The document describes an activity where students explore how adding salt affects the behavior of nanoparticles in latex paint suspensions. When salt is added, it disrupts the electrostatic repulsion between the negatively charged nanoparticles, allowing them to aggregate together. More concentrated salt solutions cause aggregation at a faster rate. Divalent salt ions like magnesium also induce aggregation more efficiently than monovalent ions due to their ability to neutralize two surface charges at once. The goal is to understand how controlling interparticle forces through salt concentrations can be used to deposit thin nanoparticle films.

1. Exploring the Behavior of Nanoparticles
By Sabrina Powell and Brian Prevo
Target grades: 7-12
Overview:
Students will learn about the behavior of small particles in solution.
Science background:
Many newly developed technologies utilize micro- and nanoparticle coatings.
This approach to materials fabrication is known as the “bottom up” approach, because
these little particles serve as building blocks from which large structures or films are
made. These particle coatings have applications in microchips, solar panels, sensors, and
a variety of other applications.
Brian Prevo is a graduate student at North Carolina State University. His research
focuses on ways to improve the “laying down” of thin-film particle coatings. Previous
techniques for forming these coatings depended on dipping the surface to be coated in a
suspension of the particles or “pouring” on the particles. Whether dipped or poured, these
previous techniques required particular environmental conditions (temperature, humidity,
etc.) for proper thin coat formation. Brian’s research has developed a faster method of
forming these coatings. In addition, his method is less wasteful of the expensive particles,
and the process does not require particular environmental conditions. To visualize how
Brian’s deposition process works, imagine that you are trying to spread a thin layer of
paint on a glass window. By loading a paintbrush with only a small amount of paint, and
applying the paint in slow, even strokes, you will create the desired thin coating. Instead,
if you load the paintbrush with as much paint as you can, and you use quick, vigorous
strokes, the resulting coating of paint will be “gloppy.” For most technology applications,
a thin, even coat is preferable to a gloppy coat.
One of the keys to Brian’s process is suspensions or dispersions of stable colloidal
particles. The word colloid refers to small particles (10-8
– 10-5
m in diameter) dispersed
in something else, often a liquid. The overall quality of a particle film will decrease if the
particles aggregate or clump together prior to deposition. Thus, Brian had to learn about
how particles in liquid suspensions behave in different conditions in order to optimize his
coating process. Today you will get to learn some of what Brian studied during his
graduate work. It is closely related to a theory known as the DLVO theory, named after
two teams of scientists who independently verified it: Derjaguin and Landau (in Russia),
and Verwei and Overbeek (in Holland). The theory is based on their respective
experimental observations and calculations regarding the mutual attraction of all particles
that is balanced by the mutual electrostatic repulsion intrinsic to all particles in water
solutions.
Activity:
Materials needed:
Test tubes (25 mL test tubes work well)
Test tube rack
Graduated cylinder (25-100 mL size works well)

2. Plastic disposable pipettes
Latex paint, flat or eggshell texture, preferably a light-colored paint
Distilled water (can be purchased at grocery store)
Sodium chloride (NaCl; table salt)
Sugar
Epsom salts (MgSO4•7H2O; available at drugstore)
1. Label 4 beakers or other containers with the following labels: 1.0 M salt, 0.1 M
salt, 0.01 M salt, and distilled water.
2. Prepare a series of salt solutions. For the first solution, dissolve 2.8 g salt (about
1/2 teaspoon) in 100 mL distilled water. Swirl or stir until all of the salt is
dissolved. This makes a 1.0 M (mole/liter) solution. The solution may be slightly
cloudy.
3. Take 10 mL of the 1.0 M salt solution and add to 90 mL distilled water in another
beaker. Swirl or stir to mix. This will make a 0.1 M solution.
4. Take 10 mL of the 0.1 M solution and add to 90 mL distilled water in another
beaker. Swirl or stir to mix. This will make a 0.01 M solution.
5. Fill a fourth beaker with distilled water only.
6. Pipette or pour 7.5 mL of the distilled water, 0.01 M salt, 0.1 M salt, and 1.0 M
salt solutions into test tubes. Label the test tubes with the concentration of the salt
solution they contain.
7. Latex paint is water soluble. Prepare a master paint dilution as follows: Transfer
18 mL distilled water into a test tube. Add about 2 mL of the latex paint (less is
better than more), and mix thoroughly by drawing and expelling the paint solution
into a pipette.
8. Use a pipette to transfer 2.5 mL of the master paint dilution into each of the
labeled test tubes. Use a clean pipette to mix the paint solution with the salt
solutions, making sure to rinse the pipette in distilled water between test tubes.
9. Observe the test tubes every 5 minutes for 15-20 minutes.
What do you observe? Why does this happen?
Why include a test tube with only distilled water?
Why use distilled water instead of tap water?
What do you think would happen if you used different colors of paint?
Why did this happen?
A polymer is something made of many (poly) units (mer) joined together. A
polymer is a large molecule which consists of many repeating units linked together (see
other Meet the Scientist profiles on the Science House website for more details).
Common examples of polymers include DNA, cellulose (paper or cotton), polyethylene
(plastic grocery bags), and polyester (clothes and beverage bottles).
Latex paint is a concentrated mixture of little polymer balls in water with some
surfactants (soapy molecules). These particles (the little polymer balls) are stable because
they all have the same negatively charged surface; each particle repels its neighbor (much
as like poles on magnets repel each other). As long as all particle surfaces have the same
charge, they remain evenly dispersed in water. When we added salt, we destabilized and

