Dissolution Dynamics of a Binary Switchable Hydrophilicty Solvent -- Polymer Drop into an Acidic Aqueous Phase

Abstract

Switchable hydrophilicity solvents (SHSs) are solvents defined by their ability to switch from their hydrophobic form to a hydrophilic form when put in contact with an acidic trigger such as $CO_2$. As a consequence, SHSs qualify as promising alternatives to volatile organic compounds during the industrial solvent extraction processes, as greener and inexpensive methods can be applied to separate and recover SHSs. Furthermore, because of their less volatile nature, SHSs are less flammable and so increase the safety of a larger-scale extraction process. In this work, we study the dynamics and in-drop phase separation during the dissolution process of a drop composed of SHS and polymer, triggered by an acid in the surrounding aqueous environment. From 70 different experimental conditions, we found a scaling relationship between the drop dissolution time and initial volume with an overall scaling coefficient $\sim$ 0.53. We quantitatively assessed and found a shorter dissolution time related with a decrease in pH of the aqueous phase or an increase in initial polymer concentration in the drop. Examining the internal state of the drop during the dissolution revealed an in-drop phase separation behavior, resulting in a porous morphology of the final polymer particle. Our experimental results provide a microscopic view of the SHS dissolution process from droplets, and findings may help design SHS extraction processes for particle formation from emulsions.