What happens to the surface tension of a solvent, say, water when we mix a solute in it? For the sake of concrete discussion, the examples that I encountered in a test were sodium chloride $(\ce{NaCl}),$ methanol $(\ce{MeOH}),$ and sodium alkyl sulphates $(\ce{ROSO3Na}).$

My guess is that since the surface tension can be defined as the force per unit length on a line drawn on the surface of the liquid, if the solute attracts $\ce{H2O}$ more than another $\ce{H2O}$ molecule, then the surface tension should increase.

Using this logic, I predict the following:

  1. Addition of methanol should decrease the surface tension because $\ce{MeOH}$ forms lower number of hydrogen bonds than pure water.

  2. Sodium chloride addition should increase the surface tension because ion dipole interactions are stronger than dipole dipole interactions or H-bonds.

  3. The $\ce{ROSO3Na}$ kind of confused me. I assume they intended for me to perceive it as detergent and I know that decreases the surface tension. They actually asked to recognize the appropriate plot of variation in surface tension with concentration and the graph I guessed attained a constant value after a certain concentration. I thought it might be due to micelle formation.

If anyone can confirm or deny my intuition, or point me in the direction of any data that would be helpful.


Davies and Rideal1 published the following graph. This graph nicely describes the action of various solutes on a solution.

enter image description here

It's clearly visible that:

  1. Addition of $\ce{MeOH}$ reduces the surface tension nonlinearly
  2. Addition of $\ce{NaCl}$ increases the surface tension linearly, but by a small amount
  3. Addition of a surfactant such as $\ce{RSO3Na}$ decreases the surface tension drastically, up until Critical Micelle Concentration is reached.

Smith and Gillham2 present a more detailed graph of the effect 1-butanol has on the surface tension of water.

enter image description here


  1. Hansen, Robert S. “Interfacial Phenomena (Davies, J. T.; Rideal, E. K.).” Journal of Chemical Education, vol. 39, no. 7, 1962, p. A552. doi:10.1021/ed039pA552.

  2. Smith, James E., and Robert W. Gillham. “Effects of Solute Concentration-Dependent Surface Tension on Unsaturated Flow: Laboratory Sand Column Experiments.” Water Resources Research, vol. 35, no. 4, 1999, pp. 973–82. doi:10.1029/1998WR900106.

  • $\begingroup$ Thank you. This is exactly what I needed. $\endgroup$ – Archimedesprinciple Jun 8 '20 at 9:14
  • $\begingroup$ You're welcome. You can accept the answer if it answers your question completely. $\endgroup$ – Aniruddha Deb Jun 8 '20 at 11:12

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