I was thinking that it must be due to weaker hydrogen bonding in ethanol than in water. But then I learnt that Raman Spectroscopy and viscosity measurements
suggest that upto a certain ethanol concentration, there would be an increase in hydrogen
bonding with increase in ethanol concentration.
Why, then, does the surface tension decrease monotonously with increasing concentration of ethanol in water-ethanol binary mixture?
Best Answer
Let me first admit this is a guess on my part, and if it is obviously factually wrong to you, then I will delete this answer as incorrect and my apologies.
Have you considered the effects of the modification of other forces, apart from hydrogen bonding, on surface tension? In particular that the introduction of ethanol may reduce the Van den Waal's force more than the increased hydrogen bonding enhances it (if indeed it does, please see the second paragraph)?
Van Der Waal's Forces
Alcohol and Water
Unfortunately, this source does not go into further explanatory details, esp. regarding the hydrogen bonding aspect.
A video that might support this statement above, regarding the surface tension of an alcohol and water mixture, can be seen at Alcohol & Water Surface Tension, although it deals with the liquids separately, not mixed together.
How does surface tension relate to viscosity?