Bulk pH-Mediated Changes in Interfacial Speciation and Hydration Structure of Propyl Paraben

Parabens are extensively used as broad-spectrum antimicrobial preservatives in food and personal care products. Bulk pH governs the ionization of phenolic OH of paraben molecules, which in turn decides the antimicrobial activity of the paraben. Notably, reduction in the antimicrobial activity of paraben molecules from acidic to neutral pH is observed, where the percentage ionization is just 3-4%, raising an important question about the underlying mechanism. Phenolic compounds such as parabens exhibit a pronounced affinity for the air-water interface, thus affecting the interfacial properties. To probe this behavior, we have employed vibrational sum frequency generation (VSFG) spectroscopy, a highly surface sensitive technique, to study the interfacial water structure around the propyl paraben molecules with varying bulk pH. VSFG data in the O-H stretch region revealed that water molecules around the paraben molecules (pH 5.6) result in strong hydrogen bonds. As the pH is changed to neutral biological pH (7.0-7.4), the interfacial hydration structure undergoes a dramatic transformation, shifting from symmetric strongly hydrogen-bonded water to asymmetric weakly hydrogen-bonded water. These changes have been ascribed to the partial dissociation of phenolic OH of the paraben molecules and possible ROH···RO^- complex formation at the air-water interface. The restructured interfacial hydration of the propyl paraben may have an influence on the antimicrobial activity of the paraben as well as its photodegradation efficiency. The experimental data on pH-dependent surface propensity suggest that the surface pK_a of propyl paraben is not significantly different from its bulk pK_a.