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Solubilization

Also known as
micellar solubilization, solubilizer

Solubilization is the process by which surfactant micelles take up a substance that is poorly soluble in water, such as an oil or fragrance, and carry it in solution. It occurs above the critical micelle concentration and produces a clear, stable liquid.

What is solubilization?

Oils and many organic compounds barely dissolve in water. Above the critical micelle concentration, however, a surfactant solution contains micelles whose cores behave like tiny droplets of oil. A poorly soluble substance can move into those cores, or into the region between the core and the head groups, and be carried in the solution. That is solubilization. The result is clear and thermodynamically stable, unlike an emulsion, which is cloudy and eventually separates.

How much can be solubilized

Solubilization capacity depends on the surfactant, the substance and the conditions. Capacity generally increases with surfactant concentration, because more micelles are available, and depends on micelle size and shape, temperature and salts. Oily substances tend to sit in the micelle core; substances with both polar and nonpolar parts tend to sit among the surfactant molecules near the surface.

Surfactants with a high hydrophilic-lipophilic balance, roughly 15 to 18 on Griffin’s scale, are typical solubilizers.

Where solubilization is used

  • Cleaning, where micelles take up oily soil and help prevent it from redepositing; see detergency.
  • Formulation, to incorporate fragrances, essential oils and oil-soluble actives into clear water-based products.
  • Pharmaceuticals and agrochemicals, to deliver poorly soluble active ingredients.
  • Environmental remediation, where surfactant solutions help mobilize organic contaminants from soil and groundwater.

Frequently asked questions

Solubilization takes a substance into micelles, producing a clear, stable solution. Emulsification disperses it as larger droplets, producing a cloudy mixture that is only kinetically stable.

Very little. It depends on micelles, so it becomes significant only above the critical micelle concentration.

  • 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.

  • Critical micelle concentration CMC

    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.

  • Detergency

    Detergency is the ability of a cleaning solution to remove soil from a surface and keep it suspended so it does not settle back. It depends on wetting the surface, loosening oily and particulate soil, and holding that soil in the wash liquid.

  • Emulsion

    An emulsion is a mixture of two liquids that do not dissolve in each other, such as oil and water, in which one is dispersed through the other as small droplets. Most emulsions separate over time unless an emulsifier stabilizes the droplets.

  • Microemulsion

    A microemulsion is a clear, thermodynamically stable mixture of oil, water and surfactant, often with a co-surfactant, that forms spontaneously. Its oil and water domains are only nanometers to tens of nanometers across, much smaller than the droplets of an ordinary emulsion.

Sources

  1. solubilization, IUPAC Compendium of Chemical Terminology (Gold Book)
  2. Surfactants and Interfacial Phenomena, 4th edition, M. J. Rosen and J. T. Kunjappu, Wiley, 2012