3. induced aggregation of the particles. The salt dissociates into positively and negatively
charged ions and prevent the particles from “seeing” each other, so they no longer repel
each other so effectively. After the salt is added, the particles can get closer and closer to
each other until eventually they stick together. Once stuck together, they become heavy
enough that they sink. We observe this precipitation as the clumps that form at the bottom
of the test tube. We tested a range of salt concentrations, but not all salt concentrations
were high enough to induce precipitation.
Extension 1:
Sugar (sucrose) is another white crystalline solid that dissolves quickly in water.
The chemical formula of sugar is C6H12O6, and sugar molecules are negatively charged
when they dissolve in water. Would you expect sugar to precipitate the latex
microspheres?
1. Make a sugar solution by dissolving 2.4 g (about 1/2 teaspoon) sugar in 100 mL
distilled water. As above, add 7.5 mL sugar solution to a labeled test tube, add 2.5
mL of the master paint dilution, and mix well with a clean pipette. Label the test
tube.
2. For comparison, make 2 additional tubes: (1) put 2.5 mL of the paint dilution into
7.5 mL distilled water, and (2) put 2.5 mL of the paint dilution into 7.5 mL of the
1.0 M sodium chloride solution. Observe the test tubes every 5 minutes for 15-20
minutes.
Does sugar precipitate the latex microspheres as well as salt does? Why do you think this
is so?
Extension 2:
Divalent salts have charges of greater than +1 or -1. Magnesium sulfate
(MgSO4•7H2O), also known as Epsom salts, is an example of a divalent salt. The
magnesium has a +2 charge, while the sulfate group has a –2 charge. Will divalent
cations precipitate latex microspheres any quicker or more efficiently than monovalent
cations? Predict what you think will happen.
1. Make a dilution series of 1.0 M, 0.1 M, and 0.01 M Epsom salts in distilled water.
The 1.0 M solution can be made by dissolving 24.6 g Epsom salts (about 2.5
tablespoons) in 100 mL distilled water. Dilute as described in steps 3-5 of the
instructions above. Note: the Epsom salts may take 5-10 minutes to go into
solution. Keep stirring until they all dissolve.
2. Repeat the experiment described above (steps 5-9) with both the sodium chloride
and the Epsom salts. Observe the test tubes every 5 minutes for 15-20 minutes.
Compare the precipitation of the latex microspheres by each solution and at each
concentration.
Did the divalent cations (Epsom salts) precipitate the latex microspheres any quicker or
more efficiently than the monovalent cations (sodium chloride)?
Why did this happen?

4. The +2 charged positive magnesium ions in the Epsom salts can occupy two
surface charges on a latex microsphere. Aggregation of the microspheres can occur at
significantly lower salt concentrations. This effect is referred to as ionic strength, which
is both a function of the salt concentration and the valency (e.g. charge, z) of an ionic
species contained in a given salt species.
Teaching notes:
•The teacher can prepare the salt solutions ahead of time, or students can prepare the salt
solutions themselves.
•In preliminary tests, glossy paint did not work well with this activity. Use flat or
eggshell latex paint instead.
•Inexpensive graduated plastic disposable pipettes that work well for this activity are
available from Carolina Biological Supply (www.carolina.com, catalog# 73-6986).
•Latex paint, the salt solutions, and the sugar solution can be washed down the drain. It is
recommended that you clean up this activity before the paint dries.
•If you have a camera with a flash, students could attempt time-lapse photography of the
precipitation: leaving the camera stationary, take a picture of the test tubes every 2-3
minutes as the latex microspheres precipitate.
References:
•http://en.wikipedia.org/wiki/Rubber
•Look at the chapters on solutions and reactions in a chemistry textbook.

