Question

Soaps consist of compounds such as sodium stearate, \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16} \mathrm{COO}^{-} \mathrm{Na}^{+},\) that have both hydrophobic and hydrophilic parts. Consider the hydrocarbon part of sodium stearate to be the "tail" and the charged part to be the "head." (a) Which part of sodium stearate, head or tail, is more likely to be solvated by water? (b) Grease is a complex mixture of (mostly) hydrophobic compounds. Which part of sodium stearate, head or tail, is most likely to bind to grease? (c) If you have large deposits of grease that you want to wash away with water, you can see that adding sodium stearate will help you produce an emulsion. What intermolecular interactions are responsible for this?

Short answer

(a) The head (charged part \(\mathrm{COO}^{-} \mathrm{Na}^{+}\)) of sodium stearate is hydrophilic and more likely to be solvated by water. (b) The tail (hydrocarbon part \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16}\)) of sodium stearate is hydrophobic and more likely to bind to grease. (c) Emulsion formation is due to hydrogen bonding and ion-dipole interactions between sodium stearate's head and water molecules, and London dispersion forces between sodium stearate's tail and grease molecules.

Step-by-step solution

Understanding Sodium Stearate Structure

Sodium stearate has a hydrocarbon part (tail) \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16}\) and a charged part (head) \(\mathrm{COO}^{-} \mathrm{Na}^{+}\). The tail is a long-chain hydrocarbon, which is hydrophobic (non-polar), while the head is ionic, consisting of a carboxylate group and a sodium ion, and it is hydrophilic (polar).

Solvation by water

(a) Solvation mainly depends on the compatibility of substances. Water is a polar solvent; thus, it solvates polar or hydrophilic parts of a compound. In the case of sodium stearate, its head (the charged part \(\mathrm{COO}^{-} \mathrm{Na}^{+}\)) is hydrophilic and more likely to be solvated by water.

Grease binding

(b) Grease, a complex mixture of hydrophobic compounds, will interact more with hydrophobic compounds or non-polar parts of a molecule. In sodium stearate, the tail (the hydrocarbon part \(\mathrm{CH}_{3}\left(\mathrm{CH}_{2}\right)_{16}\)) is hydrophobic and more likely to bind to grease.

Emulsion formation

(c) Introducing sodium stearate into a mixture of grease and water creates an emulsion, in which small droplets of grease are surrounded by sodium stearate molecules. The intermolecular interactions responsible for this process are: 1. Hydrogen bonding and ion-dipole interaction between the sodium stearate's head and water molecules. 2. London dispersion force between the sodium stearate's tail and grease molecules. These interactions result in the formation of an emulsion, thus allowing the removal of grease while washing with water.

Key concepts

Sodium Stearate

Sodium stearate is a critical component of soap that gives it the ability to clean. It is a salt derived from stearic acid and sodium hydroxide. This molecule is split into two main parts: the tail and the head.
The tail is the hydrocarbon chain, represented as \(\mathrm{CH}_{3}(\mathrm{CH}_{2})_{16}\), which is non-polar and thus hydrophobic, meaning it repels water. The head, on the other hand, is the polar, charged part \(\mathrm{COO}^{-} \mathrm{Na}^{+}\), making it hydrophilic and soluble in water.
This dual character enables sodium stearate to interact with both water and oils, making it effective in cleaning applications.

Hydrophobic and Hydrophilic Interactions

Soaps like sodium stearate exhibit both hydrophobic and hydrophilic interactions. These interactions are essential for the soap's cleaning mechanism.
The **hydrophilic head** is attracted to water molecules. This occurs because water is a polar solvent and interacts with other polar molecules through electrostatic interactions.
The **hydrophobic tail** avoids water and prefers non-polar environments, such as oils and greases. In water, hydrophobic parts tend to cluster together, minimizing their exposure to the water, which is why soap forms micelles.

• Hydrophilic interaction: The charged sodium stearate head bonds with water molecules.
• Hydrophobic interaction: The long hydrocarbon tail binds with oils and greases.

Emulsion Formation

Emulsions are mixtures of two immiscible liquids where one is dispersed in the other in the form of tiny droplets. Sodium stearate aids in creating emulsions between oil and water.
The soap molecules position themselves at the oil-water interface. The hydrophobic tails embed into the oil, while the hydrophilic heads remain in the water. This configuration stabilizes the oil droplets and keeps them suspended in water.
By reducing the surface tension between oil and water, sodium stearate enables these droplets to remain evenly dispersed, forming an emulsion, which is crucial in the cleaning process.

Intermolecular Forces

Intermolecular forces play a crucial role in the effectiveness of soap in cleaning. In the context of sodium stearate, several forces come into play.

• **Hydrogen Bonding and Ion-Dipole Interactions**: These occur between the polar heads of sodium stearate and water molecules, assisting in the solubilization of the soap in water.
• **London Dispersion Forces**: These weak intermolecular forces occur between the hydrophobic tails of sodium stearate and grease molecules.

These forces ensure that sodium stearate can interact with both water and oil, allowing it to act as a bridge between the two, facilitating the cleaning process.

Grease Removal

The process of removing grease involves the special interaction of sodium stearate with water and grease. The key to this is the amphiphilic nature of sodium stearate.
When you apply soap to a greasy area, the hydrophobic tails of sodium stearate latch onto the grease particles, effectively surrounding and loosening them from surfaces.
The hydrophilic heads remain in contact with water, allowing the whole assembly of soap and grease to be rinsed away easily.
By transforming otherwise tough oils and greases into emulsified droplets, sodium stearate makes it possible to wash them away, leaving surfaces clean.