Reef Chemistry Question of the Day #280: Which is the best analogy of ion spacing in seawater?

[mizimmer90]

That's very cool! Thanks for posting it. Are you a professional modeler of some sort?

I would just caution that the apparent spacing in 2D is closer than the reality of 3D, if you just took the ratio of water and ions and mixed them together in 2D (not sure if that's what you did or not). Extrapolating to 1D (just spacing all the water and ions on a line), the relative spacing error would much worse (about 1 ion per 55 water molecules in a line).

Thanks! Yup, computational biophysicist here!

The simulation was in 3D, but I agree that it looks closer when projected in 2D. They also look closer when the solvent isn't shown as spheres (but they would block out all the ions). Here's an updated video rotating the system, with the ions only. I should note that I updated the van der Waals radii to reflect the radius of the charged state (i.e. instead of Na, radius is Na+)

Showing everything as spheres, including water (orange and white now), the ions look more sparse.

And just because I'm having too much fun, I calculated the distance between each ion and its closest ionic neighbor for the last 10 frames of the sim (results are approx since its a short sim in a small box). As others have pointed out, ions are not uniformly distributed! I've also broken this down on a per-ion basis. For comparison, the dodecahedron box is ~6 nm in width.

 

[dennis romano]

I have re-thought this 100 times lol. Unless (which obviously there is) I'm missing something obvious - All of the analogies have some problems - BUT I'm going to change my vote to the 747.

There are different people in the plane - plus the cabin staff. Usually when the plane is nearly empty, the people (let's call them the ions) can move around freely. And the Empty seats could represent the water molecules in the shell bound to the ions - Since there (to my understanding) far more water molecules than ions in seawater. So I'm changing from the cats to B.

Sorry, I have to disagree. By definition, an ion is a positive or negative charged atom or group of atoms(molecule). So a charged atom is searching for an opposite charged atom. Negative attracts positive and vice versa

 

[mizimmer90]

Sorry, I have to disagree. By definition, an ion is a positive or negative charged atom or group of atoms(molecule). So a charged atom is searching for an opposite charged atom. Negative attracts positive and vice versa

This is partly true, however, water is a very polar molecule (containing positive and negative charge). That means that both positive and negative ions can freely associate with water molecules, which effectively screen their charge. Ions can (and do) associate with each other in solvent, but the energy difference between forming and breaking their bonds is actually pretty small. Water tends to form "cages" around ions!

How Ions Affect the Structure of Water

We model ion solvation in water. We use the MB model of water, a simple two-dimensional statistical mechanical model in which waters are represented as Lennard-Jones disks having Gaussian hydrogen-bonding arms. We introduce a charge dipole into MB waters. ...

www.ncbi.nlm.nih.gov

 

[dennis romano]

This is partly true, however, water is a very polar molecule (containing positive and negative charge). That means that both positive and negative ions can freely associate with water molecules, which effectively screen their charge. Ions can (and do) associate with each other in solvent, but the energy difference between forming and breaking their bonds is actually pretty small. Water tends to form "cages" around ions!

How Ions Affect the Structure of Water

We model ion solvation in water. We use the MB model of water, a simple two-dimensional statistical mechanical model in which waters are represented as Lennard-Jones disks having Gaussian hydrogen-bonding arms. We introduce a charge dipole into MB waters. ...

www.ncbi.nlm.nih.gov

Yes! Forgot that part of the equation. I am digging deep into the memory banks of when I had to read "Morrison and Boyd" forty plus years ago.

 

[MnFish1]

Sorry, I have to disagree. By definition, an ion is a positive or negative charged atom or group of atoms(molecule). So a charged atom is searching for an opposite charged atom. Negative attracts positive and vice versa

True - BUT - for example - doesn't the link below look like a 747 with 4 passengers and other empty seats? This is another one where Randy is keeping everyone is suspense......
https://www.usgs.gov/media/images/water-molecules-and-their-interaction-salt

 

[MnFish1]

True - BUT - for example - doesn't the link below look like a 747 with 4 passengers and other empty seats? This is another one where Randy is keeping everyone is suspense......
https://www.usgs.gov/media/images/water-molecules-and-their-interaction-salt

For the record - I'm abandoning my cat analogy. .

 

[Randy Holmes-Farley]

Thanks! Yup, computational biophysicist here!

The simulation was in 3D, but I agree that it looks closer when projected in 2D. They also look closer when the solvent isn't shown as spheres (but they would block out all the ions). Here's an updated video rotating the system, with the ions only. I should note that I updated the van der Waals radii to reflect the radius of the charged state (i.e. instead of Na, radius is Na+)

Showing everything as spheres, including water (orange and white now), the ions look more sparse.

And just because I'm having too much fun, I calculated the distance between each ion and its closest ionic neighbor for the last 10 frames of the sim (results are approx since its a short sim in a small box). As others have pointed out, ions are not uniformly distributed! I've also broken this down on a per-ion basis. For comparison, the dodecahedron box is ~6 nm in width.

This is great! Thanks for all of it!

