Hydrophilic solvent recovery from switched-on microdroplet dissolution

Switchable hydrophilicity solvents (SHSs) are a unique class of chemical compounds that can be switched between their hydrophobic and hydrophilic forms. The switchable characteristics allow SHSs to be used as emerging, green solvents for sustainable extraction and separation technology. In the production of polymeric microparticles from recycled plastics, SHSs are used to dissolve the polymer and then are switched to the hydrophilic form for separation from the generated polymeric microparticles. However, it is extremely difficult to fully recover the SHS residue from the mixtures. In this work, we will identify the key parameters that determine the level of the solvent residue during the switched-on dissolution of emulsion microdroplets. The SHS N,N-dimethylcyclohexylamine from solvent–polymer binary emulsion droplets was switched to the hydrophilic, water-soluble form, triggered by addition of an acid in the surrounding aqueous phase. By applying a sensitive detection method developed in this work, we compared the levels of SHS residue in polymer microparticles obtained under 30 different dynamical and chemical conditions for the switching processes. The quantitative analysis revealed that residue levels remained constant at varied addition rates and concentration of the trigger solution, but decreased with the increase in organic phase fractions or the decrease in the emulsion temperature. Trapped water in the drops during switched-on dissolution may have contributed to the high level of solvent residue. The understanding of the new possible mechanism for residual solvent reported in this work may help develop effective approaches for the recovery of switchable solvents in environmentally friendly separation processes.