Surface Tension and Surfactants. What’s Really Going On?

News

Posted on Tuesday 3 March 2026

For effective cleaning we add surfactants (in soaps and detergents) to lower the surface tension of water. Using a new theory, a York chemist can explain this effect more effectively, developing new insights, and hence understand why some surfactants are much better at this job than others.

The new theory. Surfactant molecules at the surface give a reduction in surface tension, while those assembled in the bulk solvent prevent it. Shimizu & Matubayasi have clarified this delicate balance with their novel theory for surface tension.

Why does surface tension decrease with increased surfactant concentration then hit a plateau? Even experts could not agree for a long time.

Seishi Shimizu (University of York) and Nobuyuki Matubayasi (Osaka University) created a new, assumption-free theory to explain the surface tension curve for all liquid solutions, including surfactants in water. They found: (1) The surface of the liquid is fully covered with surfactant molecules for much of the curve. (2) The reduction in surface tension comes from weakly-interacting surfactants that are increasing in number in the bulk. (3) Once these bulk surfactants self-assemble into micelles, their aggregation number causes the surface tension to plateau.

This new paper in Langmuir, providing new insight into such a long-established but frequently misunderstood effect, quickly became the journal’s “Most Read” article. Indeed, there are currently 3 Shimizu-Matubayasi articles in Langmuir’s ‘Top 10’, including, a novel theory for measuring surface area of materials, and a unified theory of adsorption isotherms. These papers all apply the innovative Shimizu-Matubayasi sorption theory, demonstrating exciting universality of this simple theory across wide-ranging phenomena in colloids, surfaces, materials, and liquids, and show how fundamental curiosity-led science has real-world applications.

Notes to editors:

This work has been published in Langmuir.