The electrostatics of two charged surfactant layers in aqueous media (surfactant/water/surfactant films) is investigated using molecular dynamics simulations. In the films studied (with a surfactant-surfactant distance from $\sim 35\AA$ to contact) we observe an anomalous dielectric response of water. The electrostatic potential $\varphi (z)$ inside the aqueous core of the films (containing bulk water with $\rho =1\mathrm{g}/{\mathrm{cm}}^3$) is completely different from that expected for a film containing a dielectric medium with the dielectric constant of water. In addition, our results are not consistent with a local relation between the water polarization $P_z(z)$ and the electric field $E_z(z)$. The polarization $P_z(z)$ is maximum at the interfaces (due to solvent molecules forming part of the structure of the surfactant layers) and decays from the interfaces inside the aqueous core with a decay length of order of $\sim 10\AA$.

• Received 15 December 2003

DOI:https://doi.org/10.1103/PhysRevLett.92.236102