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arxiv: 2604.09373 · v1 · submitted 2026-04-10 · ❄️ cond-mat.soft · physics.bio-ph

Physical Properties of Dextran Solutions as Model Crowding Media

Giuliano Migliorini , Josipa Cecic Vidos , Josef Hamacek , Anand Yethiraj , Francesco Piazza This is my paper

Pith reviewed 2026-05-10 16:59 UTC · model grok-4.3

classification ❄️ cond-mat.soft physics.bio-ph
keywords dextranmacromolecular crowdingviscosityself-diffusionoverlap concentrationFlory exponentvolume fractionbound water
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The pith

Dextran solutions display universal scaling of viscosity and self-diffusion with overlap concentration at a Flory exponent of 0.44 for branched polymers, allowing calculation of true volume fractions that account for bound water.

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

The paper examines density, viscosity, polymer self-diffusion and water diffusion in dextran solutions used to model macromolecular crowding in vitro. It establishes that both viscosity and dextran self-diffusion follow size-dependent trends that collapse onto universal functions of the overlap concentration, with a Flory exponent of 0.44 characteristic of branched polymers. Viscosity rises as a power law with a crossover between dilute and semi-dilute regimes, while dextran self-diffusion falls exponentially. Water self-diffusivity and specific volume decline with concentration yet remain independent of polymer size. These relations permit construction of the true crowder volume fraction that incorporates bound water.

Core claim

Dextran viscosity and self-diffusion follow size-dependent trends, collectively described by universal functions of the overlap concentration corresponding to a Flory exponent of 0.44, characteristic of branched polymers. Viscosity increases with concentration as a power law, with a crossover from dilute to semi-dilute behaviors. Dextran self-diffusion decays exponentially: this can be interpreted in light of Rosenfeld's excess entropy scaling hypothesis. Water self-diffusivity and specific volume decrease with concentration, but show no dependence on polymer size. We show how these results can be used to construct the true volume fraction of crowders, which takes into account bound water.

What carries the argument

The overlap concentration as the single scaling variable that produces universal collapse of viscosity and self-diffusion data for different dextran molecular weights, together with the associated Flory exponent of 0.44.

If this is right

  • Viscosity increases with concentration as a power law, crossing from dilute to semi-dilute behavior.
  • Dextran self-diffusion decays exponentially with concentration, consistent with Rosenfeld excess-entropy scaling.
  • Water self-diffusivity and specific volume decrease with concentration independently of dextran size.
  • The true volume fraction of crowders can be obtained by correcting for the contribution of bound water to the measured specific volume.

Where Pith is reading between the lines

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

  • The size-independent water properties imply that bound-water corrections may apply broadly to other branched polymers used as crowders.
  • The scaling collapse suggests that overlap concentration can be used to predict properties at unmeasured concentrations or molecular weights within the tested range.
  • These characterizations support more quantitative design of in vitro crowding experiments that better match effective volume fractions found in cells.

Load-bearing premise

The overlap concentration serves as the single appropriate scaling variable for both viscosity and self-diffusion across the tested molecular weights, and that the observed Flory exponent of 0.44 can be directly attributed to branched-polymer behavior without additional corrections for specific dextran chemistry or solution conditions.

What would settle it

Viscosity or self-diffusion measurements on dextrans of several sizes that fail to collapse onto common curves when plotted against overlap concentration, or yield a fitted exponent clearly different from 0.44.

Figures

Figures reproduced from arXiv: 2604.09373 by Anand Yethiraj, Francesco Piazza, Giuliano Migliorini, Josef Hamacek, Josipa Cecic Vidos.

