Surfactants, CMC, Emulsions and Foams

Critical Micelle Concentration (CMC) Determination: Measure Surfactant Concentration by Surface Tension for Anionic, Cationic and Detergent Formulations

The CMC is the surfactant concentration at which micelles start to form and surface tension stops falling. Measure surface tension across a dilution series, fit the two parts of the curve, and you obtain the CMC of your own formulation, in your own medium, rather than a value copied from a datasheet.

Who this is for: Formulation scientists, QC chemists and regulatory teams working with detergent, cosmetic, pharmaceutical and contact lens care formulations.

Where it fits: Formulation development, incoming surfactant lot checks, and CMC data for FDA 510(k) submissions for contact lens care products.

What it does not do: It does not measure micelle size, shape or aggregation number. Use scattering or fluorescence methods for those.

Method

Pendant drop surface tension across a dilution series

Standard

FDA 510(k) Appendix B (fully compliant with workflow); ISO 4311 (correlated method)

Setup

About 2 minutes

Book a 15 minute demo Request a quote

Diagram of surfactant molecules filling the air water surface and forming micelles above the CMC, in water and in a particle dispersion

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

Why a published CMC value is not your CMC

71 methods

of determining the CMC were distinguished in the NBS compilation of close to 5,000 CMC entries for 720 compounds.

Mukerjee and Mysels, NSRDS-NBS 36, National Bureau of Standards (1971)

About 3%

difference in CMC from the same data plotted two ways, larger than the uncertainty of either. How you fit the curve changes the answer.

Mukerjee and Mysels, NSRDS-NBS 36, National Bureau of Standards (1971)

300 vs 332 ppm

HTAB CMC from the University of Waterloo group's own Dropometer data, against about 332 ppm in the literature.

University of Waterloo case study, Droplet Lab (2026)

Sources: P. Mukerjee and K. J. Mysels, Critical Micelle Concentrations of Aqueous Surfactant Systems, NSRDS-NBS 36, National Bureau of Standards, 1971; Droplet Lab case study of Prof. Rajinder Pal's group, University of Waterloo, which reports the group's published Dropometer data.

Quick reference

What this CMC assessment does and what it does not

A quick reference for formulation, QC and regulatory teams checking fit before reading further.

Evidence Box

Problem this solves

Datasheet CMC values that do not match your surfactant, your medium or your temperature.

Standards

FDA's 510(k) guidance for contact lens care products describes CMC by surface tension in Appendix B. ISO 4311 covers CMC by plate, stirrup or ring.

Peer reviewed method

The Dropometer pendant drop method is published in Review of Scientific Instruments and Colloids and Surfaces A.

Customer case study

Prof. Rajinder Pal's group at the University of Waterloo reported Dropometer surface tension data in seven publications, six peer reviewed.

Customer publications

Papers from the group cover anionic surfactant binding to a cationic polymer and surfactants in nanocrystal suspensions.

Honest limit

Surface tension gives the CMC and surface activity. It does not give micelle size, shape or aggregation number.

Who this is for

What CMC question are you trying to answer?

Pick the card closest to your role. Each one points to the part of this page that answers it.

Formulation scientist

You need the CMC of a surfactant or mixed surfactant system in your actual formulation, with salts, polymers and other ingredients present. A dilution series in your own medium gives that number directly.

A pure surfactant CMC from the literature can be far from the CMC in your formulation.

QC chemist

Incoming surfactant lots vary, and impurities change how the curve looks near the CMC. A short series at fixed concentrations checks each lot against a reference.

A dip in surface tension just before the CMC usually means an impurity.

Regulatory or contact lens care team

FDA's 510(k) guidance for contact lens care products describes how to determine CMC by surface tension. The same dilution and regression workflow produces those data in house.

Validate the method within your quality system before using it in a submission.

Fit check

Is surface tension the right CMC technique for you?

