Surface tension of pure water is 72 mN/m. But it varies with the addition of acids and bases. Why?
Cohesive forces are responsible for the surface tension. Within a liquid these forces on a molecule are equal in all directions, while at the surface the molecules do not have neighbors at all sides, thus creating surface tension. The cohesive force between water molecules are particularly strong as each molecule can make four hydrogen bonds to other molecules. Other substances may have fewer bonds depending on its molecular structure. Therefore a mixture of water with other liquids can reduce the cohesive forces among the molecules and thus the surface tension.
Thank you sir. Can you suggest any method to find the surface tension of liquid? can we get accurate surface tension from pendent drop method?
There are different methods to build a tensiometer. The pendant drop method requires goniometry. A more accurate method employs force measurements, the Du Noüy-Padday method (Padday, J. F., Pitt, A. R., Pashley, R. M., "Menisci at a free liquid surface: surface tension from the maximum pull on a rod",J. Chem. Soc., Faraday Trans. I, 71(10), 1919–1931, 1974). The claimed accuracy is 0.1 mN/m.
Dear Mr Sukamanch, Prof. Schluessel explained everything. However, I have some sugestion cheaper to measure surface tension of liquids. One possibility is to measure the contact angle. There is a good book about the subject. See the link: http://www.amazon.com/Contact-Angle-Wettability-Adhesion-Volume/dp/9004158642. You can use the ImageJ software with plugin ContactAngle.jar. See the link: http://rsb.info.nih.gov/ij/plugins/contact-angle.html. Another possibility for measurement of the surface tension of liquids is to count the number of drops that are falling from a burette. You need a liquid with surface tension known. Deionized water can be used for this. On the one hand, my suggestions are very simple, therefore the accuracy can be lower. On the other hand, these suggestions can give a good idea of the surface tension. If you need excellent accuracy is better to use an appropriate device. I hope these suggestions can help you.
Peter Schluessel why is that surface tension is greater in fluid system with W/EG 60:40 than 40:60 W/EG system?
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