Surfactants, CMC, Emulsions and Foams

Emulsion Stability Mechanism: How the Emulsifier and Interfacial Tension Control Oil-in-Water and Water-in-Oil Emulsions

Every emulsion of oil and water is thermodynamically unstable and will separate in the end. How fast depends on the interfacial tension, the interfacial film and the droplet size. Measuring oil water interfacial tension for each emulsifier and concentration shows which formulations are worth a long stability test.

Who this is for: Formulation scientists and QC teams in food, cosmetics, chemicals and pharmaceuticals, and researchers working on emulsions and colloidal systems.

Where it fits: Emulsifier screening, troubleshooting phase separation, and checks on oil and emulsifier lots before emulsification.

What it does not do: It does not replace shelf life testing or measure droplet size or interfacial rheology. Use those methods alongside it.

Method

Pendant drop oil-water interfacial tension

Related standards

ISO 6889 and EN 14210 (force methods; correlated, not a compliance claim)

Setup

About 2 minutes

Book a 15 minute demo Request a quote

Illustration of oil added to water, an oil in water emulsion, and the two liquids separated into layers

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

What the research says about interfacial tension and emulsion stability

Roughly linear

is how Ostwald ripening rates depend on interfacial tension below the surfactant CMC, according to a review of ripening theory and data.

P. Taylor, Advances in Colloid and Interface Science 75:107 (1998)

About 10⁷ kT

is the energy to detach a micron sized particle from an oil water interface at 50 mN/m and a 90° contact angle. That is why particle stabilised emulsions resist coalescence.

Davies, Krüger, Coveney and Harting, arXiv 1407.0327 (2014)

3 mN/m

oil water interfacial tension measured with the Dropometer at Emory University and used to model an emulsion in Soft Matter.

Cao, Liao, Breedveld and Weeks, Soft Matter (2021)

Sources: P. Taylor, Ostwald ripening in emulsions, Advances in Colloid and Interface Science 75:107 to 163, 1998; G. B. Davies, T. Krüger, P. V. Coveney, J. Harting, Detachment energies of spheroidal particles from fluid fluid interfaces, arXiv 1407.0327, 2014; Cao, Liao, Breedveld and Weeks, Soft Matter, 2021, as reported in the Droplet Lab Emory University case study.

Quick reference

What this emulsion stability screen does and what it does not

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

Evidence Box

Problem this solves

Emulsions that cream, separate or invert, and emulsifier choices that take weeks of shelf testing to compare.

Standards

ISO 6889 and EN 14210 measure interfacial tension of surface active agents with force methods. The Dropometer uses pendant drop, a correlated method.

Peer reviewed method

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

Customer case studies

Emory University measured its own oil water tension for two Soft Matter papers; Harvard's shared facility users published seven papers with the Dropometer.

Customer publication

University of Waterloo researchers linked interfacial tension to catastrophic phase inversion of emulsions (Kim and Pal, Colloids and Interfaces, 2025).

Honest limit

Interfacial tension is one factor in stability. Droplet size, the interfacial film and the continuous phase matter as much.

Who this is for

What emulsion stability problem are you trying to solve?

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

Formulation scientist

You are choosing an emulsifier and its level for a cream, sauce or chemical emulsion. Interfacial tension against concentration shows which candidates lower the tension most at the lowest dose, before you run stability tests.

Low interfacial tension helps emulsification; it does not guarantee long term stability on its own.

QC chemist

Batches that passed before now separate or cream. Measuring oil water interfacial tension for incoming oil and emulsifier lots shows whether a raw material changed.

Measure at a fixed temperature and drop age so lots can be compared.

Researcher

Your models need the interfacial tension of your own oil and water, not a value from another study. Pendant drop measurement gives it with a few microlitres.

Make the denser liquid the drop; light oil drops climb the needle.

Fit check

Is interfacial tension the right screen for your emulsion?

It helps when the emulsifier or raw materials drive stability. It helps less when the problem is processing, droplet size or the continuous phase.

Good fit if

You compare emulsifiers, blends or concentrations.

Separation started after a change of oil or emulsifier lot.

You study phase inversion as composition changes.

You need interfacial tension values for models or reports.

