Technical note: Estimating aqueous solubilities and activity coefficients of mono- and α,ω-dicarboxylic acids using COSMOtherm

Hyttinen, Noora; Heshmatnezhad, Reyhaneh; Elm, Jonas; Kurtén, Theo; Prisle, Nønne L.

doi:https://doi.org/10.5194/acp-20-13131-2020

Articles | Volume 20, issue 21

https://doi.org/10.5194/acp-20-13131-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/acp-20-13131-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 20, issue 21

Technical note

|

09 Nov 2020

Technical note |

| 09 Nov 2020

Technical note: Estimating aqueous solubilities and activity coefficients of mono- and α,ω-dicarboxylic acids using COSMOtherm

Noora Hyttinen, Reyhaneh Heshmatnezhad, Jonas Elm, Theo Kurtén, and Nønne L. Prisle

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Final revised paper (published on 09 Nov 2020)
Supplement to the final revised paper
Preprint (discussion started on 26 May 2020)
Supplement to the preprint

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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- Supplement

RC1: 'Review of Hyttinen et al', Anonymous Referee #1, 10 Jun 2020
RC2: 'Review', Anonymous Referee #2, 26 Jun 2020
AC1: 'Response to referees', Noora Hyttinen, 02 Sep 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

AR by Noora Hyttinen on behalf of the Authors (02 Sep 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (08 Sep 2020) by Barbara Ervens

RR by Anonymous Referee #1 (14 Sep 2020)

Suggestions for revision or reasons for rejection

The authors have provided responses to the questions, comments and issues pointed out by both referees. They have carried out additional calculations and have modified the manuscript to account for suggested changes, where applicable.
For this second round of reviews of this technical note, I will focus on the replies and manuscript sections with changes. Most of the issues raised in the first round have been addressed well. I have found a few issues to be further addressed before this manuscript is finalized for publication.

The line numbers used in the following comments are those from the revised manuscript (ms version 3).

\begin{itemize}
\item line 300: Regarding the acid dissociation reaction (R2) notation for oxalic acid, it is a bit odd that you write the reaction using HA for the acid, given that oxalic acid is a diacid. It would be better to write the reaction as $\chem{H_2A} + \chem{H_2O} = \chem{HA^-} + \chem{H_3O^+}$ and perhaps also list the second dissociation reaction involving \chem{HA^-} (even if not considered by COSMO-RS-DARE). Further along these lines, from the main text alone it remains unclear whether both acid dissociation reactions were considered in the model or not; should be clarified.

\item section 3.2.2, line 282 and Supplement, Fig. S3 -- S5: The experimental data shown in these figures and related interpretation in the text needs to be corrected.
For example, in Fig. S3, 'growth' mode data from EDB experiments by Peng et al. (2001) and Choi and Chan (2002) are shown. At higher acid concentrations, under growth conditions (also listed as 'condensation' for Peng et al data), dicarboxylic acids are mostly in the effloresced/crystalline state. The measurement data do not reflect aqueous solutions and cannot be used to derive water activity coefficients. It seems the authors used those data points as if they were showing values for a single aqueous solution, which is clearly incorrect. Showing such growth data is confusing and pointless for the comparison in these figures.
Have a look at Fig. 8 from Choi and Chan (2002). It is clear from that figure that any EDB growth mode data for $a_w < 0.83$ is for the effloresced glutaric acid case, for which the aqueous phase composition, if present, is simply unknown from these measurements. Furthermore, any data for water activities less than about 0.3 are also indicative of efflorescence of the acid; therefore these data cannot be used to derive water activity coefficients. For reference, at a water activity of $\sim$~0.4, the mass fraction of solute (mfs in the Fig. 8 of Choi and Chan) is about 0.9, which corresponds to about $x_{acid}$ of 0.55 for the 'evaporation' data points. The water activity coefficient there is about 0.89 (convention I). There is no (valid) water activity coefficient data in purely aqueous solution of glutaric acid that would suggest water activity coefficients to increase substantially at high $x_{acid}$ (say for $x_{acid} > 0.6$). Therefore, the experimental data shown in Fig. S3 (and similarly in S4, S5) need to be corrected (no growth / condensation data fro EDB measurements should be shown, nor the evaporation branch data for low water activity < ~0.3, depending on the acid). The related discussion in the main text and supplement needs to be corrected as there is no measurement data supporting an increase in water activity coefficient to large values at high $x_{acid}$ for the diacids. In Fig. S3d, the shown COSMO-RS, Acid dimer variant, UNIFAC and AIOMFAC model curves are likely showing the correct trend towards high acid fraction.

