Question

Draw the structure of the sodium salt of stearic acid. How might it function to remove grease from \(r\) your clothes or your hands?

Short answer

Sodium stearate has both hydrophobic and hydrophilic regions, allowing it to effectively emulsify and remove grease.

Step-by-step solution

Understanding Stearic Acid

Stearic acid is a saturated fatty acid with the chemical formula \(C_{17}H_{35}COOH\). It consists of a long hydrocarbon chain (\(C_{17}H_{35}\)) and a carboxylic acid group (\(COOH\)).

Forming the Sodium Salt

To form the sodium salt, the hydrogen atom (H) in the carboxylic group of stearic acid is replaced by a sodium ion (\(Na^+\)). This results in the structure: \(C_{17}H_{35}COO^−Na^+\).

Structure of Sodium Stearate

The sodium stearate molecule has a long non-polar hydrocarbon chain (\(C_{17}H_{35}\)) which is hydrophobic, and a charged polar end (\(COO^−Na^+\)) which is hydrophilic. This dual nature allows it to interact with both oily and watery substances.

Function in Removing Grease

Sodium stearate works by surrounding grease particles with its hydrophobic tails and suspending them in water through its hydrophilic heads. The hydrophobic tails are attracted to oils, while the hydrophilic heads interact with water, effectively dissolving the grease.

Key concepts

Stearic Acid

Stearic acid is an interesting compound, widely recognized as a type of saturated fatty acid. Its chemical formula is represented by \(C_{17}H_{35}COOH\). This indicates it has a long hydrocarbon chain comprising 17 carbon atoms and a terminal carboxylic acid group. The hydrocarbon tail is non-polar and carbon-rich, which explains why this part is hydrophobic, meaning it does not mix well with water. The carboxylic acid group, however, is polar, giving stearic acid its ability to form reactions with bases to create salts like sodium stearate.

Stearic acid is commonly found in animal and plant fats, known for its function as a lipid component that provides energy. In industrial applications, it serves as a key ingredient in cosmetics and detergents due to its surfactant properties.

Fatty Acid

Fatty acids are essential components of lipids, such as fats and oils, found in living organisms. They can be broadly categorized into two types: saturated and unsaturated. Stearic acid belongs to the category of saturated fatty acids as it contains no double bonds between carbon atoms in its long chain. This structure contributes to its solid form at room temperature.

Fatty acids generally consist of a long hydrocarbon chain with a terminal carboxyl group (\(COOH\)). This structure allows for their reactive nature with bases, forming salts and other compounds. Because of the length of the chain and the presence of the carboxyl group, fatty acids can exhibit both hydrophobic and hydrophilic properties. This makes them useful in forming micelles, essential both in biological processes and in cleaning applications.

Amphipathic Molecule

The term "amphipathic" might sound complex but simply refers to a molecule that has both hydrophilic (water-attracting) and hydrophobic (water-repellent) parts. Sodium stearate is a prime example of an amphipathic molecule. Its structure is characterized by a long hydrophobic hydrocarbon tail and a hydrophilic head formed by the carboxylate group (\(COO^-Na^+\)).

This dual nature is what empowers amphipathic molecules like sodium stearate to act as effective cleaners. In an aqueous solution, these molecules arrange themselves in such a way that their hydrophobic parts face inwards, towards the oils or grease, while their hydrophilic ends face outwards into the water. By doing so, they facilitate the removal of oils and greases by surrounding them and making them soluble in water.

Hydrophobic and Hydrophilic Interactions

Hydrophobic and hydrophilic interactions are key concepts in understanding how detergents work. Hydrophobic interactions occur among molecules that do not mix with water. These are typically non-polar and prefer interaction with oils or other lipophilic substances. In sodium stearate, the long hydrocarbon chain exhibits these hydrophobic properties.

Conversely, hydrophilic interactions involve molecules that readily mix with or dissolve in water, often polar in nature. Sodium ions in the sodium stearate provide such characteristics. Together, these interactions establish why sodium stearate can effectively remove grease. The hydrophobic tail embeds itself into oily substances, while the hydrophilic head allows it to be washed away with water. This dual functionality is at the heart of how soaps and detergents clean.