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Chemical Reaction: Embracing Phase Diagrams
By: Steve Herman
Posted: December 5, 2006, from the December 2006 issue of GCI Magazine.
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Charles Cagniard de la Tour (1777–1859) is responsible for the deeper understanding of water. In 1822, he discovered the critical point in his famous cannon barrel experiments. He listened to discontinuities in the sound of a rolling flint ball in a sealed cannon filled with fluids at various temperatures. Above a certain temperature, the densities of the liquid and gas phases become equal, resulting in a single supercritical fluid phase. While investigating the effect of heat and pressure on liquids, Cagniard de la Tour found that there was a certain temperature for each phase above which liquids refused to remain liquid, passing into the gaseous state—no matter the amount of pressure to which they were subjected.
The square shape of Figure 1b is appropriate for two variables. For three variables (such as water, oil and surfactant), a triangle is needed. Point A is 100% oil, Point E is 100% surfactant and Point G is 100% water. There is no water along line AE, so every point along the line is a different blend of oil and surfactant. As water is added drop by drop, information along the lines AG, BG, CG, DG, EG is created that completes a picture of the system.
Friberg(1) used phase diagrams in a classic paper on emulsion science. He showed that since macroemulsions are thermodynamically unstable, the known stability could only be accounted for by the presence of liquid crystal phases in the external phase. Summarizing the power of this approach, Friberg wrote: “This hypothetical case shows how many hitherto unexplained properties of emulsions can find a reasonable explanation when ternary systems prepared under equilibrium conditions are used.”
Graham Barker(2) used phase diagrams to explain the organization of sodium stearate molecules when placed in mixtures of water and organic solvent. Sodium stearate often is used to create deodorant sticks, with specific structures needed for structural integrity. Barker’s phase diagram is a mini-course in the possible aggregate structures of amphiphilic molecules.
The sodium stearate, with a polar end and nonpolar tail, assembles based on the environment. In a predominantly polar solvent blend, the polar heads will be on the outside, either in the form of a spherical micelle, a rod or a lamellar sheet. With a nonpolar solution, the tails will stick out. The diagram shows this occurs when the dodecanol exceeds 50%.