It suits most surfactant systems and needs no probe or dye. It suits fewer cases when you need micelle structure or the surfactant is barely surface active.

Good fit if

You need the CMC of anionic, cationic or nonionic surfactant systems.

You work with mixed surfactant systems or full formulations.

You want to compare surfactant lots or suppliers.

You need CMC data for a contact lens care 510(k) submission.

Sample volume is small and you want to avoid a ring or plate.

Less relevant if

You need micelle size, shape or aggregation number.

The surfactant barely lowers surface tension, so the break is hard to see.

Solutions are so viscous that a drop will not reach a stable shape.

You need a stated ISO 4311 or ASTM D1331 compliance claim with force tensiometry.

Summary

How to determine the critical micelle concentration by surface tension

The answer in under a minute.

As concentration increases, surfactant molecules first collect at the surface, with the surfactant tail pointing out of the water, and reduce the surface tension. Once the surface is saturated, extra surfactant molecules form micelles instead, and surface tension stops falling. Measure surface tension at different concentrations, plot it against log concentration, fit a line to each part, and the CMC is where the lines cross.

The Dropometer surface tension kit measures each point by pendant drop with a few microlitres of sample; our surface tension measurement guide explains the method. ISO 4311 sets out CMC by plate, stirrup or ring, and FDA's contact lens guidance describes the same curve in Appendix B. Prof. Pal's group used the Dropometer for this work; see the University of Waterloo case study, and our analyses of papers on surfactants in nanocrystal suspensions and cationic polymer and anionic surfactant mixtures.

Expert Quote

Expert perspective on reading CMC curves

“

Surface tension versus concentration is one of the few measurements that lets you watch a surfactant reorganise an interface in real time; read the curve, not just the endpoint.

The problem

Why the CMC value on the datasheet is not the CMC in your formulation

The critical micelle concentration is one property of surfactants that formulators use every day: below it, surfactant monomers lower surface tension; above it, micelle formation takes over and cleaning, solubilising and foaming change. Published CMC values are measured for a pure surfactant in pure water at one temperature. Real surfactant systems are different. Salt lowers the CMC of an ionic surfactant, temperature shifts it, polymers can bind surfactant and move the break point, and impurities such as dodecanol in an anionic surfactant like SDS change the shape of the curve. CMC values obtained by different techniques also differ, and even the same data fitted two ways can differ by about 3 percent. So the CMC you need is the one measured in your own aqueous solution or formulation, with a stated technique and a stated fit.

Cleaning or foaming changes between surfactant lots at the same dose.

Your CMC does not match the supplier's value.

The surface tension curve dips to a minimum before the plateau.

Results differ between people fitting the same curve.

Adding a polymer or salt moves the point where performance changes.

Troubleshooting

Why CMC results go wrong and how to fix them

Why:

Traces of a more surface active impurity collect at the surface below the CMC, then dissolve into micelles above it.

How to detect:

A minimum in surface tension just before the plateau instead of a sharp break. The classic case is dodecanol in SDS, which gives a dip just below the SDS CMC.

Corrective action:

Use a purer surfactant lot or report the impurity, and compare lots against a reference. Our guide to reproducible surface tension measurement covers the checks.

Why:

At low surfactant concentrations, molecules take time to reach a new surface, so surface tension keeps falling after the drop forms.

How to detect:

Readings drift with drop age, most at the lowest concentrations.

Corrective action:

Read each drop at a fixed age, or wait until the reading stops changing, and record the age with the result.

Why:

A coarse concentration range leaves only one or two points near the CMC, so the fitted lines cross in the wrong place.

How to detect:

The estimated CMC moves a lot when you add or drop one point.

Corrective action:

Space concentrations evenly on a log scale and add extra points around the expected CMC.

Why:

Which points go into each line, and how the lines are fitted, changes the CMC estimation.

How to detect:

Two people get different CMC values from the same data.

Corrective action:

Fix the fitting rule in your SOP: which points belong to each line, and whether you use least squares or orthogonal distance regression.

