Self-Organized Surfactant Structures
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Reprinted, by permission, from J. Schnur, Science Surfactant molecules e. The membranes also organize in a variety of patterns.
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Such self-assembled structures may be swollen by adding an organic solvent to the system, which remains stable as a clear single phase. Such two-solvent composites are called microemulsions. Surfactant bilayers have been investigated for many years as model systems for biological membranes. Repulsive forces between the surfactant head groups organize the bilayers on longer length scales. This process may form lamellar stacks, sponge phases, or microemulsions, depending on whether there are one or two solvents. Even more relevant to biomolecular systems is the formation of closed-film structures called vesicles Figure 3 , which are already being employed as simple containers to transport drugs within the blood system.
Surfactants & critical micelle concentration (CMC)
The self-organizing micelles may form in different shapes depending on the specific chemistry and on solvent conditions such as pH, ionic strength, and temperature. The most common form is spherical, but cylindrical micelles also occur. In fact, entropic optimization drives long cylindrical micelles into the form of flexible polymer-like chains.
These polymeric micelles have many of the rheological properties of polymers with covalently bonded backbones, but their lengths are determined by equilibrium thermodynamics rather than being fixed. An interesting example of a self-assembling structure is the tubule, a hollow phospholipid bilayer cylinder morphologically similar to a soda straw Figure 4. The length of these ultrasmall cylinders is.
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A number of applications using both metal-clad and non-clad tubules are currently being evaluated for commercial application. The use of such microvials has led to controlled-release applications for marine antifouling release over many years and drug delivery release over several days to months. The metal-clad structures also have interesting electromagnetic properties.
Applications based on their dielectric properties miniaturized microwave circuits and as absorptive filters are currently under development in several universities and government laboratories. Vesicles: top sketch of multilamellar and unilamellar vesicles and the phospholipid bilayer of which they are made; bottom sketch of a mechanism whereby drug-containing vesicles adhere to a cell surface and present drugs for uptake by endocytosis.
Since much of biology is based on the special properties of macromolecules, it is widely anticipated that the area of polymeric biomolecular materials will be rich. For example, both polyesters and polyamides i. A current area of investigation is to understand those features of protein polymers that confer high tensile strength, high modulus, and other advantageous properties. Once those features are understood, the tools of biotechnology will make possible entirely new paradigms for synthesis and production of materials. If they can be made economically viable, these new approaches will help reduce our dependence on petroleum and furthermore will enable making materials that are biodegradable.
Schnur, R. Price, and A. Rudolph, Journal of Controlled Release Electron micrograph of nickel-coated tubules approximately 0. In the last few years it has become possible to produce designer polymers comparable to those produced naturally. Use of genetic engineering techniques to obtain new polymer molecules provides the polymer scientist with model, controllable, synthetic proteins for the first time.
Two aspects of this approach are biomimetic: targeted molecules are produced by biosynthesis, and they then self-assemble into desirable structures that have new or improved properties. Efforts in biosynthesis have been directed toward the preparation of precisely defined polymers of three kinds: 1 natural proteins such as silks, elastins, collagens, and marine bioadhesives, 2 modified versions of these biopolymers, such as simplified repetitive sequences of the native protein, and 3 synthetic proteins designed de novo that have no close natural analogues. Although such syntheses pose significant technical problems, these difficulties have all been successfully overcome in recent years.
Figure 5 shows the key steps in the in vivo synthesis of protein-like polymers under direct genetic control in bacteria. The first requirement is that the amino acid sequence of interest must be encoded into a complementary sequence of DNA. For natural proteins, the requisite DNA sequence is obtained by isolating the appropriate gene from the natural host organism.
De novo design and synthesis require chemical synthesis of an artificial coding sequence.
In either case, the isolated DNA fragment is ligated into a loop of DNA a plasmid that can be replicated in a microbial host and that includes the signals needed for controlled transcription and translation. The resulting recombinant DNA is then introduced into the host cell population, cells are grown in large numbers, and protein production is induced.