You work with water-in-oil emulsions, crude oil or fluorinated oils.

Less relevant if

Separation comes from the homogeniser, not the formulation.

You need droplet size distribution. Use light scattering or microscopy.

You need interfacial rheology. Use an oscillating drop or interfacial rheometer.

The oil and water have almost the same density, so the drop has little shape.

Summary

What controls emulsion stability and how to screen for it

The answer in under a minute.

An emulsion is a dispersion of droplets of one liquid in another, and because the oil water interface costs energy, it always tends to separate. The emulsifier slows that down by lowering interfacial tension and forming an interfacial film. Measure oil water interfacial tension for each emulsifier and concentration, and you can rank candidates before a long stability test.

The Dropometer surface and interfacial tension kit measures oil water tension by pendant drop; our surface tension measurement guide explains the method. ISO 6889 covers interfacial tension of surface active agents by force methods. Emory University used the Dropometer to measure its own oil and water; see the Emory case study and the Harvard shared facility case study. Harvard researchers also used it for functional microcapsules, and Waterloo researchers for phase inversion of emulsions.

Expert Quote

Expert perspective on interfaces

“

Plot surface tension against surfactant concentration and the interface tells you when it is saturated; the curve bends at the point where the surface can hold no more.

The problem

Why emulsions separate: the primary mechanisms

An emulsion is made of two immiscible liquids, with droplets of the dispersed phase spread through the continuous phase. Making new interface costs energy, so the system always moves towards phase separation. There are several primary mechanisms. Creaming or sedimentation moves droplets up or down because of density difference, and larger droplets move faster. Flocculation makes droplets stick together. Coalescence happens when the interfacial film between two emulsion droplets breaks and they form larger droplets. Ostwald ripening grows larger droplets at the expense of smaller droplets as the oil phase dissolves through the continuous phase. Phase inversion turns an oil-in-water emulsion into water-in-oil emulsions, or the reverse, when composition or temperature changes. The emulsifier, its concentration, the interfacial tension and the physical properties of the dispersed and continuous phases all play an important role. Most of these factors affecting the stability can be screened before a product fails a shelf test.

A cream layer forms at the top, or sediment at the bottom.

Oil appears on the surface after storage or heating.

Droplet size grows over days or weeks.

The emulsion thins or thickens suddenly as it inverts.

A new lot of oil or emulsifier gives a less stable product.

Troubleshooting

Root causes of emulsion instability and how to detect them

Why:

Emulsification creates a large interface, and the emulsifier adsorbs onto it. If there is too little, the interfacial layer is incomplete and droplets coalesce, leading to phase separation.

How to detect:

Tension at the use level is well above its plateau value for that emulsifier.

Corrective action:

Raise the level until the tension reaches its plateau, then confirm with a stability test. Our guide to reproducible surface tension measurement covers good practice.

Why:

The emulsifier tends to make its preferred phase continuous, so the wrong one gives the wrong type of emulsion formed, or an unstable one.

How to detect:

The emulsion inverts or separates soon after making; conductivity drops if water is no longer continuous.

Corrective action:

Screen emulsifiers that are more soluble in the intended continuous phase and compare their tension curves.

Why:

A new oil lot, a different phospholipid or trace impurities change the interfacial properties and can lead to instability.

How to detect:

Oil water interfacial tension differs from the reference lot. Compare pure liquids with our surface tension of common liquids.

Corrective action:

Hold the lot, check it with the supplier and record the tension with every incoming batch.

Why:

Oil from smaller droplets dissolves and moves to larger droplets. The rate rises with interfacial tension and oil solubility.

How to detect:

Droplet size grows steadily without visible flocculation, most with more soluble oils.

Corrective action:

Lower the tension, add a small amount of a less soluble oil, or narrow the droplet size distribution.

Why:

Density difference and larger droplets speed up creaming; a thin continuous phase does little to slow it.

How to detect:

A cream layer with no change in droplet size.

Corrective action:

Make smaller droplets, match densities or add a thickener or stabilizer to the continuous phase.

Why:

Some emulsifiers lower tension but form a thin film with weak viscoelastic properties, so droplets coalesce when they touch.

