Recently i made 5 dilute solution of methyl orange in water, then measured their surface tension. results showed that surface tension increased by increasing the concentration. dose any body know the reason?
Methyl orange can form a Zwitterion ion (both positive and negatively charged groups on the same molecule). It also is a flat planar molecule with two hydrophobic aromatic rings which would like to remain on the water's surface, while the charged regions, would like to be solvated by the water, however these charge regions of the methyl orange molecule can form hydrogen bond with each other producing an interlinked elastic film on the waters surface and increasing the surface tension.
Have you verified your measurement set-up. Literature indicates a drop in surface tension for methyl orange with concentration, see attached paper.
I would suspect that these surface tension effects would be highly pH dependent, using the sodium salt of methyl orange (as in the above paper) will result in an alkaline solution, and decrease nitrogen protonation. A plot of surface tension versus pH would be very interesting.
Dear Philip G Penketh Thank you very much for your help. Actually, I didn't measure the pH of my solution because I thought there was no increase or decrease in that property since I didn't add anything else to the solutions except water and methyl orange. However, I will definitely check the PH as well.
According to my tests and measurements, I am sure that the surface tension of the samples varied from approximately 73 to 76 at room temperature as the solution concentration increased from 5ppm to 200ppm. I think your answer explained the reason perfectly. Dear Philip G Penketh May I ask you to help me with any books or references I can refer to? I would appreciate it.
Dear Paul Duineveld i appreciate your help. Actually, i checked every thing (except PH) and i am sure with the surface tension increasing.
Your referenced article was helpful, thank you so much.
Dear Elnaz Ghahremani, I'm not an expert on surface tension, but I do have a relatively good general knowledge of chemistry. I' don't know of a good text to recommend on the subject. However, this does look like a very interesting phenomena/observation to investigate.
Good luck. Phil
dear Philip G Penketh You have helped me a lot so far. Thank you very much for guiding me in this matter.
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