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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.

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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

References:

  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.

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  • $\begingroup$ 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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