The vapor pressure of water nanodroplets

MATIAS FACTOROVICH; VALERIA MOLINERO; D. A. SCHERLIS

JOURNAL OF THE AMERICAN CHEMICAL SOCIETY

AMER CHEMICAL SOC

Lugar: Washington; Año: 2014 vol. 136 p. 4508 - 4514

0002-7863

Classical thermodynamics is assumed to be valid up to acertain length-scale, below which the discontinuous nature of matter becomesmanifest. In particular, this must be the case for the description of the vaporpressure based on the Kelvin equation. However, the legitimacy of thisequation in the nanoscopic regime can not be simply established, because thedetermination of the vapor pressure of very small droplets poses a challengeboth for experiments and simulations. In this article we make use of a grandcanonical screening approach recently proposed to compute the vaporpressures of finite systems from molecular dynamics simulations. Thisscheme is applied to water droplets, to show that the applicability of theKelvin equation extends to unexpectedly small lengths, of only 1 nm, wherethe inhomogeneities in the density of matter occur within spatial lengths ofthe same order of magnitude as the size of the object. While in principle thisappears to violate the main assumptions underlying thermodynamics, the density profiles reveal, however, that structures of thissize are still homogeneous in the nanosecond time-scale. Only when the inhomogeneity in the density persists through thetemporal average, as it is the case for clusters of 40 particles or less, do the macroscopic thermodynamics and the moleculardescriptions depart from each other.