Laura Cunningham and Dr. David V. Dearden, Chemistry and Biochemistry
Microsolvation is the process of attaching solvent molecules to a host molecule in the gas phase, creating a system which exhibits characteristics between that of gas-phase and solution-phase systems. Study of these microsolvated systems can help us to understand the difference between gas-phase and solution-phase behavior, and uncover the conditions under which an ion may be considered solvated. Using electrospray ionization and Fourier-transform ion cyclotron resonance mass spectrometry, our team studied the best methods for creating microsolvated systems between alkali metals and various solvents.
The ions of interest were sprayed from a solution into a vacuum chamber along with nitrogen that was saturated with the solvent we wished to attach to the ions. In the process, the original solvent evaporated. Whether or not the desired solvent then attached to the ions depended on several parameters. Much experimentation was necessary to determine the best method of creating microsolvated ions.
One parameter that had a great effect on the formation of microsolvated ions is the voltage difference between the needle where the ions originate and the tube leading to the vacuum chamber. This voltage essentially controls the energy of the ions when they enter the tube, which in turn effects their interaction with the solvent molecules. We expected that smaller voltages would be more conducive to the formation of microsolvated ions, and we found this to be generally true. If the ions had too much energy, the solvent would not be able to condense and form microsolvated ions. However, if the ions had too little energy, the signal strength would greatly decrease. Thus, the signals we obtained for microsolvated systems were typically a magnitude of order smaller than gas-phase signals for the same ions.
A very important parameter we studied is the flow rate of the saturated nitrogen. The ideal flow rate differed for every solvent we used. This is most likely due to the difference in viscosity and polarity among different solvents. If the flow was too high or too low, no microsolvated systems formed; lower flow rates tended to give better results. There are several reasons that low flow rates tend to create better microsolvation effects. First, the flow rate must be slow enough that the nitrogen gas will be saturated with the solvent as it is bubbled through. Also, at high flow rates, it is possible that once inside the vacuum chamber the solvent would condense on the ions but would re-evaporate due to the high energy content of the solvent molecules. Another hypothesis is that high flow rates may disturb the flow of ions into the chamber, meaning that the signal would be nearly zero. Of course, if the flow rate is too slow the pressure of the gas entering the vacuum chamber would be too low to allow for significant interaction between the gas and the ions.
While attempting to discover the best parameters for creating microsolvated ions, we were able to obtain some preliminary data. Microsolvation can form many different types of complexes. For example, when spraying a solution of calcium bonded to a crown ether, we found solvated calcium ions, solvated ion-crown ether complexes, and possibly solvated crown ethers, as well as other solvated metal ions present in trace amounts in the original solution. One early experiment indicated the presence of so-called “magic numbers,” or a recurrence of certain groupings of solvent molecules and metal ions over time. These may help to indicate certain conformations or positions of the ions and the solvent molecules which are particularly stable.
This study prepared the way for further microsolvation experiments, which will be able to explore the implications of our preliminary data. Because each system is different, there are no exact parameters which will work best for every experiment. However, by exploring the effects of different parameters on the formation of microsolvated systems, we were able to determine a relatively small range for each parameter that would yield the best results. We learned that microsolvation is not difficult under the right circumstances and are very excited to learn more about microsolvation.
Acknowledgements: Special thanks to Dr. David Dearden for his tremendous teaching and support, and to Sarah Meibos for her help on the project. Also, thanks to Katherine Kellersburger, Lauren Bergeson, and Craig Thulin for their help and support.