5. Student Worksheet
Name__________________________________________________________
Objective: This lab examines the behavior of small particles in solution.
Background:
Latex paint is a concentrated mixture of little polymer balls in water with some
surfactants (soapy molecules). The surface of the little polymer balls have a negative
charge. In this lab, we will add salts to latex paint. We will examine the effect of the salt
concentration on the behavior of the polymers in the latex paint.
Activity:
1. Label 4 beakers or other containers with the following labels: 1.0 M salt, 0.1 M
salt, 0.01 M salt, and distilled water.
2. Prepare a series of salt solutions. For the first solution, dissolve 2.8 g salt (about
1/2 teaspoon) in 100 mL distilled water. Swirl or stir until all of the salt is
dissolved. This makes a 1.0 M (mole/liter) solution.
3. Take 10 mL of the 1.0 M salt solution and add to 90 mL distilled water in another
beaker. Swirl or stir to mix. This will make a 0.1 M solution.
4. Take 10 mL of the 0.1 M solution and add to 90 mL distilled water in another
beaker. Swirl or stir to mix. This will make a 0.01 M solution.
5. Fill a fourth beaker with distilled water only.
6. Pipette or pour 7.5 mL of the distilled water, 0.01 M salt, 0.1 M salt, and 1.0 M
salt solutions into test tubes. Label the test tubes with the concentration of the salt
solution they contain.
7. Latex paint is water soluble. Prepare a master paint dilution as follows: Transfer
18 mL distilled water into a test tube. Add about 2 mL of the latex paint (less is
better than more), and mix thoroughly by drawing and expelling the paint solution
into a pipette.
8. Use a pipette to transfer 2.5 mL of the master paint dilution into each of the
labeled test tubes. Use a clean pipette to mix the paint solution with the salt
solutions, making sure to rinse the pipette in distilled water between test tubes.
9. Observe the test tubes every 5 minutes for 15-20 minutes.
What do you observe? _____________________________________________________
_______________________________________________________________________
Why does this happen?_____________________________________________________
_______________________________________________________________________
Why include a test tube with only distilled water?________________________________
_______________________________________________________________________

6. Why use distilled water instead of tap water?___________________________________
_______________________________________________________________________
What do you think would happen if you used different colors of paint?_______________
_______________________________________________________________________
Extension 1:
Sugar (sucrose) is another white crystalline solid that dissolves quickly in water.
The chemical formula of sugar is C6H12O6, and sugar molecules are negatively charged
when they dissolve in water.
Would you expect sugar to precipitate the latex microspheres? _____________________
1. Make a sugar solution by dissolving 2.4 g (about 1/2 teaspoon) sugar in 100 mL
distilled water. As above, add 7.5 mL sugar solution to a labeled test tube, add 2.5
mL of the master paint dilution, and mix well with a clean pipette. Label the test
tube.
2. For comparison, make 2 additional tubes: (1) put 2.5 mL of the paint dilution into
7.5 mL distilled water, and (2) put 2.5 mL of the paint dilution into 7.5 mL of the
1.0 M sodium chloride solution. Observe the test tubes every 5 minutes for 15-20
minutes.
Does sugar precipitate the latex microspheres as well as salt does? __________________
Why do you think this is so?
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
Extension 2:
Divalent salts have charges of greater than +1 or -1. Magnesium sulfate
(MgSO4•7H2O), also known as Epsom salts, is an example of a divalent salt. The
magnesium has a +2 charge, while the sulfate group has a –2 charge. Will divalent
cations precipitate latex microspheres any quicker or more efficiently than monovalent
cations?
Predict what you think will happen. __________________________________________
_______________________________________________________________________

7. 1. Make a dilution series of 1.0 M, 0.1 M, and 0.01 M Epsom salts in distilled water.
The 1.0 M solution can be made by dissolving 24.6 g Epsom salts (about 2.5
tablespoons) in 100 mL distilled water. Dilute as described in steps 3-5 of the
instructions above. Note: the Epsom salts may take 10-15 minutes to go into
solution. Keep stirring every few minutes until they all dissolve.
2. Repeat the experiment described above (steps 5-9) with both the sodium chloride
and the Epsom salts. Observe the test tubes every 5 minutes for 15-20 minutes.
Compare the precipitation of the latex microspheres by each solution and at each
concentration.
Did the divalent cations (Epsom salts) precipitate the latex microspheres any quicker or
more efficiently than the monovalent cations (sodium chloride)? ___________________
Why?__________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________

9. For the figures above, the dotted circular bounding regions indicate the interaction region
at which point two particles begin to ‘see’ or ‘feel’ each other. At low salt concentrations
they stay far apart, while at high salt concentrations they are shielded from each other and
don’t ‘feel’ each other until they are very near.
Interpreting this graph: Each curve represents the mutual repulsion and attraction
between two particles of similar surface charge in water of a given salt concentration.
Negative values mean attraction and positive values indicate repulsion. These curves
show that as particles are get closer together, at low salt concentrations they exhibit
strong repulsion so they are most likely not going to aggregate. Although, if by some
chance the particles do touch, then they will have strong attraction. This means that in

10. normal situations and temperatures the particles are stable, but as salt concentrations
increase the strength of electrostatic repulsion decreases, and eventually at sufficiently
high concentrations of salt, all repulsion is shielded and there is only attraction, the
particles will aggregate and settle out of suspension.
Combined image: