Question

Explain how (a) a soap such as sodium stearate stabilizes a colloidal dispersion of oil droplets in water; (b) milk curdles upon addition of an acid.

Short answer

(a) Sodium stearate, a soap, acts as a surfactant by reducing the surface tension between oil and water due to its dual hydrophilic and hydrophobic nature. It forms micelles around the oil droplets, with hydrophobic tails inwards and hydrophilic heads outwards, preventing droplet coalescence and stabilizing the colloidal dispersion. (b) Milk curdles when an acid is added because it increases the H⁺ concentration, neutralizing the negative charges on casein proteins. As a result, casein proteins aggregate, forming insoluble curds that precipitate out, separating milk into solid curds and liquid whey.

Step-by-step solution

(a) Stabilization of oil droplet dispersion by sodium stearate

Sodium stearate is a soap, which belongs to a class of compounds called surfactants. Surfactants are surface-active agents that act to reduce the surface tension between two immiscible liquids, like oil and water. The structure of sodium stearate consists of a long hydrocarbon tail (hydrophobic) and a sodium carboxylate head (hydrophilic). This dual nature of the soap molecule allows it to dissolve both in water and oil. When oil droplets are dispersed in water, the sodium stearate molecules orient themselves around the oil droplets, with their hydrophobic tails pointing towards the oil and the hydrophilic heads pointing towards the water. This orientation forms a structure called a micelle. The micelles create a barrier between the oil droplets and the water, which prevents the droplets from coalescing, thus stabilizing the colloidal dispersion of oil in water.

(b) Milk curdling with the addition of an acid

Milk is a colloid consisting of water, proteins, fats, minerals, and lactose. The proteins present in milk are mainly casein proteins and whey proteins, which form a colloidal suspension in water by forming micellar structures stabilized by calcium ions. In its natural state, the casein proteins are negatively charged and repel each other, maintaining a stable colloid. When an acid is added to milk, it increases the concentration of hydrogen ions (H⁺) and lowers the pH. This lowered pH neutralizes the negative charges on the casein proteins, causing them to lose their repulsive forces and aggregate with their neighboring proteins. The aggregated casein proteins become insoluble in the aqueous environment, forming curds that precipitate out of the milk. The process of curdling can also be achieved by adding enzymes like rennet, which specifically target and break down the proteins to promote aggregation. In summary, the addition of an acid to milk causes the casein proteins to lose their charge stability and aggregate, leading to the formation of curds and the separation of the milk into solid curds and liquid whey.

Key concepts

Sodium Stearate

Sodium stearate is a fascinating compound that plays a crucial role in the world of everyday chemistry. It is a type of soap and part of a larger group known as surfactants. Surfactants help bridge gaps between substances that would not normally mix, such as oil and water. An interesting fact about sodium stearate is its dual structure. It has a long tail made of hydrocarbons that's attracted to oil (hydrophobic) and a head that loves water (hydrophilic).
This dual nature allows it to act efficiently in mixtures. The magic happens when oil is introduced into water. Sodium stearate molecules surround the oil droplets. They arrange in such a way that their tails snuggle with the oil, while the heads face outward towards water. This orientation forms something known as a micelle. In the micelle structure:

• The hydrophobic tails gather around the oil, ensuring it doesn't escape.
• The hydrophilic heads face outward, interacting peacefully with water.

These micelles create a protective barrier, effectively stabilizing the dispersion and preventing the oil droplets from merging back together. This is why soaps, including sodium stearate, are so effective at cleaning and mixing oil and water.

Milk Curdling

Milk curdling can often be seen as an unwanted effect, especially when making a morning cup of tea. However, scientifically, it is a fascinating transformation. Milk, a colloid, contains proteins, fats, and minerals suspended in water. When an acid enters the milk, for instance, lemon juice or vinegar, it brings additional hydrogen ions (H⁺) into the mix. These ions lower the pH level of the milk, leading to interesting changes in the behavior of casein proteins. These proteins usually carry a negative charge, helping them repel each other and remain suspended. However, when the increased hydrogen ions neutralize these charges:

• The casein proteins lose their repelling nature.
• They start clumping together, being no longer soluble in water.

The result? Curds form and separate from the liquid, leading to the classic curdling effect. As slightly unappetizing as it might seem, curdling is critical in cheese-making and similar culinary processes.

Surfactants

Surfactants are incredible all-rounders in the field of chemistry. They are agents that alter property interfacial tension between two surfaces, like oil and water. This means they can mix things that normally don't blend. The word 'surfactant' is derived from 'surface-active agents'. What makes them so effective?

• They possess both hydrophobic (water-repelling) and hydrophilic (water-attracting) parts.
• This allows them to reduce the surface tension between different substances.

In practical terms, surfactants make it possible for oil to disperse in water and similarly facilitate the foaming and spreading action of cleaning products. From soaps to shampoos, they are the unsung heroes in many household and industrial processes. Truly, they play a significant part in ensuring that colloidal dispersions stay stable across various applications.

Casein Proteins

Casein proteins are one of the primary protein types found in milk, making up almost 80% of its proteins. They are unique in their structure and behavior. Unlike most proteins, casein forms a complex known as a micelle, much like the micelles formed by soap molecules. This structure gives them stability in liquids like milk. In their natural state, casein micelles are suspended in milk, held together by calcium ions. This suspension is usually stable due to repelling charges on the micelles. These proteins carry a negative charge, helping keep them apart and the milk liquid.
Whenever an acid is introduced:

• The pH drops, reducing the charges.
• Casein micelles begin to aggregate.

This aggregation leads to curd formation, as seen in milk curdling. Understanding casein proteins is vital not just for food science but also for nutritional studies, considering they are a crucial protein source in dairy products. By understanding more about these proteins and their behavior, we can better utilize them in various culinary and industrial processes.