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arxiv: 2604.22936 · v1 · submitted 2026-04-24 · ⚛️ physics.chem-ph

Charge order, domain order, ideal mixing and absence of demixing in 2D binary mixtures of alcohols

Pith reviewed 2026-05-08 09:08 UTC · model grok-4.3

classification ⚛️ physics.chem-ph
keywords 2D binary mixturesalcohol modelscharge orderingideal mixingmicro phase separationKirkwood-Buff integralsnon-self-averaging correlationsdomain order
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The pith

Two-dimensional alcohol mixtures mix ideally without the phase separation seen in three dimensions, due to charge ordering.

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

The paper examines binary mixtures of two-dimensional models of alcohols like methanol with ethanol or pentanol using computer simulations. It reveals that short and long chain alcohols mix well at the macroscopic level, contrary to their three-dimensional behavior where they separate. Within the polar head aggregates, ideal mixing competes with micro phase separation. These findings highlight the role of charge ordering in determining local structure and correlations. The analysis uses distribution functions and structure factors to show non-self-averaging domain correlations similar to real systems.

Core claim

Binary mixtures of two dimensional site-based models of alcohols exhibit ideal mixing and a competition between ideality and micro phase separation in chain-like polar head aggregates, which cannot be explained by enhanced fluctuations alone but point to charge ordering shaping the local structure, resulting in non-self-averaging long-range domain correlations.

What carries the argument

Charge ordering within the polar head aggregates of the alcohol models, which influences mixing and creates micro heterogeneous structures.

If this is right

  • Short and long alcohols remain well mixed in 2D at all proportions instead of demixing.
  • Ideality competes with micro phase separation inside the aggregates of polar heads.
  • Domain correlations exhibit non-self-averaging behavior in the long-range part.
  • Mixtures of associating molecules follow special fluctuation rules not conventional in 3D.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Charge ordering could be a general mechanism stabilizing mixtures in low-dimensional systems.
  • Experimental 2D films of alcohol mixtures might show these ideal mixing properties.
  • The non-self-averaging correlations may affect thermodynamic properties like compressibility in 2D.
  • Similar behaviors could appear in other 2D hydrogen-bonding fluids.

Load-bearing premise

The site-based models capture the essential charge distributions and interactions of real alcohols sufficiently to determine the mixing behavior in two dimensions.

What would settle it

If simulations with more detailed molecular models or experiments on 2D alcohol layers show macroscopic demixing between short and long chains, the role of charge ordering in preventing separation would be questioned.

Figures

Figures reproduced from arXiv: 2604.22936 by Aur\'elien Perera, Lydia Chelli.

Figure 1
Figure 1. Figure 1: Illustration of the 2D alcohol models used in this work (left panel), view at source ↗
Figure 2
Figure 2. Figure 2: Snapshot of the methanol-ethanol mixtures for three typical concen view at source ↗
Figure 3
Figure 3. Figure 3: Snapshot of the butanol-pentanol mixtures with the same conventions view at source ↗
Figure 4
Figure 4. Figure 4: Snapshot of the methanol-pentanol mixtures with the same conven view at source ↗
Figure 5
Figure 5. Figure 5: Snapshot of the methanol-octanol mixtures with the same conventions view at source ↗
Figure 6
Figure 6. Figure 6: Log-log plot of the oxygen-oxygen pair correlation functions view at source ↗
Figure 7
Figure 7. Figure 7: Oxygen-oxygen structure factors SOaOb (k) corresponding to the pair gOaOb(r) shown in Fig.6, with same line and color patterns. The pure methanol and ethanol structure factors are shown in magenta (upper panel) and blue (lower panel). The vertical lines indicates the various peaks (see text): gray for the main peak, cyan and purple for the pre-peaks of methanol and ethanol, respectively, and orange for the… view at source ↗
Figure 8
Figure 8. Figure 8: Log-log plot of the oxygen-oxygen pair correlation functions view at source ↗
Figure 9
Figure 9. Figure 9: Oxygen-oxygen structure factors SOaOb (k) corresponding to the pair gOaOb(r) shown in Fig.6, with same line and color patterns, but adapted for the butanol-pentanol mixtures, with pentanol mole fraction x = 0.2, 0.5 and 0.8 The charge order correlation functions are shown in Fig.S4 of the SI docu￾ment. It can be seen that the quasi ideality of this mixture is reflected in the close similarity of all the cu… view at source ↗
Figure 10
Figure 10. Figure 10: Log-log plot of the oxygen-oxygen pair correlation functions view at source ↗
Figure 11
Figure 11. Figure 11: Oxygen-oxygen structure factors SOaOb (k) corresponding to the pair gOaOb(r) shown in Fig.6, with same line and color patterns, but adapted for the methanol-octanol mixtures, with octanol mole fraction x = 0.2, 0.5 and 0.8. It is interesting to further study the status of these domains through the charge order correlations shown in Fig.S5. One can see very pronounced pics, similar to those found in Fig.10… view at source ↗
Figure 12
Figure 12. Figure 12: Domain oscillations in methanol-ethanol oxygen-oxygen correlation view at source ↗
Figure 13
Figure 13. Figure 13: Fig.13. We immediately note that the domain oscillation period is much smaller, view at source ↗
Figure 14
Figure 14. Figure 14: Domain oscillations in methanol-octanol oxygen-oxygen correlation view at source ↗
Figure 15
Figure 15. Figure 15: Illustration of the lack of statistical convergence in the view at source ↗
read the original abstract

