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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

T0 review · 3 major / 4 minor · reviewed 2026-08-04 · deepseek-v4-flash

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Pith's one-line read A single freeze-thaw run measures vapor pressure and enthalpies for three phases of N-methyl acetamide, including the first reported data for its low-temperature crystalline form.

desk verdict Plausible first data for crII NMA, but the Table 1 phase labels are swapped and the missing purity info makes the headline low-pressure point untrustworthy until addressed. read the letter →

arxiv 2602.02504 v1 pith:BTUDUIAZ submitted 2026-01-21 physics.chem-ph cond-mat.mtrl-scicond-mat.soft

Mohsen Salimi , Aurelien Dantan , Henrik B. Pedersen This is my paper · ORCID
classification physics.chem-phcond-mat.mtrl-scicond-mat.soft
keywords N-methylacetamidesaturationvaporpressuresublimationenthalpyvaporizationpolymorphismdynamicalmeasurementstatisticalratetheoryphasetransition
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper claims that monitoring the pressure inside a vacuum chamber as a precooled sample of N-methyl acetamide warms up can, in one continuous run, yield accurate saturation vapor pressures and vaporization/sublimation enthalpies for every phase the sample passes through: two crystalline forms (crII and crI) and the liquid. It reports the first-ever saturation vapor pressure and sublimation enthalpy for the crII phase in the range −30 to 0 °C, with internally consistent values from two independent runs. If correct, the result shows that a single dynamical measurement can replace many separate static measurements for polymorphic substances, saving time and sample while capturing each phase's thermodynamic signature.

What carries the argument

The central object is an analytical steady-state model relating chamber pressure, sample temperature, and saturation vapor pressure through an effective number of vibrational degrees of freedom (D_e,u). This model, combined with the Clausius-Clapeyron equation and a Clarke-Glew-type linear temperature dependence of heat capacities, lets the authors fit each phase's SVP and enthalpy with only two free parameters per phase. The effective DOF absorbs anharmonicity and molecular conformal effects, while the heat-capacity input (from literature, with crII values extrapolated from crI) fixes the temperature dependence of the enthalpy.

What would settle it

Measure the saturation vapor pressure of the same crII phase in a static or effusion apparatus using an independently dried and purity-certified N-methyl acetamide sample; if the SVP at 264 K deviates from 0.38 Pa by more than a few percent, the dynamical method is biased upward by residual water. Alternatively, use a mass spectrometer to sample the vapor during the run and check for a water peak that correlates with the crII-to-crI transition.

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Extended reading notes

Core claim

The authors establish that a dynamical method—cooling a sample to −30 °C, then letting it thermalize to chamber temperature while recording pressure—accurately tracks the saturation vapor pressure of N-methyl acetamide across a solid-solid transition (crII to crI near 1 °C) and a solid-liquid transition (near 30 °C). For the first time they report SVP and sublimation enthalpy for crII NMA: at 264 K the SVP is 0.38 ± 0.01 Pa and the sublimation enthalpy is 65.8 ± 2.5 kJ/mol (run 1) and 66.3 ± 0.9 kJ/mol (run 2). The two measurements agree for all three phases, supporting the claim that only one phase is present at a time during the run.

Load-bearing premise

The claim that the new crII data are accurate rests on the assumption that the sample is essentially free of water and other volatile impurities after 22 purification cycles.

