The liquid-vapour transition in monolayers of n-pentadecanoic acid at the air/water interface

Using recently developed methods for surface manometry, the first-order two-dimensional phase transition from vapour to liquid in monolayers of ultra-pure n-pentadecanoic acid at the interface between air and aqueous 10^-2 mol dm^-3 HCl has been studied at temperatures from the triple point (17 °C) to 40 °C. Contrary to previous reports, the critical temperature is well above 40 °C. It was not possible with the apparatus and methods developed to date to determine the critical temperature, which is probably in the range of 50-60 °C. The latent heat of the phase transition was obtained as a function of temperature.