Isothermal titration calorimetry measurement of enthalpy changes in monodisperse oil-in-water emulsions undergoing depletion flocculation

Isothermal titration calorimetry (ITC) was used to measure enthalpy changes resulting from titration of sodium dodecyl sulfate (SDS) micelles into 2 wt% silicone oil-in-water emulsions containing monodisperse droplets of different diameter (0.5–1.0 μm) at 30°C. The SDS micelles increased the attraction between the emulsion droplets through a depletion mechanism, which led to flocculation when the surfactant concentration exceeded a critical concentration (CFC). The presence of emulsion droplets led to an exothermic contribution to the enthalpy of mixing when SDS micelles were injected into a SDS solution below the CFC, but to an endothermic contribution when the micelles were injected into a SDS solution above the CFC. Depletion flocculation was therefore entropically favorable, but enthalpically unfavorable. This study demonstrates that ITC can be used to provide valuable information about droplet–droplet interactions in flocculated emulsions.