It turns out that different nuclei scatter neutrons differently, and you can use these differences to good effect in your experiments. An especially important difference is that between deuterium (a hydrogen atom with a nucleus that contains a proton and an extra neutron) and hydrogen. Since we studied aqueous dispersions of colloids and surfactants we would replace regular water (with hydrogen), with heavy water (with deuterium). Long nights (sometimes all night) watching the experiments gave us time to think.
We often wondered if drinking a bottle of heavy water would kill someone, or how much of the water in our bodies we could replace with heavy water without ill effects. Unlike other isotopic substitution, putting in deuterium for hydrogen actually has a chemical effect because you double the weight of the nucleus. Heavy water isn't radioactive so it wouldn't hurt you that way. We figured it would disrupt the action of important enzymes because the dielectric constant and solvent properties would be different.
An article in the Candian Journal of Physiology and Pharmacology referenced in Wikipedia actually explores some of these questions. Finally, answers for us, more than a decade later.
Bacteria and algae can handle 100% heavy water substituted for normal water.It turns out the symptoms of heavy water poisoning still looks like radiation damage, but that is because the enzyme effects we hypothesized above show up as eukaryotic cell's inability to repair DNA damage. It is too bad heavy water is expensive, $65 for 8 onces.
Protozoa can live in up to 70% heavy water.
Mammals 50% heavy water kills, 25% sterilizes, but 20% is survivable.
It would be an expensive way to poison someone, since you would need almost a quarter of the person's body weight to do it. I should have bet someone back in graduate school that I could drink a bottle and have it do no damage, because now I know it wouldn't. Too late.
A recent article in Popular Science showing heavy water ice cubes (which are denser than normal water) sinking to the bottom of the glass reminded me of this discussion, and inspired the post.
tags: heavy water, neutron scattering, science, biology