Simulations of coexistence between layered phases in planar slits

Abstract

Layering transitions of a simple fluid in a planar pore with structureless attractive surfaces are investigated, using both simulations and density functional theory. The thermodynamic conditions for coexistence between three different layered phases, denoted 1, 2 and 3 in order of increasing average density, are determined, and coexistence curves are presented. The simulations are performed in the isotension ensemble at low temperatures, while the recently developed restricted isotension ensemble was used at elevated temperatures, in order to avoid phase transitions. According to the simulation results, 1–2 and 2–3 coexistences exist between stable phases at low temperatures, but the 2-phase is truncated by a triple point at which all three phases coexist. Above this triple point, 1–3 coexistence is the only stable one. These results are not confirmed by the density functional calculations, according to which 1–3 coexistence at low temperatures is replaced by a simple thin—thick coexistence at higher temperatures. With a slightly increased surface—fluid potential, the density functional calculations do predict the existence of a 1-2-3 triple point. However, in this case the relative stability of the layered phases above and below the triple point is reversed, as compared with the simulation results. Since the differences in the coexistence quantities are small, we have to consider the possibility that this unexpected behaviour is not a true effect, but rather a result of statistical fluctuations. At any rate, the occurrence of 1–3 coexistence together with a triple point is, to the best of our knowledge, a novel observation and is confirmed by density functional theory. Furthermore, the simulations show that the 1–2, 2–3 and 1–3 coexistence curves are very similar.