By Maxier Acosta Santiago
While the recently discussed Cremer paper described efforts towards understanding the effect on the LCST of PNIPAM by salts of the Hofmeister’s series, Sakota’s article went somewhat deeper into the study of this phenomenon by taking in consideration molecular crowding. Sakota’s group decided to use a PEG polymer as a crowding agent. Crowding may affect the biding of anions from the Hofmeister’s series and the PNIPAM resulting in a change in the LCST. First, they studied the presence of PEG in a PNIPAM solution showing that the crowding agent reduces the LCST. Kosmotropic anions, that decrease the LCST, but chaotropes increase the LCST. For the Hofemeister series effect on LCST we can go back to Luis Prieto’s blog post and Cremer’s paper which explains this effect better.
When the article begins to look at the presence of PEG at different salt concentrations, they see a close correlation between the LCST and the Hofmeister series. Yet, for the chaotropes ClO4⁻ and SCN⁻, the presence of PEG lead to a larger increase in the LCST. From here they decide to apply different theories to explain the results. Within the examined theories they discuss around thermodynamics of the system. Their explanation evolves as follows, even if the organization via LCST of PNIPAM is not thermodynamically favorable, the overlapping excluded volumes of PEG and the PNIPAM particles increase the translational entropy of water molecules in solution, which makes the formation of the system possible.
Although this paper brings something new to the table to discuss (molecular crowding and LCST), I do have some concerns. When taking into consideration so many different factors like molecular crowding, salt, and the responsive system itself, we should look deeper into the behavior. To limit certain factors, they maintained certain constant concentrations throughout the paper. Yet, in the discussion, I feel they lacked additional experimental results or computational studies (using molecular and/or coarse-grained simulations) to support their theoretical thermodynamics analysis.