The crafty chemist

existence and uniqueness: thecraftychemist:I think…and I’m sorry to interrupt, but I have this...

ladyphysicist:

thecraftychemist:

I think…and I’m sorry to interrupt, but I have this terrible niggling urge to explain as I’m sure most scientific people do. Now strictly speaking there’s many different interpretations of what chemical potential really is… because. it’s. just. so. broad.

I cut the rest of your post because it’s talking about the chemical potential energy of molecular bonds and so on which is interesting

but not what the chemical potential I’m talking about is

the basic idea behind the chemical potential is that if you add mass to a system, its internal energy increases, and the chemical potential is the proportionality factor of the increase

though your post might have to do with why molecules move from higher to lower potential in a reaction I suppose but that’s more of a chemistry topic that I don’t know much about.

I don’t know if this is helpful, but just found this resource on the topic. In fact the whole lecture series is available from Cambridge here.

Chapter 2, page 40:

So the chemical potential should be thought of as the energy cost of adding an extra particle at fixed entropy and volume. But adding a particle will give more ways to share the energy around and so increase the entropy. If we insist on keeping the entropy fixed, then we will need to reduce the energy when we add an extra particle. This is why we have chemical potential < 0 for the classical ideal gas (chemically inert, low mass, mono-atomic.i.e. Helium)
There are situations where chemical potential > 0. This can occur if we have a suitably strong repulsive interaction between particles so that there’s a large energy cost associated to throwing in one extra.

Propositions and Corollaries: ladyphysicist:oh I had my E&M exam today, actuallyfunnily enough...

ladyphysicist:

oh I had my E&M exam today, actually

funnily enough there were no questions on polarization!  or magnetization, and I was expecting one or the other (there was a question about wave boundary conditions but that wasn’t really about magnetization itself).  and the radiation question was very straightforward.  but there was a really tough electrostatics question involving image charges, which I didn’t study at all.

I have thermo tomorrow too, huh.  What sort of things are going to be on your exam?

the things I’m worried about for my thermo exam are entropy and mixtures and phase transitions

Sometimes I feel like you are me in a parallel universe…then I remember that parallel universes are bullshit and all physics majors that are approximately my age across the country are learning the same things at the same time as me.

We did not really do mixtures much. Honestly, based on your posts this semester, your thermo class went much more in depth than mine did (but my E&M class seemed to do more). I struggle with anything related to the chemical potential—somewhat with phase transitions (though the Clausius-Klepron equation makes sense, at least), but mostly with Fermi surfaces. I screw up those problems somehow every time. I also just really don’t have a good grasp on what the chemical potential is/does.

Now when we talk about molecules… those Fermi surfaces sound very similar to what my physical chemistry lecturer calls ‘Potential energy surfaces’. These are the result of plotting all the allowed vibrational states, i.e. bond lengths of more than 2 atoms using the Schroedinger equation. Say you have 3 hydrogen atoms labeled a, b and c.

Ha-Hb…….. Hc.

Ha and Hb are bonded. In a reaction the Hb atom is switched with Hc. There is no change in the energy level so there is only an activation energy to overcome and the diagram is symmetrical.

In a ‘potential energy surface’ we plot the distance of Ha to Hb on the y axis and the distance of Hb to Hc on the x axis. On the z axis pointing out of the page is the amount of energy required for the atoms to stay in that position.

The atoms cannot be infinitely close (without fusion taking place) so the energy ‘cliff’ becomes incredibly steep when the distance between Ha and Hb or Hb and Hc is too small. When they are too far appart they cease stabilising each other through sharing electrons (electrons cancel out each others’ spin) and ionise. Hence, they will follow a specific path during reactions like your Fermi surfaces have points at which electrons are most stable.

This is just my interpretation of what chemical potential energy is; another’s may use entirely different methods of explaining it depending on their discipline, but as a chemist I don’t know if that helps.

Neuroscience: Researchers develop new pathway to brain for medicine

neurosciencestuff:

Stumped for years by a natural filter in the body that allows few substances, including life-saving drugs, to enter the brain through the bloodstream, physicians who treat neurological diseases may soon have a new pathway to the organ via a technique developed by a physicist and an immunologist…

Yes that’s great.

In an in vitro laboratory test with HIV-infected cells, Nair and a colleague, Sakhrat Khizroev, a professor of immunology and electrical engineering, attached the antiretroviral drug AZTTP to tiny, magneto-electric nanoparticles. Then, using magnetic energy, they guided the drug across a cell membrane created in the lab to mimic the blood-brain barrier found in the human body.

Once the drug reached its target, researchers triggered its release from the nanoparticle by zapping it with a low-energy electrical current. The drug remained functional and structurally sound after the release, according to the experiment findings.

But I’m more fascinated by the technique than the outcome.