I don't have the answer, but you can easily measure it yourself if you can find a refractometer.

It appears that self-association of some penicillins occurs in solution.

This paper describes physical characterization of penicillin V aggregates, and mentions a value for the refractive index increment of 0.082 kg/mol.

Article Self-Association of Penicillin V in Aqueous Solution

Here is another paper that gives about the same figure for aggregates of 3 other penicillins.

https://s3.amazonaws.com/academia.edu.documents/45140470/Self-Association_of_Amphiphilic_Penicill20160427-6270-1hlg8pr.pdf?AWSAccessKeyId=AKIAIWOWYYGZ2Y53UL3A&Expires=1534610350&Signature=jbhpVNv5ICQyaqulGXxpUjo6Zyw%3D&response-content-disposition=inline%3B%20filename%3DSelf-Association_of_Amphiphilic_Penicill.pdf

And this paper, if you can get it, probably has additional measurements:

Article Light scattering studies on micelle formation by some penici...

Sebastien,

check these links:

http://www.innovativesolution.it/amoxicillin.htm

http://www.chemspider.com/Chemical-Structure.31006.html

regards,

G

Interested in answers.

Thank you for you answers!

I think I am going to try to find a working differential refractometer!

Best,

Sébastien