Chapter 3 77 on the other side of the liquid/vapor boundary. This imbalance leads to a two-dimensional force at the surface, namely surface tension. The surface tension is numerically equal to the surface free energy.3 The magnitude of surface tension directly correlates with the strength of the intermolecular forces. Water has hydrogen bonds, dipole-dipole interaction, and dispersion forces between its molecules, and as a consequence the surface tension of water is rather high—72 mN/meter at room temperature. On the other hand, only dispersion forces are present between the molecules of alkanes. As a consequence, the surface tension of alkanes is relatively low— ranging 20–30 mN/meter. Surfactants comprise molecules that contain two parts: a hydrophobic segment that is expelled by water and a hydrophilic segment that interacts strongly with water. Such molecules are said to be amphipathic (amphi meaning “dual” and pathic from the same root as pathos which can be interpreted as “suffering”). Thus, a surfactant molecule “suffers” both oil and water. This dual nature confers interesting properties on surfactants in aqueous solution. At very low concentrations the surfactant is expelled to the surface, a process called adsorption. This adsorption causes the surfactant concentration at the surface to be much higher than the surfactant concentration in the bulk of the solution. At extremely low concentrations, when the surfactant molecules on the surface are located too far apart to effectively interact with each other, Traube’s Rule applies. Traube’s Rule states that the ratio of the surface concentration to the bulk concentration increases threefold for each CH 2 group of an alkyl chain.4 This ratio is called the surface excess concentration.5 According to this rule, soap with a dodecyl chain should have a surface excess concentration that is more than a half- million times its concentration in the bulk solution. At extremely low concentrations, the surfactant molecules on the surface act as a two-dimensional gas. As the concentration increases, the surfactant molecules begin to interact, but they are still mobile within the plane they behave as two-dimensional liquids. At even higher concentrations, as the surfactant saturates the surface, the chains
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