Chemical Engineering Science, Vol.61, No.9, 3009-3017, 2006
Effect of mixing protocol on formation of fine emulsions
Emulsions are usually stabilised with a mixture of surfactants with different hydrophilicity. The initial partitioning of surfactants between the dispersed phase and continuous phase, and how these phases are brought into contact, can significantly affect the emulsification processes. Dynamic-phase behaviour maps were prepared to allow for a systematic investigation of the effects of emulsification routes on emulsion properties. Six semibatch modes of additions with constant surfactant concentration across the routes were selected. For a target cyclohexane-in-water emulsion using a pair of polyoxyethylene nonylpheryl ether surfactants with a specified HLB and water volume fraction, fine droplets could form only if water dissolving the water-soluble surfactant was added to the oil dissolving the oil-soluble surfactant. This route allowed the transitional inversion to occur and as a result fine droplets were formed due to an ultra-low interfacial tension. The addition of water dissolving the water-soluble surfactant to oil dissolving the oil-soluble surfactant, direct emulsification method, produced by far large droplets because of a rather high interfacial tension. In a series of experiment, the semibatch direct and phase-inversion emulsification methods were assimilated in situ. The impeller location was used as a variable that controls which phase is added as the dispersed phase. The location of impeller in relation to the interface did not affect the emulsion drop size at a high agitation rate, but it did at a low agitation rate. Under low agitation speed and when the impeller was placed in the oil phase, the oil layer progressively, but slowly, dragged the water phase and eventually inverted to an oil-in-water emulsion, indicating that transitional-phase inversion has locally occurred in the oil layer. At a high agitation speed the mechanical energy provided by the impeller homogenised the emulsion instantaneously and did not allow the optimum formulation and the associated ultra-low interfacial tension to be reached regardless of location of the impeller. A high impeller speed increased drop size by transforming the transition inversion mechanism to a catastrophic mechanism under which the size of drops is mainly determined by the mechanical energy provided. This paper aims to show how some of the complexities involved in emulsification processes can be explained by consulting with dynamic-phase maps. (c) 2005 Elsevier Ltd. All rights reserved.