Why:

Salt and temperature change how easily ionic surfactants form micelles.

How to detect:

The CMC shifts between water and your buffer, or between days. Compare with our CMC values for common surfactants.

Corrective action:

Dilute in the product medium, control temperature, and state both with every result.

Why:

A trace of another surfactant or grease lowers surface tension at low concentrations.

How to detect:

The lowest concentrations read lower than pure water or the medium.

Corrective action:

Check the medium alone first; it should match its reference value. Clean the needle and glassware between surfactant samples.

Not sure your CMC curve is right?

Bring your surfactant samples or your experimental data to a call, and we will look at the curve together.

Book a 15 minute demo

Quality records

What a CMC determination record contains

Each determination of the CMC produces a record you can file with the batch or the study. These records can feed into your existing quality records under ISO 9001, GMP or your site's quality system.

Sample identity

Surfactant, lot, supplier, medium and any additives.

Dilution series

Concentrations prepared, how they were made and the highest concentration used.

Method settings

Needle size, drop age at reading, drops per concentration and temperature.

Readings

Surface tension values at each concentration, with the mean and spread.

Fit and result

Points used for each line, fitting method and the calculated CMC.

Drop images

Automatic edge detection gives the same reading from the same image, so a reviewer can recheck any result.

Measurements

What to measure to determine the CMC

Primary check

Surface tension plotted against surfactant concentration

Why it matters: Surface tension as a function of concentration has a break, and that break is identified as the CMC.

How to interpret: Plot surface tension against log concentration, fit the falling part and the plateau separately, and take the crossing point.

When it is not enough: A weak break, or a minimum from impurities, makes the crossing point uncertain.

Plateau

Surface tension above the CMC

Why it matters: It shows how far the surfactant can reduce the surface tension.

How to interpret: Compare the plateau between lots and suppliers; a change points to a different composition.

When it is not enough: It does not show how fast the surface reaches that value.

Slope

Slope below the CMC

Why it matters: Through the Gibbs equation, the slope gives the surface excess, how tightly molecules pack at the surface.

How to interpret: Use the slope of surface tension against ln concentration, with the right factor for ionic or nonionic surfactants.

When it is not enough: It assumes the surface is at equilibrium.

Repeatability

Spread between drops at each concentration

Why it matters: It shows whether the reading is stable enough to fit.

How to interpret: Large spread at low concentrations usually means drops need more time.

When it is not enough: Low spread does not prove the sample is free of impurities.

Validation

Validated measurement approach for CMC by surface tension

How the Dropometer itself has been validated, and where customers have published with it.

Peer reviewed method

The Dropometer pendant drop surface tension method is published in Colloids and Surfaces A, and its contact angle method in Review of Scientific Instruments.

See the validation papers

Customer evidence on surfactant systems

Prof. Rajinder Pal's group at the University of Waterloo measured surfactant and polymer solutions with the Dropometer in seven publications. Their own data gave an HTAB CMC of 300 ppm, against about 332 ppm in the literature.

Read the Waterloo case study

QC Protocol

How to determine the CMC with surface tension measurement

Six steps for the determination of CMC for one surfactant in one medium. Repeat for each new surfactant, mixture or medium.

1

Check the medium

Measure the water or product medium alone. It should match its reference value before you add any surfactant.

2

Prepare the dilution series

Make surfactant solutions of different concentrations, evenly spaced on a log scale, with extra points around the expected CMC. Our CMC experiment shows a worked series.

3

Lock the method

Fix needle size, temperature, drop age at reading and drops per concentration. Measure from the lowest concentration to the highest.

4

Measure each concentration

Measure each surfactant solution in turn. Record surface tension values for each drop and wait until readings stop changing at low concentrations.

5

Fit and calculate the CMC

Make the surface tension versus log surfactant concentration plot, fit both parts with the rule in your SOP, and calculate the CMC at the crossing point.