How to detect:

Tension is low, but coalescence still happens. Our interfacial tension glossary entry explains what tension does and does not show.

Corrective action:

Add a polymer, protein or solid particles to build the film, and check interfacial rheological behaviour with a suitable method.

Not sure why your emulsion separates?

Bring your oil, water phase and emulsifier to a call, and we will look at the interfacial tension together.

Book a 15 minute demo

Quality records

What an emulsion screening record contains

Each measurement produces a record you can file with the formulation or batch. These records can feed into your existing quality records under ISO 9001, GMP or your site's quality system, and support regulatory files where your QA team decides they apply.

Phase identity

Oil, water phase and emulsifier, with lots and suppliers.

Composition

Emulsifier concentration and any salts, thickeners or particles.

Method settings

Which liquid is the drop, needle size, densities, temperature and drop age.

Readings

Tension of each drop, the mean and the spread.

Decision

Carry forward, adjust or reject the formulation or lot, with the operator and date.

Drop images

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

Measurements

What to measure for emulsion stability

Primary check

Oil water interfacial tension at the use concentration

Why it matters: Reduction of interfacial tension makes emulsification easier and slows ripening.

How to interpret: Compare emulsifiers and levels at the same temperature and drop age. There is no universal target; link values to your own stability results.

When it is not enough: It does not show the strength of the interfacial film.

Curve

Tension against emulsifier concentration

Why it matters: It shows the level at which the interface is saturated.

How to interpret: Use a level at or above the plateau, allowing for emulsifier lost to the large interface after emulsification.

When it is not enough: Mixtures and particles can give curves without a clear plateau.

Time

Tension against drop age

Why it matters: It shows how fast the emulsifier reaches a new interface, which matters during the emulsification process.

How to interpret: A slow fall points to slow adsorption, which can leave new droplets bare.

When it is not enough: Very fast adsorption needs specialised dynamic methods.

Outcome

Stability of the emulsions over time

Why it matters: It is the result that matters for the product.

How to interpret: Track separation, droplet size and appearance, and link them to the tension record.

When it is not enough: It takes days or weeks, which is why a fast screen helps.

Validation

Validated measurement approach for oil water interfacial tension

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

Peer reviewed method

The Dropometer pendant drop 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 emulsions

Prof. Eric Weeks's group at Emory measured 3 mN/m for its TPM oil and water and about 10 mN/m for octane in water with 0.5% Tween 20, and used both in Soft Matter papers.

Read the Emory case study

QC Protocol

How to screen emulsifiers with interfacial tension

Six steps for one oil, one water phase and a set of emulsifiers. Repeat for each new oil or continuous phase.

1

Measure the clean interface

Measure the oil phase against the aqueous phase with no emulsifier. Our oil and water interfacial tension experiment shows the setup.

2

Set up the drop

Make the denser liquid the drop, formed into the lighter phase, as the Emory lab found. Enter both densities. Our post on why water forms droplets in oil explains the physics.

3

Lock the method

Fix temperature, drop volume, drop age at reading and drops per sample.

4

Measure each emulsifier and level

Measure tension across a concentration range for each surfactant or emulsifier, from low to high.

5

Rank and test

Rank candidates by tension and the level where it plateaus, then run a stability evaluation on emulsions formed with the top candidates.

6

Record and reuse

Link stability results back to the tension data, so the screen gets more useful with each project.

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 emulsion screening SOP

A two page SOP you can adapt for your oil, water phase and emulsifiers, with the method lock, screening steps, a troubleshooting table and a record sheet.

Surface tension measurement with the Dropometer

Example output

Example emulsifier screening log for an oil-in-water emulsion

Example data only. Your values will differ with oil, water phase and temperature.

How to read this log

Lower interfacial tension at the use level ranks a candidate higher, but the stability column decides. Green rows go forward, amber rows need a second look, red rows are dropped.

Example screening log

Emulsifier	Level (wt%)	Interfacial tension (mN/m)	Spread SD (mN/m)	Separation at 7 days	Result
None	0	48.5	0.4	Separated in minutes	Reference
Nonionic A	0.5	6.2	0.2	None	Carry forward
Nonionic B	0.5	9.8	0.3	Slight cream layer	Second look
Anionic C	0.5	4.1	0.2	Oil on surface	Drop: weak film

Example data, not customer data. Pendant drop interfacial tension of a light mineral oil against water at 25 °C, read at a fixed drop age.