Editorial extensions

If this is right

  • The first crII SVP and sublimation enthalpy data for N-methyl acetamide fill a gap in thermodynamic tables and can be used to refine intermolecular potentials for amides.
  • The method demonstrates that successive phase transitions can be captured in a single run, reducing measurement time and sample consumption for polymorphic low-volatile substances.
  • The significantly lower sublimation enthalpy of crII compared to crI suggests the low-temperature form is enthalpically stabilized, which may inform understanding of the polymorphism.
  • If the approach generalizes, it could provide thermodynamic data for other industrially or atmospherically relevant solids whose low-temperature crystal forms are hard to isolate.
  • The agreement between runs with different chamber temperatures indicates the phase purities are maintained during thermalization, strengthening confidence in the extracted values.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The method could be extended to detect phase transitions by the kink in the pressure-vs-temperature curve, potentially serving as a diagnostic for hidden polymorphs in other compounds.
  • The extrapolation of crI heat capacities to crII introduces systematic uncertainty that worsens at the low-temperature end; independent measurements of crII's heat capacity would tighten the reported crII enthalpy uncertainty.
  • The sensitivity of the pressure signal to water contamination suggests the method could be adapted as a purity probe, with the crII-to-crI transition region acting as a sensitive marker for residual volatiles.
  • For substances with three or more solid phases, the same single-run approach could, in principle, map all of them if the pressure sensor's dynamic range covers the full SVP span.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 4 minor

Summary. The paper reports a dynamical method for measuring saturation vapor pressures (SVP) and sublimation/vaporization enthalpies of a substance that undergoes successive phase transitions. A precooled N-methyl acetamide sample is inserted into a high-vacuum chamber at elevated temperature, and the chamber pressure is monitored as the sample thermalizes. As the sample crosses the crII→crI transition (~1 °C) and then melts (~31 °C), the measured p_V vs. T_S trace is fit phase-by-phase to a statistical rate theory model (Eq. 8) with literature heat-capacity inputs, yielding p_sat(T) and ΔH for each phase. The central new claim is the first determination of SVP and sublimation enthalpy for the low-temperature crII phase in the −30 to 0 °C range, with p_sat(264 K) = 0.38 ± 0.01 Pa and ΔH_sub = 65.8 ± 2.5 kJ/mol.

Significance. If the experimental results are accurate, the paper demonstrates a single-run route to phase-resolved SVP and enthalpy data for polymorphic substances, and it provides previously unavailable crII NMA data. The overdetermination from two independent runs with different chamber temperatures, together with agreement with literature for the crI and liquid phases, lends credibility to the method. The analysis is a proper parameter fit, not a circular derivation: the fixed inputs (α, β, D_e) come from literature heat capacities, while p*_sat and ΔH* are free parameters fitted to the measured p(T) trace. The main risks are the incomplete purification/purity reporting and internal inconsistencies in phase labeling and units, which currently prevent the reader from directly using the tabulated results.

