Surface science explainer
Exploring the Science Behind Water’s High Surface Tension: Structure and Properties of Water
Water's surface tension is about 72.8 mN/m at 20 °C, far higher than most everyday liquids (Vargaftik et al., 1983). The reason is hydrogen bonding: molecules at the surface lose bonding partners, so the liquid pays an energy cost for every bit of surface it creates.
Professor Heather Allen
Professor and Ohio State Distinguished Scholar, Ohio State University
Professor Heather C. Allen attended Saddleback College from 1989-1992, completing much of her undergraduate science and general education coursework before transferring to the University of California, Irvine in 1992. After her second year at Saddleback, she focused on the sciences, targeting chemistry as her major, winning a Saddleback College Science Scholarship Award to continue advancement toward her chemistry degree. She received her B.S. degree in Chemistry in 1993, and Ph.D. in Physical Chemistry in 1997 at the University of California Irvine working with Nobel Laureate Sherwood Rowland and Don Blake as an undergraduate, and as a graduate student with John Hemminger and Barbara Finlayson-Pitts. She continued her postdoctoral studies in 1997 at the University of Oregon advised by Geraldine Richmond. Following the Saddleback Science Scholarship, as a student and postdoctoral researcher, she was also recognized with and by the Fannie and John Hertz Fellowship, EPA, NSF Traineeship, and a NOAA Post-Doctoral Fellowship in Climate and Global Change.
Dr. Allen began her professorial career at Ohio State University in 2000, and has since been recognized for many research accomplishments, most recently the 2022 American Chemical Society Irving Langmuir Award in Chemical Physics for her cumulative work on liquids. Her prior awards include the Ohio State Distinguished Scholar Award, the Alexander von Humboldt Research Award from Germany, Alfred P. Sloan Research Fellow Award, Camille Dreyfus Teacher-Scholar Award, Fellow of the American Association for the Advancement of Science (AAAS), Research Innovation Award from Research Corp., NSF CAREER Award, and a Beckman Young Investigator Award.
Dr. Allen also has been recognized for several mentoring awards including the American Chemical Society National Award for Encouraging Women into Careers in the Chemical Sciences, an Empowered Woman Award from the City of Columbus, and an Ohio State Office of Minority Affairs Mentor Award. She is a Full Professor in the Department of Chemistry and Biochemistry, and in the Department of Pathology.
Research Overview
Dr. Allen’s research specialization is in interfacial phenomena, molecular organization, ions, ion pairing, and hydration at aqueous interfaces. Investigations of molecular organization and orientation, and chemical reaction mechanisms at gas - liquid, gas - solid, and liquid - solid interfaces are of interest. Laboratory experiments are designed to further understand organization of solvated ions and molecules such as water, lipids, and fatty acids and complexation at liquid surfaces and interfaces of biological and environmental relevance.
The Allen group utilizes and designs optical spectroscopic instruments and studies liquids and “soft” surfaces to understand fundamental processes. Applications range from ocean and atmospheric aerosol chemistry, thundercloud electrification, geochemistry, cell membrane organization, and lung lining surfactant systems.
Dr. Allen was first to design the broadband vibrational sum frequency generation (BB-VSFG) system specifically for use on liquid surfaces. (Anal. Sci. 2001, 17(11), 1325-1329; and Anal. Sci. 2001, 17(1), 137-139.) BB-VSFG, since simplified to one femtosecond amplifier, is now common-place in VSFG laboratories. VSFG is inherently interface-selective and provides molecular-level interfacial information that includes relative orientation and structure of the surface species, i.e. the surface signature.
In addition to VSFG, an array of analytical optical spectroscopic and interfacial tools is incorporated to fully understand the heterogeneous chemical processes at an interface. The Allen lab also has expertise in and uses Second Harmonic Generation, Polarized Raman Spectroscopy, Infrared Reflection Spectroscopies, Brewster Angle Microscopy and Surface Potential and Surface Tension Instrumentation.
Previous Allen group members have positions in industry, national laboratories, and academia: Battelle Corporation, Intel, American Regent, Procter and Gamble, Spartan Chemical, OSU Surface Science Facility, Syngenta, Nestle, PPG Technologies, KLA Tencor, Chemical Abstracts Service, Canadian Synchrotron Facility, United States Geological Survey (USGS), Naval Research Lab (NRL), Pacific Northwest National Laboratory (PNNL), National Center for Atmospheric Research (NCAR), and professor positions at Hebei University in Baoding, China, Ohio Wesleyan University, and University of Findlay.
Why does water have such a high surface tension?
Short answer: water molecules hold on to each other through hydrogen bonds, and a molecule at the surface has fewer neighbours to bond with. Creating more surface means breaking more of those bonds, so water resists it strongly.
Pure water at 20 °C has a surface tension of 72.75 mN/m, one of the highest of any common liquid (Vargaftik et al., 1983). Ethanol, by comparison, sits near 22 mN/m and most oils sit between 20 and 30 mN/m (Jasper, 1972). That gap explains why water beads on a waxed car while oil spreads across it.
The Droplet Lab Dropometer measures it from a pendant drop; clean water at room temperature should read close to 72 mN/m.
What causes surface tension, in plain terms?
Molecules in the bulk of a liquid experience cohesive forces from neighbours on every side, so those attractive forces balance out. Molecules at the surface have fewer neighbours and no liquid above them, so they feel a net inward pull.