Signal order

The order to check signals when an emulsion separates

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

1

Clean interface reading

Oil against water with no emulsifier matches its reference, so the oil is not contaminated.

2

Tension at use level

The emulsifier still lowers tension as much as the reference lot.

3

Droplet size

Size after emulsification matches the target; if not, check the process.

4

Type of emulsion

Conductivity or dilution confirms which phase is continuous.

5

Stability test

Creaming, coalescence and droplet growth over time.

FAQ

Common questions about emulsion stability

Questions formulation, QC and research teams ask about emulsions and interfacial tension.

What is the mechanism of emulsion stability?

Emulsions are thermodynamically unstable, so stability is about slowing separation. An emulsifier lowers interfacial tension, which makes it easier to emulsify into smaller droplets, and forms an interfacial film that keeps droplets apart; that film is crucial for the stability of most emulsions. Viscosity of the continuous phase slows creaming. The main ways emulsions fail are creaming, flocculation, coalescence, Ostwald ripening and phase inversion.

How does interfacial tension relate to emulsion stability?

Lower interfacial tension means less energy is needed to break oil into droplets, and Ostwald ripening runs more slowly, since its rate rises roughly linearly with tension below the CMC. But low tension alone does not stop coalescence; that needs a strong interfacial film. Use it as a screen, then confirm with stability tests. Our food and beverages guide covers the stability of oil-in-water emulsions in food.

What is a Pickering emulsion?

A Pickering emulsion is stabilized by solid particles at the oil water interface. Unlike emulsions stabilized by surfactants alone, it relies on particles, which are effective at stabilizing emulsions because, once at the interface, they are very hard to remove. So these emulsions resist coalescence. Particles may change interfacial tension only a little, which is why Waterloo researchers found nanocrystals weakly surface active compared with surfactants.

What causes phase inversion?

Phase inversion turns an oil-in-water emulsion into a water-in-oil emulsion or the reverse. It can be caused by adding more of the dispersed phase, changing temperature or changing the emulsifier. Catastrophic phase inversion happens suddenly at a certain volume fraction. Tension data help explain when it happens for a given surfactant.

Does this apply to water-in-oil emulsions and crude oil?

Yes. Pendant drop measurement works for water-in-oil emulsions, crude oil and water, and fluorinated oils, as long as the two liquids differ in density and you can see the drop. In oil processing and enhanced oil recovery, the tension helps judge how easily water and oil will separate or mix.

Which standards cover interfacial tension of surface active agents?

ISO 6889 covers interfacial tension by drawing up liquid films, and EN 14210 covers interfacial tension of surfactant solutions by stirrup or ring. Both are force methods. The Dropometer uses pendant drop, so it is a correlated method, not a compliance claim for either standard.

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

It should measure tension at a liquid-liquid interface as well as surface tension, let you form the drop from either phase, keep drop images and give repeatable results. Our guide to the best tensiometer for surfactant and CMC research compares options.

Business impact

What changes when you screen emulsions by interfacial tension

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

Before and with an interfacial tension screen

Metric	Before Dropometer	With Dropometer	Indicative Benchmark
Emulsifier screening	Shelf tests on every candidate	Interfacial tension first, shelf tests on the best	No published benchmark; track on your own projects
Ripening risk	Seen after weeks	Interfacial tension known up front	Ripening rate roughly linear in interfacial tension below the CMC (Taylor, 1998)
Raw material checks	Performance tests only	Interfacial tension per lot	No published benchmark; track on your own lots
Values for models	Borrowed from other studies	Measured for your own oil and water	TPM oil and water 3 mN/m, measured at Emory (Cao et al., 2021)

See the screen on your own emulsion

Bring your oil, water phase and two emulsifiers. We will show the difference on a call.

Book a 15 minute demo

Estimate the cost of emulsion formulation trials

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 oil, water phase and emulsifier cost per formulation trial.

Technician hours per iteration Your own time for prep, interfacial tension and analysis per trial.

