= Solution
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>. \b[It is an aggregation threshold, not the concentration at which every <molecule> becomes micellar.]
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