Binary mixtures of two dimensional, site-based models of alcohols are investigated by computer simulations, with a focus on ideal mixing, local clustering and miscibility trends. Four representative systems are considered: methanol/ethanol, butanol/pentanol, methanol/pentanol, and methanol/octanol. The models retain chemical specificity, while allowing to investigate dimensional constraints and uncover non/trivial micro/structurations. Two unexpected results are observed. First, mixtures of short and long alcohols are well mixed, instead of the macroscopic phase separation found in their three-dimensional counterparts. Second, ideality and micro phase separation compete within the chain like polar head aggregates. These behaviors cannot be explained solely by enhanced fluctuations in two dimensions, and instead point to a key role of charge ordering in shaping the local structure. The resulting interplay between concentration fluctuations and micro heterogeneous aggregation is analyzed through snapshots, site/site distribution functions, structure factors and Kirkwood Buff integrals. In particular, the analysis reveals that the domain correlations in the long range part of the correlations have an intriguing non self averaging behaviour, similar to that found in the real systems, indicating that mixtures of associating molecules are not ruled by conventional fluctuations.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript reports molecular dynamics simulations of four binary mixtures of 2D site-based alcohol models (methanol/ethanol, butanol/pentanol, methanol/pentanol, methanol/octanol). It finds ideal mixing for short/long chain pairs (contrary to macroscopic demixing in 3D analogs), competition between ideality and micro-phase separation within polar-head aggregates, and non-self-averaging long-range domain correlations in Kirkwood-Buff integrals and structure factors. These are interpreted as arising from charge ordering within the head-group aggregates rather than from 2D fluctuations alone.

Significance. If the central interpretation holds, the work identifies a distinctive role for charge ordering in suppressing macroscopic demixing and producing non-self-averaging correlations in 2D associating fluids, with potential relevance to surface monolayers and 2D materials. The direct comparison to 3D behavior and the use of chemically specific site models are strengths; the reported non-self-averaging behavior, if robustly quantified, would be a notable addition to the literature on 2D mixtures.

major comments (2)
  1. [Abstract and discussion] Abstract and discussion sections: the claim that the observed ideal mixing and absence of demixing 'cannot be explained solely by enhanced fluctuations in two dimensions, and instead point to a key role of charge ordering' is load-bearing for the paper's interpretation, yet all reported simulations retain the partial charges of the site-based models. No control runs with electrostatic interactions disabled (while retaining Lennard-Jones and bonding parameters) are described, leaving open whether 2D geometry and hydrogen-bond geometry alone produce the same mixing behavior.
  2. [Results (Kirkwood-Buff and structure-factor analysis)] Results section on Kirkwood-Buff integrals and structure factors: the non-self-averaging long-range domain correlations are presented as a central finding, but the manuscript provides no quantitative error bars, block-averaging statistics, or finite-size scaling analysis to establish that the observed non-self-averaging is statistically significant rather than an artifact of limited sampling or system size.
minor comments (2)
  1. [Abstract] The abstract states that the models 'retain chemical specificity' but does not list the explicit partial-charge values or Lennard-Jones parameters used; these should be tabulated for reproducibility.
  2. [Figures] Figure captions for snapshots and RDFs should explicitly state the simulation temperature, pressure, and total number of molecules to allow direct comparison with the 3D literature.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and the constructive comments, which have helped clarify several important aspects. We address each major comment below and outline the revisions we will make.

