Plastic baggies are ubiquitous in daily life, used for everything from food storage to waste disposal. Their widespread use often leads to curiosity about what can, or cannot, pass through their seemingly solid surfaces. Understanding how these common materials function at a molecular level reveals their practical applications and limitations.
What Does Permeable Mean?
A material is permeable if it allows liquids or gases to pass through. This property depends on the physical structure of the material, specifically interconnected pores or spaces. For instance, a coffee filter is permeable to water, allowing liquid to pass while retaining coffee grounds. A window screen is permeable to air, permitting circulation, but blocking larger elements like insects. Permeability is not absolute; it always relates to the specific substance attempting to pass through.
What Does Selectively Permeable Mean?
Building upon the concept of permeability, a selectively permeable material permits the passage of some substances while blocking others. This selection process typically relies on specific criteria, such as the size, electrical charge, or other molecular properties. In biological systems, cell membranes serve as prime examples of selectively permeable barriers. They allow essential molecules like water and nutrients to enter a cell while preventing larger or potentially harmful substances from passing. This selective passage can occur passively, determined by the physical dimensions of pores, allowing smaller molecules to slip through.
Is a Plastic Baggie Selectively Permeable?
A typical plastic baggie, often made from polyethylene, exhibits selective permeability in a physical sense, though not through active biological mechanisms like cell membranes. Polyethylene’s molecular structure consists of long chains packed together, forming a dense but not entirely impenetrable barrier. Microscopic gaps or free volume exist between the polymer chains.
Small gas molecules, such as oxygen, carbon dioxide, and water vapor, can diffuse through these tiny spaces. Low-density polyethylene (LDPE), commonly used for plastic bags, has relatively high permeability to oxygen and carbon dioxide, while its water vapor permeability is comparatively low. This explains why items stored in a “sealed” bag can still experience changes over time due to gas exchange.
Conversely, liquid water molecules, which are cohesive and form larger structures, generally cannot pass through these microscopic gaps, and surface tension further prevents their easy passage. Larger molecules, like starch, most solutes, and bacteria are too big to permeate the plastic. The selectivity of a plastic baggie is based on molecular size, physical state (gas versus liquid), and chemical affinity, rather than an active “selection” by the material itself.
Everyday Effects of Baggie Permeability
The selective permeability of plastic baggies has several noticeable effects in daily life, particularly concerning food storage. Bread, for example, can still go stale in a plastic bag due to the exchange of oxygen and moisture, though the bag limits this compared to open air. Staling involves changes in moisture and the crystallization of starch molecules, which plastic bags can delay by trapping some humidity. However, this trapped moisture can also create an environment conducive to mold growth.
Fruits and vegetables also demonstrate the effects of gas exchange; they ripen or spoil through processes involving gases like ethylene and carbon dioxide, which can pass through the plastic. While plastic bags can accelerate ripening by concentrating ethylene gas, they can also trap excessive moisture, potentially leading to spoilage. Despite the passage of gases and vapor, plastic baggies effectively contain liquid water. This property makes them useful for holding liquids or protecting items from external moisture, a common reason why many people might mistakenly believe they are completely impermeable.