Question

Indicate whether each of the following is a hydrophilic or a hydrophobic colloid: (a) butterfat in homogenized milk, (b) hemoglobin in blood, (c) vegetable oil in a salad dressing, (d) colloidal gold particles in water.

Short answer

(a) Butterfat in homogenized milk is a hydrophobic colloid. (b) Hemoglobin in blood is a hydrophilic colloid. (c) Vegetable oil in salad dressing is a hydrophobic colloid. (d) Colloidal gold particles in water form a hydrophilic colloid.

Step-by-step solution

(a) Butterfat in homogenized milk: hydrophilic or hydrophobic?

Butterfat is a type of lipid, which is generally hydrophobic in nature. Lipids are insoluble in water as their large hydrocarbon regions do not form favorable interactions with water molecules. Homogenized milk is created by dispersing the butterfat in small droplets throughout the liquid, making it more stable and uniform. Therefore, butterfat in homogenized milk is a hydrophobic colloid.

(b) Hemoglobin in blood: hydrophilic or hydrophobic?

Hemoglobin is a protein found in red blood cells that helps transport oxygen throughout the body. Proteins generally have both hydrophilic and hydrophobic regions, but due to their overall structure, hemoglobin is soluble in water. The hydrophilic amino acids on the surface of the protein interact with water, allowing it to remain dissolved in blood and function effectively. Therefore, hemoglobin in blood is a hydrophilic colloid.

(c) Vegetable oil in salad dressing: hydrophilic or hydrophobic?

Vegetable oil is also a type of lipid, which consists mostly of triglycerides. Like butterfat, lipids are generally hydrophobic due to their large hydrocarbon regions that do not readily interact with water molecules. For this reason, vegetable oil and water do not mix well, and when combined in salad dressings, the oil and water can separate. Therefore, vegetable oil in salad dressing is a hydrophobic colloid.

(d) Colloidal gold particles in water: hydrophilic or hydrophobic?

Colloidal gold particles have a charged surface that attracts water molecules and forms a hydration layer around the particles. This results in the stabilization of the gold particles in the aqueous solution. This interaction with water, which prevents the gold particles from aggregating or settling out, indicates that the colloidal gold particles are hydrophilic in nature. Therefore, colloidal gold particles in water form a hydrophilic colloid.

Key concepts

Hydrophilic Colloids

Hydrophilic colloids are colloidal systems where the particles are attracted to water molecules. This attraction helps stabilize the solution, making it less likely for the particles to aggregate or precipitate. This is because the particles can interact freely with water, often forming hydrogen bonds or dipole interactions.
For instance, hemoglobin in blood is a classic example of a hydrophilic colloid. The surface edges of the protein contain amino acids that are hydrophilic, meaning they easily attract and bond with water molecules. This characteristic allows hemoglobin to remain dissolved in the blood, enabling its function of oxygen transport throughout the body.
The water-loving nature of hydrophilic colloids gives them an affinity for aqueous environments, making them vital in many biological systems and industrial applications. They play a crucial role in various processes, such as stabilization of emulsions and enhancing texture in food products.

Hydrophobic Colloids

Hydrophobic colloids are systems where the particles repel water, making them less soluble in an aqueous environment. The inherent non-polar nature of the particles doesn't allow favorable interactions with water molecules, prompting them to avoid water.
In the example of butterfat in homogenized milk, butterfat, which is essentially a lipid, is naturally hydrophobic. Despite being dispersed finely in milk through homogenization, the lipid molecules resist dissolving in water, owing to their large hydrocarbon chains. These chains do not interact positively with water, so milk must be mechanically processed to keep the lipids suspended.
Hydrophobic colloids require stabilizing agents or other methods to remain mixed with water. For example, emulsifiers are added in food products like salad dressings to prevent the separation of oil and water, ensuring the hydrophobic oil droplets remain uniformly distributed.

Lipids in Colloids

Lipids are crucial components of colloids, notably as hydrophobic agents. They usually consist of long hydrocarbon chains that are non-polar and avoid water. This inherent property of lipids makes them resistant to being dissolved in water-based solutions.
Examples like vegetable oil in salad dressing and butterfat in homogenized milk demonstrate the hydrophobic nature of lipids. In salad dressings, lipids tend to separate from the water phase, forming distinct layers, unless emulsified. In homogenized milk, tiny droplets of butterfat are mechanically interspersed, yet the lipid's basic nature remains unchanged. Without stabilization, these droplets could eventually clump together due to their water-repelling nature.
Lipids' roles in colloidal systems are diverse, from providing energy and forming cellular structures in organisms to playing functional roles in food science. Understanding lipid behavior in colloids helps in the formulation of products that require consistent texture and stability.