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Influence of Surfactants on the Rheological Behavior of Nanocrystal Suspension

This study investigates how an anionic surfactant (Stepanol) and a cationic surfactant (HTAB) alter the behavior of a 1 wt% cellulose nanocrystal suspension, using pendant-drop surface tension alongside conductivity and steady-shear rheology.

At-a-Glance Summary

Primary surface measurement reported

Pendant-drop surface tension of NCC dispersion and surfactant–NCC mixtures was measured over a surfactant concentration range of 0–500 ppm.

Dropometer attribution in the paper

Surface tension was measured using the pendant drop method with a “smartphone-based pendant drop tensiometer manufactured by Droplet Lab, Markham, ON, Canada,” with drop-shape analysis based on fitting the droplet profile with the Young–Laplace equation.

How the surface-tension / contact-angle data were used in the study

Surface tension trends were compared across Stepanol–NCC and HTAB–NCC mixtures as surfactant concentration increased, and interpreted alongside conductivity and rheology. The authors describe a clear break point around 300 ppm in the HTAB–NCC surface-tension plot.

Replication / reliability statement

The measurement for each fluid was performed 30 times and an average value was calculated.

Paper Details

Title
Influence of Surfactants on the Rheological Behavior of Nanocrystal Suspension
Authors
Anuva Pal; Rajinder Pal
Journal
Preprints.org
Year
2025
Pages / Article
16 pages
DOI
10.20944/preprints202507.2147.v1

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What Was Measured

Primary surface / interfacial measurement

Surface tension of NCC dispersion and surfactant–NCC mixtures was measured by the pendant drop method across surfactant concentration increments of 50 ppm over the range 0–500 ppm.

Supporting measurements

Electrical conductivity and steady-shear rheology were measured to track surfactant-dependent changes in bulk properties alongside the surface-tension response. Dynamic light scattering (DLS) was used to determine the size distribution of cellulose nanocrystals.

Role of the Dropometer

The Dropometer is cited in the methods as a “smartphone-based pendant drop tensiometer manufactured by Droplet Lab, Markham, ON, Canada” used for pendant-drop surface tension. A pendant droplet of the aqueous phase (NCC or surfactant–NCC mixture) was generated at the tip of a stainless-steel needle (1.8 mm diameter) connected to a 500 µL Hamilton® gastight syringe (Model 1750 TPLT), dispensed using a screw-driven plunger; the droplet was imaged using a smartphone camera and analyzed with specialized software. Surface tension was calculated numerically from drop-shape analysis by fitting the droplet profile with the Young–Laplace equation.

The resulting surface-tension curves were used to compare how Stepanol versus HTAB changes interfacial behavior with concentration, and to interpret concentration regions associated with changes observed in conductivity and rheology.

Method Snapshot

Method Snapshot Table

System / series Sample composition (as prepared) Surfactant concentration program Surface measurement output Supporting measurements used alongside Instruments Conditions / notes (as stated)
NCC dispersion NCC in deionized water at fixed NCC concentration of 1 wt% (batch of approximately 1 kg) — Surface tension of NCC dispersion Steady rheology; electrical conductivity Smartphone-based pendant drop tensiometer manufactured by Droplet Lab, Markham, ON, Canada; Fann co-axial cylinder viscometer; Thermo Orion 3 Star conductivity meter Prepared at room temperature (22 ± 1 °C) using a variable-speed Gifford-Wood homogenizer (Model 1-L); cooled to room temperature before measurements; surface tension measured at room temperature
Stepanol–NCC mixtures 1 wt% NCC dispersion + sodium lauryl sulfate (Stepanol WA-100; “Stepanol”) 0–500 ppm, prepared in increments of 50 ppm Surface tension of NCC dispersion Steady rheology (power-law K and n); electrical conductivity vs Stepanol concentration Smartphone-based pendant drop tensiometer manufactured by Droplet Lab, Markham, ON, Canada; Fann co-axial cylinder viscometer; Thermo Orion 3 Star conductivity meter Prepared at room temperature; mixed at gentle speed for about 1 hour; increments made by adding more surfactant to an existing surfactant–NCC mixture and mixing; cooled to room temperature before measurements; surface tension measured at room temperature
HTAB–NCC mixtures 1 wt% NCC dispersion + hexadecyltrimethylammonium bromide (“HTAB”) 0–500 ppm, prepared in increments of 50 ppm Surface tension vs HTAB concentration Steady rheology (power-law K and n); electrical conductivity vs HTAB concentration Smartphone-based pendant drop tensiometer manufactured by Droplet Lab, Markham, ON, Canada; Fann co-axial cylinder viscometer; Thermo Orion 3 Star conductivity meter Prepared at room temperature; mixed at gentle speed for about 1 hour; increments made by adding more surfactant to an existing surfactant–NCC mixture and mixing; cooled to room temperature before measurements; surface tension measured at room temperature

Key Findings

Surface tension decreases with increasing surfactant concentration

The authors report that surface tension decreases as surfactant concentration increases, as shown in the surface-tension plots for Stepanol–NCC and HTAB–NCC mixtures.