6

Report and compare

Once you establish the CMC, report it with temperature, medium, technique and fit, and compare it with your reference lot or expected CMC.

We completed our gage R&R study on the unit and it performed very well.

Brandon Barbee

Corporate Quality Engineer, Zeus Industries, Polymer Manufacturing

Download the CMC determination SOP

A two page SOP you can adapt for your surfactant samples, with the method lock, dilution plan, a fixed method to calculate the CMC, a troubleshooting table and a record sheet.

Surface tension measurement with the Dropometer

Example output

Example CMC determination: surface tension at different surfactant concentrations

Example data only. Your values will differ with surfactant, medium and temperature.

How to read this table

Grey rows fall steeply below the CMC; green rows sit on the plateau. Fitting a line to each group gives a calculated CMC of about 6.5 mM for this example. Your fitting rule should be fixed in your SOP before you run samples.

Example surface tension versus concentration

Concentration (mM)	Drops	Surface tension (mN/m)	Spread SD (mN/m)	Region
0 (water)	5	72.0	0.2	Medium check
1	5	56.1	0.6	Below CMC
2	5	49.3	0.5	Below CMC
4	5	42.4	0.4	Below CMC
6	5	38.6	0.4	Below CMC
10	5	37.6	0.3	Plateau
15	5	37.5	0.3	Plateau

Example data, not customer data. Pendant drop surface tension of an anionic surfactant in water at 25 °C, read at a fixed drop age.

Signal order

The order to check a CMC curve

Check these in order. Each one rules out a problem before you look at the next.

1

Medium reading

Water or medium matches its reference, so nothing is contaminated.

2

Drop stability

Changes in surface tension at the lowest concentrations have stopped.

3

Curve shape

A clean break, not a minimum before the plateau.

4

Points near the break

Enough points on each side to fit both lines.

5

Fit and reference

The calculated CMC sits close to your reference lot.

FAQ

Common questions about critical micelle concentration and surfactant concentration

Questions formulation, QC and regulatory teams ask about CMC determination.

What is the critical micelle concentration?

The critical micelle concentration is the concentration of a surfactant above which surfactant molecules form micelles. Below it, any added amount of surfactant goes mostly to the surface and lowers surface tension. The formation of micelles starts above a certain concentration, which is why it is called critical. Above it, the surface is saturated and surface tension stays almost flat. See our glossary entry on the critical micelle concentration and our guide to surfactants.

How do you determine the CMC from surface tension?

Measure surface tension at a range of surfactant concentrations, plot it against log concentration, and fit one line to the falling part and one to the plateau. The CMC of the surfactant is where the lines cross, and the CMC can be calculated from the two fitted equations. A low CMC means the surfactant forms micelles at small doses, which matters for dosing and cost.

Should I use least squares or orthogonal distance regression?

You can calculate the CMC using either method. Ordinary least squares assumes error only in surface tension. Orthogonal distance regression allows for error in both concentration and surface tension, which can matter when dilution errors are large. Either is fine if your SOP fixes the method and the points used, because the fit itself can shift the CMC by a few percent.

Does the method work for anionic, cationic and nonionic surfactants?

Yes. Surface tension works for anionic surfactant, cationic surfactant and nonionic surfactant systems, and for mixtures. Ionic surfactants are sensitive to salt, so measure them in the medium they will be used in. Conductivity only works for ionic surfactants, which is one reason surface tension is widely used.

Why does my curve show a minimum before the CMC?

A minimum usually means a more surface active impurity is present, for example a fatty alcohol left in an anionic surfactant. The impurity sits at the surface below the CMC and moves into micelles above it. Report it, and use the break, not the minimum, for the CMC.

Which standards cover CMC by surface tension?

