Just in time for Halloween, we'll make some slime. Everyone has a
feeling about how slime should act; of course, it should be slimey! It
should stick togther and drip, but it should not come apart. So there
should be something about slime that keeps it stuck together, but still
allows it to move and sag. The principle of slime is in the
cross-linking, or connecting, of molecules that are long "chains" to
begin with. Gelatin (the stuff used to make Jello) is a good example of
a long chain molecule. In this case, the
cross-linking agent is heating/dissolving in water, followed by
cooling. The water used in the mixing of Jello is
suspended within a matrix of cross-linked gelatin molecules, and does
not easily evaporate as it would if it were simply sitting in a cup.
Another useful long chain is Poly Vinyl Acetate, contained in
conventional "white glue" or "Elmer's" glue. In this case, a chemical
additive - simple, supermarket-style "Borax" (sodium
tetraborate decahydrate, Na2B4O7.10H2O)
- serves as the cross-linking agent. And again, water is entrapped, or
suspended within the matrix of cross-linked
Poly Vinyl Acetate molecules.
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(photo: see reference 6)
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Steps to Slime
1. Thoroughly mix ½ cup of water and ½ cup of white glue
("Elmer's School Glue")
in a tall paper or plastic cup.
2. Add a few drops of food coloring to the water/glue mixture, again
mixing thoroughly. This will be your "Glue Solution".
3. In a separate cup, mix 1/2 cup of water and 1/2 teaspoon of
"Borax". The objective is to make a saturated Borax solution, that is,
one in which the Borax will not continue to dissolve after
mixing. This will
be
your "Borax Solution". See the notes below in the "chemistry"
section regarding the borax solution.
4. Now you are ready to slimify. Get ready to pour a small amount of
Borax Solution into your Glue Solution, being prepared to mix the two
very rapidly with a big flat tongue depressor or similar flat stirrer.
5. For very firm slime, add a large amount of Borax Solution to your
Glue Solution. For drippy slime, add a small amount of Borax Solution
to the Glue Solution and mix very rapidly and thoroughly.
Some Observations
1. Place the slime in a bowl and let it sit overnight. It
will most likely be a lumpy mess when you place the slime in the bowl,
but by morning, the slime will have settled nicely into the bowl,
forming a level surface, with a collection of surface bubbles from air
that was
trapped within the slime when it was mixed.
2. Gently lay a fork (prongs down) onto the surface of the
slime. In several hours, the fork will have sunk into the
slime, and the slime will encapsulate the fork. The fork will
tear the slime if it is removed rapidly. Similarly, if pulled
slowly, slime can be stretched into thin sheets or
strings. But if pulled quickly, the slime can be forced to break
or fracture along a relatively flat plane.
3. The biochemist in our group contends that the cross-linking
reaction is endothermic; that is, that the linking reaction sucks
energy from the surrounding volume of slime and making the slime cool
to the touch. However, the physicist in our group points out that
the whole blob of slime is largely water, imperfectly trapped within a
matrix of polymer, so water evaporation goes on continuously over the
surface of the slime, cooling the surface. Additionally, because
of the high water content of the slime, the thermal conductivity of the
slime is essentially that of water, and feels cool to the touch, even
if it is the same temperature as the ambient. So, like good
scientists, we'll have to
get out our thermometer and actually TEST the temperature of the slime
to see if it ended up at a lower temperature than when it started.
Some Chemistry
Now that we've made a mess, let's see if there is some science
behind all of this. The chemistry that we'll describe is
complicated, but some of our readers will want to read about all of
these
details. What is nice about this is that there is a little bit of
science for younger children, while there is also some considerable
detail left open for study by even the most advanced chemists.
None of us are organic chemists, so we're depending
on the literature and the web for accurate information, hopefully
derived from more-or-less primary sources. There is a lot of
"vapor" on the web, so we've tried to filter out most of it in our
search. We would appreciate corrections and comments from our
readers.
First we
see that we've been a little lax about our nomenclature. The
long-chain, cross-linked polymer that we've made from PolyVinyl Acetate is actually called "Gluep"
or "Gack", while the polymer made from Poly Vinyl Alcohol is called "Slime".
Volumes have been written about the mechanism
responsible for the cross-linking of PVAlcohol, while the web is
substantially less complete about the details of the chemistry
responsible for cross-linking PVAcetate.
Cross-Linking PolyVinyl
Alcohol
So we start with the
single Vinyl Alcohol molecule (called an alcohol by the addition of the
hydroxyl group OH- to the vinyl group CH2-CH)
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and string a bunch of them together to form Poly Vinyl Alcohol.
