Well you cannot.

Dear Zahra Damirchi , I´m afraid you cannot prepare a solution of bPEI 10 MN from a bPEI 60 MN.

My advice is to order the right bPEI 10 MN and once you get it make the 75mg/ml solution you need. I know that means to spent more money and waiting more time, but it is the shortest path. May be you could save some time and money asking if some other group or lab at your university have the bPEI 10 MN, they probably will give you enough to prepare a small volume of your solution. Another option would be to explain the mistake to your local chemical provider of bPEI and if the wrong bPEI bottle is not open they may be could replace it by the right one, paying the price difference.

1) Why can´t you prepare the 75mg/ml of bPEI 10MN solution from bPEI 60MN??

It looks like weird at first sight because both polymers are polyethyleneimine (PEI) both are branched versions of that polymer (bPEI). What makes they different are their average molecular weight. Generally speaking, polymers are mistures formed by the same kind of polymer chain (linear or branched) but with different sizes and proportions of each size chain, that is known as polydispersity. Polydispersity happens naturally while they are synthesized. This makes complicated to define a molecular weight, it will be easy if your polymer was formed by just chains of the same size, but that is not the case, so we have to use statistics to get a value, an average value for describing the molecular weight of the polymer.

Both the distribution shape (how many molecules of each size there are in your polymer bulk) and the average molecular weight influence the properties of a polymer. And that has a lot to do with your 2) point and observations about the viscosity of bPEI 60MN.

There are different ways to calculate the average molecular weight of a polymer, but always the molecular weight of apolymer must be calculated from the molecular weights of all the chains in the sample and their proportions in the sample.

One of these average molecular weight is the Number Average Molecular Weight (Mn or MN) is the statistical average molecular weight of all the polymer chains in the sample and is defined by the sum of the number of chains of length i (Ni) times the molecular weight of the chains of length i (Mi), divided by the sum of all the chains in the sample .

Let me draw a simple example:

Imagine this hyphen - represents a monomer of your polymer, with a molecular weight of 1, and that you have a polymer bulk sample with the following distribution of length chains:

---

---

----

----

----

----

-----

------

------

-------

So you have

2 chains of 3 monomers or Ni=N3=2, Mi= M3=3

4 chains of 4 monomers or Ni=N4=4, Mi=M4=4

1 chain of 5 monomers or Ni=N5=1, Mi=M5=5

2 chains of 6 monomers or Ni=N6=2, Mi=M6=6

1 chain of 7 monomers or Ni=N7=1, Mi=M7=1

and therefore the Number average molecular weight, Mn will be:

Mn= (2x3)+(4x4)+(1x5)+(2x6)+(1x7)/(2+4+1+2+1)

Mn=46/10= 4.6

There are not chains of size 4.6 in your polymer, you cannot cut a monomer and add 0.6 of it to a chain of 4 monomers. This number is just an average, but someway it describes your polymer bulk, where the more abundant chain length is 4 and despite there are chains of 3 they are less than the chains of 5, 6 and 7 monomers, so the number average molecular weight is bigger than the molecular weight of the more common chain length.

Now imagine another bulk sample of the same polymer, this time with the following distribution of size lengths:

---

----

------

------

----------

----------

----------

----------------

----------------

---------------------

1 chain of 3 monomers or Ni=N3=1, Mi=M3=3

1 chain of 4 monomers or Ni=N4=1, Mi=M4=4

2 chains of 6 monomers or Ni=N6=2, Mi=M6=6

3 chains of 10 monomers or Ni=N10=3, Mi=M10=10

2 chains of 15 monomers or Ni=N15=2, Mi=M15=15

1 chain of 20 monomers or Ni=N20= 1, Mi=M20=20

Mn= (1x3)+(1x4)+(2x6)+(3x10)+(2x15)+(1x20)/ (1+1+2+3+2+1)

Mn= 99/10=9.9

Note that both samples have 10 polymer chains but the second sample have chains longer than the first sample, so its number average molecular weight should be larger than the first sample. Once again, the Mn value is telling us that chains around 10 monomers are the more common in this sample.

So the average size in the first case is 4.6 and the second is 9.9, so almost the double.

In your case bPEI 10Mn compared with bPEI 60Mn would be 6 times bigger chains in the bPEI 60Mn, that longer chains make the misture thicker than the bPEI 10Mn. In this case, because the PEI is branched, and not linear, causes a stronger viscosity.

Back to your question 1) can you prepare your bPEI 10Mn from bPEI 60Mn?

as you can see they are completely different and the only way to prepare it would mean to separate the right amount and the right sizes of polymer chains of bPEI 60Mn to get a bPEI 10Mn, what is experimentally difficult and costly and probably impossible if some short chain lengthd are not present in the bPEI 60Mn.

Note 1: probably your bPEI are not bPEI 10Mn and bPEI 60Mn but bPEI 10,000 MN and bPEI 60,000 MN, given that the molecular weight of the monomer (43.07).

Note 2: probably your bPEI 60 MN, 50wt is bPEI 60,000 Mn, 50wt% aq. solution from Thermo Scientific Chemicals, selled by Fisher Scientific:

https://www.fishersci.se/shop/products/polyethyleneimine-m-n-60-000-50-wt-aq-solution-branched-thermo-scientific/10720881

This note 2 brings us back to your 2) question: Can you dissolve bPEI 60,000 Mn 50wt% aqueous solution in water?

So your bPEI 60Mn is already dissolved in water, actually half of its weight is just water. Fisher sells 100g, 250g and 1kg bottles, so for every 100g of product you have 50g of bPEI 60,000Mn and 50g of water.

Adding more water should be possible and doing so the mixture would be more fluid, but that is not changing at all the fact that you cannot use this mixture to make your 75mg/ml solution of bPEI 10MN from this bPEI 60Mn.

So be sure of the bPEI you need, probably bPEI 10,000Mn instead of bPEI 10Mn. Order it from your provider or try to return the wrong one * or well try to borrow the small amount you need to prepare your solution 75mg/ml from another lab.

* From the previous link, Fisher says this item is not returnable, but may be if the bottle was not opened and you bought it from a local provider, may be they could take the wrong one and give you the right one.

Hope this helps, good luck with your research!

Best,

Manuel

Dear Manuel Gomez,

Thank you very much for your complete answer.

I need PeI for amine modification of my silica nanoparticles. So,now I am trying to substitute the bPEI 60000 to bPEI 10000 with loweing the concentration

(I dividend the concentration to 6).

Thanks for your comment.Buying bPEI 10000,is my last choice.

Hi Zahra,

You´re welcome. I hope the polymer change will work for your experiments. Good luck.

Regards,

Manuel

It depends on how outside the box you might want to try. First is a real shame you are using branched PEI. I looked up the easier to work with linear PEI, which obviously you are not using but it claims PEI (assuming linear polyethyleneimine) is soluble in hot water, at low pH, in methanol, ethanol, or chloroform. Now I used to make solutions where I had two components that would not mix. I however added acetone and was able to dissolve both into acetone. Then using low vacuum and a magnetic stir place I removed the acetone leaving a nicely mixed mixture of A and B. Now acetone for you is out of the question. But if you can use something that high a higher vapor pressure then maybe you could mess around with a small side project of trying to play with your concentration. I agree with the others, the correct answer is get the correct solution first so go buy the good stuff. But you might find a bit of play time to see what you can do with the other solution that might sit on your shelf for years of no use or you might find another use that you don't have to mess around to get the right concentration. Just wanted to share a bit of an idea that might or might not help in a brain storm with you. But buy the right stuff. Good luck.