287057 From Tailored Membranes to Crystal Design: How Physicochemical Properties of Polymeric Surfaces Affect the Nucleation of Organic Molecules
There is today an increasing interest in developing solid films acting as heteronucleants as a way to control the crystallization process of active pharmaceutical ingredients (APIs). While micro-fabrication methods have been developed so far to produce on lab-scale micropatterned supports with peculiar surface topography , here we describe routes for the production of macroscopic and handily films (membranes) from polymeric materials with a wide range of chemical functionalities. It is today well accepted that polymeric membranes used as heteronucleant surfaces can represent a promising alternative to metal, inorganic or biomaterials, since their structural and chemical properties are easily tunable by a variety of established preparation methods .
In this work, tailored membranes exhibiting different morphological characteristics and chemical functionalities were prepared by phase inversion technique from various polymers [poly(vinylidenefluoride-co-hexafluoropropylene), polyimide, sulphonated polyetheretherketone at different sulphonation degree, sulfonated polyetherethersulfone, polypropylene, polyethersulfone, cellulose acetate, polydimethylsiloxane]. These polymeric films, displaying specific surface patterning at the nanometric range (morphology characterized by SEM and AFM) and tailored physicochemical properties, were used as solid substrates for the heterogeneous nucleation of three representative APIs: acetaminophen, acetylsalicylic acid and glycine. The wide experimental investigation clarifies in which extent the chemical nature of the surface determines the possibility that a membrane acts as a nucleation-active substrate. It was found that, among various morphological parameters, roughness positively or negatively affects nucleation rate depending on the extent of interaction between the crystallizing solution and the surface (hydrophobic/hydrophilic behavior). In the case of hydrophilic surfaces, a significant fraction of interacting solute molecules is likely to accumulate at the solid-solution interface via adsorption, where molecular recognition events may induce partial realignment in the solute enriched layers (concentration polarization). In addition, molecules can further concentrate by physical entrapment in certain heterogeneous domains; in this case, increasing roughness would enhance the probability of molecular entrapment at the membrane surface, thereby leading to increased supersaturation level and accelerated nucleation rate. For hydrophobic supports, the Wenzel equation predicts an increased apparent contact angle at higher roughness; moreover, experimental results and theoretical analysis based on the Classical Nucleation Theory approach confirm a reduced tendency of organic molecules to nucleate .
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