Contents
Partially compliant

ISO 4311: determination of the critical micellization concentration by surface tension

ISO 4311 finds the critical micelle concentration (CMC) of anionic and non ionic surfactants from surface tension against log concentration. The Dropometer is partially compliant: it gets the same CMC from a pendant drop surface tension series, with ±0.03 mN/m accuracy (Dropometer product specification). Pendant drop is an alternative method, not the plate, stirrup or ring method the standard specifies; if a contract or customer requires an ISO 4311 result, run a correlation study on your own samples first.

What brings you here?

Who this is for
Surfactant makers, cosmetics and household product formulators and contact lens care makers who need the CMC of anionic or non ionic surfactants.
Compliance tier
Partially compliant: the Dropometer gets the CMC from a pendant drop surface tension series, an alternative to the plate, stirrup or ring method ISO 4311 specifies.
Measurement
Pendant drop surface tension up to 75 mN/m, ±0.03 mN/m accuracy, 0.01 mN/m resolution, static and dynamic (Dropometer datasheet).
Setup
About 2 minutes. Battery powered, 1 kg, runs on a lab bench or beside the mixing tank.
Last updated
October 20, 2026
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Free one page checklist for ISO 4311: how to build the dilution series, the settings to lock, how to fit the break point and what to record. Made for surfactant QC and formulation benches.

Talk to an expert

Book a 15 minute demo on your surfactant

Tell us your surfactant, your solvent and your current CMC check. We will run a short series live and show you plainly where pendant drop fits beside the plate or ring tensiometer ISO 4311 specifies.

Book a 15 minute demo
View the official standard
Gurdeep-Saini-Photo
Written by
Gurdeep Singh Saini
Holds a BASc in Mechanical Engineering from Ryerson University (now Toronto Metropolitan University) and an MASc from York University. He develops the automatic edge and baseline detection behind the Dropometer's analysis.
COO at Droplet Lab
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Droplet-Lab logo
Technical Review by
Droplet Lab Team
Droplet Lab builds precision instruments and software for surface science measurement, specialising in contact angle analysis and surface tension characterisation. Used by researchers across materials science, pharmaceuticals, coatings, and advanced manufacturing, Droplet Lab's Dropometer has contributed to studies published in peer-reviewed journals including Advanced Functional Materials (Impact Factor 19). The team combines instrument engineering with deep domain knowledge in wettability science with a focus on practical accuracy.
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Verified against ISO 4311
Gurdeep-Saini-Photo
Written By

Gurdeep Singh Saini

COO at Droplet Lab

Holds a BASc in Mechanical Engineering from Ryerson University (now Toronto Metropolitan University) and an MASc from York University. He develops the automatic edge and baseline detection behind the Dropometer's analysis.

Droplet-Lab logo
Reviewed By

Droplet Lab Team

Droplet Lab builds precision instruments and software for surface science measurement, specialising in contact angle analysis and surface tension characterisation. Used by researchers across materials science, pharmaceuticals, coatings, and advanced manufacturing, Droplet Lab's Dropometer has contributed to studies published in peer-reviewed journals including Advanced Functional Materials (Impact Factor 19). The team combines instrument engineering with deep domain knowledge in wettability science with a focus on practical accuracy.

Featured Studies & Publications

Research Paper Contact angle measurement with a smartphone
Research Paper Surface tension measurement with a smartphone using a pendant drop

Evidence box

Standard

ISO 4311:1979, Anionic and non-ionic surface active agents: determination of the critical micellization concentration by measuring surface tension with a plate, stirrup or ring. An international standard from the International Organization for Standardization (ISO/TC 91), last reviewed and confirmed in 2022. The FDA recognises it for contact lens care products (recognition number 10-99).

Compliance tier

Partially compliant. The Dropometer measures the surface tension of each solution in a series by pendant drop, and you find the CMC from the plot against log concentration, as ISO 4311 does. The standard specifies a plate, stirrup or ring to ISO 304, so pendant drop is an alternative method: if a contract or customer requires an ISO 4311 result, run a correlation study on your own samples first. The Dropometer has no temperature control.