major comments (3)
  1. [Table 1; §1; §4] The phase labels in Table 1 are reversed relative to the text. The Introduction and Conclusion state that the low-temperature phase is crII, transforming to crI at ~1 °C when heated, and that the headline result is crII SVP in the −30 to 0 °C range. However, Table 1 labels the row for −20–0 °C as 'crI' and the row for 2–29 °C as 'crII' in both measurements. This makes it impossible to tell which fit parameters and results correspond to crII, directly obscuring the paper's claimed new data. Figure 4's caption likewise refers to 'crI–crII and crII-liquid phase transitions' in the order appropriate only if the labels were reversed. Please correct Table 1, all figure captions, and any associated text to use a single consistent assignment (crII = low-temperature phase, crI = high-temperature phase).
  2. [Table 1, units of β_u and α_u] The units stated in Table 1 for β_u and α_u appear to be wrong by orders of magnitude. The header lists β_u in kJ/mol and α_u in kJ/mol/K, but the numerical values (e.g., β = −33.8, −39.9, −60.4 and α = −0.2, −0.2, −0.02) are only physically plausible if β is in J/mol/K and α is in J/mol/K². Since these parameters enter the exponent in Eq. (5), a reader using the printed units could not reproduce the reported p_sat curves. This is not only a typographical issue: the manuscript reports thermodynamic constants and must present them in unambiguous, correct units. Please re-label the columns (or, if the intended units are actually kJ/mol, correct the values by three orders of magnitude).
  3. [§2.2; Fig. 2(b); Table 1 crII row] The sample purity is reported as 'certified purity of XXX%' — an unresolved placeholder — and the paper provides no post-purification chemical analysis, e.g., residual water content. The method's own Fig. 2(b) demonstrates that insufficient purification overestimates p_V and gives a contamination signature in the crII→crI transition region. The new crII SVP values are the lowest-pressure data in the paper (0.38 Pa at 264 K), and the SVP of water at that temperature is orders of magnitude higher; even a small residual water fraction would dominate the total pressure measured by the absolute capacitance sensor. The convergence of the purification curve after 22 cycles is reassuring but does not prove water is absent. In addition, the chamber outgassing rate quoted in §2.1 (10⁻² Pa/h) is not negligible relative to the lowest reported SVP. Please provide the actual certified purity, report a
minor comments (4)
  1. [§2.3, Eq. (8)] The left-hand side of Eq. (8) is typeset as p_V^{D_e,u}; the exponent D_e,u on p_V appears to be a typo, as the right-hand side gives a model for p_V itself. Please clarify the notation.
  2. [Captions] Figure 4 caption says 'crI–crII and crII-liquid phase transitions' for increasing temperature; the correct sequence is crII→crI and crI→liquid. Similarly, check Figure 5 and Figure 6 axis labels and legends for consistency with the corrected phase nomenclature.
  3. [Throughout] There are several typos and misspellings: 'eniantropic' (Introduction), 'abrupty' (§3), 'evaporization' (Table 1 header), 'occured' (§3), 'surmize' (§2.2). A careful proofreading pass is needed.
  4. [§2.3] The paper states that for crII the heat capacity is taken as an extrapolation of crI data. This assumption is reasonable given the narrow temperature range, but it should be acknowledged in the uncertainty budget or at least stated explicitly in the Table 1 notes, since β and α are fixed inputs to the fit.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity: SVP/enthalpy values are free fit parameters to measured p(T) data using a literature-derived thermodynamic model.

full rationale

The paper's reported SVP and enthalpies are obtained by fitting the measured p_V vs. T_S trace to Eq. (8) with p*_sat,u and ΔH*_u as the only free parameters; all other quantities (β, α, D_e) are fixed from literature heat capacities or from prior method papers, not from the target results. Eq. (8) is a steady-state SRT expression that relates chamber pressure to SVP through a known characteristic function, and at T_V = T_S it reduces to p_V = p_sat, so the fit is not circular. The new crII data are first-time values, not a renaming of known data; the comparison against external literature SVPs for crI and liquid provides independent validation of the method in this paper. The manuscript does rely on the authors' prior method papers, but those are independent, externally validated method developments and do not contain the target NMA result; thus no load-bearing self-citation reduction occurs. The purity placeholder 'XXX%' and lack of a final water assay are correctness/uncertainty concerns, not circular derivation steps. Overall, no step reduces a claimed prediction to its input by construction.

Assumptions & free parameters 6 free parameters · 6 assumptions · 0 invented entities

The measurement depends on six fitted reference parameters (p* and ΔH* per phase) and on standard thermodynamic relations plus three domain assumptions: linear Cp corrections, the SRT steady-state model, and sample purity. No new physical entities are introduced; the effective vibrational DOF D_e is a model parameter fixed by Eq. (7), not an invented entity.

