Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 1 / 4 / c

The critical micelle concentration is the approximate total surfactant concentration above which additional surfactant predominantly enters micelles. Below it, most surfactant is present as separate molecules; above it, the free-monomer concentration changes much more slowly while the micelle population increases.

The crossover occurs when aggregation balances the unfavorable exposure of hydrophobic tails to water against head-group repulsion and the loss of molecular translational entropy. It is an aggregation threshold, not the concentration at which every molecule becomes micellar.

A longer hydrophobic tail makes transfer from water into a micelle more favorable, lowering the critical micelle concentration within this homologous series. Ranking the four quoted concentrations therefore givesThis assignment uses the chain lengths and concentration ranking printed in the question; it does not identify the compounds by substituting an external table of measured concentrations.