Cristalização

Crystallisation Techniques

Crystal Quality

The most promising crystals are transparent and sharp edged with the preferred dimensions 0.1 to 0.4mm. Acceptable crystals may be produced serendipitously from the preparative route. The first visual inspection is also used to check the consistency of the sample. Different colours or shapes of crystal may indicate unreacted starting material or by-products and more information about the preparative method may be needed. If the sample is not of sufficient quality it may be necessary to use a different crystallisation technique.

Crystal Growing

The aim is to grow single crystals of suitable size in at least two of the three dimensions. The size of the crystals can be influenced by a number of factors, for example the solubility of the sample in the chosen solvent, the number of nucleation sites and time. If possible a solvent should be chosen in which the sample is moderately soluble. The crystal-growing vessel should be clean because dust provides numerous nucleation sites and may initiate interfering crystal growth. It is important to avoid disturbance of the vessel. Vibration or frequent movement to check the sample tends to lead to poor quality crystals.

Techniques

Commonly used techniques include solvent evaporation; slow cooling of the solution, solvent/ non-solvent diffusion, vapour diffusion and sublimation and many variations on these themes. The choice of technique may be dictated by the amount of sample.

Solvent Evaporation

This is the simplest technique for air stable samples. A near saturated solution is prepared in a suitable solvent. The sample can then be left in a sample vial that has a perforated cap. The size of the perforations is an experimental variable that depends to some extent on the volatility of the sample. It is desirable to incline the tube so that some of the crystals grow on the side of the tube. This facilitates easier removal of delicate crystals without damage. Other variations on this method are to transfer some of the solution to a crystallisation dish and cover with perforated Al foil, or to trap some of the solvent between microscope slides.

Slow Cooling

This is good for less soluble solute-solvent systems where the boiling point of the solvent is in the range 30 - 90 0C. Prepare a saturated solution where the solvent is heated to just below the boiling point. Transfer the solution to a stoppered tube and this tube to a Dewar containing water at a temperature also just below the boiling point of the solvent (be very sure this temperature is below the boiling point.). The water level should exceed the solvent level but be below the stopper. The Dewar should be capped and left for several days.

Both the above techniques MAY be improved by using solvent mixtures. Better crystals may form or be induced where none were formed from single solvent solutions. The method may also be used to tailor crystal form. Very fine needles or extremely thin plates give poor diffraction data. Variation of solvent composition may inhibit or promote growth of particular crystal faces and hence yield crystals of suitable morphology.

Solvent Diffusion

This method is applicable to mg quantities of sample that are air and/or solvent sensitive. A solution is placed in a sample tube then a second less dense solvent is carefully dripped down the side of the tube using either a pipette or a syringe to form a discreet layer. A good solvent combination is CH2Cl2/EtOH provided the sample is near insoluble in the ether. Crystals form at the boundary where the solvents slowly diffuse.

Vapour Diffusion

This method is similar to the previous method and also applicable to mg quantities of sample. In this case a solution of the sample contained in a small sample tube is placed in a larger tube containing a second less efficient solvent and this tube then sealed. The method works best if the solution solvent is the less volatile and thus predominately the second solvent diffuses into the sample solution.

Vacuum Sublimation

A large number of compounds can be sublimed to form excellent crystals. There are numerous variations of this method using either static or dynamic vacuum. A small amount of sample sealed under vacuum in a Pyrex tube can be subject to a temperature gradient in any number of ways. A simple method that often succeeds is to put the tube on a warm oven. The small temperature gradient can be sufficient to produce crystals in hours or possibly weeks depending on the volatility of the sample and the quality of the vacuum. Dynamic vacuum works better for less volatile samples. Vacuum sublimation is ideal for very air sensitive compounds as the tubes can be loaded in dry boxes.

Growing Crystals That Will Make Your Crystallographer Happy

Paul D. Boyle

Department of Chemistry

North Carolina State University

Box 8204

Raleigh, NC, 27695

With Contributions from:

Clarence Pfluger ([e-mail]); Anthony Linden ([e-mail]); Chuck Barnes ([e-mail]); Andrea Sella ([e-mail])

Note: This document is geared toward graduate students and other researchers wanting to grow crystals suitable for X-ray structure determinations. If you are a grade school, middle school, or high school student looking for help in growing crystals for a school project, you can try reading Crystal Growing for Students first. It will probably be more helpful to you.

Contents

• The Goal

• General Considerations

o Solvent

o Nucleation

o Mechanics

o Time

• Techniques

o Slow Evaporation

o Slow Cooling

o Variations on Slow Evaporation and Slow Cooling

o Vapor Diffusion

o Solvent Diffusion

o Reactant Diffusion

o Sublimation

o Convection