Tweaking Behavior of Hydrogen Bond Donor in Choline Chloride-Based Deep Eutectic Solvents for Regulating the Phase Transition of Poly(N-vinylcaprolactam): A Sustainable Medium for an Early Hydrophobic Collapse

Abstract

Deep eutectic solvents (DESs) are recognized as a "green" alternative to conventional ionic liquids and organic solvents owing to their specific properties. In this study, the influence of DESs containing choline chloride (ChCl) as a hydrogen bond acceptor (HBA) and urea, ethylene glycol (EG), and lactic acid (LA) as hydrogen bond donors (HBD) on the thermoresponsive behavior of poly(N-vinylcaprolactam) (PVCL) is investigated using various techniques. Spectroscopic investigations indicate biased interactions of the HBD group present in the DESs with a hydration layer of PVCL. Dynamic light scattering and temperature-dependent fluorescence spectroscopy results clearly show a decrease in the lower critical solution temperature of PVCL in the presence of the DESs. The hydrophobic collapse of PVCL in the presence of the DESs follows the order ChCl:urea > ChCl:EG > ChCl:LA. It is proposed that the presence of DESs does not interfere with the functional groups present in PVCL; however, it ruptures the hydrogen bonding between PVCL and the water molecules and destabilizes the water gradient around PVCL. The DESs have provided an alternative platform for low-temperature dehydration of PVCL, which can be useful as a drug carrier agent.

Temperature-Switchable Polymer: Uniting Deep Eutectic Solvents with Poly(N-isopropylacrylamide) and Poly(N-vinyl caprolactam)

“…To confirm that the changes in the absorption spectra of PNIPAM are only due to the variation in the ionicity of the solution in the presence of the DESs, the UV−visible spectra of the ANS− DES solution were studied, which clearly showed the absence of absorption peaks in the respective wavelength regions. 42 Therefore, this observation also supports that the variations in the absorption spectra are due to the indirect involvement of the DESs in changing the hydrogen bonding network around the polymer backbone. Steady-state fluorescence spectroscopy revealed an increase in the fluorescence intensity of the ANS− PNIPAM solution after adding the cholinium-based DESs.…”

“…Comparative dynamic light scattering data. (a) Volume percentages and (b) correlation functions for PNIPAM and PVCL aqueous solutions in the presence of 100 mM DESs at 25 °C.…”

“…Comparison of (a) size variations (at 30 °C) and (b) changes in LCST of PNIPAM and PVCL aqueous solutions in the presence of 100 mM ChCl/LA.…”

“…The water contents, percentage purities of the starting material, and successful characterizations of the synthesized DESs using FTIR spectroscopy have already been explained in our previous report. 42…”

“…The phase transition temperature of PVCL decreased to a greater extent than that of PNIPAM in the presence of similar DESs, which can be attributed to the presence of a more hydrophobic lactam ring in the former. 42 Overall, HBD groups and the polymeric backbone are two important factors for regulating the transition temperature in the presence of ChCl-based DESs.…”