\item section 3.2.2, line 287: Correct the sentence; "supersaturated" and "crystalline" have different meaning and the phrasing here confuses this. An acid solution cannot be supersaturated and "crystalline" at the same time; in the presence of a crystalline phase, the remaining aqueous solution will automatically be saturated under equilibrium conditions (but not to be confused with the composition at the solubility limit).

\item Abstract, line 8: correct phrasing: "fitting parameters" should be "use of fit parameters". Also, this sentence would make more sense if it were stated for what purpose the fit parameters were introduced. Of course, with a sufficient number of fit parameters and associated functions, one could use all kinds of models to fit thermodynamic data.

\item line 89: Rephrase the following: "Pseudo-chemical potential has recently been used in molecular level solvation thermodynamics as a replacement to chemical potential". "replacement" is not a good description since the pseudo-chemical potential is not replacing the meaning or use of the chemical potential in thermodynamics; it simply expresses a different quantity, which is why naming it "pseudo-chemical" remains an unfortunate choice by Ben-Naim.

\item line 98 and Eq. (4): This sentence is unclear: "In LLE, the standard chemical potential ($\mu$) of a compound is equal in both of the liquid phases..." and also on line 101: "The standard chemical potential of compound $i$ in a solution is defined using the standard chemical potential at the reference state.". In both sentences, the authors seem to confuse the chemical potential with the term "standard chemical potential" -- they are equivalent. The word "standard" has very specific meaning in thermodynamics and should not be confused with "regular" or "usual". In the second sentence, it should be "The standard chemical potential of compound $i$ in a solution is defined using the chemical potential at the reference state" (where, in convention I, the second term on the right hand side vanishes).

\item line 106, Eq. (6): The notation of this equation is wrong. The phase should be specified for each activity (e.g. as superscript; $a_i^\alpha$) since that is the point of the isoactivity condition among different phases. The same should also be corrected for Eq. (4). It is also potentially misleading to state the mole fraction in parenthesis, because this equation could be misunderstood as activity times mole fraction = ...

\end{itemize}

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RR by Anonymous Referee #2 (16 Sep 2020)

Suggestions for revision or reasons for rejection

The authors have revised the manuscript according to the comments and suggestions of the reviewers. With these revisions, the manuscript is substantially improved and can be published subject to minor revisions outlined below:

Line 13: do you mean lack of feasibility and sample availability? If yes, the sentence needs to be reformulated.

Line 45: It is not clear to what "previously" refers in the new context of the revised manuscript.

Lines 89 – 90: The benefit of working with pseudo-chemical potentials should be explained.

Line 100: Why do you use in this equation the standard chemical potential and not the pseudo-chemical potential?

Line 279: the convention III is explained in the supplement but not in the main text. Either an explanation should be added to the main text or there should be a reference to the supplement here.

Lines 282 – 299: In this discussion, “COSMOtherm” is sometimes used to refer to COSMO-RS and COSMO-RS-DARE and sometimes only to refer to one of these implementations. This is confusing. E.g. line 286: what is meant here with both models? UNIFAC and COSMOtherm or COSMO-RS and COSMO-RS-DARE? Line 293: “with COSMOtherm slightly overestimating the experiments”: Do you mean here COSMO-RS and COSMO-RS-DARE or just one of these? Line 297 – 299: do you mean just COSMO-RS or also COSMO-RS-DARE? The authors should consider to only use “COSMOtherm” when both implementations (COSMO-RS and COSMO-RS-DARE) are meant.

Lines 300 – 309: here activity coefficients are calculated including dissociation of oxalic acid. However, it is not stated whether this is done for COSMO-RS or COSMO-RS-DARE. Also, the explanation of the dissociation calculation given in the supplement is obscure. Why do you assume a pH value of 7.0 of the solution? The oxalic acid solution should be acidic. As starting point of the calculation, the equilibrium equation should be formulated. In Fig. S6 only the dissociation of all confs are given as dashed lines. But what are all confs? What are the 0 H-bond confs?

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ED: Publish subject to minor revisions (review by editor) (17 Sep 2020) by Barbara Ervens

AR by Noora Hyttinen on behalf of the Authors (23 Sep 2020) Author's response Manuscript

ED: Publish as is (01 Oct 2020) by Barbara Ervens

AR by Noora Hyttinen on behalf of the Authors (01 Oct 2020)

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Short summary

We present aqueous solubilities and activity coefficients of mono- and dicarboxylic acids (C₁–C₆ and C₂–C₈, respectively) estimated using the COSMOtherm program. In addition, we have calculated effective equilibrium constants of dimerization and hydration of the same acids in the condensed phase. We were also able to improve the agreement between experimental and estimated properties of monocarboxylic acids in aqueous solutions by including clustering reactions in COSMOtherm calculations.