HTAB surface-tension curve shows a break point near 300 ppm

A clear break point is described for the HTAB–NCC surface-tension plot around 300 ppm surfactant concentration. The authors state that surface tension rises at 300 ppm, indicating surfactant migration to the surface of the cellulose nanocrystals.

Stepanol surface-tension response is smooth over 0–500 ppm

For Stepanol–NCC mixtures, the surface-tension versus concentration plot shows no break point over the tested range.

Conductivity trends align with the interfacial transition for HTAB

The conductivity of HTAB–NCC mixtures increases linearly up to about 350 ppm HTAB and then shows a change in slope; the authors link the slower conductivity increase above this point to surfactant molecules migrating to the nanocrystal surface and charge neutralization.

Break points are summarized as consistent with rheology changes

In the conclusions, the authors state that conductivity and surface tension plots clearly exhibit break points around the HTAB surfactant concentration of 300 ppm in agreement with changes in rheological properties.

Thresholds / Regimes

The authors identify transition behavior from break points in conductivity and surface-tension plots as surfactant concentration increases. They also contextualize the HTAB transition region using a reported cmc (critical micelle concentration) for pure HTAB solutions.
System / condition Threshold name Value Units How determined in the paper Figure / section Column 7
HTAB–NCC mixtures Surface-tension break point ~300 ppm Clear break point in surface tension versus surfactant concentration plot Figure 15; Section 3.4 The surface tension rises at 300 ppm; interpreted as surfactant migrating to the surface of the cellulose nanocrystals
HTAB–NCC mixtures Conductivity slope change ~350 ppm Change in slope of conductivity plot Figure 14b; Section 3.4 Conductivity increases slowly above 350 ppm
Pure HTAB solutions (reported value) cmc (critical micelle concentration) 0.91 mM (332 ppm) mM; ppm Reported value cited by the authors Section 3.4 Used by the authors to contextualize concentration region where HTAB–NCC rheological changes occur
Stepanol–NCC mixtures Break point behavior (conductivity and surface tension) — — Summarized from plots and conclusions Figure 14a; Figure 15a; Conclusions Conductivity and surface-tension plots are described as exhibiting no break points over the tested Stepanol concentration range

Figures & Visuals

Figure 1 — Dropometer setup reference

What it shows

Shows the smartphone-based pendant drop tensiometer used for the pendant-drop surface-tension measurements.

Figure 14 — Conductivity context for interpreting surface tension

What it shows

Shows conductivity versus surfactant concentration for Stepanol–NCC and HTAB–NCC mixtures, including the HTAB slope change around 350 ppm.

Figure 15 — Primary Dropometer-derived surface-tension output

What it shows

Shows surface tension versus surfactant concentration for Stepanol–NCC and HTAB–NCC mixtures, including the HTAB break point around 300 ppm.

Figure 13 — Rheology comparison used alongside interfacial data

What it shows

Compares consistency index K and flow behavior index n for Stepanol–NCC and HTAB–NCC mixtures to support interpretation of concentration-dependent changes.

Why It Matters

The paper frames NCC as a rheology modifier used across a wide range of formulations and notes that many commercial products combine thickeners and surfactants, motivating the need to understand additive interactions in aqueous systems. Within this study, surface tension and conductivity measurements are described as being carried out simultaneously to track how surfactant addition influences NCC suspensions.

In the results and conclusions, the pendant-drop surface-tension curves help distinguish the Stepanol system from the HTAB system that exhibits a clear transition around 300 ppm. That interfacial transition is discussed as consistent with conductivity and rheology changes and interpreted in terms of surfactant migration to the nanocrystal surface and charge-neutralization effects.

Practical Takeaways

Use Figure 15 to compare surfactant charge effects at the interface.

The surface-tension curves provide a direct side-by-side view of Stepanol–NCC and HTAB–NCC behavior over 0–500 ppm.

Watch the ~300 ppm HTAB transition

The paper highlights a clear surface-tension break point around 300 ppm in HTAB–NCC mixtures, discussed alongside strong changes in rheology and a conductivity slope change.

Pair surface tension with conductivity for interpretation

The authors interpret surface-tension behavior together with conductivity trends (Figure 14) to discuss surfactant migration and charge neutralization.

Anchor concentration changes to the reported cmc context

The paper cites a cmc for pure HTAB solutions (0.91 mM, corresponding to 332 ppm) to contextualize where pronounced changes occur in HTAB–NCC mixtures.

Replicate structure is explicitly reported for surface tension

The pendant-drop surface-tension measurement for each fluid was performed 30 times and an average value was calculated.

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