar solvent to a chloroform solution of 1 leads to the formation fibrous aggregates, as shown in the SEM image below. Optimization of the formation of fibers, as well as similar studies on the derivatives with two or four branched chains (2, 3) are underway.
One of the intermediate products in the synthesis of our amphiphilic catenanes was utilized as a starting material for a novel family of biscationic, single-chained surfactants. While a range of amphiphilic structures are known, surfactants with multiple head groups and a single tail have only recently been reported in the literature. They have been found to have unique properties compared to those of traditional (one head, one tail) amphiphiles. These include diminished aggregation number, severely bent chain conformations and increased insensitivity to the addition of electrolytes such as KBr. Our compounds are different from those reported in the literature; while they also have multiple (two) head groups and a single tail, they also incorporate a rigid aromatic spacer between the head groups. Karolina Roszak (JMU '07) developed the original synthesis of these surfactants; during the summer of 2007, Addie Hill, a visiting undergraduate student from Winston-Salem State University worked on this project. She scaled up the preparation of three surfactants in this family, with chain lengths ranging from 10 to 18 carbons. Initial studies on these new molecules in water show that they self assemble into aggregates, as indicated by a sharp break in the plot of log(concentration) versus conductivity. Addie determined the critical micelle concentrations (cmc) for several of the derivatives in water using this method. The C18H37 derivative also forms needle-like fibers in solution, suggesting a model with a high degree of either interdigitation or significant bending of the alkyl chains. We are currently preparing and studying the properties of two additional derivatives.
Finally, we have also prepared a series or chiral propargylic alcohols in an effort to produce self-assembled fibers in non-polar media using the same philosophy as the amphiphilic catenanes. This project is a collaborative effort with Dr. Lin Pu and Dr. Michal Sabat at the University of Virginia, who have recently published the crystal structures of several chiral propargylic alcohols. In the solid-state, these molecules form hexamers (through hydrogen-bonding and aromatic stacking) which then stack on top of each other to form pseudo-infinite channels. Our original intention was to modify the original structure by the addition of a para alkoxy chain to one of the aromatic rings in an effort to hinder aggregation lateral to the channel axis, thus forming supramolecular fibers. We have since found that we can induce many of our derivatives (including the parent compound) to form fibers and/or gels in non-polar media. We have studied the structural characteristics of these gels using scanning electron microscopy (SEM) and powder X-Ray diffraction. Thermal stability tests have also been performed using the ball-drop tests (a physical method) as well as differential scanning calorimetry (DSC). Additional structural studies utilizing ATR-IR are underway. We are also continuing to explore the properties and potential utility of a large range of propargylic alcohol derivatives. Two undergraduate students (Ashleigh Borges, JMU '07 and Michelle Lum, Harvey Mudd College '07) developed the synthesis and performed the initial gelation studies. A third undergraduate (Clayton Dingle, JMU '08) continued these studies during the summer of 2007.
The students who worked on this project who have now graduated have moved on to medical school (Ashleigh Borges), physician's assistant school (Karolina Roszak) and employment in the biochemical industry (Michelle Lum). Two of the three current seniors are applying to PhD programs in chemistry (Nicholas Henrich and Addie Hill).
