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PHYSICAL CHEMISTRY MOLECULAR RULERS PROBE INTERFACES Method profiles polarity changes across different liquid-liquid junctions MAUREEN ROUHI Many chemical and biological phenomena de-pend on transport through liquid-liquid interfaces. Such systems are difficult to study, and methods to test assumptions about how properties change across an interface have not been available. Now, a method to profile polarity changes across water-oil interfaces has been developed. It reveals the width of interfaces and shows that some interfacial environments differ dramatically from predictions based on the properties of the bulk liquids. Steel Walker Robert A. Walker, an assistant professor of chemistry at the University of Maryland, College Park, and graduate student William H. Steel developed the method, which is based on use of "molecular rulers" [Nature, 424, 296 (2003)]. The technique and what it reveals have implications for phase-transfer catalysis, drug bioavailability, and other interfacial phenomena. A molecular ruler is a surfactant in which a solvent-sensitive hydrophobic probe and an anionic head group are separated by a spacer. In the new study, the hydrophobic probe is based on p-nitroanisole, the head group is a sulfate anion, and the separation length is varied with different numbers of methylene linkers. The wavelengths at which the probe absorbs light correlate with the polarity of its environment. For example, p-nitroanisole absorbs at 293 nm when dissolved in cyclohexane, at 316 nm in water, and at 308 nm at a water-cyclohexane interface. At water-oil interfaces, the sulfate head groups anchor the molecule to the aqueous phase. Only the hydrophobic probe can extend into the less polar organic phase. By monitoring the absorption wavelengths of molecular rulers of various lengths, Steel and Walker profiled polarity changes at two types of interfaces. The water-cyclohexane system is an example of a weakly associating interface. The probe reports a polarity in between that of cyclohexane and water at a ruler length of C2—that is, with two methylenes between the probe and head group. With C4, the polarity moves closer to that of cyclohexane. And by C6, the probe sees only a cyclohexane environment. A fully extended C6 ruler is about 9 Ĺ long, less than the diameter of three water molecules, Walker says. So the water-cyclohexane interface is sharp, and polarity changes abruptly. These results confirm previous findings of simulations and X-ray scattering experiments. More surprising are the results with the water-1-octanol system, an example of a strongly associating interface. The C2 ruler reports an environment that is much less polar than either water or 1-octanol. As the ruler lengthens, the polarity approaches that of 1-octanol, and by C8 the probe reflects the polarity of 1-octanol. But it never reports a polarity in between that of water and 1-octanol. These results can be explained, Walker says, by parallel packing of the carbon chains of the 1-octanol molecules as their respective hydroxyl groups are trapped by hydrogen bonding to water molecules in the interface. The packing creates a greasy wall between the very polar water phase and the moderately polar alcohol phase, a phenomenon that Walker says has not been observed previously. "The interface can have properties that are not reflected by either liquid," he concludes |
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