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Critical micelle concentration

Abbreviation
CMC
Also known as
critical micellization concentration, c.m.c.

Critical micelle concentration (CMC) is the surfactant concentration above which surfactant molecules begin to assemble into micelles. Below it, added surfactant lowers surface tension; above it, surface tension stays roughly constant and additional surfactant forms more micelles.

What is the critical micelle concentration?

The critical micelle concentration (CMC) marks the point at which a surfactant in solution changes behavior. Below it, surfactant molecules exist individually and crowd onto surfaces and interfaces, lowering surface tension as they do. Above it, the interfaces are effectively saturated, and additional surfactant molecules associate into micelles in the bulk liquid.

IUPAC’s definition is careful about what the number means. Micelle formation does not switch on at one exact concentration; it happens across a small range, below which virtually no micelles are detected and above which virtually all added surfactant forms micelles. Many properties of the solution change at a different rate on either side of that range. Extrapolating the trend of one such property from both sides until the lines cross gives the CMC. IUPAC also uses the name critical micellization concentration, with the abbreviation cmc.

How the CMC is measured

The CMC is found by measuring a property of the solution across a range of concentrations and locating the break in the trend. Common methods include:

  • Surface tension (tensiometry). Surface tension falls steadily as concentration rises, then levels off. The CMC is taken at the break, usually on a plot against the logarithm of concentration.
  • Electrical conductivity. For ionic surfactants, conductivity rises more slowly with concentration once micelles form, because counterions bind to the micelles. The change in slope marks the CMC.
  • Fluorescence probes and light scattering. Probe molecules that respond to the oily environment inside micelles, or scattering from the micelles themselves, reveal when aggregates appear.

Because each method responds to a different property, the values they give are close but not identical. IUPAC recommends stating the method whenever a CMC is reported.

What changes the CMC

  • Hydrophobic tail length. A longer hydrocarbon tail lowers the CMC substantially, because the molecule is less comfortable dissolved in water.
  • Head group. Nonionic surfactants generally have much lower CMCs than ionic surfactants with tails of similar length, because their head groups do not repel one another electrically.
  • Dissolved salts. Added electrolyte lowers the CMC of ionic surfactants by screening the repulsion between charged head groups.
  • Temperature. Temperature shifts the CMC, by amounts and in directions that depend on the surfactant type.

Why the CMC matters in practice

The CMC is a measure of efficiency: it shows how much surfactant is needed before interfaces are saturated and micelles begin to form. A surfactant with a low CMC reaches that point at a lower dose. Rosen distinguishes this efficiency from effectiveness, the lowest surface tension the surfactant achieves, which is read from the surface tension at the CMC. Comparing surfactants fairly means looking at both, measured the same way.

The CMC measured in pure water is also only a starting point. In use, surfactant is lost to the surfaces it adsorbs onto, such as rock, soil, pigment or fabric, and the concentration left in solution can fall below the CMC even when the dose added was well above it.

CMC values are reported either as molar concentrations (mol/L or mmol/L) or as mass concentrations (mg/L). Converting between them requires the surfactant’s molecular weight, so comparisons across data sheets should be made in the same units. For reference, the widely used anionic surfactant sodium dodecyl sulfate has a CMC of about 8 mmol/L in water at 25 °C, according to the National Bureau of Standards compilation by Mukerjee and Mysels.

CMC and Locus biosurfactants

Locus lists a low critical micelle concentration among the properties of its SUSTAIN frac surfactant and its Amphi glycolipid ingredients, and presents it as one reason those products are effective at low use rates.

Frequently asked questions

For efficiency, usually: a lower CMC means fewer molecules are needed to saturate interfaces. But the CMC alone does not show how far a surfactant lowers surface tension, and it says nothing about wetting, foaming, compatibility or stability in a particular formulation. It is best compared alongside the surface tension at the CMC and performance tests in the actual system.

It stays roughly constant. Once micelles form, the concentration of free, unassociated surfactant molecules stays close to the CMC, so the interface does not become much more crowded as more surfactant is added. The extra surfactant goes into micelles instead.

Yes. IUPAC’s preferred name is critical micellization concentration, with the symbol cM and the abbreviation cmc. Critical micelle concentration is the more common form in industry and in product literature.

  • Micelle

    A micelle is a nanometer-scale cluster of surfactant molecules that forms spontaneously in water once the surfactant concentration passes the critical micelle concentration. The molecules' water-repelling tails gather in the core and their water-attracting heads face outward.

  • Surface tension

    Surface tension is the energy per unit area, or force per unit length, at the surface of a liquid where it meets a gas such as air. It arises because molecules at the surface have fewer like neighbors to attract them than molecules inside the liquid.

  • Surfactant

    A surfactant, short for surface-active agent, is a compound that lowers the surface tension of a liquid, or the interfacial tension between two phases, by adsorbing at the boundary between them. Each molecule has a water-attracting head and a water-repelling tail.

  • Interfacial tension IFT

    Interfacial tension (IFT) is the energy per unit area, or force per unit length, at the boundary between two liquids that do not mix, such as oil and water. Surfactants lower it by adsorbing at that boundary.

  • Hydrophilic-lipophilic balance HLB

    Hydrophilic-lipophilic balance (HLB) is a number that describes how strongly a surfactant is attracted to water relative to oil. On Griffin's scale for nonionic surfactants it runs from 0 to 20: low values favor oil, high values favor water.

  • Sophorolipid

    A sophorolipid is a glycolipid biosurfactant made by certain yeasts, principally Starmerella bombicola. Each molecule joins the sugar sophorose to a hydroxylated fatty acid, and it occurs in two main forms: an open acidic form and a closed ring, or lactonic, form.

  • Rhamnolipid

    A rhamnolipid is a glycolipid biosurfactant made mainly by Pseudomonas bacteria. Each molecule joins one or two units of the sugar rhamnose to one or two hydroxylated fatty acids, giving mono-rhamnolipids and di-rhamnolipids.

  • Surfactant adsorption

    Surfactant adsorption is the attachment of surfactant molecules to a surface, such as reservoir rock, soil, pigment or fabric. It is how surfactants do much of their work, but in oilfield and soil applications it also removes surfactant from solution before it reaches its target.

Sources

  1. critical micelle concentration (C01395), IUPAC Compendium of Chemical Terminology (Gold Book)
  2. Critical Micelle Concentrations of Aqueous Surfactant Systems (NSRDS-NBS 36), P. Mukerjee and K. J. Mysels, U.S. National Bureau of Standards, 1971
  3. Surfactants and Interfacial Phenomena, 4th edition, M. J. Rosen and J. T. Kunjappu, Wiley, 2012