STEM ED/CHM Nanotechnology
Self-Assembly of Crystals Guide
Introduction
Sodium ions (Na+1) and chloride ions (Cl-1) in solution will self-assemble into
regularly shaped crystal lattices if water slowly evaporates from a solution of table salt.
When water evaporates rapidly, sodium ions and chloride ions self-assemble into less
ordered structures.
Activity Goals:
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Form very small cubic crystals by evaporating water from a solution of sodium
chloride.
Use a USB microscope to measure the dimensions of self-assembled sodium
chloride crystals.
Use dimensions expressed in scientific notation to compare the crystals you
grow with crystals that have nanoscale dimensions.
Materials
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Watch glasses or shallow curved glass dish
Table salt
Tweezers or a probe to move crystals that form.
Warming tray or hot plate
USB microscope and computer
Petri dish with a cover
Day One
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Make or obtain a saturated solution of sodium chloride.
Pour some of the saturate solution into two “watch glasses” or dishes with a
slightly concave surface until they are approximately half of the area of the dish
is full.
Put the dishes in a location where the water can slowly evaporate. You can also
heat the dish very gently.
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During the Next Few Days
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Make and record observations of the self-assembly of crystals.
Adjust the rate of evaporation of water as needed.
Use a magnifier and a probe to isolate some of the sodium chloride crystals that
form as the volume of water gradually decreases. Let them dry.
Store crystals in a covered petri dish. Put the dis on a piece of paper that
includes the names of people in your group.
Size Matters
There are many ways to study the effect of the size of an object on the rate at
which it will undergo a physical or chemical change. For example, you can compare the
rate of equal masses of crushed ice and large ice cubes. The crushed ice will melt much
faster that the large ice cube even there are the same number of water molecules in the
crushed ice and in the ice cube.
Question 1: Why does crushed ice melt faster than an equal mass of large ice cubes?
The crushed ice has more surface area than the large ice cube. This means that a larger
number of molecules of the crushed ice can gain thermal energy from the surrounding to
change from a solid to a liquid.
Question 2: What are some other examples of the effect of the size of object on a rate
of physical or chemical change? A stick of butter cut into small pieces will melt more
rapidly than a large piece of butter that has the same mass. Small splinters of wood will
catch for more rapidly than large pieces of wood that has the same mass.
Perimeter to Total (P/T) and Surface Area to Volume (SA/V) Ratios
The Size Matter activity on the Nanotechnology Institute web site guides students
through a study of Perimeter to Total (P/T) Ratios. In that activity, students use cards
and blocks to determine the P/T Rations for two-dimensional and three-dimensional
structures.
P can also equal the number of atoms, ions or molecules on the surface of a
structure. T can also equal the total number of atoms, ions or molecules in a structure.
However, it would be a is challenge to count or determine the number of atoms, ions or
molecules on the surface of a structure and the total number of atoms, ions or
molecules in a very small structure like a small crystal of sodium chloride.
For this reason, nanoscale scientists and engineers use a Surface Area to Volume
Ratio to reveal the dramatic change in the ratio between the number of atoms, ions or
molecules on the surface of a structure and the total number of atoms, ions or
molecules in the entire structure.
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Calculate Surface Area to Volume Ratios of NaCl Crystals.
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Use or construct a data table to record the length, width, and depth of several
sodium chloride crystals. Include columns for the volume, total surface area, and
the calculated Surface Area to Volume Ratio (SA/V) for each crystal.
Connect a USB Microscope to a computer.
Calibrate the USB microscope to determine the relationship between the
dimension of an image of an object on the computer’s monitor and the
dimension of the object on the USB’s viewing platform.
Record the dimensions (in centimeters) of small sodium chloride crystals that
you have collected.
Calculate the Surface Area to Volume Ratio (SA/V) for NaCl Crystals.
Students have several options for calculating the surface area, volume and SA/V
Ratio of a NaCl crystal.
 They can use calculators.
 They can also use an on-line Surface Area to Volume Ratio calculator at:
http://www.cod.edu/people/faculty/chenpe/sa-ratio.html
Question 3: How might a decrease in the size of a NaCl crystal affect the value for the
Surface Area to Volume Ratio for the crystal? There should be an increase in the ratio.
Question 4: How would an increase in the Surface Area to Volume Ration of NaCl crystal
affect the rate at which NaCl crystals would dissolve in water? The rate at which NaCl
dissolves should increase.
Surface Area to Volume Ratios at the Nanometer Scale:
You used a centimeter ruler to analyze the Surface Area to Volume Ratio of salt
crystals. A Surface Area to Volume Ratio can also be determined for a nanoscale
structure. As an example, a nanoscale cubic crystal has a width of 4.5 nanometers. 4.5
nanometers is equal to 4.5 x 10-9 meters.
Question 5: How is 4.5 nanometers equal be expressed in centimeters? 4.5 x 10-7 cm.
Question 6: What would be the Surface Area to Volume Ratio for a cuboid structure
that is 16.5 nanometers wide, 120.0 nanometers long and 4.5 nanometers thick? 1.82 x
106. Important Note: The units associated with the ratio (cm2/cm3) cannot be used to
mathematically calculate a rate of a chemical or physical change.
Questions 7: What can you conclude about the Surface Area to Volume Ratios for
nanoscale structures? The ratio is dramatically larger than larger scale structures.
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