 

[IslandLifeReef]

True - BUT - for example - doesn't the link below look like a 747 with 4 passengers and other empty seats? This is another

Reef Chemistry Question of the Day #280:

Which of the following is the best analogy of ion spacing in seawater?

A. The ions in seawater are spaced like a 20 ducks on a 1 acre pond
B. The ions in seawater are spaced like 4 people on a 747 jet
C. The ions in seawater are spaced like 2 ice cubes in a glass of water
D. The ions in seawater are spaced like 3 kittens in a single carrier.

Yes, I know, it's an odd way to ask the question, but I think many reefers have little intuition of what the relevant spacing is and I hope we can learn from it.

Hint: There was a relevant Reef Chemistry question of the day about 5 years ago. lol


Previous Reef Chemistry Question of the Day:

Reef Chemistry Question of the Day #279: Ions sticking to calcium carbonate

Reef Chemistry Question of the Day #279 Which of the following two ions are least likely to stick to a bare calcium carbonate surface in a reef aquarium? A. Lithium B. Magnesium C. Calcium D. Strontium E. Phosphate F. Nitrate Good luck! Previous question of the day...

www.reef2reef.com

So, what is the answer?

 

[Randy Holmes-Farley]

So, what is the answer?

Didn’t give it yet. I usually wait a few days.

 

[Miami Reef]

Didn’t give it yet. I usually wait a few days.

Are we ready for the answer?

 

[BaliReefBox]

How do people have sooooo much spare time to do this when they own a reef tank..

 

[TokenReefer]

Lol I spend way more time reading about reef tanks these days than doing anything to my own

 

[Randy Holmes-Farley]

And the answer is:

Which of the following is the best analogy of ion spacing in seawater?

C. The ions in seawater are spaced like 2 ice cubes in a glass of water


As both Jim and I calculated with slightly different assumptions, the spacing between ions in seawater is about 3 water molecules (suprisingly close, IMO). That makes it closes to 2 ice cubes in a glass of water, which might be spaced out by about 3 times their own size

Reef Chemistry Question of the Day #241 How Many Water Molecules Between the Ions in Seawater?

Reef Chemistry Question of the Day #241 On average, how many water molecules are directly between one ion in seawater, and its next nearest neighbor? For example, for a given magnesium ion, how far away is the nearest other ion (sodium, calcium, chloride, sulfate, etc.) in terms of the number...

www.reef2reef.com

Happy Reefing!

 

[MnFish1]

And the answer is:

Which of the following is the best analogy of ion spacing in seawater?

C. The ions in seawater are spaced like 2 ice cubes in a glass of water


As both Jim and I calculated with slightly different assumptions, the spacing between ions in seawater is about 3 water molecules (suprisingly close, IMO). That makes it closes to 2 ice cubes in a glass of water, which might be spaced out by about 3 times their own size

Reef Chemistry Question of the Day #241 How Many Water Molecules Between the Ions in Seawater?

Reef Chemistry Question of the Day #241 On average, how many water molecules are directly between one ion in seawater, and its next nearest neighbor? For example, for a given magnesium ion, how far away is the nearest other ion (sodium, calcium, chloride, sulfate, etc.) in terms of the number...

www.reef2reef.com

Happy Reefing!

I was under the impression that: "The result: the hydration shells of the different ions contain between two and 21 water molecules. The exact number depends, for instance, on the size of the ion and its valency", based on a couple of papers? Can you explain, how you do a calculation (like you and Jim did), when you don't know the size of the ice cubes related to the size of the glass/container. In other words, how do you calculate 'how close' the ice cubes are together when you don't know their size? Once again - you've provided an interesting discussion!

 

[Randy Holmes-Farley]

I was under the impression that: "The result: the hydration shells of the different ions contain between two and 21 water molecules. The exact number depends, for instance, on the size of the ion and its valency", based on a couple of papers? Can you explain, how you do a calculation (like you and Jim did), when you don't know the size of the ice cubes related to the size of the glass/container. In other words, how do you calculate 'how close' the ice cubes are together when you don't know their size? Once again - you've provided an interesting discussion!

In this specific scenario, we were measuring ion to ion, not hydrated sphere to hydrated sphere. I counted water molecules between the ions, whether strongly interacting with the ion or not, as water between them.

The hydration sphere is typically about one water molecule thick. 21 molecules sounds like a lot but that is likely only for a very large multi-atom ion (such as sulfate, where it might have about 12) in three dimensions.

 

[Sean Clark]

The spacing of ions in seawater is equivalent to Nicholas Cage B movies among all other releases in any given year.

 

[MnFish1]

In this specific scenario, we were measuring ion to ion, not hydrated sphere to hydrated sphere. I counted water molecules between the ions, whether strongly interacting with the ion or not, as water between them.

The hydration sphere is typically about one water molecule thick. 21 molecules sounds like a lot but that is likely only for a very large multi-atom ion (such as sulfate, where it might have about 12) in three dimensions.

AHHHHHHHHHHH.... Well then.... I'll just make the point that the plane (to me) was better. Different people (different ions), I was picturing the seats as the water ions. My bad. Granted I was wrong on the cats lol