Loaded lab rate ($/hr)

Trials avoided with interfacial tension screening (%) 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 interfacial tension cannot tell you about emulsion stability

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

Low interfacial tension does not guarantee stable emulsions.

It does not measure droplet size or the droplet size distribution.

It does not measure interfacial film strength or rheological properties.

Liquids of nearly equal density give drops with little shape to fit.

Emulsifiers that adsorb slowly need readings at a fixed, stated drop age.

Results in a model system may differ from the full emulsion with its thickener.

Tension data rank emulsifiers and flag raw material changes. Droplet size, interfacial rheology, continuous phase rheology and shelf life still need their 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.

Taylor, P. Ostwald ripening in emulsions. Advances in Colloid and Interface Science 75(2):107 to 163, 1998. https://bibbase.org/network/publication/taylor-ostwaldripeninginemulsions-1998

2.

Davies, G. B., Krüger, T., Coveney, P. V., Harting, J. Detachment energies of spheroidal particles from fluid fluid interfaces. arXiv 1407.0327, 2014. https://arxiv.org/abs/1407.0327

3.

Kim, D., Pal, R. Influence of cellulose nanocrystals and surfactants on catastrophic phase inversion and stability of emulsions. Colloids and Interfaces 9:46, 2025. Customer publication that used the Dropometer; the link goes to our analysis. https://dropletlab.com/validation/citations/analysis/influence-of-cellulose-nanocrystals-and-surfactants-on-catastrophic-phase-inversion-and-stability-of-emulsions/

4.

Cao, Liao, Breedveld and Weeks, Soft Matter, 2021, and Cheng et al., Soft Matter, 2024: oil water interfacial tension measured with the Dropometer, as reported in our Emory University case study. https://dropletlab.com/validation/case-studies/emory-university-emulsion-interfacial-tension/

5.

ISO 6889. Surface active agents: determination of interfacial tension by drawing up liquid films.

https://dropletlab.com/industry-standards/iso-6889/

6.

EN 14210. Surface active agents: determination of interfacial tension of solutions of surface active agents by the stirrup or ring method.

https://dropletlab.com/industry-standards/en-14210/

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.

Emory University case study

11.

Harvard shared facility case study

Emulsion Stability Mechanism: How the Emulsifier and Interfacial Tension Control Oil-in-Water and Water-in-Oil Emulsions

Featured Studies & Publications

What the research says about interfacial tension and emulsion stability

Roughly linear

About 10⁷ kT

3 mN/m

What this emulsion stability screen does and what it does not

Evidence Box

What emulsion stability problem are you trying to solve?

Formulation scientist

QC chemist

Researcher

Is interfacial tension the right screen for your emulsion?

Good fit if

Less relevant if

What controls emulsion stability and how to screen for it

Expert perspective on interfaces

Why emulsions separate: the primary mechanisms

Root causes of emulsion instability and how to detect them

Not sure why your emulsion separates?

What an emulsion screening record contains

Phase identity

Composition

Method settings

Readings

Decision

Drop images

What to measure for emulsion stability

Oil water interfacial tension at the use concentration

Tension against emulsifier concentration

Tension against drop age

Stability of the emulsions over time

Validated measurement approach for oil water interfacial tension

How to screen emulsifiers with interfacial tension

Measure the clean interface

Set up the drop

Lock the method

Measure each emulsifier and level

Rank and test

Record and reuse

Download the emulsion screening SOP

Example emulsifier screening log for an oil-in-water emulsion

Example screening log

The order to check signals when an emulsion separates

Clean interface reading

Tension at use level

Droplet size

Type of emulsion

Stability test

Common questions about emulsion stability

What changes when you screen emulsions by interfacial tension

Before and with an interfacial tension screen

See the screen on your own emulsion

Estimate the cost of emulsion formulation trials

Estimate saved iterations and lab cost.

What interfacial tension cannot tell you about emulsion stability

Standards, evidence and related surfactant workflows

ISO 6889

EN 14210

Emory University

Kim and Pal (2025): our analysis

Dropometer

Surfactants, emulsions and foams

Foam control and foam quality

Transformer oil interfacial tension

How this page was created

Drafting assistance

Technical review

Verification steps

Updates

Report a correction

References