Microemulsion and macroemulsion behaviour of systems containing oil, water and nonionic surfactant
In this thesis, attempts have been made to correlate some equilibrium properties of microemulsions with the formation and stability of macroemulsions. Studies have been mainly limited to water-in-oil (W/O) systems stabilised by pure nonionic surfactants of the poly-oxyethylene alkyl ether (CnEm) type. Initially however, a brief account is presented of the behaviour of W/O microemulsions stabilised by commercial nonionic surfactants of the type used in foods.A detailed study of the equilibrium behaviour of W/O microemulsions stabilised by tetra-oxyethylene mono-n-dodecyl ether, C12E4,in hydrocarbon oils is presented. Aggregates form above a certain surfactant concentration in the oil, designated the critical microemulsion concentration, cμc. Changes in the monolayer curvature induced through changing the temperature, salt concentration and the nature of the oil phase, are discussed in terms of an effective surfactant molecular geometry. The effect of solubilisation of protein into the dispersed aqueous phase is also presented.The corresponding W/O macroemulsions prepared with these systems at concentrations in excess of the cμc breakdown primarily by droplet sedimentation. There appears to be an optimum surfactant concentration at which maximum stabilisation is achieved. This is approximately equal to the cμc of the surfactant in the oil phase. Addition of microemulsion droplets to the oil dramatically increases the rate of oil phase resolution. For a fixed concentration of aggregated surfactant, the acceleration rate is rather insensitive to the size of the microemulsion droplets. These findings are reasonably consistent with a depletion flocculation mechanism in which microemulsions are assumed to behave as hard spheres and are excluded from the intervening region between approaching emulsion droplets.Microemulsion phase behaviour is also reported for oil phases containing different concentrations of medium and long chain length triglycerides (MCT, LCT) in heptane. The critical microemulsion concentration is observed to increase dramatically upon increasing the concentration of triglyceride. Additionally, the enthalpy change upon transfer of a mole of C12E4 monomers from the oil phase to the preferred microemulsion droplet curvature becomes significantly less exothermic as the concentration of MCT in the oil phase increases.The breakdown of W/O macroemulsions produced with these triglyceride systems is also consistent with a depletion flocculation mechanism. Emulsions prepared with 75% MCT as the oil phase undergo coalescence once an approximately closepacked droplet volume fraction is achieved. It is shown that as the cμc increases (through the addition of MCT or increasing the temperature), the stability of the emulsion dramatically decreases.Using a technique of time-resolved fluorimetry, the rate of exchange of probe molecules between microemulsion droplets has been determined. The exchange rate is always slowest at the solubilisation phase boundary where inter-droplet attractions are a minimum. The rate constants at this phase boundary are similar for aggregates formed in both heptane and in an equal volume ratio mixture of heptane : MCT. The possible relevance to the magnitude of the exchange rate constant of energies associated with bending the surfactant monolayer and desorbing surfactant from the droplet surface are considered.This technique has also been applied to three surfactant systems for which monolayer bending rigidities have previously been reported. Microemulsion exchange rates measured at equivalent positions on the solubilisation phase boundary increase as the monolayer rigidity decreases. The stability of the corresponding W/O macroemulsions is also observed to follow this order, the fastest water resolution rates being observed for the systems with the lowest monolayer rigidities.