Coloides

Colloids
This supplementary reading for Chem 110H describes colloids. You should begin by reading Section 13.6 of the text for a general background on colloids. Then read the more detailed description below and solve the problems at the end of the supplement. We begin by reviewing standard homogeneous and heterogeneous mixtures, and then move on to colloids. A simple definition of a solution isa mixture of two or more different types of molecules in which the different types are distributed evenly amongst each other. As we will see below, we also call a solution a molecular dispersion, since the two types of molecules are evenly distributed, or ‘dispersed’ in one another. As pointed out in Table 13.1 of the text, solutions can be made out of 7 of the 9 combinations of states. A fewexamples of solutions are air, which is a gas-gas mixture of ~80% N2 and ~20% O2, and brine, which is a solid-liquid mixture of sodium chloride dissolved in water. We call the substance that is the major component of the solution the solvent, and the substance that is the minor component the solute. In this picture, we imagine that as we examine smaller and smaller volume elements of the solution, wewill see the same proportions of the different molecule types. Moreover, we often assume that they will be randomly distributed in each other. Hence, we also refer to a solution as a homogeneous mixture. This characteristic of a solution will typically hold as long as the chosen volume element remains slightly above molecular dimensions. We say ‘typically hold’ because interesting exceptions existsuch as a ‘homogeneous’ mixture of methanol (CH3OH) and water. At the macroscopic level, methanol and water are miscible in all proportions. However, at the molecular scale, the methanol molecules form small clusters, or ‘icebergs’, wherein the methyl groups are oriented towards each other and their hydroxyl groups are pointed outward towards, and hydrogen bond with, the surrounding water; thus,the water and methanol molecules are not randomly distributed at the molecular level, and both the methanol and water are somewhat organized.1 Nonetheless, we will refer to such mixtures as solutions since when we deliver them with standard macroscoptic methods on the mL to µL scale, such as pipetting, we deliver a fixed amount of solute molecules. At the other end of the spectrum areheterogeneous mixtures, in which the two or more different molecule types are unevenly distributed amongst each other. In these cases, as we examine smaller and smaller volume elements of the sample, we see different and unpredictable proportions of the different molecule types; indeed, this heterogeneous composition is often observable in relatively large volume elements, sometimes being noticeable to thenaked eye. A few examples of heterogeneous mixtures are a salad of lettuce and tomatoes, and sand. (Please consult pp.8-9 of your text for a review of homogeneous and heterogeneous mixtures). Now that we have reviewed homogeneous and heterogeneous mixtures, we will discuss a special type of homogeneous mixtures called colloids in which a solutelike substance is dispersed in a solventlike substance.What makes colloids special is that one of these two particles is especially
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Dixit, S., Crain, J., Poon, W. C., Finney, J. L., and Soper, A. K. (2002) Molecular segregation observed in a concentrated alcoholwater solution. Nature 416, 829-832.

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large—much large than the small-molecule solutes we typically associate with solutions, such as sodium chloride or sugar. We call the largemolecule a colloidal particle. Colloids are also known as colloidal dispersions because the solutelike substance is said to be evenly spread out or ‘dispersed’ in the solventlike substance. As described in Table 13.6 in the text and Table 13.6, colloids can be made out of all 9 of the possible combinations of solids, liquids, and gases. A few examples of colloids include aerosols such as smoke,...