Problem 90 - Chemistry: The Central Science

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.

Step-by-Step Solution

Answer

Sodium stearate stabilizes a colloidal dispersion of oil droplets in water by forming micelles with polar heads interacting with water and nonpolar tails interacting with oil. The charged surfaces of micelles repel each other, preventing coalescing of oil droplets. Milk curdles upon acid addition as it decreases pH, neutralizing kappa-casein that stabilizes casein micelles. Casein micelles aggregate, forming a network trapping milk components, resulting in curd formation.

Step 1: (a) Understanding the structure of sodium stearate)

Sodium stearate is a soap, and its molecules have a polar "head" and a nonpolar "tail". The polar head is attracted to water, while the nonpolar tail is attracted to oil. This structure allows it to interact with both water and oil.

Step 2: (a) Formation of micelles)

When sodium stearate is mixed with water and oil, the soap molecules form structures called micelles. Micelles are spherical aggregates of soap molecules with the nonpolar tails pointing inward, forming the core, and the polar heads pointing outward, interacting with the water molecules. The oil droplets become encapsulated within the micelles, with the nonpolar tails of the soap molecules interacting with the oil.

Step 3: (a) Stabilizing the colloidal dispersion)

The presence of micelles in the mixture prevents the oil droplets from coming together and coalescing because the negatively charged polar heads on the surface of the micelles repel each other. This repulsion keeps the oil droplets dispersed in the water, creating a stable colloidal dispersion.

Step 4: (b) Structure of milk proteins)

Milk contains proteins called caseins, which form micelle-like structures in the milk. The proteins in milk also have polar and nonpolar regions, allowing them to interact with both water and fat molecules. The casein micelles are stabilized by a negatively charged protein called kappa-casein, which keeps them in suspension.

Step 5: (b) Role of acid in curdling)

When an acid is added to milk, the pH of the milk decreases. This decrease in pH causes the negatively charged kappa-casein to become neutral, losing its ability to stabilize the casein micelles. The casein micelles lose their charge and begin to aggregate together.

Step 6: (b) Formation of curds)

As the casein micelles aggregate, they form a network that traps other components of the milk, such as fat and water. This network leads to the formation of solid curds, which is the curdling process. The liquid remaining after curdling is known as whey. In conclusion, both phenomena are explained by the different chemical properties and interactions between species in the mixtures. Sodium stearate stabilizes the colloidal dispersion of oil droplets in water through the formation of micelles and repulsion between their charged surfaces. On the other hand, milk curdles upon the addition of an acid due to the neutralization of kappa-casein and the aggregation of casein micelles, which form a network that leads to the formation of curds.