Terahertz Time domain Spectroscopy (THz-TDS) measurements are performed on
reverse micelles, prepared with the non-ionic surfactant Igepal CO-520, and the results
are combined with previously performed measurements in the GHz range. With this
technique the complex permittivity εˆ is measured and the orientation dynamics of water
are studied for three systems: spherical micelles of neat water
(water/Igepal/cyclohexane), spherical micelles with acid (1M HCl/Igepal/cyclohexane),
and cylindrical micelles of neat water (water/Igepal/n-hexane). It is found that the static
permittivity of water in micelles is lower than that of bulk water, but slightly higher than
was calculated by using an effective medium approximation (EMA). The thickness of the
layer of water molecules that is affected by the nano-confinement is approximately 5.3 ±
0.4 Å, which corresponds to two water molecules. These molecules on the surface of the
micelle have a larger re-orientation time constant than bulk water, and the value of those
constants is dependent on the size of the micelle.
The dynamics of protons in micelles with acid differs from the dynamics of protons in
bulk water. As a result of the nano-confinement, the protons give rise to a resonance,
which is similar to a Debye relaxation process. This is in contrast to protons in bulk
water, where the contribution to the permittivity is purely imaginary and inversely
proportional to the frequency.
For cylindrical micelles it is found that the ratio between surface water and bulk water
changes. In cylindrical micelles there are relatively more surface water molecules than in
spherical micelles. This is also expected when considering the geometrical surface to
volume ratio of a cylinder and a sphere.