Frictional effects in solute-solvent interactions

Dr. Neil Canter, Contributing Editor | TLT Tech Beat January 2009

Researchers developed an empirical procedure to study friction using atoms as solutes instead of molecules.

KEY CONCEPTS
• A sodium atom is produced from a sodium anion by a different mechanism than a sodium cation. The two processes differ at the microscopic level.
• The strong repulsive force generated when a sodium cation gains an electron more rapidly accelerates the reaction as compared to the diffusion process that is needed to convert the sodium anion to the neutral atom.
• Dielectric and mechanical friction are generated during both processes.

Interest in a better understanding of how friction occurs on the microscopic scale is necessary to enable us to better deal with the real-world lubrication problems we encounter on the macroscopic scale. In a previous TLT article, the concept of molecular friction in solution was described (1). This phenomenon is an important consideration because most lubricants are liquids that contain a basestock or solvent such as mineral oil and additives that can be designated as solutes.

The previous work done by professors Stephen Bradforth, Richard Stratt and co-workers was to evaluate the formation of a cyano radical in solution. In this research, the cyano radical rotated over an unexpectedly long time frame. The researchers felt that solvent molecules would rush in more quickly to dissipate the energy of rotation. But the rapid rotation of the cyano radicals generated a bubble around them because they were able to quickly push away some of the nearby solvent atoms. Eventually, rotation of the cyano radicals declined and the solvent molecules rush back. Then the physical bumping of the solvent molecules into the cyano radical slows down the rotation in a manner analogous to molecular friction.

A different view of the frictional effects between solutes and solvents can be obtained if solutes are evaluated as atoms instead of as molecules. Benjamin Schwartz, professor of chemistry and vice chairman of the department of chemistry & biochemistry, at UCLA, says, “In evaluation of solute-solvent interactions, spectroscopic analysis of a molecule is very complicated. The use of atoms is far simpler because they cannot vibrate or rotate. In essence, atoms are perfectly clean.”

Schwartz has been looking to better understand how solvents respond to changes in the size and the electronic charge distribution of solutes. During this process, the electronic charge is moving in a solvent system from a donor to an acceptor. Schwartz says, “We have been striving to better understand the interplay between dielectric and mechanical friction generated when the solute undergoes a change in size or electronic state.”

FORMATION OF NEUTRAL SODIUM
Schwartz developed an empirical procedure using sodium ions as the substrate. He says, “We decided to evaluate the preparation of neutral sodium from its two ionic forms, the anion and the cation. Sodium is very easy to analyze spectroscopically, and we are very familiar with how to prepare its anion and cation.”

Schwartz wished to investigate whether the transformation of the anion and cation of sodium to the neutral atom would be consistent with the linear response theory. Schwartz adds, “This theory states that a system moved out of equilibrium by a perturbation such as a chemical reaction should relax in an identical fashion to the relaxation of the natural fluctuations in equilibrium.” In other words, Schwartz was trying to test whether or not the friction felt by the sodium atom was dependent on the initial arrangement of the surrounding solvent molecules.

The anion and cation of sodium were prepared by known methods. In the former’s case, the researchers dissolved sodium metal in tetrahydrofuran (THF) by adding a crown ether. These crown ether molecules serve to complex the sodium cations in solution, enabling free sodium anions to exist. Schwartz says, “Upon adding the crown ether, the liquid changes very dramatically from colorless to dark blue.”

Excitation of sodium anions with a red laser in a process known as electron photodetachment generates neutral sodium and an electron. Schwartz explains, “This reaction takes place readily and, most important, we have determined that the electron moves far away from the neutral sodium atom. Thus, the only species present near the neutral sodium atom are the THF solvent molecules.”

Synthesis of the neutral sodium atom is a more difficult process from the sodium cation. Schwartz says, “A typical diffusion-based process for the formation of sodium cations takes longer than the 5-10 picoseconds it takes for the solute-solvent system to relax. This means that we needed to devise a process that will allow us to place electrons on to sodium atoms in a time faster than the diffusion limit. We chose to take electrons from iodide ions because sodium cations and iodide anions pair up in THF.”

Photoinduced electron transfer was used to excite the iodine anion, enabling it to release an electron. With the sodium cation in close proximity, the electron was very quickly attracted to the cation. Schwartz estimates that the neutral atom is formed in approximately 2 picoseconds.

Although both routes create the same neutral sodium species, the two processes used to produce the neutral sodium atom are different on the microscopic level. A sodium anion is 20% larger in size than the neutral atom. In turn, Schwartz estimates that the sodium cation is 20% smaller than the neutral atom because it no longer has an electron in its outer shell.

The size difference between the sodium anion and cation ensure that these species are converted to the neutral atom by different pathways. Approximately six to eight molecules of THF surround one sodium anion. Once the electron is released and the sodium atom converted to a neutral state, the size reduction means that only four to six THF molecules can be adjacent to one neutral sodium atom. Schwartz says, “This process amounts to waiting for diffusion to squeeze several of the THF molecules out of the way so that the rest can move in toward the sodium atom, which is smaller by 20%.”

In contrast, the motion of the THF solvent molecules surrounding the conversion of a sodium cation to the neutral atom is much different. Addition of an electron leads to a rapid size increase in the sodium atom causing a strong repulsion between the solute and the electrons in the THF molecules. This force pushes the reaction at a more rapid pace as compared to the motion of molecules accompanying the formation of the neutral atom from the anion, which relies mainly on diffusion.

Schwartz indicates that the time needed for the solvent system to re-equilibrate is approximately 10 picoseconds for the sodium anion reaction, which is twice as long as the corresponding sodium cation reaction. The two reaction pathways are illustrated in Figure 1.


Figure 1. Sodium anion and cation solutes in the solvent THF form neutral sodium atoms by different pathways. Both dielectric friction and mechanical friction occur during these reactions. (Courtesy of UCLA)

In producing a neutral sodium atom, the fact that the mechanism for solvent relaxation is different for the two different routes means that the linear response theory does not apply. Schwartz speculates that it is the interplay between dielectric friction and mechanical friction that is responsible for the breakdown of the linear response in this system. He says, “Dielectric friction is involved due to the change in charge when the neutral atom is created and mechanical friction is involved because of the change in atomic size upon neutralization.”

Future work concerns conducting computer simulation studies to determine exactly what the solute and solvent molecules are doing during the reactions. Schwartz’s objective is to have the simulations generate the same answer as was seen empirically.

The differences in the reaction of the sodium anion and cation in THF provide additional information about the molecular origins of friction. Further information can be found in a recent paper (2).

REFERENCES
1. Canter, N. (2006), “Diminished Molecular Friction in Solution,” TLT, 62 (7), pp. 10–12.
2. Bragg, A., Cavanagh, M., and Schwartz, B. (2008), “Linear Response Breakdown in Solvation Dynamics Induced by Atomic Electron-Transfer Reactions,” Science, 321 (5897) pp. 1817–1822.

Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.