Single-run determination of the saturation vapor pressure and enthalpy of vaporization/sublimation of a substance undergoing successive solid-solid and solid-liquid phase transitions: the case of $N$-methyl acetamide

TL;DR

This work tackles the challenge of measuring the saturation vapor pressure (SVP) and phase-enthalpies for a low-volatile, polymorphic substance, N-methyl acetamide (NMA), over a wide temperature range. It employs a single-run dynamical approach using the ASVAP apparatus, where a precooled NMA sample equilibrates in a static vacuum and the chamber pressure $p_V$ is monitored to extract $p_{\text{sat}}(T)$ and enthalpies for crI, crII, and the liquid phase via a steady-state rate-theory model that accounts for temperature-dependent enthalpies. The analysis yields the SVP and $\Delta H^*$ values for all phases, including the first SVP and sublimation enthalpy data for crII in $-30^\circ$C to $0^\circ$C, and confirms consistency with prior measurements. The method demonstrates a powerful single-run path to thermodynamic data for polymorphic, low-volatile substances and has potential applicability to a broader class of materials with solid-solid and solid-liquid transitions.

Abstract

We report on the dynamical measurement of the saturation vapor pressure of $N$-methyl acetamide in the temperature range $-30^\circ$C to $34^\circ$C. This is achieved by monitoring the pressure inside a vacuum chamber in which a precooled sample of the substance slowly thermalizes to the chamber temperature, undergoing first a phase transition between two crystalline structures around $1^\circ$C and then a solid-liquid phase transition around $30^\circ$C. Such a measurement provides in a single run accurate data for the saturation vapor pressure and the enthalpies of sublimation and vaporization of the different phases of the investigated substance.

Figures (6)

• Figure 1: $T_s$ and $p_V$ as a function of time for measurement 1 (red, initial sample temperature of $-20^\circ$C and chamber temperature of 34.5$^\circ$C) and measurement 2 (orange, initial sample temperature of $-34^\circ$C and chamber temperature of 20.5$^\circ$C).
• Figure 2: (a) Example of variation of the pressure in the load chamber under successive cycles of evacuation and static vacuum. (b) Examples of $p_V$vs$T_S$ measurements after short (5 cycles, magenta) and long (22 cycles, red) purification sequences in the conditions corresponding to those of measurement 1.
• Figure 3: Variation of the heat capacities versus temperature. The pink, magenta and grey thick lines show the values of the liquid, solid (crI) and gaseous heat capacities at constant pressure reported in the literature. The black dashed lines show the results of the linear interpolations used to determine $\beta_\textrm{u}$ and $\alpha_\textrm{u}$ in the different temperature regions corresponding to the different phases. The vertical dashed lines show the positions of the phase transitions when increasing the temperature. The bottom horizontal black lines indicate the temperature ranges used in the experimental measurements 1 and 2.
• Figure 4: (a) $p_V$vs$T_S$ for measurement 1 (red), together with the results of fits with Eq. (\ref{['eq:pVDe']}) for each phase (thin black lines). (b) Fit residuals.
• Figure 5: (a) $p_{\textrm{sat}}$vs$T_s$ for measurements 1 (red line) and 2 (orange line), together with previously reported results Gopal1968Kortum1970Aucejo1993Zaitseva2019Zaitseva2019bStejfa2020. (b) Relative deviation of the measured SVPs with respect to the Cox parametrization performed in Ref. Stejfa2020. The thin red lines show the $\pm 1\sigma$ confidence interval for measurement 1. The symbols show the results of Refs. Gopal1968Kortum1970Aucejo1993Zaitseva2019Zaitseva2019bStejfa2020.