Instrument performance

Pendant drop surface tension up to 75 mN/m, accuracy ±0.03 mN/m, resolution 0.01 mN/m, static and dynamic, from the Dropometer product specification.

Peer reviewed method

The drop shape method is validated against a KRUSS DSA100E in two peer reviewed papers. See our validation publications.

Customer evidence

A 2025 paper in Polymers used the Dropometer to measure pendant drop surface tension of anionic, non ionic and other surfactant solutions mixed with polymers, and estimated aggregation points from slope changes. Read our analysis of the paper.

What it does not tell you

The CMC tells you where micelles start to form in that solvent at that temperature. It does not tell you cleaning, foaming or wetting performance on its own, and it changes when salts or other ingredients are added. For wetting on a solid, measure contact angle and surface free energy as well.

Executive summary

What this standard measures, who uses it and how the Dropometer fits

ISO 4311 determines the critical micelle concentration (CMC) of anionic and non ionic surfactants by measuring the surface tension of a dilution series and finding the break in the curve against log concentration. Surfactant makers, cosmetics and household product formulators and contact lens care makers use it to set dosing and to check raw material lots. The Dropometer is partially compliant: it gets the CMC the standard is after from a pendant drop surface tension series and the same break point against log concentration. Pendant drop is an alternative method, not the plate, stirrup or ring method the standard specifies; if a contract or customer requires an ISO 4311 result, run a correlation study on your own samples first. The Dropometer has no temperature control, so record room temperature with every point.

The standard reads each solution on a plate, stirrup or ring tensiometer, following the determination of surface tension in ISO 304, plots surface tension against the logarithm of concentration and takes the CMC at the sharp change of slope. ISO notes that a single sharp value is often not possible, so a narrow concentration range around the break is an acceptable result. For the terms, see the critical micelle concentration glossary entry and our guide to understanding surfactants.

The Dropometer pendant drop tensiometer measures surface tension up to 75 mN/m with ±0.03 mN/m accuracy, which covers water at room temperature and every point in a surfactant series. For the full workflow, see CMC assessment and surfactant efficiency, and practise it with Experiment 9: CMC from surface tension against concentration. A university chemical engineering lab uses the Dropometer for pendant drop surface tension of polymer and surfactant solutions, and a 2025 study of polymer and surfactant mixtures read slope changes from its surface tension data. Formulators can read the cosmetics guide for context.

Further reading: surface tension measurement guide, reproducible surface tension measurement, CMC reference values for common surfactants and choosing a tensiometer for surfactant and CMC research.

How the Dropometer fits the surface tension method

The ISO 4311 measurement, step by step, in our own words

From stock solution to a CMC with its range

These steps outline the procedure and show where the Dropometer differs from the method's apparatus. They do not replace the standard. Buy the current edition and follow its water quality, series and reporting rules.

±0.03 mN/m

Surface tension accuracy

From the Dropometer product specification. Resolution is 0.01 mN/m.

Up to 75 mN/m

Surface tension range

Covers water at room temperature and every point of a surfactant series.

About 2 min

Setup

Base plate, phone holder, LED and syringe. No PC needed.

1

Make the stock and the series

Weigh the surfactant and make a stock solution in water of the quality the standard specifies. Dilute it into a series spaced evenly on a log scale, with the concentrations bracketing the expected CMC. Add extra points near the break.

2

Check the water first

Set up the Dropometer for pendant drop: level the base plate, mount the phone and LED, and fill a clean syringe with the same water. A pendant drop of clean water should read close to the reference value for your room temperature. A low reading means contamination.

3

Measure from dilute to concentrated

Start with the most dilute solution and rinse the syringe with each new solution before filling. Form a pendant drop, enter the solution density, and fit the profile with the Young-Laplace model.

4

Wait for a steady value

Dilute solutions take time to reach equilibrium, so track the dynamic surface tension and record the value once it stops changing. Use a fresh drop for each replicate, and use the same rule for every solution.

5

Plot and fit the break

Plot the mean surface tension against log concentration. Fit one straight line to the falling part and one to the plateau. The CMC is where they cross. Also note the range between the last point below the break and the first point above it.

