Question

(a) Many proteins that remain homogeneously distributed in water have molecular masses in the range of 30,000 amu and larger. In what sense is it appropriate to consider such suspensions to be colloids rather than solutions? Explain. (b) What general name is given to a colloidal dispersion of one liquid in another? What is an emulsifying agent?

Short answer

(a) Protein suspensions in water with molecular masses greater than 30,000 amu are considered colloids because their large size prevents them from dissolving completely and remains homogeneously distributed in water. (b) A colloidal dispersion of one liquid in another is called an emulsion. An emulsifying agent is a substance that stabilizes emulsions by reducing the surface tension between the two immiscible liquids, preventing them from coalescing back into separate phases, such as lecithin in egg yolks or natural gums in salad dressings.

Step-by-step solution

(a) Protein Suspensions: Colloids or Solutions?

Protein suspensions in water usually have molecular masses larger than 30,000 amu. Suspensions can be classified into two categories: colloids and solutions. Solutions are homogeneous mixtures where the solute particles are evenly distributed and are typically very small in size, less than 1 nanometer. Colloids, on the other hand, typically have larger particles with size ranging from 1 nanometer to 1000 nanometers. Since the protein particles in question have molecular masses greater than 30,000 amu and remain homogeneously distributed in water, they would be considered colloids. The large size of these protein particles prevents them from dissolving completely, maintaining a colloidal state.

(b) Colloidal Dispersion of One Liquid in Another

A colloidal dispersion in which one liquid is dispersed in another liquid while maintaining separate phases is called an emulsion. Examples of emulsions include milk (fat dispersed in water), mayonnaise (oil dispersed in vinegar), and salad dressing (oil and vinegar).

(c) Emulsifying Agents

An emulsifying agent is a substance that is added to stabilize emulsions. Acting as a surface-active agent, they reduce the surface tension between the two immiscible liquids and prevent the droplets from coalescing back into separate phases. Examples of emulsifying agents include lecithin (found in egg yolks and used in making mayonnaise) and natural gums (used in salad dressings).

Key concepts

Protein Suspensions

Protein suspensions in water often lead to an interesting categorization: colloids rather than solutions. When proteins are suspended in a fluid like water, they do not dissolve completely but remain evenly distributed across the liquid. The molecular masses of these protein particles are usually above 30,000 amu. This large size is key in classifying them as colloids.
The distinction between solutions and colloids relies on particle size. Solutions have very small particles, generally less than 1 nanometer in size, allowing complete dissolution. In contrast, colloids contain particles ranging between 1 nanometer and 1000 nanometers, contributing to their stability and homogeneity without true dissolution.
Thus, due to their particle size and distribution pattern, protein suspensions exhibit the characteristics of colloids, maintaining a stable suspension without dissolving completely.

Emulsions

An emulsion represents a specific type of colloid where two immiscible liquids, like oil and water, are combined with one dispersed in the other. This colloidal system is crucial in numerous everyday products.
Common examples include:

• Milk, where fat droplets are dispersed in water.
• Mayonnaise, a blend of oil in vinegar.
• Salad dressings, which often involve oil and water phases.

The operation of emulsions is based on maintaining distinct liquid phases, while still appearing as a stable mixture, thanks to the minute size and distribution of droplets within the mixture. The properties of emulsions enable these products to possess unique textures and functionalities, crucial in food science and manufacturing.

Emulsifying Agents

The success of many commercial and culinary emulsions owes much to emulsifying agents. These substances play a pivotal role by stabilizing emulsions, ensuring that the dispersed liquid droplets remain distinct rather than merging back into separate layers.
An emulsifying agent operates by reducing the surface tension between the immiscible liquids, acting as a bridge to keep them from separating. For instance:

• Lecithin found in egg yolks is often used in mayonnaise, stabilizing oil-in-vinegar mixtures.
• Natural gums are frequent in salad dressings to maintain a smooth, blended consistency.

These agents are surface-active, or amphiphilic, meaning they can bond with both water and oil components. Their ability to maintain emulsions stable is crucial for diverse applications in both domestic kitchens and industrial settings.