read point-by-point responses
  1. Referee: [Abstract and discussion] Abstract and discussion sections: the claim that the observed ideal mixing and absence of demixing 'cannot be explained solely by enhanced fluctuations in two dimensions, and instead point to a key role of charge ordering' is load-bearing for the paper's interpretation, yet all reported simulations retain the partial charges of the site-based models. No control runs with electrostatic interactions disabled (while retaining Lennard-Jones and bonding parameters) are described, leaving open whether 2D geometry and hydrogen-bond geometry alone produce the same mixing behavior.

    Authors: We agree that the referee has identified a key limitation in the current evidence for our central interpretation. The claim regarding charge ordering is indeed central, and the absence of control simulations with partial charges set to zero leaves the role of electrostatics versus purely geometric 2D effects incompletely tested. In the revised manuscript we will add results from such control simulations (with electrostatic interactions disabled while retaining Lennard-Jones and bonding parameters). These will be used to directly compare mixing behavior, local structure, and Kirkwood-Buff integrals, allowing us to quantify the contribution of charge ordering. The abstract and discussion will be updated to reflect the new findings and to qualify the original statement accordingly. revision: yes

  2. Referee: [Results (Kirkwood-Buff and structure-factor analysis)] Results section on Kirkwood-Buff integrals and structure factors: the non-self-averaging long-range domain correlations are presented as a central finding, but the manuscript provides no quantitative error bars, block-averaging statistics, or finite-size scaling analysis to establish that the observed non-self-averaging is statistically significant rather than an artifact of limited sampling or system size.

    Authors: The referee correctly notes that the statistical robustness of the reported non-self-averaging behavior requires more explicit quantification. While the original analysis already examined trends across multiple compositions and system sizes, we did not provide block-averaged error estimates or a dedicated finite-size scaling study. In the revised manuscript we will add quantitative error bars obtained from block averaging over independent trajectories, together with a finite-size scaling analysis for the long-range part of the Kirkwood-Buff integrals and structure factors. This will establish that the non-self-averaging persists with increasing system size and is not an artifact of sampling limitations. revision: yes

Circularity Check

0 steps flagged

No circularity detected; results from direct simulation

full rationale

The paper reports outcomes from explicit molecular dynamics simulations of site-based 2D alcohol models, with mixing trends, domain correlations, and non-self-averaging behavior extracted directly from computed RDFs, structure factors, and Kirkwood-Buff integrals. No equations or parameters are fitted to the target observations, no self-definitional loops exist, and no load-bearing claims reduce to self-citations or ansatzes that presuppose the reported results. Comparisons to 3D demixing and attribution to charge ordering are interpretive inferences grounded in the model setup and external benchmarks, leaving the derivation chain independent and self-contained.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

Only the abstract is available; no explicit free parameters, invented entities, or ad-hoc axioms are stated. The work implicitly relies on standard statistical-mechanical assumptions for molecular simulations and on the domain assumption that the site-based models capture essential alcohol chemistry.

axioms (2)
  • standard math Standard assumptions of statistical mechanics for molecular simulations (ergodicity, adequate sampling of phase space)
    All simulation-based claims presuppose that the generated trajectories represent equilibrium ensembles.
  • domain assumption The site-based models retain sufficient chemical specificity to represent real alcohol behavior in 2D
    Stated directly in the abstract as the basis for investigating dimensional constraints.

pith-pipeline@v0.9.0 · 5512 in / 1557 out tokens · 90905 ms · 2026-05-08T09:08:17.971348+00:00 · methodology

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