PHYS110 First Year Seminar in Physics

Biological Physics

Lab 6: Crystal Growing – a competition

Intro

Growing quality crystals is important in semiconductor industry, pharmaceutical

industry, physics, chemistry, biochemistry and other sciences and industries. For

example, the processor of your computer has been manufactured lithographically on a

silicon wafer, a high quality slab of single crystal silicon. Crystals are also useful

because they diffract x-rays. X-ray diffraction is a method for molecular structure

determination with an atomic resolution –not only of small inorganic molecules, but also

of large macromolecules such as proteins and nucleic acids. Pharmaceutical companies

depend on crystallographers to grow their crystals of their proteins so that they can study,

for example, how particular compounds bind to active sites of enzymes.

In this lab, you will try your hand at growing crystals. This lab will not be graded, but

bonus points may be awarded as described in the syllabus.

Physics of Growth

For crystals to grow, compound must first be dissolved into solution. For example, if you

add some sugar to your coffee, it might dissolve entirely, but if you keep adding large

amounts, at some point no more sugar will dissolve and the sugar will simply remain

solid. When the solubility limit is reached, the solution is said to be saturated. The only

way to grow a crystal is to dissolve more solute into the solution than the saturation limit.

This may seem like a contradiction in terms at first – how can you dissolve more

compound than allowed by the solubility limit? The resolution is that the “solubility

limit” refers to a state of the solute-solution system in equilibrium. It is possible to

dissolve more sugar into your coffee (very carefully) than allowed by the solubility limit,

with the understanding that eventually this sugar will precipitate out, once the

equilibrium is reached after some time. When more compound is dissolved than allowed

by the (thermodynamic) solubility limit, the solution is said to be supersaturated – this

state of the system is said to be out of equilibrium. If you push the system far out of

equilibrium, the solute will precipitate violently and the equilibrium will be reached

quickly.

The trick to successful crystal growth is to carefully supersaturate the solution, but not

too much, else the compound precipitates quickly in a haphazard fashion, forming tiny

polycrystalline crystals or powder. When you supersaturate the solution carefully, so that

it does not precipitate instantly, the solution is said to be metastable. So the idea is first

to make a solution metastable, and later to push the system slowly far out of equilibrium

to get the crystal growing.

An example of a solubility diagram. The solid line is the solubility line for a sample compound. The dotted line is the metastable line for the compound.

Saturation Control A solubility diagram can help you establish the path (not shown) that you might take as

you push your system to supersaturation. As shown in the sample figure above, solubility

for most compounds increases with rising temperature. To learn how to control

supersaturation, consider the following example. If you dissolve 225 g of the compound

in 100 g of water at about 65 °C, your compound will be entirely soluble (see point C on

the diagram). To bring your system into a metastable state, you need to “push it” toward

supersaturation along a path on the diagram, for example to point A or point B. To get to

point A, you would need to dump more solute into the solution (to increase the

concentration), or allow the water to evaporate (also increases concentration).

Alternatively, you could go to point B by cooling the solution to 10 °C. Or you could

choose any other path by cooling, heating, and evaporating the solution all at the same

time. How you want to control your supersaturation I leave up to you.

Procedure Choose to grow a crystal of copper II sulfate, potassium ferricyanide, potassium

ferrocyanide, potassium alum, chrome alum, or magnesium sulfate. The solubility of

these compounds at about room temperature is written on some bottles, and for others,

you will have to find it out from other sources.

The following recipe is not guaranteed to work. You may need to experiment to optimize

your protocol

1. Weight out about 30% more of the compound than its solubility and place it into a

plastic vial. Pour in water to 100mL.

2. Sonicate at temperature 30°C, or so, until most of it dissolves (20-30 minutes).

3. Allow the solution to cool to room temperature. (This will make the solution

supersaturated.)

4. Transfer 1/3 of this solution to a glass jar or another empty plastic bottle. Be

careful not to transfer any solids, just the liquid from top. Save the capped

leftover 2/3 solution.

5. Allow the 1/3 solution to evaporate for several days, until crystals appear at the

bottom of the jar. You may need to check on the progress daily.

6. Take one crystal out and tie the crystal seed to a nylon thread (or dental floss).

See Figure 1 on page 6 below.

7. Insert the seed into 2/3 solution and suspend the thread above the bottle opening.

See Figure 2 on page 6 below. Leave open to evaporate and wait another few

days.

Some Tips:

Growing crystals is often regarded as an “art” rather than an exact science for several

reasons. One, there are many paths on the solubility diagram you could take to

supersaturate your solution, and the outcome of the growth often depends on the path.

Two, the speed at which your solute-solution system moves on a path matters: move too

quickly and you will get powder; move too slowly and you will never get crystals. Three,

the labile region is an “unstable” equilibrium for your system, if you cross into the

region, the “house of cards” suddenly falls. So how close you are to the dashed line in

the diagram also matters: be too close and you get powder; be too far and you will never

get crystals. In fact, sometimes if you are too close to the labile region, it only takes a

tap, or a scratch on a beaker and the powder will fall out. Four, the purity of your

compound and the solution matters too. The presence of foreign molecules in the

solution increases the likelihood of growth defects in your crystal.

So as you grow your crystals, keep careful record of how much salt you have dissolved

and how much water evaporates (e.g. by weighing your flask). Do not unnecessarily

jostle your solution as you make your observations, and try keeping the solution at

constant temperature (e.g. by placing it in a Styrofoam box).