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 mum) at 30 degreesC. 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. (C) 2001 Elsevier Science B.V. All rights reserved.