COLL 147 |
| Bin Lin1, Alon McCormick1, H. Ted Davis1, and Reinhard Strey2. (1) Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave SE, Minneapolis, MN 55455, (2) Institute for Physical Chemistry, University of Cologne, Luxemburger Str. 116, Koeln, 50939, Germany |
| Sodium soaps are a class of surface-active materials that have been widely used in cleaning, thickening, lubricating products and drug formulations. When the hydrophobic aliphatic chain has more than 11 carbons, sodium soap has limited solubility in water at room temperature; it crystallizes to form a dispersion of crystallites with lamellar structure. Therefore, it is of practical interest to seek a simple but effective way to induce a microstructure transformation from crystalline lamellae to spherical micelles so as to depress the soap Krafft point and enhance the soap solubility. Interestingly we recently found that tetraalkylammonium bromides (TAABs) with four symmetric short hydrocarbon chains can serve as such structure-changing electrolytes. We systematically examined various ternary sodium soap/TAAB/water systems at the molecular, microscopic, and macroscopic levels using cryogenic electron microscopy, X-ray diffraction, and video-enhanced light microscopy. We observed that tetraalkylammonium ions significantly influence the aggregation behavior of soap molecules. For instance, with the addition of enough TAAB, the soap aqueous crystalline dispersion becomes a stable homogenous spherical micellar solution. The solubility investigations of sodium soaps in aqueous salt solutions by direct visual phase behavior observations show that TAABs have an excellent ability to depress the soap Krafft point. A very intriguing phase behavior was found for the ternary water-tetrapentylammonium bromide (TPeAB)-sodium myristate (NaMy, one of the common soaps) system. In addition to an upper miscibility gap with lower consolute point (like the cloud point phenomenon for nonionic surfactant systems), we also observed the characteristic three-phase region usually observed in microemulsion in water-oil-surfactant systems around the optimal state. The significance of these observations will be discussed. |
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Symposium in Memory of Arthur W. Adamson
2:00 PM-5:40 PM, Monday, March 29, 2004 Marriott -- Orange County 4, Oral
Division of Colloid and Surface Chemistry |