Spectroscopic and Thermodynamic Investigations of a Mixed Surfactant/Polymer System at the Oil-Water Interface
Steen, Collin J.
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Liquid-liquid interfaces are prevalent in many biological and environmental systems. Certain naturally-occurring molecules, known as surfactants, move to and remain at interfaces, thereby altering the surface chemistry of these systems. Polymers are another broad class of naturally-occurring molecules that are essential to many life and environmental processes. Specifically, polymers are known to interact with surfactants in unforeseen ways. Vibrational sum frequency spectroscopy (VSFS) and interfacial tensiometry were employed to discern the molecular level behavior, structure, and orientation of a mixed surfactant-polymer system at the oil-water interface. Cetrimonium bromide (CTAB), a ubiquitous cationic surfactant, was observed to quickly move to the buried oil-water interface, adopt a net orientation, and reduce the interfacial tension of the system. Polyacrylic acid (PAA), a common carboxylate-containing polymer, was shown to not be interfacially active at neutral pH. However, in a mixed system of CTAB and PAA, the molecular behavior and dynamics were noticeably different from that of either individual component. In the mixed system with CTAB, PAA was not immediately detected after interface formation. However, over time, carboxylate and methylene vibrational modes corresponding to the polymer appeared in the spectrum. This suggests that while CTAB moves to the interface immediately, PAA, despite not being interfacially active, gradually accumulates at the interface and is present and oriented within 24 hours. These findings will expand our existing knowledge of the structure and dynamics of surfactant-polymer systems at interfaces and may have implications for various environmental, biological, and industrial applications.