ISO 4311 covers CMC of anionic and nonionic surface active agents by plate, stirrup or ring. ASTM D1331 covers surface tension of surfactant solutions by force tensiometry, and EN 14370 covers surface tension of surface active agents. FDA's 510(k) guidance for contact lens care products describes determination of critical micelle concentration by surface tension in Appendix B. The Dropometer runs a pendant drop method, so for ISO 4311 and ASTM D1331 it is a correlated method, not a compliance claim.

What should we look for in a tensiometer for CMC work?

It should measure small volumes, track readings with drop age so you can see equilibrium, keep drop images and give repeatable results. Our guide to the best tensiometer for surfactant and CMC research compares options, and our chemicals guide covers surfactant work more widely.

Business impact

What changes when you measure CMC in your own formulation

Typical changes in how teams work. Benchmarks are given only where a published source exists.

Before and with in house CMC determination

Metric	Before Dropometer	With Dropometer	Indicative Benchmark
CMC value used	Datasheet or literature	Measured in your own medium	CMC values in the NBS compilation span 71 methods (Mukerjee and Mysels, 1971)
Fitting	Varies by analyst	Fixed rule in the SOP	Same data fitted two ways differed by about 3% (Mukerjee and Mysels, 1971)
Lot to lot checks	Performance tests only	Short series against a reference lot	No published benchmark; track on your own lots
Agreement with literature	Unknown	Checked against a known surfactant	HTAB 300 ppm against about 332 ppm (University of Waterloo case study)

See a CMC run on your own surfactant

Bring a surfactant sample and your expected CMC. We will show the curve on a call.

Book a 15 minute demo

Estimate the cost of formulation iterations

Enter your own numbers. The result is an estimate, not a Droplet Lab claim.

Instrument cost ($) Each Dropometer unit is $5,000; the default models 1 unit.

Iterations / batches / trials per month

Material cost per iteration ($) Your own surfactant and dilution series prep cost per run.

Technician hours per iteration Your own time for dilution, measurement and fitting per run.

Loaded lab rate ($/hr)

Iterations avoided with in house CMC data (%) Use your own records; start low if unsure.

Result

~0

Iterations saved / month

~0

Monthly savings

~0

Payback period

~0

First year net benefit

Monthly savings = materials saved + technician time saved from reduced iterations.

Limits

What CMC and surface tension measurement cannot tell you

Honest limits, so you know when to reach for another technique.

It does not give the size, shape or aggregation number of surfactant micelles.

Impurities can create a minimum that hides the true break.

Low concentrations of surfactants need time to reach equilibrium.

Weakly surface active surfactants give a shallow break that is hard to fit.

Very viscous samples may not reach a stable drop shape.

A CMC measured in water does not transfer to a formulation with salts or polymers.

Surface tension gives the CMC, the plateau and the surface excess. Micelle size, shape, aggregation number and how the formulation performs in use still need other methods and your own tests.

Book a 15 minute demo Request a quote

How this page was created

Editorial and technical transparency notes for this page.

Transparency Details 4 checklist items

01

Drafting assistance

Drafted with Claude Opus 5.5 (Anthropic) using web search for sources, then edited by the Droplet Lab team.

02

Technical review

Reviewed and edited for technical accuracy by the Droplet Lab Team.

03

Verification steps

Standard identifiers, units, thresholds and key procedural claims are checked against cited sources before publication.

04

Updates

Reviewed every 12 months or when the underlying method changes.

Report a correction

Spotted an issue in this summary? Send a correction request and our team will review it.

Correction Request

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.

Contact us to report a correction

Sources

References

1.

Mukerjee, P., Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems. NSRDS-NBS 36, National Bureau of Standards, 1971. https://nvlpubs.nist.gov/nistpubs/Legacy/NSRDS/nbsnsrds36.pdf

2.

US Food and Drug Administration. Premarket Notification (510(k)) Guidance Document for Contact Lens Care Products, 1997, Appendix B: Determining critical micelle concentration by surface tension. https://www.fda.gov/regulatory-information/search-fda-guidance-documents/premarket-notification-510k-guidance-contact-lens-care-products

3.

