Colloidal complexation of a macromolecule with a small molecular weight natural polyphenol: Implications in modulating polymer functionalities

Natural polyphenolic compounds show interesting complexation behavior with various macromolecules due to their unique structural characteristics that enables molecular transition such as electronic delocalization and conjugation and intra- and intermolecular hydrogen bonding. Here, we report on the preparation and characterization of novel colloidal complexes (size range of 56-116 nm) based on the spontaneous interactions of a small molecular weight polyphenol (tannic acid) with an industrially relevant macromolecule (methylcellulose). The binding stoichiometry obtained from isothermal titration calorimetry suggested that 33 molecules of tannic acid were bound to one molecule of polymer. The values of ΔH (-11.4 kJ mol^-1) and ΔS (-35.5 J K^-1 mol^-1) suggested that the interaction was enthalpy driven and the relatively low value of ΔH further indicated the non-covalent nature (i.e. hydrophobic interaction and hydrogen bonding) of the interaction. Effects of this complexation on the functionalities of methylcellulose were investigated in terms of the loss of thermoreversible gelling (due to the irreversible association of colloidal complexes at high temperature), improvement of the emulsifying property (because of the interfacial localization of hydrophobic colloidal complexes further contributing to the gelling of the interfacial film) and enhancement of the foam stabilizing property (based on the absorption of colloidal complexes on the air-water interface and resulting enhancement of the interfacial stiffness due to surface gelation invoked by the colloidal complexes). These findings will be of wide interest to researchers and industrial scientists working in the field of polymer chemistry and material science, especially because methylcellulose is one of the most commonly used polymers for a range of industrial applications.