Questions

1 The amphiphilic molecule sodium dodecyl sulphate (SDS) has the following molecular characteristics:

length of hydrophobic tail lc = 1.67 nm volume of hydrophobic tail v = 0.35 nm3 area per head group a0 = 0.57 nm2.

Assuming that SDS forms spherical micelles, calculate the micelle radius and the mean aggregation number. Check for the consistency of this result.

Answers

1 The spherical micelle has a volume and area given by



Therefore



and



The radius that has been found is larger than the maximum chain length, lc, so molecules with these parameters would not form spherical micelles. In fact, this can also be checked by calculating



which is larger than 1/3 and smaller than 1/2. So a molecule with these parameters will form cylindrical aggregates.