Sun, C. C., Pal, R. Effect of polymer concentration on the rheology and surface activity of cationic polymer and anionic surfactant mixtures. Fluids 10:253, 2025. Customer publication that used the Dropometer; the link goes to our analysis. https://dropletlab.com/validation/citations/analysis/effect-of-polymer-concentration-on-the-rheology-and-surface-activity-of-cationic-polymer-and-anionic-surfactant-mixtures/

4.

Pal, R., Sun, C. C. Influence of added surfactants on the rheology and surface activity of polymer solutions. ChemEngineering 9:105, 2025. Customer publication that used the Dropometer; the link goes to our analysis. https://dropletlab.com/validation/citations/analysis/influence-of-added-surfactants-on-the-rheology-and-surface-activity-of-polymer-solutions/

5.

Pal, A., Pal, R. Influence of surfactants on the rheological behavior of nanocrystal suspension. Preprints.org, 2025. Customer preprint that used the Dropometer; the link goes to our analysis. https://dropletlab.com/validation/citations/analysis/influence-of-surfactants-on-the-rheological-behavior-of-nanocrystal-suspension/

6.

ISO 4311. Anionic and non ionic surface active agents: determination of the critical micellization concentration, method by measuring surface tension with a plate, stirrup or ring. https://dropletlab.com/industry-standards/iso-4311/

7.

Surface tension measurement with a smartphone using a pendant drop. Colloids and Surfaces A: Physicochemical and Engineering Aspects.

https://www.sciencedirect.com/science/article/abs/pii/S0927775717307744

8.

Droplet Lab validation papers

9.

Customer publications that cite the Dropometer

10.

FDA 510(k) Appendix B explained

11.

University of Waterloo case study

Critical Micelle Concentration (CMC) Determination: Measure Surfactant Concentration by Surface Tension for Anionic, Cationic and Detergent Formulations

Featured Studies & Publications

Why a published CMC value is not your CMC

71 methods

About 3%

300 vs 332 ppm

What this CMC assessment does and what it does not

Evidence Box

What CMC question are you trying to answer?

Formulation scientist

QC chemist

Regulatory or contact lens care team

Is surface tension the right CMC technique for you?

Good fit if

Less relevant if

How to determine the critical micelle concentration by surface tension

Expert perspective on reading CMC curves

Why the CMC value on the datasheet is not the CMC in your formulation

Why CMC results go wrong and how to fix them

Not sure your CMC curve is right?

What a CMC determination record contains

Sample identity

Dilution series

Method settings

Readings

Fit and result

Drop images

What to measure to determine the CMC

Surface tension plotted against surfactant concentration

Surface tension above the CMC

Slope below the CMC

Spread between drops at each concentration

Validated measurement approach for CMC by surface tension

How to determine the CMC with surface tension measurement

Check the medium

Prepare the dilution series

Lock the method

Measure each concentration

Fit and calculate the CMC

Report and compare

Download the CMC determination SOP

Example CMC determination: surface tension at different surfactant concentrations

Example surface tension versus concentration

The order to check a CMC curve

Medium reading

Drop stability

Curve shape

Points near the break

Fit and reference

Common questions about critical micelle concentration and surfactant concentration

What changes when you measure CMC in your own formulation

Before and with in house CMC determination

See a CMC run on your own surfactant

Estimate the cost of formulation iterations

Estimate saved iterations and lab cost.

What CMC and surface tension measurement cannot tell you

Standards, evidence and related surfactant workflows

ISO 4311

FDA 510(k) Appendix B

University of Waterloo rheology lab

Cationic polymer and anionic surfactant: our analysis

Dropometer

Surfactants, emulsions and foams

Foam control and foam quality

Drilling and fracturing fluid surfactants

How this page was created

Drafting assistance

Technical review

Verification steps

Updates

Report a correction

References