= Solution
The <amphiphile packing parameter> is $p=v/(a\ell)$: hydrocarbon volume divided by the product of effective head-group area and maximal tail length. For a spherical core of radius $R$, $v/a=R/3$; requiring $R\leq\ell$ gives \b[spheres for $p\lesssim1/3$]. Cylindrical cores instead have $v/a=R/2$, favouring cylinders or rods for roughly $1/3<p\lesssim1/2$. Lower-curvature bilayer structures are favoured towards $p\simeq1$; intermediate ranges can include flexible or closed bilayer morphologies.
The charged sulfate head groups make the effective area sensitive to electrostatic repulsion, salt and counterion binding. Screening can reduce that area, increasing $p$ and favouring growth from spheres towards elongated <micelles>. These are geometric selection rules, not exact phase boundaries or a claim that every morphology is stable at any concentration.
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