Hydrophobic molecules do not dissolve well in water. Given that water is a very commonly used solvent, this makes certain processes very difficult: washing oily food residue off dishes, cleaning up spilled oil, keeping the oil and water phases of salad dressings well mixed, and carrying out chemical reactions that involve both hydrophobic and hydrophilic components.
Surfactants are a class of amphipathic compounds that includes soaps, detergents, and emulsifiers. With the use of surfactants, hydrophobic compounds can be suspended in aqueous solution by forming micelles (see Fig. $2-7$ ). A micelle has a hydrophobic core consisting of the hydrophobic compound and the hydrophobic "tails" of the surfactant; the hydrophilic "heads" of the surfactant cover the surface of the micelle. A suspension of micelles is called an emulsion. The more hydrophilic the head group of the surfactant, the more powerful it is - that is, the greater its capacity to emulsify hydrophobic material.
When you use soap to remove grease from dirty dishes, the soap forms an emulsion with the grease that is easily removed by water through interaction with the hydrophilic head of the soap molecules. Likewise, a detergent can be used to emulsify spilled oil for removal by water. And emulsifiers in commercial salad dressings keep the oil suspended evenly throughout the water-based mixture.
There are some situations in which it would be very useful to have a "switchable", surfactant: a molecule that could be reversibly converted between a surfactant and a nonsurfactant.
(a) Imagine such a "switchable" surfactant existed. How would you use it to clean up and then recover the oil from an oil spill?
Liu et al. describe a prototypical switchable surfactant in their 2006 article "Switchable Surfactants." The switching is based on the following reaction:
(b) Given that the $\mathrm{p} K_{2}$ of a typical amidinium ion is $12.4,$ in which direction (left or
right $)$ would you expect the equilibrium of the above reaction to lie? (See Fig. $2-16$ for relevant $\left.\mathrm{p} K_{2} \quad \text { values. }\right) \quad$ Justify $\quad$ your $\quad$ answer. $\quad$ Hint: $\quad$ Remember $\quad$ the $\quad$ reaction $\mathbf{H}_{2} \mathbf{O}+\mathbf{C O}_{2} \rightleftharpoons \mathbf{H}_{2} \mathbf{C O}_{3}$
Liu and colleagues produced a switchable surfactant for which $\mathrm{R}=\mathrm{C}_{16} \mathrm{H}_{33}$. They do not name the molecule in their article; for brevity, we'll call it s-surf.
(c) The amidinium form of s-surf is a powerful surfactant; the amidine form is not. Explain this observation.
Liu and colleagues found that they could switch between the two forms of s-surf by changing the gas that they bubbled through a solution of the surfactant. They demonstrated this switch by measuring the electrical conductivity of the s-surf solution; aqueous solutions of ionic compounds have higher conductivity than solutions of nonionic compounds. They started with a solution of the amidine form of s-surf in water. Their results are shown below; dotted lines indicate the switch from one gas to another. .
(d) In which form is the majority of s-surf at point A? At point B?
(e) Why does the electrical conductivity rise from time 0 to point $\mathrm{A} ?$
(f) Why does the electrical conductivity fall from point A to point B?
(g) Explain how you would use s-surf to clean up and recover the oil from an oil spill.