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:
Addition of methanol should decrease the surface tension because $\ce{MeOH}$ forms lower number of hydrogen bonds than pure water.
Sodium chloride addition should increase the surface tension because ion dipole interactions are stronger than dipole dipole interactions or H-bonds.
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.