Polystyrene, a common plastic in items like disposable cups and protective packaging, interacts uniquely with acetone. When these two substances meet, solid polystyrene appears to vanish or transform into a gooey residue. This observation sparks curiosity about the underlying chemical principles. Understanding this interaction involves delving into the molecular characteristics of both polystyrene and acetone.
The Principle of Dissolution
The ability of one substance to dissolve another is governed by the principle of “like dissolves like.” This means substances with similar molecular properties, particularly polarity, tend to dissolve in each other. Molecules are categorized as polar or nonpolar based on their electrical charge distribution. Polar molecules have an uneven electron distribution, creating regions of partial positive and negative charge. Nonpolar molecules have a more even electron distribution, lacking distinct charged poles.
Dissolution occurs when attractive forces between solvent and solute molecules are comparable to forces within the solute and solvent. For example, water, a polar solvent, dissolves other polar substances like salt because their molecular attractions overcome the internal forces holding salt crystals together. Water does not mix with nonpolar substances like oil, as the strong attraction between water molecules is not overcome by weaker attractions to oil molecules.
Polystyrene’s Molecular Structure
Polystyrene is a synthetic polymer composed of repeating styrene monomers linked into long chains. These monomers primarily consist of carbon and hydrogen atoms, arranged in a hydrocarbon backbone with a benzene ring attached to every other carbon atom.
The chemical bonds within polystyrene are predominantly nonpolar carbon-hydrogen bonds. While benzene rings contain some electron delocalization, the overall structure of polystyrene is largely nonpolar. This explains why polystyrene does not dissolve in polar solvents like water; the attractive forces between polystyrene chains are weaker than the strong attractions within water molecules, preventing separation.
Acetone’s Molecular Structure and Solvent Action
Acetone, also known as propanone (CH₃COCH₃), is a common organic solvent used in applications like nail polish remover. Its molecular structure features a central carbon atom double-bonded to an oxygen atom, forming a carbonyl group (C=O), flanked by two methyl (CH₃) groups.
The oxygen atom in the carbonyl group is more electronegative than carbon, pulling electrons towards itself and creating partial negative and positive charges. This uneven electron distribution makes the carbonyl group polar, giving acetone a dipole moment. However, the two methyl groups, composed of carbon and hydrogen, contribute nonpolar characteristics. This dual nature, with both a polar segment and larger nonpolar regions, allows acetone to interact effectively with a wide range of substances.
The Dissolution Process Explained
When polystyrene contacts acetone, acetone molecules interact with the long polystyrene chains. The solvent molecules, with their mixed polar and nonpolar characteristics, overcome the weaker intermolecular forces, primarily van der Waals forces, holding the polystyrene chains together. This is a physical process, not a chemical reaction where new substances are formed. Acetone molecules effectively penetrate and surround the polystyrene chains.
As acetone molecules wedge between the polystyrene chains, they disrupt existing attractions within the solid plastic. New, favorable intermolecular attractions form between acetone molecules and polystyrene chains. This allows the polymer chains to separate and disperse throughout the acetone, leading to dissolution. For foamed polystyrene, such as Styrofoam, the dramatic shrinking observed is largely due to the release of trapped air (about 95% of its volume) as the solid plastic structure collapses into a more compact, viscous gel.
Practical Applications and Safety Considerations
Acetone’s ability to dissolve polystyrene has several practical applications. The resulting mixture can act as an effective adhesive for bonding polystyrene components, used in plastic modeling or for repairing items. Acetone is also a common laboratory solvent for cleaning glassware, dissolving many residues.
Despite its utility, acetone requires careful handling due to safety concerns. It is highly flammable, and its vapors pose a fire hazard, so it must be kept away from ignition sources. Acetone can cause eye and skin irritation, and prolonged exposure can lead to dryness by removing natural oils from the skin. Therefore, use acetone in well-ventilated areas and wear appropriate personal protective equipment, such as chemical-resistant gloves and eye protection, to minimize exposure.