Hydroxypropyl Cellulose–NaCl as an Aqueous Two‐Phase System: Insights From Variable‐Temperature NMR and PC‐SAFT Modeling

Aqueous two‐phase systems (ATPSs) are widely employed in various applications, including liquid–liquid separation and selective partitioning of high‐value biomolecules. This study investigates ATPSs comprising concentrated hydroxypropyl cellulose (HPC) and NaCl aqueous mixtures using a combination of variable‐temperature nuclear magnetic resonance and perturbed chain‐statistical associating fluid theory (PC‐SAFT). NaCl lowers the demixing temperature, leading to the formation of a single aqueous phase and a viscous semi‐solid precipitate composed exclusively of HPC. The demixing process involves subtle structural and dynamic changes in different regions of the HPC polymer and alters the mobility of Na + and Cl − ions; these experimental insights corroborate the thermodynamic predictions obtained from PC‐SAFT modeling. The proposed methodology provides a valuable framework for studying other types of ATPS.