Molecular Understanding of Interfacial Interactions in Anti-Biofouling, Oil-Water Separation and Pharmaceutical Crystallization

Abstract

Molecular behaviors at interfaces govern the separation and aggregation of materials in proximity to phase boundaries, mediating the properties of the materials. Understandings of interfacial molecular structures are therefore necessary for the development of functional materials with desired properties. In this thesis, polymer/water (solid/liquid) and oil/water (liquid/liquid) interfaces pertaining to different research fields were analyzed using an interface-sensitive technique: sum frequency generation (SFG) vibrational spectroscopy. Molecular mechanisms of antifouling materials, which can resist biomolecule and organism adhesion, were investigated first. SFG studies on antifouling zwitterionic hydrogels and amphiphilic polypeptoids revealed that the strongly hydrogen-bonded interfacial water plays a significant role in the antifouling performances of these materials. It was also found that the presence of hydrogen-bonding functionality profoundly impacts the surface hydration and antifouling activity of polypeptoid coatings (chapter 2). In addition to the hydrophilic/amphiphilic antifouling polymers, SFG has also been applied to study non-fouling hydrophobic slippery liquid-infused porous surfaces (SLIPS), which are constructed by infusing porous nanostructured solid substrates with lubricant oils that adhere strongly to the substrates. SFG analyses demonstrated that water molecules on SLIPS are very disordered. This finding provides in-depth understanding on the intermolecular interactions between the lubricant oil and a repelled liquid droplet, and how such interactions lead to the excellent liquid-repellent property and durability of SLIPS. (chapter 3). The extremely strong separation of oil and water observed on SLIPS is difficult to achieve in corn oil refining industry. The purification and collection of corn oil from water is severely hindered by the presence of surface-active species with emulsifying property in corn. Oil-in-water dispersion can then be produced, reducing the oil separation efficiency from water. Zein protein, the major storage protein in maize, can function as such an emulsifying agent due to its amphiphilic nature. With SFG characterization, the migration of zein protein molecules to the oil-water interface was observed, evidenced by the detected interfacial amide I group signal from the oil-water interface. The addition of three industrial surfactants into the zein-oil-water mixture was demonstrated to result in the decrease of the amide I group ordering at the oil-water interface. The disordering was also accompanied by corn oil coalescence, i.e., reduction of the emulsifying effect. Therefore, the zein protein-induced emulsification was closely associated with the ordered interfacial zein structure. The disruption of the protein ordering by surfactants contribute significantly to the enhanced oil-water separation efficiency (chapter 4). In addition to the antifouling applications, polymer surfaces were also applied to function as heteronuclei for controlling the crystallization of pharmaceutical compounds. A newly discovered solid form of the anti-diabetic drug: pioglitazone hydrochloride, was crystallized using halogenated polymer surfaces. This new solid form was revealed to be more thermodynamically stable than the marketed form (chapter 5). In summary, this thesis focuses on characterizations of various interfaces, which provide fundamental understandings of molecular mechanisms of antifouling, oil-water separation, and drug crystallization. Strong surface hydration and ordered interfacial water molecules are critical to the antifouling properties of hydrophilic/amphiphilic materials. In contrast, disordering of interfacial water molecules is closely associated with the extraordinary water-repellent property of hydrophobic SLIPS. Additionally, interfacial behavior of zein protein and surfactant greatly impacts oil-water separation. This thesis also highlights the necessity of thorough exploration of pharmaceutical solid forms.