The effect of temperature on in situ β-cyclodextrin inclusion complex aggregate films at liquid-liquid interfaces

The stability of β-cyclodextrin (β-CD)-oil inclusion complex (IC) interfacial films observed on emulsion droplets is enhanced by the binding affinity (or formation constant, K_f) of the 'guest' oil molecule to the β-CD cavity. Changes to the surrounding environment, such as temperature increase or dilution, are expected to shift the complexation equilibrium leading to interfacial film desorption/dissolution. It is postulated that the relative response of the films is due to the enhanced host-guest binding strength counteracting shifts to the complexation equilibrium that lead to destabilisation, thus providing a framework for carefully tuning emulsion stability.

Oil-in-water emulsions stabilised by β-CD were formulated for a series of oil types with increasing alkane chain length and structural complexity (providing a range of K_f values) across increasing β-CD concentrations. Emulsion behaviour during temperature increase was examined quantitively using ion-exchange chromatography and under direct observation in cryo-scanning electron microscopy (cryo-SEM). Interfacial shear rheology equipped with a double wall ring (DWR) geometry, was then used to examine the elastic response of the interfacial films to temperature increase.

β-CD ICs laden films showed a decrease in interfacial shear elastic modulus with increasing temperature indicating that the film desorbs/dissociates once a critical temperature is reached. This effect is diminished by increasing the β-CD concentration or by utilising oils of higher K_f value (eucalyptol > hexadecane > n-dodecane > n-decane). The corresponding improved emulsion thermal stability was further linked to a higher adsorption of available ICs to the interface, which retain their particle-like configuration upon heating within the range of temperatures tested.