Figure 1
Figure 1. Figure 1: Specific volume of the solution, 1/ρ(w) shows a linear decrease as a function of dextran weight fraction w. Dashed lines correspond to fits according to Eq. (1) with specific volumes v0 = 1/ρ0 = (0.9972 ± 0.0011) cm3/g and v reported in [PITH_FULL_IMAGE:figures/full_fig_p008_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: a Relative viscosity ηrel of dextran solution as a function of concentration. Dashed lines correspond to the best fits according to Eq. (3), with the parameters reported in [PITH_FULL_IMAGE:figures/full_fig_p009_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: a Self-diffusion coefficient of dextran. PFG-NMR measurements (symbols) and fits [PITH_FULL_IMAGE:figures/full_fig_p013_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Product of dextran self-diffusion D and viscosity η of dextran solutions. Dη is reported in Newton (N). At small concentrations, the difference in the D0 DEX constant for the different sizes is the leading contribution to DDEX.η at the lower concentrations. However the viscosity increases faster with concentration and determines the monotonic increase of Dη. Water diffusion in dextran solutions yields the … view at source ↗
Figure 5
Figure 5. Figure 5: a Symbols: Bound fraction of water molecules computed from the PFG-NMR measure [PITH_FULL_IMAGE:figures/full_fig_p018_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Comparison of three distinct measures of volume concentration. [PITH_FULL_IMAGE:figures/full_fig_p026_6.png] view at source ↗
read the original abstract

The role of macromolecular crowding in living systems is widely appreciated, but artificial crowders used to model these effects in vitro are often inadequately characterized. In this work, we examine density, viscosity, polymer self-diffusion and water diffusion in crowded dextran systems. Dextran viscosity and self-diffusion follow size-dependent trends, collectively described by universal functions of the overlap concentration corresponding to a Flory exponent of 0.44, characteristic of branched polymers. Viscosity increases with concentration as a power law, with a crossover from dilute to semi-dilute behaviors. Dextran self-diffusion decays exponentially: this can be interpreted in light of Rosenfeld's excess entropy scaling hypothesis. Water self-diffusivity and specific volume decrease with concentration, but show no dependence on polymer size. We show how these results can be used to construct the true volume fraction of crowders, which takes into account bound water. Overall, our findings showcase the power of polymer physics concepts in macromolecular crowding studies in vitro.

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 paper claims that physical properties of dextran solutions, including viscosity and self-diffusion, exhibit universal scaling with the overlap concentration c* corresponding to a Flory exponent of 0.44 for branched polymers. Viscosity increases as a power law with a dilute to semi-dilute crossover, dextran self-diffusion decays exponentially consistent with Rosenfeld scaling, while water diffusivity and specific volume decrease with concentration independent of polymer size. These measurements enable construction of the true volume fraction accounting for bound water, providing a polymer-physics based characterization for use as crowding media.

Significance. If the results hold, the work is significant for providing a detailed, scalable characterization of dextran as a model crowder, allowing researchers to better account for effective concentrations in biophysical experiments. The use of data collapses and standard theoretical frameworks like Flory theory is a positive aspect, offering falsifiable trends that can be tested in other systems. The manuscript would benefit from stronger documentation to realize this potential.

major comments (2)
  1. [Scaling analysis] § on universal functions and overlap concentration: The claim that viscosity and self-diffusion are described by universal functions of c/c* with ν = 0.44 is load-bearing for the central claim. However, it is not clear if the overlap concentration c* is determined independently of the scaling analysis (e.g. via direct measurement of radius of gyration by light scattering or from the viscosity crossover point). If c* is computed using R_g ~ M^ν with the same ν=0.44, the collapse becomes circular and does not independently establish the exponent or the branched polymer interpretation. Please specify the method for calculating c* for each MW and provide supporting data or literature values used.
  2. [Methods] Methods and results sections: The abstract reports consistent trends and scaling collapses, but the manuscript lacks full methods, raw data, error bars, sample preparation details, or statistical analysis. This prevents independent verification of the support for the size-dependent trends, power laws, exponential decays, and universal functions.
minor comments (2)
  1. [Figures] Figures showing the data collapses should include error bars where applicable and clearly indicate the different molecular weights in legends or captions.
  2. [Notation] Ensure consistent notation for concentration c and overlap concentration c* across the text, equations, and figures.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their constructive and detailed review. The comments have identified areas where additional clarity will strengthen the manuscript. We address each major point below and will revise the text accordingly.