6

Record the method you used

Record the CMC, its range, the surface tension at the plateau, the surfactant lot and purity, the water, the room temperature and the number of drops. State that surface tension was measured by pendant drop, an alternative to the plate, stirrup or ring method of ISO 304 that ISO 4311 specifies, and name any correlation study you ran. See reproducible surface tension measurement for good bench practice.

7

Watch the full measurement

This video shows a complete Dropometer pendant drop surface tension measurement, from setup to result. It covers the steps above in one run.

Dropometer pendant drop surface tension, start to finish.

Validated measurement approach

Independent benchmarking and publication-based validation references.

Peer reviewed method

The Dropometer drop shape method is validated against a KRUSS DSA100E in two peer reviewed papers, one in Review of Scientific Instruments and one in Colloids and Surfaces A. Published surface tension accuracy is ±0.03 mN/m.

Read the validation papers

Customer evidence

We have no published client paper that runs ISO 4311 itself. A 2025 paper in Polymers used Dropometer pendant drop surface tension to follow how surfactant solutions with added polymer change with concentration, and read aggregation points from slope changes.

Read our analysis

Set critical micellization concentration limits from your own lots

ISO 4311 · CMC · surfactant QC and formulation

ISO 4311 publishes no pass or fail CMC. It asks you to state the operating conditions with the result, including product purity and, as far as known, the impurities, because temperature, metal salts and organic compounds all move the CMC (source: ISO 4311:1979 introduction, preview pages).

Check the system first. Deionised water at 22.5 °C should read about 72.4 mN/m (source: IAPWS R1-76(2014)). If your water reads low, clean the syringe, needle and glassware before you test surfactants.

Set the accept range for a raw material lot from your own capability study. Measure several good lots with the same water, series and room conditions, then set limits around their CMC. Compare with the published values in our CMC reference values. If you need results that match a plate or ring tensiometer, run a correlation study on your own samples first.

Example output: a surfactant dilution series

Example data for illustration only. These are not measured results. Your own surfactant, water, room temperature and limits decide the numbers.

Example: non ionic surfactant in deionised water, room temperature 22.5 °C

Concentration (mmol/L) log10 c Drops (n) Mean surface tension (mN/m) Std dev (mN/m) Region
0.01-2.00551.60.12Below CMC: falling
0.02-1.70545.40.10Below CMC: falling
0.05-1.30537.50.09Last point below the break
0.2-0.70531.40.06First point above the break
0.5-0.30531.10.05Above CMC: plateau
1.00.00531.00.05Above CMC: plateau

How to read this table

Each row is the mean of five pendant drops, read once the value stopped changing. A straight line fitted to the first three points meets a line fitted to the last three at 0.098 mmol/L, the example CMC, where the surface tension is about 31.6 mN/m. The yellow rows bracket the break.

Example output chart

The same example data as the table, plotted against log concentration with the two straight line fits. The lines cross at the example CMC.

Line chart of example surface tension against log concentration for a non ionic surfactant, falling from 51.6 to 37.5 mN/m and then levelling at about 31 mN/m, with two fitted lines crossing at an example CMC of 0.098 mmol/L

Example data, not measured results.

Business impact

What changes when the CMC series runs on a pendant drop bench

Metric Before Dropometer With Dropometer Results
Where the series runs Shared plate or ring tensiometer in a central lab On a bench beside formulation or QC, on battery for up to 8 h Dropometer product specification
Result format One force based reading per solution A drop image and surface tension per drop, 0.01 mN/m resolution, ±0.03 mN/m accuracy Dropometer product specification
Liquid per reading A vessel of solution for the plate or ring The solution in one syringe and a pendant drop No published benchmark
Report that names ISO 4311 Plate, stirrup or ring tensiometer Pendant drop CMC, after a correlation study on your own samples ISO 4311 title and scope

See a CMC series on your own surfactant

Bring a lot that performed well and one that did not. We will measure a short series of both live and plot them against log concentration.

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ROI estimate

Estimate the cost of the surfactant lot and dosing failures this test can catch

Enter your own numbers. The defaults are examples, not Droplet Lab results.

Quality cost snapshot

Estimate what wetting related failures cost you.

Example only. Use what you see in a trial on your own samples.
Your own figure.
Your own estimate.

