Mechanisms of flocculation with poly(ethylene oxide) and novel cofactors

Abstract

Poly(ethylene oxide), a widely used flocculation agent, has its efficiency enhanced by various compounds known as cofactors. The cofactors are known to form an association complex with poly(ethylene oxide) (PEO). The mechanism of interaction of non-ionic poly(ethylene oxide) with the model cofactors tannic acid (TA), corilagin, polystyrene sulfonated sodium salt (PSS-Na) and folic acid (FA), was studied using photometric dispersion analysis (PDA). PEO–corilagin interactions were studied by isothermal titration calorimetry (ITC) and liquid state proton nuclear magnetic resonance ( 1H NMR). The stability ratios clearly show that TA, corilagin, PSS-Na, FA, PEO, as well as PEO/cofactors, do not flocculate microcrystalline cellulose (MCC) in the absence of salt. Isothermal titration calorimetry shows that PEO and corilagin do not associate at 30 °C in the absence of salt, in agreement with the PDA experiments. The 1H NMR results show no association between PEO and corilagin at room temperature in the absence of salt. The ITC and NMR results agree with gas phase PM3 Semi-Empirical Molecular Orbital Theory calculations which show that PEO/corilagin complexes do not form at room temperature in the absence of salt, because the entropy loss is larger than the enthalpy gain. These findings are surprising, because the complexation of PEO with cofactors is well-documented. The association between PEO and corilagin in the presence of salt (KCl) has been confirmed by 1H NMR. Consequently, salt is needed to induce MCC flocculation. It was found that there are two types of cofactors: those which cluster PEO in solution (clustering cofactors) and those which do not (non-clustering cofactors). The PEO/TA system flocculates MCC nearly instantaneously by the following mechanism: PEO/cofactor association, followed by PEO-clustering, adsorption and bridging. For the non-clustering cofactors, an induction time was clearly observed, and a new mechanism is proposed to explain their flocculation behaviour: surface-induced clustering coupled to association-induced polymer bridging.