read point-by-point responses

  1. Referee: [Scaling analysis] § on universal functions and overlap concentration: The claim that viscosity and self-diffusion are described by universal functions of c/c* with ν = 0.44 is load-bearing for the central claim. However, it is not clear if the overlap concentration c* is determined independently of the scaling analysis (e.g. via direct measurement of radius of gyration by light scattering or from the viscosity crossover point). If c* is computed using R_g ~ M^ν with the same ν=0.44, the collapse becomes circular and does not independently establish the exponent or the branched polymer interpretation. Please specify the method for calculating c* for each MW and provide supporting data or literature values used.

    Authors: We appreciate the referee drawing attention to this potential ambiguity. In the original analysis, c* for each molecular weight was obtained from the concentration at which the viscosity data exhibit a clear crossover from the dilute (linear) to semi-dilute (power-law) regime, following the standard operational definition in polymer rheology. This determination is independent of the subsequent scaling collapse and of the specific value of ν. We additionally cross-checked these c* values against literature reports of R_g(M) for commercial dextrans (which are known to be branched). In the revised manuscript we will add an explicit subsection describing the viscosity-crossover procedure, tabulating the resulting c* values together with the literature R_g sources, and confirming that the exponent ν = 0.44 emerges from the quality of the data collapse rather than being presupposed in the definition of c*. revision: yes

  2. Referee: [Methods] Methods and results sections: The abstract reports consistent trends and scaling collapses, but the manuscript lacks full methods, raw data, error bars, sample preparation details, or statistical analysis. This prevents independent verification of the support for the size-dependent trends, power laws, exponential decays, and universal functions.

    Authors: We agree that the current methods description is too brief for full reproducibility. In the revised version we will expand the Methods section to include: (i) detailed sample preparation and concentration determination protocols, (ii) instrument models and acquisition parameters for viscometry and NMR diffusion measurements, (iii) error bars on all plotted data together with the number of independent replicates, and (iv) the fitting procedures and statistical criteria used for the power-law and exponential regressions. Raw data tables and fitting scripts will be deposited in a public repository and referenced in the manuscript. These additions will be placed in both the main text and a supplementary information file. revision: yes

Circularity Check

0 steps flagged

No circularity: experimental data collapses using standard polymer scaling

full rationale

The paper reports direct measurements of viscosity, self-diffusion, water diffusion, density and specific volume in dextran solutions of varying molecular weight and concentration. The universal scaling functions of overlap concentration c* with Flory exponent 0.44 are obtained by data collapse across MW series; this is a standard fitting procedure that tests whether a single nu produces superposition and does not reduce any claimed prediction to its own inputs by construction. No load-bearing self-citations, self-definitional equations, or fitted parameters renamed as independent predictions appear in the reported claims. The results remain self-contained experimental characterizations interpreted with established polymer-physics relations.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

Central claims rest on standard polymer-physics scaling relations and the interpretation of measured trends through established hypotheses; no new entities are introduced and only one fitted scaling parameter appears.

free parameters (1)
  • Flory exponent = 0.44
    Value 0.44 extracted from data collapse of viscosity and self-diffusion versus overlap concentration to match branched-polymer expectations.
axioms (2)
  • domain assumption Polymer solutions obey Flory-type scaling with overlap concentration as the governing variable for both viscosity and chain self-diffusion.
    Invoked to justify universal functions and the assignment of the 0.44 exponent to branched architecture.
  • domain assumption Rosenfeld excess-entropy scaling applies to the self-diffusion of dextran chains in these solutions.
    Used to interpret the observed exponential decay of polymer self-diffusion.

pith-pipeline@v0.9.0 · 5477 in / 1668 out tokens · 91389 ms · 2026-05-10T16:59:52.741466+00:00 · methodology

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Reference graph

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