To date, this approach has been used successfully to produce a variety of natural proteins, as well as dozens of wholly artificial protein-like polymers. It is now relatively routine to use bacterial hosts to produce multigram quantities of polymers with degrees of polymerization up to and molecular weights up to , The first commercial product based on this technology, an artificial cell attachment protein that can be used to coat polystyrene culture dishes, has already appeared. Although the anticipated structural regularity has been thoroughly demonstrated in several instances, mutations have also occurred that have led to an altered chain sequence.
There have also been examples of enzymatic degradation, which leads to chain length heterogeneity. Such problems have as yet unknown consequences for targeted secondary and tertiary structures. Biological synthesis is also being used to prepare important classes of polymers other than proteins. Bacterial polyesters have attracted particular attention because of their biodegradability. Mobley, ed. Key steps in the in vivo synthesis of protein-like polymers under direct genetic control in bacteria. First a DNA sequence is produced that codes for the desired polymer. This DNA is then inserted into a plasmid.
Self-Organization in the Nonionic Surfactant Systems
When the plasmid is inserted into E. Reprinted with permission from J. Tirrell, M. Fournier, T. Mason, and D. Costs are still high in comparison to those for commodity polymers, but increasing volume seems likely to bring production costs down to levels that are economically competitive in a wider range of applications.
Self-organized Surfactant Structures hardcover
Furthermore, it has recently been demonstrated that the production of bacterial polyesters can be transferred to plants, opening the way for their production from biomass. Natural polysaccharides are under development for biodegradable packaging applications. Several companies have recently commercialized starch-based polymer blends that can be processed using existing thermoplastic methods.
Biology often combines very different materials in closely interwoven patterns to develop unique properties that stem from the co-continuous nature of the assembly. The liquid-crystalline state in polymers, first viewed as unusual, is now becoming accepted as a normal state occurring between the crystalline and isotropic liquid states. High-performance fibers based on solution processing of lyotropic liquid-crystalline polymer solutions e. Certain insects, including spiders, take advantage of the low viscosity in the liquid crystalline regime.
Spider dragline silk is a versatile engineering material that performs several demanding functions. The mechanical properties of dragline silk exceed those of many synthetic fibers. Moreover, dragline silk exhibits the unusual behavior that the strain required to cause failure actually increases with increasing deformation.
Spiders extrude an aqueous solution of silk protein to spin the molecules into oriented fibers. The female garden cross spider can use seven different glands, each containing silk with a unique amino acid sequence, to. Kaplan, pp. Kaplan, W. Adams, B. Farmer, and C.
Viney, eds. American Chemical Society, Washington, D. Figure 6 illustrates a proposed model for the structure of spider silk.
There is renewed interest in the structure-property relationships of such structural biological materials because the fibers have outstanding tensile as well as compressive properties. Work is under way to fully characterize the molecular weight and sequence distribution; the nature of the in vivo solution speculated by some to be liquid crystalline ; the structure, size, and orientation of the crystalline regions; and their interconnection to the amorphous regions. The consensus crystalline repeat in two silk proteins has recently been identified.
Diagram of the proposed model for the molecular arrangement of alanine residues in a fiber of spider dragline silk. In reality, the glycine-rich matrix composes about 70 percent of the fiber; in this drawing it has been largely suppressed for clarity. Reprinted, by permission, from A.
Simmons, C. Michal, and L. Jelinski, Science —87 Another important use of polymers is adding them to hydrocarbon-and water-based fluids to control rheological properties. Ionomers, oil-soluble polymers containing ionic functionality such as sulfonate or carboxylate groups, have been synthesized in a variety of architectures including random, block, and telechelic structures. In hydrocarbon solvents, the polar ionic groups interact to create self-assembled or aggregated polymer chains, which appear to have much higher molecular weights than the non-functionalized polymers.
Conversely, hydrophobically associating polymers are water-soluble but contain a small amount of oil-soluble or hydrophobic functionality.
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These two classes of polymers have been called associative thickeners. The association of the functional groups in fluids enables these synthetic polymers to mimic the secondary and tertiary interactions and structures found in biomolecules such as proteins and polysaccharides. The inter-and intra-molecular interactions of the polymers define their conformation and assembly in solution, and in turn, the solution's rheological properties.