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 w t% silicone oil-in-water emulsions containing monodisperse droplets of diff erent diameter (0.5-1.0 mum) at 30 degreesC. The SDS micelles increased the attraction between the emulsion droplets through a depletion mechanism, wh ich led to flocculation when the surfactant concentration exceeded a critic al concentration (CFC). The presence of emulsion droplets led to an exother mic 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 flo cculation was therefore entropically favorable, but enthalpically unfavorab le. This study demonstrates that ITC can be used to provide valuable inform ation about droplet-droplet interactions in flocculated emulsions. (C) 2001 Elsevier Science B.V. All rights reserved.