The liquid responds by keeping its surface as small as possible. That is why free droplets become spheres and why a drop hanging from a needle has a predictable shape.
Surface tension, written γ, can be read as a force per unit length acting along the surface or as the energy needed to make one square metre of new surface. It is reported in mN/m, which is the same as mJ/m² and the older unit dyn/cm. The same idea applies at the interface between water and an immiscible liquid such as oil, where it is called interfacial tension. For a full walkthrough of the methods, see the surface tension measurement guide.
How does the structure of water raise its surface tension?
Each water molecule can donate two hydrogen bonds, one through each of its hydrogen atoms, and can accept two more through its oxygen. In the bulk liquid, molecules form a constantly shifting network of these bonds.
A molecule at the surface cannot complete that network, because as much as one-quarter of its potential partners are missing. The missing bonds are an energy cost, and surface tension is the measure of that cost per unit area.
Liquids held together only by weaker dispersion forces, such as hexane, pay a much smaller cost. Their surface tension is correspondingly lower.
How does water compare with other liquids?
Water value from Vargaftik et al. (1983). Other values are typical literature values at 20 °C from the Jasper (1972) compilation; published values for glycerol and ethanol vary by about 1 mN/m between sources.
| Liquid | Surface tension at 20 °C (mN/m) | Main attraction between molecules |
|---|---|---|
| Water | 72.8 | Hydrogen bonding |
| Glycerol | about 64 | Hydrogen bonding |
| Diiodomethane | 50.8 | Dispersion (strong, polarisable iodine) |
| Hexadecane | 27.5 | Dispersion |
| Ethanol | about 22 | Hydrogen bonding, short chain |
| n-Hexane | 18.4 | Dispersion |
Why does water's surface tension fall as it warms up?
Heat makes molecules move faster and breaks hydrogen bonds more often. With a weaker network, there is less energy lost by sitting at the surface.
The IAPWS reference correlation gives about 72.0 mN/m at 25 °C and about 58.9 mN/m at 100 °C (IAPWS, 2014). Near room temperature, that is a drop of roughly 0.15 mN/m for every degree Celsius.
In practice, a 5 °C swing in a lab changes the reading by close to 1 mN/m. Record the liquid temperature with every surface tension measurement.
What lowers or raises water's surface tension?
Surfactants lower it strongly. Soap molecules compete for surface sites and replace water molecules there, notably changing the hydrogen bonding network at the surface. The guide to surfactants covers this in detail, and Expérience 2 lets students measure it.
Contamination does the same thing by accident. If pure water reads well below 72 mN/m at 20 °C, suspect a dirty syringe, needle or container before you suspect the instrument.
Common salts such as sodium chloride raise it slightly, because those ions are depleted from the outermost layer of the water. Larger, softer ions such as iodide behave differently and can gather at the surface (Jungwirth and Tobias, 2006).
Which properties of water come from its surface tension?
- Round droplets. A free water droplet pulls into a sphere, the shape with the smallest surface area for its volume.
- Capillary action. In a narrow glass tube, water climbs the wall and forms a curved meniscus, because it is attracted to glass more strongly than to itself. How high it rises depends on surface tension, tube radius, density and contact angle.
- Walking on water. The surface can bend under a light load like an elastic membrane, which is how water striders stand on it.
- Wetting. Whether water beads or spreads on a solid depends on the balance of forces where the liquid and the surface meet, measured as the contact angle. Detergents improve wetting and help with cleaning by lowering the surface tension.
How do you measure surface tension of pure water?
The pendant drop method images a drop hanging from a needle and fits its outline with the Young-Laplace equation. Force methods, such as the Wilhelmy plate and du Noüy ring, measure the pull of the liquid on a probe.
Pure water is the standard check for any of these methods. If your instrument reads close to the reference value at the recorded temperature, your needle, optics and fit are working.
Droplet Lab's smartphone pendant drop method was validated against a benchtop instrument in peer review. The papers are listed in the validation library.
Expert Quote
A pendant drop gives you surface tension only when the Young-Laplace fit follows the whole outline of the drop and you control how old the surface is. Read a fresh drop of a surfactant solution too early and you are measuring the clock, not the liquid.
Where can you go deeper?
These pages cover the methods, standards and data behind this post.
- Buying guide: Best Tensiometer for Teaching Labs
- Glossary: Surface tension
- Classroom experiment: Exp 3: Surface tension of water & temperature effect
- Open dataset: Surface Tension of Common Liquids
- Tool: Free Contact Angle Tool
- Related post: How Surface Tension Lets Insects Walk on Water
Measure water's surface tension in your own lab
See how a pendant drop measurement works on a phone based instrument, from drop to number, in a short call.
Foire aux questions
What is the surface tension of water at room temperature?
About 72.8 mN/m at 20 °C and about 72.0 mN/m at 25 °C (Vargaftik et al., 1983; IAPWS, 2014). Always quote the temperature with the value.
Is surface tension a force or an energy?
Both. For a liquid, force per unit length (mN/m) and energy per unit area (mJ/m²) are the same quantity in different units.
Why does soap lower water's surface tension?
Soap molecules gather at the surface with their water-loving heads in the water and their oily tails pointing out. They replace water molecules at the surface, so fewer hydrogen bonds are broken.
Does salt change the surface tension of water?
Sodium chloride raises it slightly, because its ions are depleted from the outermost layer of the water. Some larger ions, such as iodide, gather at the surface instead (Jungwirth and Tobias, 2006).