Structure and dielectric properties of polar fluids with extended dipoles: results from numerical simulations

The strengths and shortcomings of the point dipole model for polar fluids of spherical molecules are illustrated by considering the physically more relevant case of extended dipoles formed by two opposite charges +/-q separated by a distance d (dipole moment mu = qd). Extensive molecular dynamics simulations on a high-density dipolar fluid are used to analyse the dependence of the pair structure, dielectric constant epsilon and dynamics as a function of the ratio d/sigma (sigma is the molecular diameter), for a fixed dipole moment mu. The point dipole model is found to agree well with the extended dipole model up to d/sigma similar or equal to 0.3. Beyond that ratio, epsilon shows a non-trivial variation with d/sigma. When d/sigma > 0.6, a transition is observed towards a hexagonal columnar phase; the corresponding value of the dipole moment is found to be substantially lower than the value of the point dipole required to drive a similar transition.