where a0 is the surface area of the hydrophilic head, ν0 is the tail volume of the monomer and ℓ is the length of this tail. Based on the values of P one can intuitively infer the shape of the self-assembling object. However, even relatively simple surfactants, such as sodium dodecylsulfate, can under certain conditions (that is, high ionic strength) self-assemble in ways that take the concept of packing parameter to its limit. Implicit in the idea of the packing parameter is the assumption of sequestration of the hydrophobic and hydrophilic moieties of each amphiphile into two spatially well-defined regions, the first being a buried part (the so-called core), protected from the solvent by the second one (shell ). The existence of core-shell structures has indeed been shown in numerous experiments, prominently using small-angle scattering (SAS). Assemblies do not need to be spherically symmetric, but depending on the detailed balance of the core/shell contributions to the free energy, organizations with ellipsoidal, cylindrical, or even lamellar/vesicular shapes may also form. . Provided that the interaction parameters between the tail, headgroup, and solvent are rather large, the micellar structure is well-segregated into distinct core and shell regions. For more weakly interacting systems or micelles in mixed solvents , however, significant solvent penetration into the core is found, a feature which can be utilised for micellar catalysis. O O H Whereas surfactants with a simple hydrophobic/hydrophilic linear molecular structure have generally found to form core-shell organizations, less is known about the assemblies of surfactant molecules with more complex molecular geometry. An example is Triton X-100 (TX-100) which contains a bulky aromatic tetramethylbutyl-phenyl group as a hydrophobic tail connected to a hydrophilic oligoethylene oxide (EO) of 9 or 10 units in length (Figure , inset). This amphiphilicity causes the molecules to form micellar structures above a relatively low critical micelle concentration (CMC) of 0.2 to 0.3 mm at 20 • C. TX-100 is one of the most widely used non-ionic detergents for biochemical applications, due to its selective perturbation of the lipid membrane The structure of the TX-100 micelles has been extensively debated in the literature, without reaching a clear consensus. Robson and Dennis suggested different packing morphologies based on geometrical considerations. Due to the small hydrophobic tail of TX-100 relative to the large hydrophilic EO chain, they concluded that if the Triton X-100 micelles are spherical, there must be some degree of staggering of the EO group and the hydrophobic p-tertoctylphenyl group, meaning that no clear core-shell regions would be defined. If such overlap does not exist, an oblate micelle would be the optimal morphology.