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Now we have some polymer ready for cross-linking. Our objective
will be to add something that will bind several of these
long molecules together side-by-side, something like the rungs of a
ladder. We start by dissolving some Borax (Na2B4O7.10H2O)
in water, which dissociates into sodium ions and tetraborate
ions. The tetraborate ion reacts with water (hydrolyzes) to
produce boric acid and the
OH- ion.
B4O7-2(aq) + 7 H2O
<—> 4 H3BO3(aq) + 2 OH-(aq)
The boric acid further reacts with water to form the
borate anion.
H3BO3(aq) + 2 H2O
<— > B(OH)4-(aq) + H3O+(aq)
So now we have the form of the borate ion that we
need. The B(OH)4– ion is shaped like
a tetrahedron (four sides, each side is an equilateral triangle) with
the boron in the center and the OH groups at each corner. Hydrogen bonds
form between the borate ion and the OH groups on the
sides of the Poly Vinyl Alcohol.
These bonds are not very strong, and are broken and reformed easily,
allowing the slime to sag and move slowly under stress. A
particularly good picture is available in Reference 3.
Let's study this scrambled mess. The green borate ion is in the
middle, the PVA chains are on the outside and are bent and twisted,
water molecules are dispersed randomly throughout the assembly and
dotted blue lines (hydrogen bonds) make connections between Oxygen and
Hydrogen, especially to those Oxygen and Hydrogen that are paired up
and sticking out the
sides of the PVA chains. There are some important features in
this
picture:
a) the polymer chains distort to allow these hydrogen bonds to
occur, and
b) water molecules dispersed throughout the mess add to the
hydrogen bonding.
Reference 2 and others describe the cross-linking in a way that does
not
emphasize the importance of hydrogen bonding, and by imagining that
the
water remaining in the slime really doesn't have much of a role to
play. They
draw a simplified picture like this;
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Notice that this picture describes the bonding as if a water molecule
had simply been removed from each bond site. This treats the bond
as a covalent bond, and not the weaker hydrogen bond which we believe
is
responsible for the true nature of the relatively weak
cross-linking. This is supported by the discussion in Reference 1.
Cross-Linking
PolyVinyl
Acetate
So what about Poly Vinyl Acetate, the long chain that we've used
to make our "Gluep", "Gack" or whatever we're calling it? The
acetate groups (CH3-CO=O)
attached to the Vinyl groups make the long chains look a little
different.
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Resources on the web are very vague about what happens to the Poly
Vinyl Acetate to allow cross-linking. At least one reference (see
Ref. 5) indicates
that the Poly Vinyl Acetate (Elmer's glue) in water tends to lose some
of the acetate groups into solution, only to have these groups replaced
by O-H groups as in Poly Vinyl Alcohol. Then, cross-linking
occcurs in the same way as before. However, if this is the case,
then Slime and Gluep should be pretty similar to each other. But
other sources point out
that the borate ion in the middle of everything is still charged, and
that adding acid to the cross-linked polymer can pull the borate
out of the links (the hydronium ion attracts the borate ion) and
dissolve the slime back into an aqueous PVA
solution again. So we're going to have to do some experiments to
find out what is going on.
Note About the Borax
Solution.
At room temperature, a saturated solution is about 4% by weight of
Borax to water (the density of Borax is about 1.7 gm/ml; see Ref.
4). As the
temperature rises, the amount of Borax
that can
be dissolved in water will increase. Heating the water (below the
boiling point!) will help the borax to
dissolve, but make sure you allow it to cool before using it. As the
temperature falls, excess Borax will precipiate out of the solution and
sit on the bottom of the cup. Use the saturated solution for your
experiments and leave the undissolved crystals sitting on the bottom of
the cup.
References
Please
e-mail us if you cannot find these references.
1. Casassa, E.Z., Sarquis, A.M., and Van Dyke, C.H., "The
Gelation of Polyvinyl Alcohol with Borax," Journal of Chemical
Education, Vol. 63, #1, January, 1986, p.57-60.
2. www.rohmhaas.com/company/plabs.dir/htmldocs/itstheslime.html
3. www.madsci.org/posts/archives/2003-12/1070294201.Ch.r.html
4. www.borax.com/pdfs/dist/Profile_Borax_Decahydrate.pdf
5. www.chemistry.lsu.edu/webpub/demo-2-silly-putty.pdf
6. www.psrc.usm.edu/org/Polymer%20Demos/Pdf/slime.pdf
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