Result

~0
Estimated annual cost today
~0
Estimated annual cost avoided
~0
Payback period

Annual exposure combines audit/CAPA cost, customer rejections, and recall probability; avoided exposure applies the risk reduction percentage.

Measurement settings for each surfactant solution

Lock these before you compare CMC results between lots, shifts or sites

Parameter Recommended Setting Technical Rationale
Solvent Water of the quality the standard specifies, or your stated aqueous medium The method covers distilled water or other aqueous systems. Impurities in the water shift the curve.
Concentration series Even steps on a log scale, bracketing the CMC, extra points near the break The CMC is read from the plot against log concentration, so the points must sit on both sides.
Surface tension method Pendant drop, an alternative method; name it in the report The standard specifies a plate, stirrup or ring to ISO 304. Correlate on your own samples before you report an ISO 4311 result.
Reading rule Value once the dynamic surface tension stops changing, same rule for every solution Dilute solutions reach equilibrium slowly. A fixed rule keeps points comparable.
Fit model and density Young-Laplace, with the solution density entered The pendant drop result depends on the density you enter. Keep the value and its source the same.
Replicates Several fresh drops per concentration The mean and spread at each point show whether the break is real.
CMC calculation Two straight line fits, same points chosen by the same rule Changing which points go into each line moves the CMC.
Temperature Record room temperature for every point The CMC moves with temperature. The Dropometer runs at 10 to 45 °C and has no temperature control.

Is ISO 4311 the right test?

Pick the method by the question you need answered

Start: What do you need to know about your surfactant?

CMC for formulation or lot QC

Signals:

You need the CMC of an anionic or non ionic surfactant in your own water or medium, for screening, dosing or lot to lot checks.

Rule-out:

Run the same series and break point on the Dropometer by pendant drop. This gets the CMC ISO 4311 is after by an alternative method, so name pendant drop in your records.

A report that must name ISO 4311

Signals:

Your customer, regulator or contract requires the CMC measured exactly as ISO 4311 specifies.

Rule-out:

Pendant drop is an alternative method, not the plate, stirrup or ring method the standard specifies. Run a correlation study on your own samples against a plate or ring tensiometer to ISO 304 or EN 14370 first, or report from that tensiometer. For contact lens care submissions, see the FDA 510(k) CMC page.

Cationic surfactants

Signals:

Your surfactant is cationic, so it falls outside the ISO 4311 scope.

Rule-out:

The same pendant drop series still shows the break in house. State the method you used, because ISO 4311 does not cover it.

Tension between two liquids

Signals:

You need the tension between an oil and a surfactant solution, for emulsions or cleaning.

Rule-out:

The Dropometer measures interfacial tension by pendant drop of one liquid in the other with the optional cuvette. ISO 4311 covers surface tension against air only. See ASTM D1331 for an interfacial method.

Decision tree at a glance

Start from the question you need answered. Blue boxes are the parts of this work the Dropometer runs.

Decision tree for choosing between a pendant drop CMC series on the Dropometer, a plate or ring tensiometer or a correlation study for reports that name ISO 4311, a pendant drop series for cationic surfactants, and pendant drop in the optional cuvette for tension between oil and water

Is ISO 4311 the right test?

Interpreting the surface tension curve

What the curve tells you, and what it does not

The falling line is adsorption: Below the CMC, more surfactant packs into the surface, so surface tension drops with each step up in concentration.
The plateau means micelles: Above the CMC, extra surfactant forms micelles in the bulk, so the surface tension barely changes.
Report a range as well as a value: ISO notes a single sharp value is often not possible. Give the CMC and the bracket between the points either side.
A dip near the break means impurities: A minimum below the plateau near the CMC usually points to a surface active impurity in the lot.
Expert perspective

Why surface tension levels off at the CMC

The critical micelle concentration is not a number you look up; it is where the surface-tension curve flattens, and the only honest way to know it is to measure it.

