Ethan Clarke
The solubility of plastic in water depends on its chemical composition. Generally, conventional plastics do not dissolve in water. However, some plastics are soluble in organic solvents. For example, Polyvinyl Chloride (PVC) is soluble in acetone, and polystyrene can be dissolved in limonene or petrol. The solubility of a plastic depends on its polarity, with polar thermoplastics being more soluble in water than non-polar plastics such as polyethylene. The carbon-to-oxygen ratio also plays a role in solubility, with polyethers containing more oxygen atoms exhibiting higher solubility. However, there are exceptions, such as POM, which has a high oxygen-to-carbon ratio but is insoluble in water.
| Characteristics | Values |
|---|---|
| Plastic solubility in water | Conventional plastics do not dissolve in water. |
| Polyvinyl alcohol (PVA) solubility in water | Insoluble unless the water is heated to near-boiling to break hydrogen bonds. |
| Starch-based plastics | Some are designed to easily dissolve in water, while others are impervious to acids, bases, and powerful organic solvents. |
| Polar thermoplastics | More easily dissolved in water. |
| Non-polar plastics | Do not dissolve easily. |
| Polypropylene | Hard to dissolve in organic solvents at room temperature. |
| Polypropylene solvents | Xylenes (or dimethylbenzenes) can dissolve polypropylene, but not at room temperature. |
| Polyethylene glycol (PEG) solubility in water | Soluble in water due to the presence of slightly negatively charged oxygen atoms. |
| Polyoxymethylene (POM) solubility in water | Insoluble in water despite having a higher oxygen/carbon ratio than PEG. |
| PEG and POM solubility difference | Attributed to induction effects, where POM oxygen atoms withdraw less electron density from carbon atoms due to "sharing." |
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What You'll Learn
The carbon/oxygen ratio of the polymer
The solubility of a polymer in water depends on its interaction with water molecules, which is influenced by the polymer's carbon/oxygen ratio. Polymers with more oxygen atoms and fewer carbon atoms tend to dissolve better in water. This is because oxygen atoms are negatively charged, making them hydrophilic and attracted to the water molecules.
Polyether molecules, for example, generally follow this trend of increased solubility with higher oxygen content. PEG (polyethylene glycol), with the repeating unit -CH2-CH2-O--, is highly soluble in water due to the presence of negatively charged oxygen atoms. PPG (polypropylene glycol), with a similar structure but fewer oxygen atoms (-CH2-CH2-CH2-O-), is less soluble.
However, there are exceptions to this trend, such as POM (polyoxymethylene), which has the highest possible oxygen/carbon ratio among plastics but is completely insoluble in water. To understand this anomaly, researchers from the University of Amsterdam and the Max Planck Institute for Polymer Research in Mainz conducted experiments using advanced techniques like femtosecond-infrared spectroscopy and quantum calculations.
They discovered that the solubility of polymers is not solely determined by the number of oxygen atoms but also by their ability to withdraw electron density from neighbouring carbon atoms, known as induction effects. In PEG, each oxygen atom has two carbon atoms to withdraw electron density from, resulting in a higher negative charge. In contrast, the oxygen atoms in POM have to "share" carbon atoms, leading to a lower negative charge and reduced solubility.
By altering the oxygen charges in POM to match those in PEG through computer simulations, researchers confirmed that the difference in oxygen partial charge was the key factor influencing solubility. This discovery helps solve the mystery of why certain plastics with high oxygen content, like POM, are insoluble in water despite expectations to the contrary.
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Non-polar plastics are not easily dissolved
The solubility of plastics in water depends on their chemical composition. A common saying in chemistry is that "like dissolves like", meaning that polar solvents tend to dissolve other polar things, while non-polar solvents tend to dissolve other non-polar things.
Non-polar plastics, such as polyethylene, polypropylene, and polystyrene, are examples of plastics that do not easily dissolve in water. This is because they are non-polar hydrocarbon polymers, with polystyrene having phenyl pendant groups and polypropylene having methyl pendant groups. The polymer chains in these plastics are entangled by a large number of inter-molecular secondary bonds, which makes them resistant to dissolution in water.
On the other hand, polar plastics with chlorine, such as vinyl, tend to be more soluble in water due to their polarity. Additionally, some starch-based plastics are designed to easily dissolve in water, while others are impervious to acids, bases, and organic solvents.
It is important to note that while conventional plastics do not dissolve in water, they may undergo a process called leaching, where the plastic slowly breaks down and releases chemicals into the water over time. Therefore, it is recommended to avoid storing water in plastic containers for extended periods.
Furthermore, certain organic solvents can be used to dissolve plastics. For example, acetone is a powerful solvent for many thermoplastics, and it can also swell thermosets. Other solvents like limonene and xylenes (or dimethylbenzenes) can dissolve polystyrene and polypropylene, respectively, but only at higher temperatures.
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Starch-based plastics are designed to dissolve in water
Plastic is a polymer, and its solubility is determined by its interaction with water. The carbon-oxygen ratio of the polymer plays a significant role in solubility. For instance, polyethers tend to dissolve better in water when they contain more oxygen and fewer carbon atoms. However, there are exceptions to this trend, such as the widely used plastic POM, which has the highest possible oxygen-to-carbon ratio yet is insoluble in water.
