= Solution
An \b[<amphiphile>] has a solvent-compatible polar head and a hydrophobic tail region. The three geometrical parameters are the tail volume $v$, effective head-group area $a_0$ at the interface, and maximum effective tail length $\ell_c$. Their dimensionless combination is the \b[<amphiphile packing parameter>], $P=v/(a_0\ell_c)$.
\Image[/past-exam-of-the-mphil-in-micro-and-nanotechnology-enterprise-course-of-the-university-of-cambridge/2019/paper-2-amphiphile-packing.png]
{title=Head area, hydrophobic volume and tail length control preferred aggregate curvature}
{source=paper-2-amphiphile-packing.py}
The head area is an effective interfacial area affected by solvent, electrostatic repulsion and composition; it is not simply a fixed atomic cross-section.
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