Impact of emulsifier microenvironments on acid–base equilibrium and activity of antioxidants

Mechanisms by which emulsifiers influence the activity of antioxidants in emulsions were investigated. To evaluate the importance of the partitioning between the different phases, the proportions of ferulic acid (FA) and isoferulic acid (iFA) at the o/w interface, in the coexisting micellar phase and in the aqueous phase were experimentally determined. An increase of the total emulsifier concentration and hence the micellar concentration led to an increased solubilisation of FA and iFA by the emulsifier pseudophases. From the point of view that the solubilisation sites of the antioxidants have different properties and thereby affect their activity, the microenvironment of each antioxidant was characterised in terms of local pH values and electrostatic potential. The degree of dissociation of both FA and iFA decreased in the order CTAB > SDS ≈ Brij. Information about the relative locations of the antioxidants was deduced from the extents to which the dissociation equilibria were affected by their environment. It was concluded that (1) the proximity to the polar headgroup of SDS, (2) a high degree of dissociation in a CTAB environment and (3) a deeper intercalation into the headgroup region of Brij were favourable for the antioxidant activity.