Starch-based plastics, also known as thermoplastic starch (TPS), are designed to be biodegradable and environmentally friendly alternatives to conventional plastics. They are made from naturally occurring and inexpensive materials, making them low-cost and easy to implement. TPS is highly versatile and can be tailored to various applications by blending it with other polymers, whether natural or synthetic.
One of the key advantages of TPS is its biodegradability, which addresses the environmental concerns associated with conventional plastics. TPS can decompose when exposed to consistent heat, oxygen, and bacteria that consume the monomers. However, the decomposition process can be slow, ranging from a few years to several hundred years, similar to regular plastic.
While TPS offers a sustainable alternative, it also has some disadvantages. Pure starch-based materials have lower mechanical properties and are sensitive to moisture. To overcome these challenges, blends and composites have been developed, such as combining TPS with polymers like PVA or PLA. These blends improve the mechanical and thermal properties of TPS while maintaining biodegradability and reducing costs.
The starch-based blends can also be strengthened with natural fibers from plants, such as cellulose nanofibers, which enhance tensile strength, improve water resistance, and provide better thermal resistance. Additionally, starch-based plastics can be designed with different levels of porosity to control water absorption. While larger pores may be desirable for specific applications, such as medical devices releasing drugs into the skin, most packaging and disposable utensils require minimal porosity to prevent water absorption and escape.
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Polyvinyl alcohol (PVA) can dissolve in water
Most plastics do not dissolve in water due to differences in polarity. In chemistry, there is a saying that "like dissolves like", meaning that polar solvents tend to dissolve polar solids, liquids, and gases, while non-polar solvents tend to dissolve other non-polar substances.
However, there are exceptions to this rule. Polyvinyl alcohol (PVA) is a synthetic, water-soluble polymer. It is commonly supplied as beads or as solutions in water. PVA can be dissolved in hot or cold water, although the process is faster with hot water due to the high speed of rotation of the rotor, which creates a powerful suction that draws the PVA powder into the water. The solubility of PVA in water is influenced by the carbon-to-oxygen ratio of the polymer. PVA has excellent film-forming, emulsifying, and adhesive properties, and it is resistant to oil, grease, and solvents. It is also biocompatible, with low toxicity and low protein adhesion, making it suitable for various medical applications such as vascular stents, cartilage replacements, contact lenses, and embolisation particles for peripheral hypervascular tumors. In the food packaging industry, PVA is used as a coating agent due to its water solubility, biodegradability, and oxygen barrier properties. It is also used in papermaking, textiles, detergents, and cosmetics.
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Plasticizers impede dissolution
The presence of plasticizers in conventional plastics impedes their dissolution in water. Plasticizers are additives that are added to plastics during the manufacturing process to increase their flexibility, durability, and range of functionality. These additives do not easily dissolve in water, and their presence in plastics prevents the polymers from interacting with water molecules.
Polyvinyl alcohol (PVA), for instance, is a water-soluble polymer. However, extensive hydrogen bonding makes it insoluble in water unless the water is heated to near-boiling temperatures. Upon cooling, PVA remains dissolved as the hydrogen bonding with water molecules minimizes the intermolecular and intramolecular hydrogen bonding of PVA.
Polyether molecules, such as polyethylene glycol (PEG) and polyoxymethylene (POM), also exhibit varying solubilities in water. PEG is soluble in water due to the presence of slightly negatively charged oxygen atoms, which makes them hydrophilic. On the other hand, POM, despite having a higher oxygen-to-carbon ratio, is insoluble in water. Researchers from the University of Amsterdam and the Max Planck Institute for Polymer Research in Mainz found that the difference in solubility between PEG and POM is due to the oxygen partial charge.
The carbon-to-oxygen ratio of a polymer strongly influences its solubility in water. Quantum calculations revealed that this relationship is not due to the distance between oxygen atoms in the polymer chain but rather the induction effects. In PEG, each oxygen atom can withdraw electron density from two neighboring carbon atoms, while in POM, the oxygen atoms must "share" the carbon atoms, resulting in reduced electron withdrawal. This understanding of induction effects will make it easier to predict solubilities in the future.
Additionally, the polarity of thermoplastics plays a role in their solubility. Polar thermoplastics, such as vinyl plastics containing chlorine, are more easily dissolved in water than non-polar plastics like polyethylene. The principle of "like dissolves like" applies, where polar solvents dissolve other polar substances, and non-polar solvents dissolve non-polar substances.
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Frequently asked questions
Conventional plastics do not dissolve in water because they are non-polar and therefore cannot dissolve in polar solvents like water.
Starch-based plastics are designed to easily dissolve in water. Poly (vinyl alcohol) is another water-soluble polymer.
Acetone is a great solvent for many plastics (thermoplastics). Other plastics can be dissolved in organic solvents like xylenes (or dimethylbenzenes) and limonene.
The solubility of a plastic in a solvent depends on the interaction between the two substances. The rule of thumb is "like dissolves like", meaning polar substances dissolve other polar substances and non-polar substances dissolve other non-polar substances.