free parameters (6)
  • p*_sat,crII (low-T form per text) = 0.38 ± 0.01 Pa at T*=264 K (both measurements)
    Reference SVP for the low-temperature solid phase fitted via Eq. (8); reported in Table 1 under a swapped label 'crI'.
  • p*_sat,crI (high-T form per text) = 9.7 ± 0.2 Pa (M1), 9.9 ± 0.2 Pa (M2) at T*=294 K
    Reference SVP for the high-temperature solid phase fitted via Eq. (8); reported in Table 1 under the label 'crII'.
  • p*_sat,liquid = 32.5 ± 0.7 Pa at T*=307 K (M1 only)
    Reference SVP for liquid NMA from a single 31–34.5 °C window.
  • ΔH*_crII (low-T form per text) = 65.8 ± 2.5 kJ/mol (M1), 66.3 ± 0.9 kJ/mol (M2) at T*=264 K
    Sublimation enthalpy at reference temperature, a free parameter of the Eq. (8) fit.
  • ΔH*_crI (high-T form per text) = 71.4 ± 1.4 kJ/mol (M1), 71.6 ± 1.6 kJ/mol (M2) at T*=294 K
    Sublimation enthalpy at reference temperature, a free parameter of the Eq. (8) fit.
  • ΔH*_liquid = 63.1 ± 13.5 kJ/mol at T*=307 K
    Vaporization enthalpy at reference temperature; very weakly constrained because the liquid temperature window is only ~3.5 K.
assumptions (6)
  • standard math Ideal-gas Clausius-Clapeyron relation, Eq. (1)
    Assumes ideal-gas vapor and the standard d(ln p)/d(1/T) = -ΔH/R form; standard thermodynamic relation cited to Wark (1988).
  • domain assumption Clarke-Glew linear approximation for ΔCp = Cp,g - Cp,u, Eq. (3)
    Assumes the heat-capacity difference between gas and each condensed phase is a linear function of temperature over each fitted range; a standard but non-exact approximation.
  • ad hoc to paper crII heat capacity equals extrapolation from crI data
    Section 2.3: 'In absence of reported values for the crII heat capacity we use values extrapolated from the crI phase data.' This directly fixes β_crII and α_crII used when fitting the new crII SVP and enthalpy data, with no experimental validation.
  • domain assumption Steady-state flux-balance condition of the SRT model, Eqs. (6)-(8)
    Assumes evaporation and condensation rates are much faster than sample thermalization so that measured chamber pressure equals p_sat times a characteristic function; model imported from the authors' prior method papers.
  • domain assumption Effective vibrational degrees of freedom D_e,u from gas heat capacity, Eq. (7)
    Assumes molecular vibrations with an effective count D_e dominate the SRT characteristic function; D_e is computed from literature Cp,g, not fitted to the present data.
  • domain assumption Purification converges to a water-free sample
    Section 2.2 relies on convergence of load-chamber pressure over purification cycles; no independent chemical purity measurement is reported (certified purity is a placeholder 'XXX%'), and the authors themselves show insufficient purification overestimates SVP.

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Pith. "Pith review of 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." pith.science (2026). https://pith.science/paper/BTUDUIAZ

@misc{pith2026260202504,
  author       = {Pith},
  title        = {Pith review of: 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},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/BTUDUIAZ}},
  note         = {Machine review of arXiv:2602.02504}
}
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

Figures reproduced from arXiv: 2602.02504 by the authors.

Figure 1
Figure 1. 𝑇𝑠 and 𝑝𝑉 as a function of time for measurement 1 (red, initial sample temperature of −20◦C and chamber temperature of 34.5◦C) and measurement 2 (orange, initial sample temperature of −34◦C and chamber temperature of 20.5◦C). First Author et al.: Preprint submitted to Elsevier Page 4 of 4 [PITH_FULL_IMAGE:figures/full_fig_p005_1.png] view at source ↗
Figure 3
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 𝛽u and 𝛼u in the different temperature regions corresponding to the different phases. The vertical dashed lines show the… view at source ↗
Figure 5
Figure 5. (a) 𝑝sat vs 𝑇𝑠 for measurements 1 (red line) and 2 (orange line), together with previously reported results Gopal and Rizvi (1968); Kortüm and Biedersee (1970); Aucejo et al. (1993); Zaitseva et al. (2019a,b); Štejfa et al. (2020). (b) Relative deviation of the measured SVPs with respect to the Cox parametrization performed in Ref. Štejfa et al. (2020). The thin red lines show the ±1𝜎 confidence interval for measure… view at source ↗
Figures from the paper (1 more)
Figure 6
Figure 6. Figure 6: Enthalpies of sublimation and vaporization for 𝑁- methyl acetamide determined in this work for the three phases as a function of temperature. The single points with error bar indicate the used reference temperature (𝑇 ∗ u ) and show the 1𝜎 confidence interval. The blac…

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Reference graph

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