Why does Methanol have a higher surface tension than Ethanol? [closed]








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I am researching the surface tension of different alcohols and found something odd: the surface tension of Methanol was higher than that of Ethanol, which doesn't make sense to me. My understanding is that surface tension depends on the strength of intermolecular forces--Both alcohols have a hydrogen bond, but Ethanol has a stronger london-dispersion force(this is evidenced by the boiling/melting point), so shouldn't Ethanol have a higher surface tension? Well, I know that this is wrong, but I can't find a good explanation to why Methanol has a higher surface tension.


The only theory I currently have in mind is that the LDF of Ethanol somehow cancels out the strength of its hydrogen bond, but I am not sure if this is correct.




As I pointed out in my comment, the strengths of hydrogen-bonding may play a role in difference in the surface tensions between ethanol and methanol. Methanol is a smaller molecule compared to ethanol and certainly has stronger H-bonding comparatively.


This is because methanol has only one carbon atom in its main carbon chain but ethanol has two. This extra methylene group in ethanol in addition to methyl group of methanol $(\ce{CH3CH2})$ would be able to form London Forces with methyl groups in ethanol molecules, making the formation of hydrogen bonding within the molecules comparatively less prominent, thus can be considered less strong than that in methanol. In methanol, the strongest inter-molecular force between molecules are only hydrogen bonding.


Aside hydrogen bonding, the difference between activity coefficients of methanol and ethanol has playing bigger role according to published data (Ref.1). According to Ref.1:



> 
> The study of the influence of the activity coefficient on the surface tension behavior is of particular importance. [...] Taking the activity coefficient into account leads to a substantial decrease in the surface tension (except for the $\ce{KCl}$ electrolyte and the case of the $\ce{NaCl}$ electrolyte where the activity coefficient significantly increases the surface tension at high salinity).
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The activity coefficient of ethanol is $\gt 1$ while that of methanol is $\lt 1$ in ethanol-methanol binary system, according to Ref.2:


[![activity coefficients of EtOH & MeOH](https://i.stack.imgur.com/4DP7D.png)](https://i.stack.imgur.com/4DP7D.png)



> 
> Perhaps the most interesting of these three binaries is the methanol-ethanol system, which is generally assumed to be ideal. Figure 4 shows that the components when present in low concentration do not behave ideally. The activity coefficients of methanol at low methanol concentrations are less than 1.00, and the coefficients for ethanol at low ethanol concentrations are greater than 1. To support all of above arguements, the ref.3 states clearly that:
> The behavior of ethanol-methanol binary system is far from ideal because both the hydrogen bond donating and accepting strengths are seem not to be identical.
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**References:**


1. Philippe Leroy, Arnault Lassin, Mohamed Azaroual, and Laurent André, "Predicting the surface tension of aqueous 1:1 electrolyte solutions at high salinity," *Geochimica et Cosmochimica Acta* **2010**, *74(19)*, 5427-5442 (DOI: <https://doi.org/10.1016/j.gca.2010.06.012>).
2. H. H. Amer, R. R. Paxton, and Matthew Van Winkle, "Methanol-Ethanol-Acetone: Vapor-Liquid Equilibria," *Ind. Eng. Chem.* **1956**, *48(1)*, 142–146 (DOI: <https://doi.org/10.1021/ie50553a041>).
3. K. Nilavarasi, Thejus R. Kartha, and V. Madhurima, "Evidence of anomalous behavior of intermolecular interactions at low concentration of methanol in ethanol-methanol binary system," *Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy* **2018**, *188(1)*, 301-310 (DOI: <https://doi.org/10.1016/j.saa.2017.07.015>).


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