STEM ED/CHM Nanotechnology
Self-Assembly of Crystals
Introduction
Sodium ions (Na+1) and chloride ions (Cl-1) in solution
will self-assemble into regularly shaped crystals if water
slowly evaporates from a solution of table salt. If water
evaporates rapidly, sodium ions and chloride ions will selfassemble into less ordered structures.
Activity Goals:
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Form very small cubic crystals of sodium chloride.
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 and water
Tweezers or a probe to move crystals as they 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 a 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.
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.
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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?
Question 2: What are some other examples of the effect of the size of object on a rate
of physical or chemical change?
Perimeter to Total (P/T) and Surface Area to Volume (SA/V) Ratios
The rate at which physical and chemical changes take place usually increases as a
sample of a material is divided into smaller pieces. A Perimeter to Total Ratio (P/T) can
relate changes in the chemical or physical properties of a structure to the size of the
structure.
Number of atoms, ions or molecules on the surface of a structure = P/T Ratio
Total number of atoms, ion or molecules in the structure.
However, it is challenging to count the number of atoms, ions or molecules on the
surface of a structure and the total number of atoms, ions or molecules in the volume of
a structure. For this reason, nanoscale scientists and engineers often use a Surface
Area to Volume Ratio (SA/V) to relate changes in the chemical or physical properties of
a structure to the size of the structure.
Surface Area of an object
Volume of the object
= SA/V Ratio
The Surface Area to Volume Ratio can be used to make general statements about
the relationship between the properties of a material and the size of the pieces of a
material.
 The units that are used to measure the dimensions of an object will determine
the value of the ratio. For example, a ratio calculated when measuring
dimensions in centimeters would be different from a ratio calculated when
measuring dimension in meters.
 The unit of the ratio cannot be used to correlate ration rates with the value for
the ration. This is because the units for the ration will be cm2/cm3.
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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.
You have several options for calculating the surface area, volume and SA/V Ratio of
a NaCl crystal.
 You can use calculators.
 You 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?
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?
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?
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?
Questions 7: What can you conclude about the Surface Area to Volume Ratios for
nanoscale structures?
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