Thomas Graham's System of Substance Classification

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

Thomas Graham was the first scientist to classify substances as colloids or crystalloids. In this lesson, we will learn how he described these categories and how those categories have been further defined.

Colloids and Crystalloids

When we define colloids we typically call them large, but then we define 'large' as being 1-100 nm, which is smaller than our eyes can even see! So, what makes these molecules large? We are describing them as large because they cannot pass through a semipermeable membrane. Also, they do not truly dissolve into a mixture, causing any mixture they're in to be opaque.

Anything smaller than a colloid is a crystalloid. When crystalloids are dissolved, they form a clear solution that can pass through a semipermeable membrane.

Thomas Graham developed a classification method based on the size of particles
Thomas Graham image

Thomas Graham was the first scientist to discover this distinction between crystalloids and colloids. He developed a system of classification based upon the size of the particles. Specifically, he found that crystalloids are compounds that readily diffuse through the semipermeable membrane (such as those found in our body), while colloids will not, or will only diffuse through very slowly. Crystalloids include sugar, salts, and urea, while colloids include starch, gelatin, and proteins.

Types of Colloids

There are several different ways to group and define colloids, including the type of suspended particles, the type of molecule interactions, and how well the particles interact with their solvents.

Type of Particles

Colloids can be further defined based on what the particles are suspended in. These groupings include:

  • Sol: solid particles dispersed in liquid
  • Aerosol: any particles dispersed in gas
    • Fog: liquid particles dispersed in gas
    • Smoke: solid particles dispersed in gas
  • Emulsion: liquid particles dispersed in liquid

Although fog and smoke may look similar, they are differentiated based on whether they have liquid or solid particles dispersed in the gas

Molecule Interactions

Colloids can also be defined based on how the particles interact with each other and how many different types of particles are present. These groupings include:

  • Multimolecular colloids
  • Macromolecular colloids
  • Associated colloids (also known as micelles)

Multimolecular colloids take individual molecules that would typically be considered a crystalloid and combine them together until they form a molecule in the colloid range. For example, gold and sulfur will do this. Individual gold or sulfur molecules are very small, small enough to easily travel through a semipermeable membrane. But they will combine together to form large molecules, and this is what forms the multimolecular colloids.

Macromolecular colloids are simply colloids where the individual molecule is large enough to be considered a colloid. This includes starch, cellulose, and proteins.

Associated colloids are ones that form crystalloid solutions at small concentrations, but in large concentrations they begin to aggregate together forming large molecules within the colloid range. This includes soaps and detergents.

Interaction with Solvent

Colloids will either mix easily with the solvent, or require some additional energy to get them to mix together to form a solution.

When colloids mix easily together, they are called lyophillic. Lyophilic means solvent or liquid loving. These are colloids where the dispersed particles easily interact with the liquid around them. In order to make these types of colloids, we simply mix the colloid and solvent together. This category includes starches and gels.

When colloids don't mix together easily, they are called lyophobic. Lyophobic means solvent or liquid fearing. These colloids need additional energy (in the form of mechanical, electrical, or chemical energy) in order to get them to mix with the solvent. This category includes inks and metal colloids.

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