Volume 67, Issue 3

Experimentation with Chemistry

by Dr. Bruce Hrnjez

Everyone can relate to the idea that one’s surroundings can have a profound affect on one’s behavior and, inevitably, on the course of one’s life.  So it is also with molecules.  A molecule’s surroundings, the medium in which it is dissolved, can exert subtle, or not so subtle, effects on the way the molecule vibrates, rotates, or interacts with light, and the way its chemical bonds break or form in a chemical reaction.  Generally speaking, we refer to these phenomena as “solvent effects,” and their study is among the most pressing concerns of those who want to understand how molecules behave.

Typically, those who study solvent effects will choose a particular chemical system and then examine the behavior of that system in many different solvents.  In principle, this is a perfectly acceptable approach, but interpretation of the results is enormously complicated because the number of variables is very large.  The name of the game in science is to minimize the number of variables and rigorously control how they change.  We have a way of circumventing the troublesome number of variables that accompany the change of solvent.  Essentially, we use supercritical fluids as tools to probe solvent effects. A supercritical fluid exists in an unusual state that is not gas, liquid, or solid.  It is a fluid, best known commercially for when supercritical carbon dioxide is used to decaffeinate coffee (look for “natural effervescence” on the label).    In using a supercritical fluid, we rely on the fact that we can continuously vary the bulk density of the fluid at constant temperature with small changes in pressure.  Bulk solvent properties such as viscosity and dielectric constant are sensitive functions of solvent density.  The beauty of this simple approach is that we can study solvent effects without changing the solvent.  The immediate result is that the number of variables is dramatically reduced, giving us hope that that we can contribute to the understanding of solvent effects at a fundamental level.

Dr. Hrnjez is a tenure-track chemistry professor at Yeshiva College.  This is his first semester at Yeshiva.  Previously, he taught organic chemistry at North Dakota State University, and performed research at Harvard Medical School.

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