With liquid microjet technology, precisely mixed chemicals flow rapidly through a fused silica capillary shaped to a fine tip, a nozzle with an opening only a few micrometers (millionths of a meter) in diameter. The resulting liquid jet travels through a few centimeters of vacuum inside the beam chamber and is intersected by the synchrotron's x-ray beam, then collected by a cold skimmer and condensed out, to prevent any liquid molecules from contaminating the pristine vacuum of the synchrotron.
The great advantages of the system, says Saykally, are that in a vacuum the soft x-ray beam encounters only the liquid target -- there's no interference from air or windows or the like -- and that the rapid passage of the sample through the beam minimizes x-ray damage, which otherwise can be severe.
"In our NEXAFS experiment, the x-ray beam kicks the lowest-energy core electrons of the carbon on the carboxylate group up into the lowest empty antibonding molecular orbitals," Uejio explains. "The more tightly bound the cation is to the carboxylate, the more energy it takes to promote the electron. Therefore, the x-ray absorption spectra tell you about the relative binding energies of sodium, potassium, and lithium."
Kosmotropes versus chaotropes
The results confirmed Vrbka et al's models and further supported the Law of Matching Water Affinities -- thus lending weight to a growing trend in the interpretation of the venerable Hofmeister series. Saykally explains that the ions in the Hofmeister series are traditionally divided into kosmotropes, which bind strongly to water and supposedly structure it, and chaotropes, which bind only weakly to water and destructure it.
"There has been a widely held view that the Hofmeister series reflects changes in the bulk structuring of water -- that salting-out results when ions orient water molecules over a long range, reducing their densi
|Contact: Paul Preuss|
DOE/Lawrence Berkeley National Laboratory