Effects of polymer additives on the crystallization of hydrates: a molecular-level modulation.

The purpose of this study is to elucidate the effects of polymer additives on the growth behavior and final internal structure of hydrate crystals. The polymers used were poly(ethylene) glycol (PEG), ethyl(hydroxyethyl) cellulose (EHEC) and hydroxylpropyl methylcellulose (HPMC). Rheological characterization of the polymer solutions was carried out. Nitrofurantoin and carbamazepine were crystallized separately from a mixture of acetone and polymer/water solution through evaporative crystallization. The crystallization process and final crystals were characterized by light microscopy, XRPD and FT-IR spectroscopy. PEG showed a minor viscosity increase, but polymer network associations were found in the EHEC and HPMC solutions. With no additive, nitrofurantoin crystallized as a mixture of plate-like (monohydrate I) and needle-like (monohydrate II) crystals. All polymers affected the crystalline phase, and HPMC and EHEC also changed the crystal morphology from needle-like to dendrite-like. H-Bonding was found to exist between these polymers (O-H group) and nitrofurantoin monohydrate (CO== group), while this was not the case with carbamazepine hydrate. Additives were able to modify the crystal phase and morphology due to H-bonding and polymer network properties. We envisage that the improved understanding of additives' role provided by this study can be utilized when designing control strategies for the crystallization process.