Dr. Alidad Amirfazli

Department of Mechanical Engineering, York University, Co-founder and Scientific Advisor, Droplet Lab

Common pitfalls and limits of the method

Honest limits of the method and of the Dropometer

Pendant drop is an alternative method, not the plate, stirrup or ring method ISO 4311 specifies. If a contract or customer requires an ISO 4311 result, run a correlation study on your own samples first.
The Dropometer has no temperature control. Condition solutions at room temperature and record it, because the CMC moves with temperature.
Dilute solutions reach equilibrium slowly, and a long lived drop can evaporate. Use the same reading rule for every point.
Traces of surfactant left in the syringe raise the apparent concentration of the next solution. Run dilute to concentrated and rinse between solutions.
The pendant drop result depends on the density you enter. Use the density of each solution, or state the value you used for the whole series.
FAQ

Frequently asked questions

What does ISO 4311 measure?

It specifies a method for the determination of the critical micellization concentration (CMC) of anionic and non ionic surfactants in water or other aqueous solutions. You measure the surface tension of a series of solutions with a plate, stirrup or ring, plot it against log concentration, and take the CMC where the slope changes sharply.

Can the Dropometer run ISO 4311?

Partly. The Dropometer gets the CMC ISO 4311 is after: it measures the surface tension of each solution by pendant drop with ±0.03 mN/m accuracy (Dropometer product specification), and you find the break on the same plot against log concentration. Pendant drop is an alternative method, not the plate, stirrup or ring method the standard specifies; if a contract or customer requires an ISO 4311 result, run a correlation study on your own samples first.

Is pendant drop an accepted alternative to the ring or plate in ISO 4311?

Not as written. Its full title names a method by measuring surface tension with a plate, stirrup or ring to ISO 304, and we found no allowance for other methods. Both routes measure surface tension, but do not treat the results as interchangeable until a correlation study on your own samples shows they agree. See EN 14370 for the plate and ring route.

How do I find the CMC from my surface tension data?

Plot surface tension against log concentration. Fit one straight line to the falling part and one to the plateau, and take the CMC where they cross. In our example data, the lines cross at 0.098 mmol/L. Report the bracket between the points either side as well.

Does ISO 4311 apply to cationic surfactants?

No. It covers anionic and non ionic surfactants that dissolve in water. Non ionic products need a cloud point at least 5 °C above the test temperature, and anionic products a Krafft point at least 5 °C below it (source: ISO 4311 field of application).

Is ISO 4311 still current, and who recognises it?

Yes. ISO 4311:1979 was last reviewed and confirmed in 2022. The FDA lists it as a recognised consensus standard for contact lens care products. See our FDA 510(k) CMC page.

Legal note

This page summarises ISO 4311 in our own words and does not reproduce the standard. Buy the current edition from ISO before you write a procedure. The Dropometer is partially compliant: it gets the CMC by pendant drop, an alternative to the plate, stirrup or ring the standard specifies. Droplet Lab does not certify compliance. Your own method validation and correlation study do.

View the official standard Book a 15 minute demo

How this page was created

Sources, review and tools

Transparency 4 checklist items
01

Sources

ISO 4311:1979 listing (ISO); ISO 4311:1979 preview pages (ANSI webstore); ISO 4311:1979 listing (ANSI webstore); ISO 304:1985 listing (ISO), the Dropometer product specification and Droplet Lab validation papers.

02

Drafting assistance

Drafted with Claude (Anthropic) from the sources listed, then reviewed by Droplet Lab.

03

What we did not do

We did not copy text from the standard. Buy the current edition for the full procedure.

04

Last checked

October 2026

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We work hard to keep this page accurate and up to date. If you spot an error (wrong revision/year, missing requirement, incorrect interpretation, or broken link), tell us and we'll review it.

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References

1. ISO 4311:1979. iso.org/standard/10177.html
2. ISO 4311:1979 preview pages (ANSI webstore). webstore.ansi.org/preview-pages/ISO/preview_ISO+4311-1979.pdf
3. ISO 4311:1979 listing (ANSI webstore). webstore.ansi.org/standards/iso/iso43111979
4. ISO 304:1985 listing (ISO). iso.org/standard/4238.html
5. Droplet Lab. Peer reviewed validation papers in Review of Scientific Instruments and Colloids and Surfaces A. dropletlab.com/validation/publications/
6. Droplet Lab. Dropometer product specification, 2026. dropletlab.com/products/dropometer/