Thin Film Formed by Solvent Evaporation

FIGURE 8.22 A solid may be prepared by dissolving it in an organic solvent, placing it on a salt crystal (left), and allowing the solvent to evaporate such that the film of the solid is cast on the surface of the salt plate, which can then be placed in the path of the light (right). 

So-called disposable IR cards are also available for this. Such cards have polyethylene windows on which the sample solution may be applied and the solvent evaporated. Unless they are subtracted out, IR spectra from such cards would show the polyethylene absorption bands. 

The solvent evaporation method is especially useful for polymers that can be cast onto the surface of the salt plate. Casting involves dissolving the polymer in an appropriate solvent at an elevated temperature and evaporating the solvent on the salt plate by heating the salt plate on a hot plate or under a heat lamp. 

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